PROJECT SUMMARY How Is Rifting Exhuming the Youngest HP/UHP Rocks on Earth?

Syracuse U., Boston U., Columbia U., U.C. Santa Barbara, U. Texas Austin. Collaborators from Papua New Guinea, New Zealand, Australia, Canada, Germany

Intellectual Merit: At the Woodlark rift of eastern Papua New Guinea, an active oceanic spreading center terminates along strike into a continental rift. In this region of active and rapid continental extension, Late Miocene-Pliocene eclogites have been exhumed at cm/yr rates within the lower plates of metamorphic core complexes (MCCs). We propose a 5-year multidisciplinary project to address two key questions in continental dynamics: How does lithosphere rupture? How are HP/UHP terranes formed and exhumed? We will use eastern Papua New Guinea as a field laboratory to examine how the lithosphere has evolved petrologically, rheologically, and thermally during the transition from subduction to rifting and seafloor spreading. Geologic fieldwork will document the spatial distribution, temporal sequence, and kinematics of ductile to brittle fabrics in metamorphic, magmatic and sedimentary rocks of the conjugate rifted margins, the MCCs west of the active seafloor spreading rift tip, and the inferred youngest MCC (Dayman Dome). Analytical work will document the pressure–temperature–time–deformation evolution of exhumed rocks, as well as the melting conditions and sources that have given rise to volcanic rocks. A passive seismic deployment will image the crust and mantle beneath and north of MCCs where partially exhumed HP/UHP rocks may occur at depth. Rock physical properties modeling will integrate petrophysical lab measurements of HP/UHP field samples, electron back-scatter diffraction studies of lattice preferred orientations, and quantitative mineral-physical-properties calculations for determining seismic wave speeds of rocks. GPS surveys will provide direct measurements of horizontal strain rates associated with HP/UHP exhumation and microplate kinematics. Geodynamic modeling will numerically simulate MCC evolution and exhumation of HP/UHP rocks. All data will be integrated by tectonic synthesis to test end-member models proposed for the tectonic evolution of the region and address the two key questions above. The eastern Papua New Guinea HP/UHP terrane will be compared to similar terranes globally to assess whether exhumation mechanisms documented in this region can be applied to other HP/UHP terranes. Results will contribute to an improved understanding of the processes governing the origin, structure, composition, and dynamic evolution of continental building blocks. These processes include deformation localization within the crust, exhumation of UHP rocks, and the fundamental processes by which the Earth’s tectonic plates rift and rupture. Broader impacts: This collaborative study brings together the talents of a multidisciplinary group of seismologists, structural geologists, thermochronologists, petrologists, geochemists, geodesists and geodynamicists, and will provide significant research and training opportunities for postdoctoral scholars, graduate students and undergraduates. Women comprise a significant proportion of the team. Our research plan broadens the participation of underrepresented groups, with Papua New Guinea students and faculty participating in data acquisition, data processing, and data analysis. PIs will contribute to University of Papua New Guinea undergraduate curricula through lectures and short courses. Outreach activities include educating local communities about the volcanic and seismic hazards of their home area.

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PROJECT DESCRIPTION Table 1. Budget summary indicating annual and total project costs for each institution.

Institution Year 1 Year 2 Year 3 Year 4 Year 5 Total Syracuse Univ. $293,717 $377,881 $423,650 $280,279 $220,778 $1,596,305

Boston Univ. $33,075 $436,559 $419,476 $158,117 $145,569 $1,192,796 LDEO–Columbia Univ. $16,225 $52,877 $220,059 $241,913 $175,741 $706,815

UT Texas Austin $192,967 $124,765 $82,503 $18,900 $1,200 $420,335 UC Santa Barbara $67,352 $98,633 $66,173 $55,603 $57,333 $345,094

$603,336 $1,090,715 $1,211,861 $754,812 $600,621 $4,261,345 INTRODUCTION Statement of problems to be addressed

Eastern Papua New Guinea has long been heralded as one of the best natural laboratories on Earth to study the transition from continental rifting to seafloor spreading (Fig.1). The recent discovery of very young ultrahigh-pressure (UHP) and high-pressure (HP) rocks in the same area now makes this the best place on Earth to address two key questions in continental dynamics: How does lithosphere rupture? How are UHP terranes formed and exhumed?

The discovery of UHP polymorphs of silica (coesite; Chopin, 1984; Smith, 1984; Wang et al., 1989) and carbon (diamond; Xu et al., 1992; Dobrzhinetskaya et al., 1995) in collisional orogens such as the Alps, Norwegian Caledonides and Dabie Shan have led to a new paradigm in Earth sciences in which buoyant continental crust is subducted to mantle depths (> 100 km) and subsequently exhumed. More than 20 known or suspected UHP terranes have now been identified worldwide (see Chopin (2003) and Liou et al. (2004) for recent summaries). The number and volume of recognized UHP terranes imply that subduction and exhumation of continental crust has had a major effect on Earth’s evolution, including the exchange of material between the crust and mantle, the generation and collapse of mountain belts, the formation and recycling of continental crust, plate-tectonic motions and plate-boundary processes. The relative youth of this field of inquiry, however, means that there are many fundamental questions about UHP tectonics that have not been definitively addressed. The recent discovery of Late Miocene–Pliocene eclogite, including coesite eclogite, in Papua New Guinea (Baldwin et al., 2004a, 2005a; Monteleone et al., in press) provides a fabulous new opportunity to examine UHP processes in the context of active tectonics. Eastern Papua New Guinea as a Continental Dynamics Field Laboratory

The proposed research will attack to key questions in the active Australian–Woodlark plate boundary zone of eastern Papua New Guinea, where the youngest (8–2 Ma) eclogites known on Earth have been exhumed at plate tectonic (cm/yr) rates (Baldwin et al., 2004a; Monteleone et al., in press). The terrane has not yet been incorporated into a collisional belt and subjected to the usual array of obfuscatory syn- and post-collisional metamorphism, magmatism, and deformation. Thus, eastern Papua New Guinea offers the best field laboratory globally for studying HP/UHP formation and exhumation, and the best possibility for finding HP/UHP rocks at depth in the process of being exhumed to the surface.

Eastern Papua New Guinea is an active tectonic environment in which previously subducted continental crust can be studied in a transient plate-boundary zone where rifting of a subduction complex changes along strike into a sea-floor spreading system. One of the attractions of this region is the temporal and spatial variation in geological features and lithospheric architecture created by the westward propagation of the spreading center. The Australian–Woodlark plate

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boundary zone provides an unprecedented opportunity to study how lithosphere evolves petrologically, rheologically, and thermally during the transition from subduction to rifting and seafloor spreading. Collaborative Research Proposed

We propose a five-year, interdisciplinary, collaborative study involving a team of Earth scientists from the US, Papua New Guinea, New Zealand, Australia, Germany, and Canada, including seismologists, geodesists, structural geologists, thermochronologists, petrologists, geochemists, and geodynamicists that builds on existing geological and geophysical databases to resolve outstanding questions of first-order importance to plate tectonics. Geologic fieldwork will document the spatial distribution, temporal sequence, and kinematics of ductile to brittle fabrics in metamorphic, magmatic and sedimentary rocks of the southern (Pocklington Rise) and northern (Woodlark Rise) rifted margins, the metamorphic core complexes west of the active seafloor spreading rift tip, and the inferred youngest metamorphic core complex in the Dayman Dome. Analytical work will document the pressure, temperature, time, deformation (P-T-t-D) and geochemical evolution of these exhumed rocks. A passive seismic deployment will image the crust and mantle beneath the region of metamorphic core complexes and the region north of them where partially exhumed HP/UHP rocks likely reside at depth. GPS surveys will provide direct measurements of horizontal strain rates associated with HP/UHP exhumation, and critical information regarding the modern-day microplate kinematics in the Woodlark Basin. Geodynamic modeling will simulate metamorphic core complex evolution and exhumation of HP/UHP rocks. We will build upon previous studies of the region, by integrating new and existing data sets to document the along-strike variation in the lithospheric evolution of this transient plate boundary zone. Furthermore, the goals of this collaborative research dovetail with a recent IODP Mission Proposal that identifies the Woodlark Basin as an IODP target site (see letter of support from John Hopper).

TECTONIC SETTING, GEOLOGY AND GEOPHYSICS OF THE WOODLARK RIFT The present tectonic setting of Papua New Guinea is the result of rapid (10–11 cm/yr)

oblique Australian–Pacific plate convergence (Fig. 1). The Australian–Pacific plate boundary zone is complex, has considerable along-strike variation, and has been a site of microplate formation and rotation, short-lived subduction and intraplate deformation (Tregoning et al., 1998; Hall and Spakman, 2003; Wallace et al., 2004; 2005). Since ~6 Ma the Woodlark Basin spreading center has propagated westward (Taylor et al., 1995). The spreading ridge forms the plate boundary between the Australian plate and the Woodlark microplate, and at its western end is adjacent to the most rapidly extending (~20–40 mm/yr at 151.5°E) continental rift system on Earth (e.g., Taylor et al., 1995, 1999; Abers, 2001). Mutter et al. (1996) recognized geometrical similarities between continental breakup in the western Woodlark Basin and propagation of oceanic rifts within oceanic lithosphere. Results of a marine geophysical survey (Taylor et al., 1995) led Taylor et al. (1999) to propose that continental breakup in the Woodlark Basin proceeds by successive phases of rifting localization, spreading-center nucleation, and propagation, followed by a lateral jump of the spreading axis to the next site of localized rifting. Taylor et al. (1999) argued that continental breakup is fundamentally controlled by a “centerline” of rheologically weak continental crust. They further suggested that the locus of active rifting is in the process of jumping ~50 km southward to the next site of localized rifting within the Goodenough Basin.

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Figure 1. a) Tectonic and geologic map of eastern PNG after Baldwin et al. (2004a). Inset shows microplates of the Australian–Pacific plate-boundary zone. Eclogites occur in the core zone of the lower plates of Goodenough and Fergusson Island MCCs. White dashed line indicates Bruhnes chron. Abbreviations are: NBP=North Bismark Plate; SBP=South Bismark Plate, SP=Solomon Plate; D=Dobu volcano; DD=Dayman Dome; DI=D’Entrecasteaux Islands: G= Goodenough Island; F=Fergusson Island; N=Normanby Island; GB=Goodenough Bay;

GV=Goropu volcano; MB=Milne Bay; MI=Misima Island; ML=Mt. Lamington; MS=Moresby Seamount; MV=Mt. Victory; NB=New Britain;TI=Trobriand Islands; TP=Trobriand Platform; WI=Woodlark Island; WR=Woodlark Rise; PR=Pocklington Rise; OSFZ=Owen Stanley Fault Zone; HP=Huon Peninsula; PUB=Papuan Ultramafic Belt; SI = Solomon Islands; Sub=subduction; OC=oceanic crust, age as indicated; seds=sedimentary rocks; GS=greenschist; BS=blueschist; EC=eclogite; AM=amphibolite. b) Cross section from Little et al. (2004). Hangingwall stratigraphy after Fang (2000).

The Cenozoic evolution of the Australian–Woodlark plate-boundary zone involved northward subduction of the Australian passive continental margin beneath a Late Paleogene–Early Eocene island arc built on oceanic lithosphere (Davies and Jacques, 1984). This collision led to HP metamorphism (Worthing, 1988; Davies and Warren 1992; Hill and Baldwin 1993) of Jurassic–Cretaceous sediments and basalts (i.e., Owen Stanley metamorphic rocks), and southward obduction of oceanic crust and mantle (i.e., the 16–21 km-thick ophiolite of the Papuan Ultramafic Belt; Davies, 1971, 1980). The Papuan Ultramafic Belt is regionally extensive (Davies and Smith, 1971; Davies, 1980) and is well exposed west of 149°E, where the bounding Owen Stanley Fault system intersects the coast. Its continuation to the south and east remains controversial. The presence of Eocene dolerite and gabbro in drill cores as far east as 152°E (Monteleone et al., 2001) has led some to suggest that the entire rift is overlain by 15 km of mafic and ultramafic rock (Martinez et al., 2001), but rocks recovered from the drill core are geochemically distinct from the Papuan Ultramafic Belt (Brooks and Tegner, 2001). Recent

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crustal tomography (Ferris et al., 2006) and dispersion patterns of correlated seismic noise (Ferris and Abers, 2006) do not show any evidence for the expected high velocities.

Metamorphic Core Complexes (MCCs) of eastern Papua New Guinea Fergusson and Goodenough Islands MCCs. The Fergusson and Goodenough Island MCCs

(Davies and Warren, 1988; Hill et al., 1992), along with Normanby Island, comprise the D’Entrecasteaux Islands and are located in a zone of extension west of the Woodlark Basin spreading center (Fig. 1). They consist of a lower plate of complexly deformed gneissic domes (up to 2.5 km a.s.l.) separated from an upper plate of largely undeformed mafic and ultramafic rocks by shear zones and detachment faults. On Fergusson and Goodenough Islands, metamorphic basement can be divided into two structural zones: A core zone that consists of eclogites, migmatites, gneisses and mylonitic rocks; and an outer shear zone (Hill 1994). Although structural fabrics in the core zone are highly variable, an early deformation event (D1) resulted in gneissic layering that predates peak metamorphism.

Mafic eclogites preserve peak HP/UHP mineral assemblages and occur as variably retrogressed lenses/layers in quartzofeldspathic gneisses and dikes that crosscut the D1 gneissic fabric (Davies and Warren, 1992; Hill and Baldwin, 1993; Monteleone and Baldwin, 2006). Major- and trace-element abundances are comparable with average MORB (Davies and Warren, 1992). The integrated field, structural and P-T-t data sets indicate that mafic eclogites and their enclosing felsic gneisses underwent eclogite-facies metamorphism from 8–2 Ma, and have been subsequently exhumed at plate-tectonic (cm/yr) rates (Hill, 1994; Baldwin and Ireland, 1995; Baldwin et al., 2004a; Little et al., 2005; Monteleone et al., in press). Raman spectroscopy confirms the presence of coesite in a 7.9 Ma eclogite from Fergusson Island (Baldwin et al., 2005a, b, in prep.; Fig. 2) indicating that some rocks were subducted to mantle depths (~100 km) and subsequently exhumed.

A second generation of deformation (D2) postdated peak metamorphism and resulted in folding and boudinage of the gneissic layering and formation of retrograde fabrics associated with development of the outer shear zone. The earliest retrograde fabrics formed during amphibolite-facies shearing (7–11 kbar, 570–730°C; Hill and Baldwin, 1993). Plio–Pleistocene cooling and exhumation histories of the core zone and shear zone rocks are constrained by 40Ar/39Ar muscovite, biotite, and feldspar, and apatite fission-track and (U-Th/He) ages (Fig. 3; Baldwin et al., 1993; manuscripts in prep.). Exhumation was facilitated by km-scale mylonitic shear zones.

Normanby Island MCC. Recent field and analytical data indicate that Normanby Island is a MCC containing two distinct regions with exhumed lower plates and a central region of relatively unmetamorphosed upper-plate rocks and sediments (Fig. 3). Northwestern Normanby Island is a high-grade domal core of quartzofeldspathic gneisses that was rapidly exhumed in the Pliocene. Its lithologic evolution is comparable to the lower plates on Fergusson and Goodenough Islands, although mafic eclogites have not yet been identified within Normanby’s quartzofeldspathic gneisses. Central Normanby Island comprises an upper plate consisting of serpentinized ultramafics with possible correlations with the Papuan Ultramafic Belt. The serpentinized ultramafic rocks are intruded by granodiorites and volcanics (Davies 1973). 40Ar/39Ar analyses of K-feldspar, biotite, white mica, and hornblende from the central and northwestern regions document volcanism, granodiorite intrusion, and exhumation-related cooling of felsic gneiss from ~2.2–1.8 Ma (Monteleone et al., 2006). Granodiorite emplacement and rapid cooling is synchronous with granodiorite intrusion on Fergusson and Goodenough Island (Baldwin et al., 1993).

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Figure 2: a) Photomicrograph of eclogite showing ~150 µm diameter SiO2 inclusion. Partial transformation to palisade quartz accompanied radial fracturing of its omphacite host grain. b) Five Raman spectra for this bimineralic SiO2 inclusion yielded diagnostic Raman bands (Liu et al., 1997) for coesite and quartz. c) Tera–Wasserburg plot with in situ U/Pb ion probe results. Individual analyses are plotted with 1σ errors. Regression (solid line) of U/Pb data, uncorrected for common Pb, indicates a 238U/206Pb age of 7.9 ± 1.9 Ma (2σ; MSWD= 9.2). Red lines indicate the 2σ error envelope on the regression. Figures are from Baldwin et al. (in prep.) and Monteleone et al. (in press).

Currently located < 30 km from the active seafloor spreading rift tip, the Prevost Range of eastern Normanby Island has a fundamentally different P-T-t history and geomorphology than the Fergusson and Goodenough Islands lower plates. The Prevost Range MCC consists of an elongate dome of mafic, calcareous, and pelitic schist bounded by continental transform faults on both sides (Little et al., 2004; in press). The lower plate of this north-vergent MCC has been uplifted as much as ~1100 m above sea level and exposes a > 1 km thick carapace of blueschist-derived mylonites that have a subhorizontal top-to-the-NNE shear fabric. Blueschists within the Prevost Range were exhumed within mylonitic shear zones from ~20 km depth. Temperatures were 400–500˚C during deformation and cooled below ~350°C by 3 Ma (Monteleone et al., 2006; Little et al., in press). Onshore Normanby Island, the variably back-tilted, inactive detachment fault capping these mylonites has been eroded, but its corrugated geomorphology is preserved in the landscape of the asymmetric dome. Corrugations and stretching lineations throughout the Prevost Range are parallel to Woodlark–Australia plate motions from 3.6–0.52 Ma (Fig. 3). The lower plate of this MCC was likely unroofed via a rolling-hinge mechanism, with active half-grabens offshore to the north indicating present-day activity on this detachment system. North of the island, the detachment fault is preserved as a ~12° N-dipping scarp on the seafloor. The northern, submarine part of the Woodlark rift contains several active half-grabens, and has been the site of several > Mw 6.0 earthquakes on planes dipping 23–30° N. Focal depths in this part of the rift are < 8–9 km (Abers et al., 1997). A plate-motion change at 0.5 Ma reactivated the bounding continental transform faults on either side of the MCC as oblique normal faults (Little et al., in press).

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Figure 3. Structural map of the D’Entrecasteaux Islands (after Little et al., 2004) and temperature-time plots for selected localities from Goodenough and Fergusson Islands. Multiple chronometers suggest that exhumation of the Goodenough Island MCC occurred later than exhumation of the Fergusson Island MCC. Selected 40Ar/39Ar spectra shown for Normanby Island illustrate the range of ages obtained for variably recrystallized white mica. Note that these ages overlap with U-Pb zircon ages from Goodenough and Fergusson Islands. Age spectra for volcanic and intrusive rocks on Normanby Island document magmatic activity coeval with exhumation of the D’Entrecasteaux MCCs.

Dayman Dome MCC. The Suckling–Dayman Dome of the Papuan Peninsula provides the clearest geomorphic expression of recent faulting and MCC formation in eastern Papua New Guinea (Fig. 1). The dome is > 50 km long, reaches ~3.7 km a.s.l., and occupies the footwall of a possibly active detachment system, the southern Owen Stanley fault zone (Davies and Warren, 1988; Mann et al., 2004). Metamorphosed basalt in the footwall grades from prehnite-pumpellyite-facies through pumpellyite-actinolite-facies with some high P/T minerals (lawsonite, stilpnomelane and sodic amphibole), to greenschist-facies and includes some contact metamorphic rocks close to the granite and monzonite intrusions (Davies, 1980). Preliminary field observations by N. Dazcko indicate that metamorphic rocks of the Dayman Dome are

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largely epidote-amphibolite and greenschist-facies with local glaucophane and garnet. It is inferred that these rocks are retrogressed blueschists and that greenschist-facies exhumation fabrics have largely overprinted the earlier assemblages. The overprint is most extensive near shear zones and more pristine blueschists may occur at highest elevations within the dome.

Adjacent sedimentary rocks exposed on the Papuan Peninsula and at shallow depths in Goodenough Bay (Fig. 1) likely represent deposits of a Late Miocene(?)–Holocene supradetachment basin that was transferred to and elevated with the footwall block (Mann et al., 2004). These rocks, originally mapped and dated by Davies and Smith (1971), are well exposed across a broad, deeply incised coastal zone. Holocene reef studies have shown that the most rapid surface uplift (~3 mm/yr) occurs on the footwall of the Owen Stanley fault zone near Goodenough Bay (Mann et al., 2004). Surface uplift rates decrease eastward toward the tip of the peninsula. The exhumed fault surface defining the flank of the Dayman Dome contrasts with the more subdued, heavily-eroded topography of the D’Entrecasteaux Islands (Fig. 4).

Figure 4. SRTM topography of the Dayman Dome and Goodenough Island. Dayman Dome is an incipient MCC with retrogressed blueschist-facies rocks in the footwall and is characterized by the spectacular exposure of an active low-angle normal fault.

Regional GPS surveys (Tregoning et al., 1998) show that about 20 mm/yr of NNW–SSE opening is presently occurring across the eastern Papuan Peninsula and D’Entrecasteaux Islands. This opening direction is consistent with roughly E–W-oriented normal faults observed in offshore areas (Mutter et al., 1996; Fang, 2000), normal-fault earthquake focal mechanisms (Abers et al., 1997; Ferris et al., in prep.), and possible oblique striations observed on the exhumed fault surface exposed across the Dayman Dome.

Misima Island MCC. Along the southern rifted margin of the Woodlark Basin (i.e., Pocklington Rise) recent studies confirm that Misima Island, is also a MCC with top-to-the–NE sense of shear (Figs. 1 & 5; Peters et al., 2004, 2005, dissertation in progress). A normal fault dips ~25° to the NE and is the dominant structural feature on Misima Island, splitting it into three distinct geological domains. Amphibolite-facies felsic and mafic gneisses in the lower plate are juxtaposed against greenschist-facies schists and overlying unmetamorphosed conglomerates, volcanics, and other sedimentary rocks in the upper plate. Separating these two domains is a > 3 m-thick layer of fault gouge. The measured NNE slip direction on the fault is parallel to the Australian–Woodlark plate vector between 500 ka and 3.5 Ma calculated from seafloor spreading data (Taylor et al., 1999), showing that the development of this structure was directly related to extension of the Woodlark rift. A 2–3 km thick zone of gneisses beneath the fault has NNE-trending stretching lineations commonly defined by elongate hornblende crystals, and a shallowly dipping foliation sub-parallel to the overlying fault plane. Structurally deeper rocks farther to the west retain E–W trending lineations on a sub-horizontal foliation, which we

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interpret as an older, collisional fabric. Garnet amphibolites from the lower plate yielded 40Ar/39Ar amphibole and mica ages interpreted to record cooling and exhumation from ~13–8 Ma. Quartz crystallographic preferred orientations show that deformation beneath the detachment involved a component of simple shear parallel to the lineation with a top-to-the-NE sense-of-shear. High-temperature quartz and feldspar deformation mechanisms, and minimal retrogression suggest that the lower plate tectonites preserve an early snapshot of extensional fabrics in the lower crust. Recent surface uplift of the island is indicated by six coral reef terraces preserved in the geomorphology on the south side of the island. Exhumation commenced prior to the earliest record of seafloor spreading, but the detachment fault may still be active.

Figure 5. Simplified structural map of Misima Island from Peters (dissertation in progress). Three structural domains are documented: 1) Older, high-grade fabrics associated with contraction; 2) Younger detachment fault zone associated with top-to-the-NE normal sense of shear; 3) Variably metamorphosed upper-plate rocks preserving fabrics associated with contraction. Compiled thermochronologic data for selected samples show that cooling and exhumation of metamorphic tectonites on Misima Island predate the formation of the oldest known oceanic crust (~ 6 Ma) in the Woodlark Basin.

Magmatism associated with MCCs. Post-Miocene volcanoes erupted throughout the region, with a general younging of volcanism to the west, culminating in the historic eruptions of Mts. Lamington, Victory and Goropu on the mainland. Most of the post-Miocene volcanism has been calc-alkaline, ranging from basalt to rhyolite, and recording trace-element and isotopic characteristics typical of subduction-zone magmas (Hegner and Smith, 1992; Stolz et al., 1993). Igneous activity is related spatially and temporally to the MCCs on Goodenough and Fergusson Islands (Figs. 1 & 4; Hill et al., 1995). An active volcano (Goropu) sits astride the boundary fault of the Dayman Dome, and another (Dobu) protrudes from Dawson Strait between Fergusson and Normanby Islands. Pleistocene volcanic rocks on Lusancay Island (Trobriand Platform, Fig. 1) appear to have originated as eclogite melts (Smith et al., 1979; Hasche and Ben Avraham, 2005), and abundant Pliocene high Mg# andesites (e.g., adakites) in the D'Entrecasteaux Islands (Hegner and Smith, 1992) may also originate as eclogite melts in the mantle. Volcanic ashes drilled in the Woodlark Basin shifted in composition from calc-alkaline to peralkaline during the past several 100 ka, (Lackschewitz et al., 2003). This shift, also recorded in Fergusson Island

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volcanics (Smith, 1976; Stolz et al., 1993) has been interpreted as a transition from subduction-modified sources to extensional magmatism in the region.

Summary of recent findings concerning the evolution of eastern Papua New Guinea MCCs

Relict HP/UHP mineral assemblages occur within the lower plates of the eastern Papua New Guinea MCCs. Eclogite-facies metamorphism occurred from 8–2 Ma. Exhumation of MCC lower plates was diachronous from east to west, occurring prior to, and synchronous with seafloor spreading in the Woodlark Basin. Mylonites preserve top-to-the-north shear indicators with stretching lineations parallel to Plio–Pleistocene plate motion. These results are significant in that they document that the youngest HP/UHP terrane known on Earth has been exhumed at plate-tectonic rates in an area of rifting west of the active Woodlark Basin seafloor spreading system. Geophysical Characterization

Crust and Mantle Structure. Regional gravity surveys indicate overall compensation of high topography in the Papuan Peninsula, but with large, positive gravity anomalies along its NE side (St. John, 1967, 1970). Related work (Milsom, 1973) showed that the Eocene(?) Papuan Ultramafic Belt, mapped as an ophiolite (Davies, 1971), includes a 15 km-thick, NE-dipping section of mafic and ultramafic rock between 147°E and 149°E. Crustal refraction confirmed a high-velocity upper crust in this region and provides the only direct constraints on Papuan Ultramafic Belt crustal structure (Finlayson et al., 1976; 1977). Since independence from Australia in 1975, the amount of large-scale geophysical data collection in Papua New Guinea dropped precipitously, with some exceptions. For example, Abers and Roecker (1991) simultaneously inverted travel times for structure and hypocenters, and (Hall and Spakman, 2002) could find no evidence for a high-velocity subducting slab beneath the Peninsula or Woodlark Rift using teleseismic imaging.

Seismicity. Early investigations based on a very sparse local seismic network and global catalogs show that shallow seismicity follows the Woodlark Rift and D’Entrecasteaux Islands on a E–W trend. There is some seismicity 100–200 km deep beneath the Papuan Peninsula but only NW of 9°S, 148°E (Denham, 1969; Johnson and Molnar, 1972; Ripper, 1982); the origin of these earthquakes remains unclear. There is some suggestion that they define the southern limb of a doubly dipping slab of Solomon Sea oceanic lithosphere subducting southward beneath New Guinea (Cooper and Taylor, 1987; Pegler et al., 1995). However, most earthquakes form a narrow pencil near 150 km depth, with no clear dip, and may relate to NE-dipping subduction beneath the south coast of the Peninsula (Abers and Roecker, 1991; Johnson and Jaques, 1980).

Few, if any, shallow earthquakes can be clearly associated with the thrust systems on either side of the peninsula. Most shallow activity is associated with a well-developed series of rift-related normal-faulting earthquakes that follow the trend of Woodlark seafloor spreading from the rift tip at 152°E to 148°E (e.g., Abers et al., 1997; Weissel et al., 1982). While the exact locations of these events suffer from poor monitoring (the ISC catalog is probably complete only above mb = 5.2; Abers, 2001), the majority of events show normal faulting with N–S slip vectors, following the trend of the spreading axis, the southern margin of Goodenough Bay, or the northern flanks of Dayman Dome (Abers et al., 1997). Thus, extension is active throughout the region of MCCs, and it is possible that the MCCs are still being exhumed along active faults. Indeed, the D’Entrecasteaux Islands region is one of the only places on Earth where normal-faulting earthquakes repeatedly show slip on planes as low as 25–30°, and are thus critically relevant to the low-angle normal fault debate (Abers, 1991; Abers et al., 1997).

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1999–2000 Imaging Experiment. The first seismic imaging since the early 1970’s was conducted in 1999–2000 as WoodSeis2000, a joint PASSCAL and OBS project centered on the rift tip and eastern D’Entrecasteaux Islands (Figs. 6 & 7; Abers et al., 2002). Although only five broadband seismographs were deployed, the experiment demonstrated the ability of modern instrumentation to be deployed in this logistically difficult setting, and provides several critical results:

a. The crust thins toward the center of the rift, from 30–35 km thick beneath the end of the Papuan Plateau to 20–25 km beneath the D’Entrecasteaux Islands, and then thickens again farther north. This thinning is greatest where the upper crust has been unroofed over the MCCs, suggesting that thickening of the lower crust (for example by flow) is insufficient to compensate the tectonic denudation (Abers et al., 2002).

b. The mantle shows slow P-wave velocities, and hence high temperatures, also beneath the axis of the D’Entrecasteaux Islands and spreading center (Fig. 7). Gravity anomalies indicate that the resultant mantle buoyancy roughly compensates the crustal thinning. The overall picture is one of a thinned, probably removed, mantle lithosphere extending at least 150–200 km west of the region of seafloor spreading, indicating that mantle spreading may at least partly precede crustal rifting (Abers et al. 2002).

c. The crust shows continent-like velocity structure except within 25–50 km of the oceanic rift tip (Fig. 6). Thus, thick sequences of ophiolites or mafic intrusions cannot make up much of the crust around the D’Entrecasteaux Islands. An enigmatic layer of Vp = 7.2–7.5 km/s, intermediate between crust and mantle values, underlies the receiver-function Moho beneath much of the D’Entrecasteaux Islands (Ferris et al., 2006).

Figure 6. A) P velocity, and B) Vp/Vs at 23.5 km depth from inversion of local earthquake travel times (Ferris et al., 2006). Dashed line shows limit of resolved structure. High Vp, low Vp/Vs belt extending from oceanic rift tip WNW is inferred locus of crustal thinning associated with rift propagation.

d. Most microearthquakes lie along a single fault system, extending WNW from the rift tip to the north flanks of the Fergusson Island MCC (Fig. 8). The simplest interpretation is that the dominant structure lies north of the MCCs or along their northern bounding faults, similar to what has been inferred from sparse teleseismic data. Some secondary seismicity is distributed south, around Goodenough Bay, indicating that secondary structures may become more important westward. Four intermediate-depth earthquakes (out of > 2100) have been observed where Trobriand Trough subduction might be expected.

e. The geometry of this network (land stations at 11 sites, 6–12 ocean bottom seismometers) precludes making definitive statements about seismicity or structure on the Papuan Peninsula or anywhere west of Goodenough Island. Clearly, a transition from continental crust to ophiolite must exist. Also, the upper 5–8 km of crust beneath Goodenough Bay could not be imaged except though novel techniques relying on correlation of seismic noise (Ferris and Abers, 2006), making it difficult to definitively test the density-inversion hypothesis of Martinez et al. (2001).

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In summary, the WoodSeis2000 experiment provided the first information about the subsurface at tectonically useful resolution, and showed that the D’Entrecasteaux Islands strongly resemble a continent in the last stages of rifting. However, the limited resolution obscures the relationship between the Moho, mantle structures and crustal shear zones. Also, compared with the present proposal, the coverage is centered on the rift tip at 151.5°E, east of the MCCs, and thus provides little data on deep structure beneath where the HP/UHP rocks were discovered.

Figure 7. Imaging results from WoodSeis2000 passive seismic experiment. Inversion of teleseismic data for mantle velocity perturbation, at 55 km depth, and crustal thickness from receiver functions (colored circles, key) (Abers et al., 2002). Note slow, presumably hot mantle beneath D’Entrecasteaux Islands, where crust is thinnest.

Figure 8. Seismicity from joint relocation with velocities, WoodSeis-2000 (Ferris and Abers, in prep.). Map: triangles show stations; dashed lines show estimated detection limits as function of magnitude (e.g. ML=1.5 detected only within second contour). Most seismicity follows a narrow WNW-trending band from the rift tip.

ENDMEMBER MODELS FOR THE TECTONIC EVOLUTION OF EASTERN PAPUA NEW GUINEA

Recent findings, summarized above, have prompted us to re-evaluate previously proposed models for the tectonic evolution of the region. Is continental lithosphere rupturing here due to back-arc rifting (e.g., Honza et al., 1987), or because a paleo-subduction zone is being unzipped via microplate rotation (Baldwin et al., 2004b; Webb et al., 2005)? Below we briefly describe these two models, shown schematically in Figure 9, and then outline respective predictions and the methods by which we plan to test these hypotheses (summarized in Table 2). Note that Figure 9 is a schematic diagram illustrating the present day lithospheric structure, and as such does not show HP/UHP rocks at depths where they formed (i.e., it shows where exhumed HP/UHP rocks occur at the surface and where partially exhumed HP/UHP rocks may occur at depth).

The back-arc model. In this model, orogenic thickening of the crust of the Papuan Peninsula occurred in response to a continent–arc collision. Northeast subduction of the Australian margin led to accretion of the Owen Stanley metamorphic belt beneath a Late Paleocene–Early Eocene island arc. Subduction of Australian-affinity continental lithosphere choked the subduction zone, causing ophiolite emplacement (Papuan Ultramafic Belt; Davies, 1968) and initiation of south-dipping subduction at the Trobriand Trough (Davies et al., 1984). Miocene–Holocene magmatism (e.g., Smith and Milsom, 1984) is interpreted to be associated with Trobriand

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subduction (Hamilton, 1979; Taylor et al., 1999). Seafloor spreading in the Woodlark Basin began ca. 6 Ma and progressed discontinuously by stepping of a westward-migrating rift such that it remained within the “rheologically weak” continental domain and not in the Solomon or Coral Seas to the south and north (Taylor et al., 1999). Exhumation of the HP and UHP rocks is being driven locally by a crustal density inversion (dense ophiolite overlaying less-dense continental crust) that led to MCC formation in the D’Entrecasteaux Islands (Martinez et al., 2001).

Figure 9. Comparison of endmember tectonic models for eastern Papua New Guinea: The back-arc model (left; after Martinez et al., 2001) and the microplate rotation model (right). DD=Dayman Dome; DI=D’Entrecasteaux Islands; GB=Goodenough Bay; OSM=Owen Stanley metamorphic belt; PUB=Papuan Ultramafic Belt.

The microplate rotation model. As with the back-arc model, northward subduction of the northern Australian continental margin led to continent–arc collision. HP and UHP metamorphism, which U-Pb zircon ages indicate continued into Pliocene time (Monteleone et al., in press), overlapped temporally with extension and sea-floor spreading along strike in the Woodlark Basin. The microplate rotation model is in part based on the fact that several authors have linked extension and seafloor spreading in the Woodlark Basin to slab-pull forces associated with subduction of Solomon Sea oceanic lithosphere at the New Britain and San Cristobal trenches (Fig. 1; Weissel et al., 1982; Hall, 2001; Westaway, 2005). Inception of NE-dipping subduction at the New Britain Trench marked a polarity reversal believed to be related to the Late Miocene–Pliocene collision of the Ontong Java plateau with the Northern Melanesian Arc System (e.g., Coleman and Kroenke, 1981; Petterson et al., 1999; Mann and Taira, 2004; and references therein). Wallace et al. (2004) highlighted the observation that the beginning of rapid seafloor spreading in the Woodlark Basin at ~3.5 Ma (Taylor, 1979) is contemporaneous with the arc-continent collision on the Huon Peninsula at ~3.5–4 Ma (Abbott et al., 1994). The collision led to rapid rotation of the South Bismark plate (Wallace et al., 2005), accelerated subduction at the New Britain Trench, and counterclockwise rotation of the Woodlark microplate relative to Australia. The microplate rotation model hypothesizes that counterclockwise rotation of the Woodlark plate is being accommodated by exploitation of the former subduction thrust between Australian crust and the Papuan Ultramafic Belt. Tectonic reconstructions of the model are in accord with the documented southeastward transition from convergence, to sinistral strike-slip faulting, to divergence along the plate boundary; calculations suggest that removal of the upper plate via counterclockwise rotation and synchronous exhumation of lower plate rocks from

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beneath mylonitic shear zones is sufficient to exhume HP/UHP rocks from > 100 km depths (Webb et al., 2005, in prep.). Evidence for and against subduction at the Trobriand Trough

At the heart of the ongoing debate concerning the tectonic significance of the Trobriand Trough (Hall and Spakman, 2002; Taylor and Huchon, 2001; Westaway, 2005) is whether this bathymetric feature represents an active, or recently active southward-dipping subduction zone (Fig. 1). In the “back-arc model” this bathymetric feature is the trench of a S-dipping subduction zone, whereas in the “microplate rotation model” the Trobriand Trough represents the isostatic expression of the present northern limit of subducted Australian continental crust. Ultimately, the context of UHP/HP metamorphism and exhumation is closely tied to the large-scale mantle dynamics taking place here. As discussed above, seismicity is equivocal and lacks an organized (shallow to deep) pattern indicative of a S-dipping subduction thrust at the Trobriand Trough. Seismic reflection images of the Trobriand Trough show weakly deformed sediment and abundant normal faulting, allowing no more than a few mm/yr convergence (Kirchoff-Stein, 1992; Davies et al., 1984; Lock et al., 1987). Although teleseismic tomography shows no evidence of a high-velocity slab (Hall and Spakman, 2002) resolution is poor in the upper 200 km. Plate-motion models requiring closure indicate that any movement along the Trobriand Trough should be transcurrent to extensional (Wallace et al., 2004). Preliminary analysis of gravity data (Abers et al. 2002) indicates a S-dipping Moho consistent with flexure during underhrusting, but it is unclear whether the data require more than a few km of convergence throughout the Cenozoic history of the Trough. Thus, existing geophysical evidence for subduction is weak and ambiguous.

Geological and geochemical data. The active, calc-alkaline volcanoes Mt. Lamington, Victory and Goropu have also been used as evidence for active subduction beneath the Papuan Peninsula. These volcanoes have not been studied recently (Taylor, 1958; Arculus et al., 1983; Smith, 1981), but existing data show high Ba/La (> 20) typical of arc volcanoes, and abundant hydrous phenocrysts in the explosive 1951 Lamington eruption. On the other hand, Lamington lavas fail to show the expected 10Be excess of recently subducted sediments (Gill et al., 1993) as is present in the unambiguous nearby New Britain arc. To further complicate the situation, virtually every volcanic rock in the study area possesses some kind of subduction signature, even those demonstrably related to extension. For example, basalts from Dobu volcano, within the active MCC region, possess Nb anomalies (Stolz et al., 1993) and even Woodlark Basin basalts have water contents more typical of back-arc basin basalts than MORB (~0.8 wt% H2O; Danyuschevsky et al., 1993). Whether these features reflect on-going subduction, or mantle polluted by prior subduction, is a major question, one that has been difficult to answer with the current lack of age control and complete geochemical datasets for post-Miocene volcanism in the area.

Summary: What distinguishes these models?

Subducted plates. The back-arc and microplate rotation models make fundamentally different predictions regarding the nature of the Trobriand Trough and the lithospheric structure of eastern Papua New Guinea. If southward subduction of Solomon Sea oceanic crust occurred and was sufficiently long-lived to produce extensive arc volcanism since the middle Miocene, one would expect an oceanic slab extending to depths of several hundred km beneath the Papuan Peninsula. The microplate rotation model, however, hypothesizes the presence of NE-dipping

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Australian crust beneath oceanic crust of the Solomon Sea. Seismic tomography will be an important tool to resolve this issue.

Northern vs. southern rift margin. Previously, it has been inferred that the Pocklington and Woodlark Rises are conjugate rifted margins and that the geology of both regions should be correlative (i.e. Australian basement and cover affinities ± Papuan Ultramafic Belt; Taylor et al., 1995). The microplate rotation model predicts a fundamentally asymmetric geology, so that any correlative rocks of Australian affinity on the Woodlark Rise, or at depth, should be higher pressure equivalents of those on the Pocklington Rise. Alternatively, the Woodlark Rise may represent the hanging wall of the former NE-dipping subduction zone, such that basement rocks exposed at the surface are of Papuan Ultramafic Belt affinity rather than Australian affinity. Geologic mapping, geochemical and isotopic studies, comparison of P-T-t-D paths, and geophysical data will be critical to testing for correlations between these rifted margins.

Magmatic trends. The back-arc model interprets the active volcanoes (Fig. 1) to represent the magmatic arc above southward-subducting Solomon Sea oceanic lithosphere, whereas the microplate rotation model suggests that these volcanoes have formed in response to rifting ahead of the westward-propagating rift tip. In this case, the evolution from calc-alkaline to rift-related magmas reflects the gradual replacement of subduction-modified mantle by fresh asthenosphere (Smith, 1981). The southern and northern volcanic belts would then represent an earlier magmatic arc, and more recent rift-related volcanism, respectively. The presence of lavas with adakitic signatures may reflect melting of eclogites similar to those being exhumed, but they remain weakly studied. Structural, petrological, geochemical, and geochronological studies of volcanoes along this arc line will be critical to discriminating between these alternative hypotheses. Model predictions: Hypotheses and tests The main goal of this proposal is to understand how lithosphere ruptures and how HP/UHP rocks are exhumed, particularly at the high rates observed in eastern Papua New Guinea. Given this tectonic history and uncertainties, several alternative hypotheses can be formulated for their origin and mechanism of exhumation:

• The protoliths of the HP/UHP rocks are Australian crust and cover sequences • The HP/UHP rocks are related to southward subduction of Solomon Sea oceanic crust • The HP/UHP rocks are fragments of terranes comprising the Papuan Peninsula • Exhumation takes place by large-scale simple shear, perhaps reactivating subduction

thrusts • Exhumation takes place by diapirism in the mantle • Exhumation through the crust takes place by density-driven lower-crustal flow

Field studies will be guided by testing these endmember models and alternatives for the exhumation of the HP/UHP rocks (e.g., Platt, 1986; Hacker and Peacock, 1995). Several tests are outlined in Table 2.

The microplate rotation model predicts that the HP and UHP rocks formed in a subduction-zone environment and were exhumed predominately as a coherent slab. It further suggests that counter-clockwise rotation of the Woodlark plate relative to the Australian plate reactivated the subduction thrust as a top-to-the-N normal-sense shear zone. Therefore exhumation occurs at plate tectonic rates, and a N-dipping slab should be present; structures are asymmetric at all scales. Increasing pressure and temperature gradients are expected from south to north in Australian affinity rocks as is implied by regional geologic maps (Fig. 1). A secondary gradient

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might be expected from west to east where the magnitude of extension (and exhumation) is expected to be greatest.

Table 2. Predictions and tests of endmember models.

Backarc model predictions Microplate motation model

predictions Methods by which

to test Nature of Trobriand Trough

S-dipping slab; Thrust faulting & associated seismicity (if still active).

NE-dipping slab: Isostatic expression of N-limit of AUS crust at depth; Dominated by normal faulting.

Geophysics: Moment tensor solutions; Tomography.

Nature of the Woodlark Rise

Conjugate margin to Pocklington Rise part of footwall during AUS subduction; Predicts that the lithospheric-scale structure of the Woodlark Rise comprises a PUB–AUS–Solomon Sea sandwich.

If conjugate margin, any rocks with AUS affinities should be higher P-T equivalents to PR rocks; If WR was in hanging wall during AUS subduction, basement of Woodlark Rise is PUB with possible cover of arc rocks coeval with AUS subduction; PUB = Solomon Sea oceanic crust and mantle.

Mapping of WR & Pocklington Rise; P-T-t-D studies; Geochemical correlations; Geophysics.

Linear array of Miocene-Recent volcanic rocks

Volcanic arc due to subduction at Trobriand Trough; Modified mantle signature of AUS mantle due to Trobriand subduction.

Asthenospheric upwelling ahead of the propagating rift tip; Arc signature (?) is due to melting of AUS subduction complex; Increasing mantle component with time and younging of the trend to WNW.

Geochemical and isotopic signatures of volcanic rocks; geochronology.

Geologic structure

Various scenarios, including: 1) ~100 km AUS crust thinned by pure shear (though N-dipping low angle faults recognized from seismic studies); 2) MCC formation due to density inversion.

Reactivated subduction zone; Bulk of exhumation associated with top-to-N non-coaxial shear, some component of coaxial stretching at deep structural levels; Regional pattern in stretching lineations consistent with rotation and plate motion vectors.

Mapping, kinematic analysis and comparison to plate motion vectors; Microstructures

Regional isograds

Various scenarios, including: 1) symmetric P-T gradients to N & S of MCCs due to collapsed isograds 2) chaotic due to accretionary wedge origin

Main gradient is increase in P-T toward the north; possible secondary gradient of increasing P&T to the east.

Mapping & P-T-t-D studies; Seismic velocities; Petrologic modeling.

Ophiolite thickness

~15 km thick based on mapped PUB thicknesses & gravity modeling.

< 6 km thick based on tomography (Ferris et al., 2006); underlain by eclogitized AUS crust (?).

Geophysics; Vp/Vs modeling.

Note: AUS=Australian Plate; PR=Pocklington Rise; PUB=Papuan Ultramafic Belt; WR=Woodlark Rise The back-arc model implies that the UHP rocks would have to have formed at the base of

anomalously overthickened (> 100 km thick) continental crust, or that the UHP rocks were exhumed to midcrustal levels prior to onset of southward subduction at the Trobriand Trough. Final exhumation of the HP/UHP rocks would be associated with pure shear (Martinez et al., 2001), although the predominance of north-dipping normal faults in the Woodlark rift is recognized (Taylor et al., 1999). Pure shear thinning of the overthickened continental crust would essentially collapse the isograds and therefore predict symmetrical P-T gradients to the north and south of the rift. Alternatively, the mafic eclogites could have formed in the accretionary wedge associated with southward-subduction and therefore could have Solomon Sea affinities.

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DESCRIPTION OF RESEARCH WORK PLAN AND RATIONALE FOR COLLABORATIVE APPROACH

A focused multidisciplinary study within the bounds of NSF’s Continental Dynamics program is warranted because the problem of the origin and exhumation of HP/UHP rocks naturally requires many approaches, from petrology, geophysics, regional tectonics and geodynamics. The fundamental questions outlined in this proposal include:

• What are the controlling structures and mechanisms of exhumation? • How and where are HP/UHP rocks preserved in the wake of rifting?

These broader questions can be broken into several issues specific to the regional tectonics: • Is HP/UHP exhumation driven by density inversion or by microplate rotation? • What roles have magmatism or ductile flow in the crust, or heating or thinning of the

lithospheric mantle played in the geodynamics of these MCCs? • How do structural fabrics preserved in the lower plates of Plio–Pleistocene MCCs relate

to present-day kinematics of the region and known plate motions derived from seafloor spreading data in the Woodlark Basin?

In the context of solving the UHP problem, this work will also shed light on the fundamental question of the nature of rifting, and its transition to seafloor spreading, the processes by which 200 km of extension have been distributed across the Papuan margin since 4.5 Ma, the extent to which rifting is synchronous with seafloor spreading or is time transgressive, and the overall mechanics of rifting. Distinguishing between backarc and microplate models has implications for whether or not deep rock exhumation is driven by “bottom-up” vs. “top-down” tectonic processes. GPS offers an unprecedented opportunity to measure present-day crustal deformation associated with deep rock exhumation, while structural geology and geochronology can extend this present-day snapshot to geologic time frames, and provide direct evidence of the rheology of the exhumed rocks. Geodynamic modelling, constrained by field and P-T-t-D data, will tie together the different timescales and will be integrated with worldwide studies of HP/UHP terranes to develop new models for their exhumation. We describe below the contributions of each PI, along with a description of integrative activities. International Collaboration The national and international collaboration we propose will be effective in surmounting any potential logistical difficulties, in sharing costs for data collection in the field and laboratories, and foster interaction among many of the leaders in past work in Papua New Guinea. Hugh Davies (University of Papua New Guinea) has over 40 years’ prior investigation in the geologic, petrologic and tectonic evolution of Papua New Guinea and will sponsor our research visas. Ian Smith (Auckland University) has contributed significantly to understanding the region’s magmatic evolution. An essential part of our collaborative research will involve training young Papua New Guinea scholars in methods and techniques of geologic, geophysical and geodetic fieldwork, and subsequent data and rock analysis. The parallel projects of Daczko, Wallace, Little, and Tregoning provide additional field and geophysical data that will enhance international scientific exchange and cooperation and will be shared for our collaborative analytical work and petrotectonic interpretation. Our international collaborators from NZ and Australia have already secured, or intend to secure funding from their respective funding agencies for participation in this project (see letters of support). Joint publications of the scientific data and interpretations will be published in the Earth science literature.

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1. GEOLOGIC AND STRUCTURAL FIELD STUDIES

Focus Areas: Dayman Dome (UTAustin, Macquarie University); D’Entrecasteaux Islands (SU, UCSB, Victoria University of Wellington,); Pocklington Rise (SU, Victoria University of Wellington); Woodlark Rise (SU, Victoria University of Wellington)

Fieldwork will focus on the metamorphic, magmatic and sedimentary rocks of the southern rifted margin (Pocklington Rise), the northern rifted margin (Woodlark Rise) and the extended region west of the active rift tip (MCCs on Fergusson and Goodenough Island and newly recognized MCC of Normanby Island; Little et al., 2004), and the inferred youngest MCC in the Dayman Dome (Fig. 10). A series of transects across the MCCs and islands on the Pocklington and Woodlark Rises will be undertaken to determine key structural and petrologic relationships and to sample comprehensively for P-T-t-D and geochemical studies. We will ascertain the regional extent of Late Cenozoic HP/UHP rocks in eastern Papua New Guinea and determine when and under what conditions these continental and oceanic rocks were metamorphosed and by what process(es) they were exhumed.

Figure 10. Oblique digital elevation model from GeoMapApp showing target localities (in yellow) of the proposed field studies. D=Dobu volcano; DD=Dayman Dome; FI=Fergusson Island; G=Goropu volcano; GI=Goodenough Island; LA=Louisiade Archipelago; MI=Misima Island; ML=Mt. Lamington; MV=Mt. Victory; PR=Pocklington Rise; TT=Trobriand Trough; WI=Woodlark Island; WR=Woodlark Rise. Goropu volcano intrudes the hanging wall of the Owen Stanley Fault Zone in the Dayman Dome. Dobu volcano is located in the Dawson Strait between Normanby and Fergusson Islands.

On the Papuan Peninsula mapping will focus on the low-angle, oblique–normal fault (i.e., Owen Stanley fault zone) west of Goodenough Bay and along the edge of Dayman Dome. The level and amount of late Holocene displacement along the fault will be determined from offset geomorphic features including stream channels, river terraces and microtopography. Kinematic indicators along exposed fault planes will be measured to determine the sense of shear along the fault and its variability along strike. Young sedimentary rocks deformed in the fault zone, particularly young marine basins that appear to have been entrained as “riders” on the actively upthrown footwall block will be mapped and the age, depositional facies, and water depth of these strata determined to constrain the late Neogene history of fault movement. These basins are

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sparsely sampled but were previously determined to be Plio–Pleistocene in age (D’Addario et al., 1975; Davies and Warren, 1988); a suite of sedimentary samples will be collected in a systematic manner for micropaleontological study to better constrain the ages and water depths of these basins. Ages will be constrained using the 87Sr/86Sr isotopic ratios of microfossils from various levels of exposed successions. These data will be integrated with results from a pilot magnetostratigraphic study of interbedded mudstone and sandstones in order to appropriately match findings to the late Neogene portion of the geomagnetic polarity time scale (Cande and Kent, 1992, 1995) and 87Sr/86Sr isotopic time scale (Farrell et al., 1995). Conglomerate and sandstone provenance studies will further constrain the timing of exposure of notable lithologies (i.e., blueschist) within the adjacent Dayman Dome. Existing marine seismic reflection data from Goodenough Bay (Fang 2000) will be compiled. These data, available in digital format through the NSF Seismic Data Center at UTIG, constrain the offshore position of active faults and young fault-controlled basins beneath Goodenough Bay and considerably expand the scale of observation for the Owen Stanley fault zone in the region where the fault trace appears to extend just offshore of the Papuan Peninsula. By documenting the age and sedimentary facies of entrained upper Neogene basinal rocks along the Owen Stanley fault zone, and incorporating existing marine geophysical data in the adjacent Goodenough Bay we will constrain the locations and characteristics of active and inactive faults in this offshore area.

Additional fieldwork is necessary in the D'Entrecasteaux Island MCCs because: 1) the earlier published work (e.g., Hill, 1994) was undertaken largely at outcrop scale without documentation of microstructures and deformation mechanisms, and 2) in the wake of recent geophysical data sets and the discovery of coesite, the relationship of the MCCs to lithospheric extension and exhumation of HP/UHP rocks has become an increasingly central and controversial issue. In order to address the role of the shear zones in exhuming HP/UHP rocks and lithospheric extension we will collect oriented specimens for microfabric analysis on both the mylonitic rocks and their high-P gneissic substrate from several transects across the domes of Goodenough and Fergusson Islands. The microstructures and deformational fabrics of these specimens will be analyzed with electron-backscatter diffraction (EBSD) and also dated using a range of thermochronologic methods and will build on previous results. The results will constitute an important and straightforward step in resolving the evolutionary history and geodynamic role of the D’Entrecasteaux Islands shear zones in accommodating continental extension throughout eastern Papua New Guinea and in understanding the processes by which UHP rocks are exhumed on Earth.

At the mesoscopic scale, special care will be devoted to finding overprinting relationships and tracing fabrics between exposures, recognizing strain gradients, and documenting the thickness, kinematics and bulk shear sense of high-strain zones. Where brittlely extended supracrustal sequences are encountered, including the upper plate of any detachment faults, we will bracket the timing of normal faulting using cross-cutting relationships, and will document the extension direction. Geologic kinematic data will be compared to velocities known from seafloor-spreading data, and geodetic studies will reveal how strain is currently being distributed among active fault systems. Fault-slip data will be collected at localities where brittle faulting is observed. Sense of slip will be deduced based on offset markers or features on the fault plane such as steps, Riedel shears, or fibers. The nature of the fault surface, such as coating or specific fibrous mineral growth associated with a slip direction, will be noted and can be useful in the correlation of data between outcrops or multiple slip events. Data for each locality will first be

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carefully pooled if overprinting relationships are observed or other evidence for more than one slip event exists. The pooled data will then be used to calculate principal “stress” orientations for each reactivation event (see Angelier (1984) and Twiss and Unruh (1998) for detailed discussions of this technique). Results of fault-slip analyses will be integrated with P-T-t-D studies and compared with data from GPS and seismic studies. 2. P-T-t-D STUDIES (SU, UCSB, VUW, WSU, UCLA)

Using an integrated petrologic, thermochronologic, and structural approach we will document the P-T conditions and timing of subduction and subsequent exhumation of metasediments and metabasalts on the southern rifted margin, within the D’Entrecasteaux Islands and within the Dayman Dome (Fig. 1). Targets include blueschist- and greenschist-facies metamorphic rocks (P = 7–12 kbar) exposed in the footwall of the Owen Stanley fault system and lowermost greenschist–prehnite-pumpellyite grade metasediments and metabasalts on the southern rifted margin (i.e., the Calvados Schist of the Pocklington Rise) for which no radiometric data exist (Fig.1; Davies, 1980). The multiply deformed schists of the Pocklington Rise are intruded by gabbro and dolerite dikes and overlain by basic and intermediate volcanics whose age is constrained by whole-rock K-Ar dating of pyroxene andesite at 11 Ma (Smith, 1973). On Woodlark Island, the largest above-sea-level exposure of the Woodlark Rise, the basement rocks are pre-early Miocene (Cretaceous-Eocene?) low-K tholeiitic basalts, dolerite and gabbro sills, and minor cherts and limestones thought to be correlative with the Papuan Ultramafic Belt (Ashley and Flood, 1981; Lindley, 1994). These are unconformably overlain by early Miocene limestone and volcaniclastic sediments and Miocene high-K volcanic and intrusive rocks. We will target the low-K tholeiites and high-K igneous rocks from Woodlark Island (Ashley and Flood, 1981; Lindley, 1994). In order to assess the role of magmatism in the rifting process, and exhumation of HP/UHP rocks we will petrologically characterize and determine the age/thermal history of intrusive and extrusive igneous rocks to elucidate the spatial and temporal evolution of magmatism.

Microstructural observations on oriented thin sections combined with mineral compositions obtained from electron microprobe analysis will aid in the interpretation of thermochronologic data for multiply deformed metamorphic rocks and igneous rocks. Petrographic and electron microprobe analysis will be used to determine the P-T history for metamorphic and magmatic rocks, and to identify zircon, apatite and K-bearing phases suitable for thermochronologic analysis. We will employ U/Pb ion probe, 40Ar/39Ar laser and step heating, fission track, and (U-Th)/He techniques to document provenance ages, metamorphic, magmatic, and uplift/exhumation histories of the rocks exposed in the islands and on the Papuan Peninsula. Combining temperature-time measurements, thermobarometry on mineral assemblages, and detailed structural and microstructural studies will enable the determination of P-T-t-D paths of rocks exhumed within the eastern Papua New Guinea rift. The data set will be used to determine rates of convergent plate-boundary processes such as subduction burial and exhumation rates. Together with geologic and structural constraints, we will assess the relative contributions of normal faulting, ductile flow, diapirism, and erosion, in exhuming these HP/UHP rocks.

Electron back-scatter diffraction (EBSD) will be used to determine the lattice preferred orientations (LPOs) of individual crystals in thin section. This information will be used in multiple ways, to: i) assess deformation mechanisms (e.g., predominance of grain-boundary sliding over dislocation creep); ii) qualitatively determine deformation temperature (from activated slip systems); iii) determine sense of shear and strain symmetry (using asymmetry of

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the fabric skeleton); iv) reveal preferred growth orientations in undeformed, unmetamorphosed rock; v) determine the abundance of each phase (needed for calculating physical properties and assessing phase-transformation rates); vi) characterize the distribution of grain sizes and shapes (can be used, for example, as measures of strain or sense of shear and for paleopiezometry); and vii) identify polymorphs of phases such as calcite/aragonite, quartz/coesite, graphite/diamond etc. For some examples of these applications see Hacker et al. (2003a), Walsh (2003), Mehl et al. (2003), Johnston and Hacker (2004), Young (2005), and Young et al. (in review). 3. GEOCHEMISTRY (BU, SU, AND STUDENTS, IN COLLABORATION WITH WSU, STANFORD, UCLA, HANNOVER UNIVERSITY).

3.1 Geochemistry of Volcanic Rocks. There are two thrusts to the volcanic rock studies: bulk rock geochemistry, and volatile measurements. Previous work has identified geochemical signatures useful in unraveling sources of volcanic rocks in Papua New Guinea. Nd–Pb–Hf isotopes are essential for distinguishing Woodlark mantle from ancient Australian lower crust and Solomon Sea plate sources (Hegner and Smith, 1992; Woodhead et al., 1998; Lackschewitz et al., 2003; Dril et al., 1997). Some subduction tracers (e.g., Ba/La) illuminate mantle affected by subduction vs. that recently melted during extension in the Papua New Guinea region (Stolz et al., 1993). The presence of high Mg#-andesites (aka adakites) throughout the D’Entrecasteaux Islands may require eclogite sources, by analogy with classic slab eclogite melts that have reacted with mantle peridotite during buoyant melt migration (Kelemen et al., 1993). The combination of major elements and REE patterns are very powerful in identifying eclogite melts (Smith et al., 1979). Sampling targets will include volcanic rocks collected in context with the geological and structural studies outlined above from the Woodlark Rise, D’Entrecasteaux Islands, and Dayman Dome as well as from the active volcanoes on the mainland (sharing helicopter time with the seismic campaign there). We hope to involve a student from University of Papua New Guinea in fieldwork, and in subsequent labwork at BU. We will also have access to the extensive sample collection of I.E.M. Smith, who was among the first to sample and study volcanic rocks from most of these areas (see letter of support). Major element (ICP-ES at BU), trace element (ICP-MS at BU) and Nd–Pb–Hf isotopic (at WSU) analysis of well-dated volcanic rocks across the region will provide constraints on the magma sources in a 4-D view across the area.

Volatile measurements will also be carried out to constrain the melting process. Extension drives mantle melting through decompression, while subduction drives melting through the introduction of water into the mantle. Resolving the contributions of decompression vs. wet melting is clearly of utility in a region as complex as Papua New Guinea, where primary questions are aimed at the extent of subduction and the progress of rifting. We have been working on exactly this problem in back-arc basins around the world (Kelley et al., 2006), using conservative elements such as Ti to estimate melt fraction, combined with water concentrations to determine wet melting productivity and mantle temperature. While arguably the most important chemical component, water is very difficult to measure in magmas. The only subaerial samples that are not degassed are melt inclusions trapped in phenocrysts at high pressure (> 1 kb) and then well quenched in tephra. The high rates of chemical weathering in Papua New Guinea limit this work to historical tephra, thus Lamington, Victory and Goropu volcanoes (Figs. 1 & 10). Goropu is located on the Dayman Dome, while Mt. Lamington is west of known extension, and so these volcanoes traverse a region of interest. Lapilli and ash deposits from the early to climactic phases of the 1951 eruption of Mt Lamington look promising, as they contain abundant

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hydrous phenocrysts (hornblende and biotite; Taylor, 1958). Melt inclusions will be subjected to SIMS ion probe, electron probe and laser-ICPMS analyses to determine volatile elements (H2O, CO2, S, Cl, F), non-volatile subduction tracers (Ba, Th, Sr, Pb) and melt fraction proxies (Ti, Zr, Nb). This parallels our work with Erik Hauri at Carnegie in several active arcs around the world (e.g., Wade et al., 2005, 2006). We will also estimate water content using a hygrometer we have been developing (Plank et al., 2004), based on the Eu anomaly in clinopyroxene phenocrysts as a proxy for plagioclase saturation temperature (which is highly dependent on pH2O). The clinopyroxene hygrometer may permit us to obtain water estimates in older mafic volcanic rocks (e.g., from the D’Entrecasteaux Islands), and so assess the role of water as both a subduction tracer and driver for melting in the active region of core-complex formation.

3.2 Hydrogen isotope-based paleoelevation reconstructions. We plan to constrain the timing of surface uplift in the footwall of the D’Entrecasteaux Island MCCs by analyzing the stable isotopic composition of waters that interacted with shear-zone mylonites. Stable isotope-based paleoelevation reconstructions exploit the fact that precipitation and surface waters become depleted in 18O and D with increasing elevation (Dansgaard, 1964) and are significantly different from the isotopic composition of sea water. The Papuan core complexes represent a good target for such a study because the change in isotopic composition with elevation is relatively well known (Chivas et al. 1984). Determining the stable isotope record of paleoprecipitation necessitates formation and preservation of mineral proxies for ancient and modern meteoric waters. The approach proposed here, determining paleoprecipitation from synkinematic hydrous minerals in mylonitic shear zones that interacted with surface waters during uplift and exhumation is novel (Mulch et al. 2004).

Extensional mylonitic shear zones and associated normal faults bounding metamorphic core complexes facilitate percolation of meteoric fluids into actively deforming crust, in some cases even penetrating the brittle–ductile transition (e.g. Wickham and Taylor 1987, Fricke et al. 1992, Morrison 1994, Morrison and Anderson 1998). These detachments commonly have a protracted deformation and recrystallization history, thus hydrous minerals in the detachment record the interaction with meteoric waters on the timescales of tectonic processes. The hydrogen isotope record in detachment white mica is likely to provide a reliable proxy for the isotopic composition of pristine meteoric water because hydrogen is a trace element in silicate rocks and the D/H ratio of synkinematic white mica will be strongly controlled by the D/H ratio of meteoric water. Comparison of preliminary hydrogen isotope and oxygen isotope data for one shear zone (Morima Shear Zone on Fergusson Island; Fig. 3) with data from modern stream waters indicates that the oldest exhumation-related mylonites recrystallized in the presence of seawater, with the upper crustal fault systems still being sub-marine. Over time, we observe a trend toward lower δD values consistent with meteoric water being sourced from on-land sites with increasing elevation. Knowledge of the exact timing of when the first ductilely deformed rocks were exposed at the surface further constrains rock exhumation rates and adds an important boundary condition to models of surface uplift and exhumation of UHP rocks.

3.3 Oxygen isotope thermometry. The absence of phengite or kyanite within eclogites has frustrated efforts to obtain precise and accurate P-T estimates for HP/UHP metamorphism in eastern Papua New Guinea (Monteleone and Baldwin, 2006). Temperature estimates based on Fe2+-Mg partitioning between garnet and omphacite vary ≥ 100°C within individual eclogite samples depending on assumption of Fe3+/Fe2+ in omphacite, and are thus of limited reliability. Determining the temperature of mineral equilibration under HP/UHP conditions is critical to complement the thermochronological data, particularly within the shear zones where the

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distinction between cooling and crystallization ages may provide valuable insight into the relative timing of advective cooling and exhumation. We plan to measure oxygen-isotope fractionation between cogenetic mineral pairs in the HP/UHP rocks and the associated mylonitic shear zones to determine temperatures of oxygen-isotope exchange at eclogite-facies conditions and during retrograde metamorphism. UHP rocks from various localities on Earth have shown remarkably large variations in their oxygen-isotopic composition depending on the fluid environment in which they crystallized (see e.g. Sharp et al. 1993, Masago et al. 2003, Zheng et al. 2003, Zhang et al. 2005), and oxygen-isotope thermometry has proven to be one of the most reliable thermometers in those rocks. Despite the relatively high temperatures of equilibration of the Papua New Guinea eclogites, their very rapid post-Late Miocene exhumation should minimize any post-crystallization diffusive oxygen isotope exchange. To better characterize the thermal evolution of footwall-bounding shear zones and HP/UHP rocks in the lower plates of the Papua New Guinea core complexes, we plan to measure the temperature-dependent oxygen isotope fractionation and compare the thermometry results with those obtained using new thermometers based on concentrations of Ti in zircon and Zr in rutile (Zack et al., 2004; Watson et al., 2006; Monteleone and Baldwin, 2006). Our preliminary zircon and rutile thermometry results suggests increasing temperatures of eclogite-facies metamorphism with decreasing age of metamorphic zircons from ~8–2 Ma, reflecting an increase in geothermal gradient during and after the peak of HP/UHP metamorphism. Oxygen isotope thermometry will provide additional constraints on peak temperatures of metamorphism that can be compared to HP/UHP terranes globally (Hacker, in press). 4. GEODESY (IGNS, ANU AND PAPUA NEW GUINEA SURVEY TEAMS).

The use of Global Positioning System (GPS) technology presents us with a unique opportunity to directly measure the horizontal strain rates associated with the exhumation of HP/UHP rocks in the Woodlark region. Moreover, regional GPS measurements will provide us with the modern-day kinematic boundary conditions we require to better understand the overall tectonic evolution of the Woodlark Basin and to determine the relationship between the spatial pattern of exhumation rates inferred for the past 5 Myr and the present-day velocity field. A sparse campaign GPS network installed by P. Tregoning exists in the Woodlark region; however, the current distribution of GPS sites is insufficient to address the questions we aim to answer in this proposal. We will conduct repeated campaign-style GPS measurements at (1) sites within relatively dense networks that span regions of documented rapid exhumation of HP metamorphic rocks and (2) measurements at sites within a broader, comprehensive network in the Woodlark region. GPS measurements within the denser networks will give us insight into which structures accommodate extension in the on-land portion of the Woodlark rift, as well as documenting the rate of horizontal strain associated with active exhumation of HP/UHP rocks. Regional GPS measurements will allow accurate measurement of the modern-day microplate kinematics and quantification of the total rates of extension occurring in the Woodlark Basin. The regional GPS data will be integrated with other kinematic data (e.g., earthquake slip vectors, transform orientations, and spreading rates) using a similar approach to that implemented previously in Papua New Guinea by Wallace et al. (2004) to obtain the best possible estimate of the regional microplate kinematics.

Funding for geodesy is separate from this proposal. Paul Tregoning of Australian National University (ANU) already has Australian Research Council funding to observe the skeleton, regional GPS network that he has established. Observations from this regional network will form

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a basis for the densified regional network that he and our New Zealand collaborators (Laura Wallace, Tim Little and Susan Ellis) propose to install using the Marsden Fund of the Royal Society of New Zealand (Fig. 11). Tregoning’s existing network will also provide data that can be used in the first couple of years of this project as constraints on potential tectonic models for the Woodlark region, as well as helping us pinpoint where to target the densified GPS network. 5. SEISMOLOGY (BU, LDEO, AND PAPUA NEW GUINEA FIELD ASSISTANTS).

The seismic portion of this experiment will directly image crust and mantle rocks where we anticipate finding HP/UHP rocks at depth that are likely in the process of being exhumed. Constraints on seismic velocities provide information on the composition, metamorphic assemblages and hydration of HP/UHP rocks at depth, and place first-order bounds on the extent of mantle hydration, melting and thermal structure beneath the MCCs and volcanoes. A variety of broadband imaging techniques will map in detail the structures associated with exhumation at the MCCs and the associated faulting. Specifically, images of velocities and interfaces below and north of the MCCs, at Moho and upper mantle depths, should easily discriminate between the endmember exhumation models (Fig. 9). Obvious diagnostics include a flat or dipping Moho north of the MCCs and a N- or S-dipping structure in the upper mantle beneath the Trobriand platform. Velocities and anisotropy provide constraints on composition and fabric in both crust and upper mantle. Data from WoodSeis2000 showed a thinning of the crust along the D’Entrecasteaux Islands (Abers et al., 2002), but given the sparse station geometry it is not clear whether crust thins directly under the core complexes, thin crust is offset to the north, or if crustal thinning is a broader regional feature (Ferris et al., 2006). Also, the along-strike variations in structure will provide insight into the depth extent and significance of NE-striking crustal transfer (strike-slip) structures and whether the MCCs represent an upper crustal phenomenon or a penetrative crustal feature.

5.1 Deployment (Abers, Gaherty, post-doc, graduate student, and PNG field assistants). To accomplish these objectives, we will deploy a dense array of broadband seismographs for 15 months. A 31-station, on-land deployment will be concentrated along two transects sampling the core-complex system at two endmember limits of extension. The eastern transect crosses the MCCs at Fergusson Island where extension is the largest without rupture to sea floor spreading. The western transect samples the Dayman Dome region, the MCC closest to the pole of plate motion and hence the least overall extension, perhaps where UHP rocks are least exhumed. Offshore, 8 ocean-bottom seismographs (OBS’s) in the deepwater portions of the region (e.g., Goodenough Basin, Cape Vogel Basin) will provide crucial constraints on deep structure beneath the D’Entrecasteaux Islands from earthquakes to the north. While a transect across Goodenough would be desired, the shallow, reef-infested waters north of there makes access impossible. The instrument spacing will be 10–25 km, sufficient to provide crossing rays from teleseisms at lower crustal depths and deeper (Fig. 12). These studies are in part a continuation of WoodSeis2000, which was sparser (25–50 km instrument spacing) and centered on the rift tip farther east. All data from that previous experiment, covering the active rift tip to the east, will be integrated with the proposed study. All data will be archived at the IRIS DMC.

Roads and airstrips are generally absent. We plan as many sites as possible to be near coastal settlements so that they can be accessed by boat; we plan to charter the 50-foot dive boat Marlin I for much of that fieldwork (http: //www.marlin1charters.com.au/). Work will be based out of Alotau, the only place with serviceable infrastructure and regular air access. Two in-land stations on the Dayman Dome transect will be serviced by helicopter out of Port Moresby; it is a c. 1 hr.

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transit from Port Moresby to these sites, and installation is designed to minimize the total cost. Stations will be deployed for 15 months total, with nominal service visits at months 2, 5, 8, 11 after deployment. Our experience is that 3 months unattended is about the limit over which stations will operate in PNG without environmental damage. Where possible, the sites will be located close to nearby GPS sites, providing additional opportunities for servicing and instrument checks. A critical component to this entire field plan will be full participation by collaborators from the University of Papua New Guinea (see support letter) including UPNG students.

Figure 11. Location of proposed GPS, OBS and on-land (PASSCAL) broadband seismometers, and volcanoes to be sampled. Seismometers form 2 NNE-trending arrays, ~200 km long, spanning the Trobriand Platform and crossing the MCC belt, at the Dayman Dome or across Fergusson Is. Stations are limited to known islands or OBS’s at water depths exceeding 400 m (blue contour).

The marine deployment will utilize 4-component broadband OBS borrowed from the Ocean-Bottom Seismograph Instrumentation Pool (OBSIP; see supplementary documentation). These instruments employ a fully broadband 3-component seismometer, as well as a differential pressure gauge, in an autonomous package that can be deployed on the seafloor for up to 15 months, coincident with the onshore deployment. At frequencies of 0.02–0.1 Hz oceanic noise levels are low and both surface and body waves are readily observed on well-coupled ocean bottom seismometers (e.g., Collins et al., 2001). Seafloor noise is produced in large part by ocean waves and tidal-induced and other currents (Collins et al., 2001; Webb, 1998). We anticipate that seafloor conditions at our chosen sites should be relatively quiet: wave height is usually low in Goodenough Bay and Cape Vogel Basin due to lack of fetch, and tidal fluctuations recorded on the north coast of the Papuan Peninsula (Aigura Point) are generally < 1 m peak-to-peak (http://tbone.biol.sc.edu/tide/). The planned site locations are all in water depths of >400 m (>1000 m in Goodenough Basin), further limiting wave and tidal noise. There is no deep trawling in either Cape Vogel or Goodenough basins (Neil Stanton, Nako Fisheries, pers. comm.), and thus with deep-water deployments there is minimal risk of the instruments being disturbed by fishing or other activities. The locations are all sited along MCS profiles shot during a 1992 survey (Mutter et al., 1996). These data are available from the NSF Marine Geoscience Data system (http://www.marine.org) and will provide useful constraints on sediment and basement structure.

The deployment and recovery operations will be performed by one or more co-PIs, in conjunction with graduate students and OBSIP personnel. Deployment and recovery of eight

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instruments can be easily accommodated on an intermediate class vessel. With the exception of PI and student travel, the costs associated with OBS deployment and recovery are funded directly through OBSIP and are not included in this proposal (see OBSIP and UNOLS documents in Supplementary Information).

Figure 12. Predicted seismic sampling. Circles: expected Ps receiver function sampling of Moho for proposed array (blue) and for WoodSeis2000 (red), based on observed event distribution in 1999-2000, in 30° azimuthal bins. The new array has multiple-station sampling of the Moho, leading to unaliased 2D mapping beneath the MCCs, unlike WoodSeis2000. Color background and scale bar predicted formal error in hypocenter depth (km) for crustal earthquakes, for typical data quality. Errors less than 3 km are attained for the entire continental rift. Purple line: 2 km contour for same calculation, WoodSeis2000 (Fig. 8).

5.2 Seismic Analyses Travel-time and attenuation inversions (BU). The proposed array should have crossing rays

to allow imaging mid-crustal depths through the upper 250 km of the mantle. Currently we use a joint velocity-hypocenter inversion based on finite-difference travel time calculators and fully nonlinear hypocenter relocation (Roecker et al., 2004). A similar scheme will be used to invert for seismic attenuation, a parameter with high sensitivity to temperature relative to composition (e.g., Stachnik et al., 2004). These regional-signal inversions will be compared with or combined with teleseismic inversions, which provide relative velocities only but deeper sampling (e.g., Abers et al., 2002b). The combination of parameters, Vp, Vp/Vs, Qs, and potentially Qp/Qs, should help separate out effects due to temperature from those due to composition (Karato, 2003; Roth et al., 2000); see Section 6.

Scattered waves and receiver functions (BU). Simple measurements from scattered P coda, i.e. receiver functions, provide clear measurements of Moho depth beneath stations to accuracy of a couple of km (Fig. 7), and analysis of multiples provides constraints on Vp/Vs of the crust. Recent advances in imaging techniques now allow full migration of this P-wave coda (e.g., Rondenay et al., 2005); examples show that coherent P-S scattered energy can be extracted and migrated to depths exceeding 100 km and can provide images of structure at resolution of a few km (Fig. 13). As applied here, we will be able to image any relict subducted crust now at UHP conditions, and generate a dense image of the Moho. At greater depths, we hope to be able to image the base of the lithosphere (Rychert et al., 2005) and any subcrustal interfaces associated with deeply subducted material. For example, the two models in Fig. 9 suggest very different upper-mantle structure north of the MCCs. Analysis of these signals within the crust could help constrain crustal anisotropy, and thus allow a better understanding of shear zone structure at depth (e.g., Wilson et al., 2004). Figure 12 shows nominal ray receiver function sampling points of the Moho by the P-to-S conversion for the proposed experiment, and shows that the stations are sufficiently close to allow multiple sampling for unaliased imaging of the Moho and deeper. Similar resolution along a linear profile from Central Alaska clearly shows the downgoing plate to 130 km depth, the Moho, and internal structure of the overlying crust (Fig. 13).

Surface waves (LDEO/BU). Surface waves provide a natural complement to body waves, in that they are sensitive to shallow structure with good vertical resolution, and their horizontal

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propagation provides an alternative sampling of seismic properties. Integrating surface- and body-wave observations into a single modeling study significantly decreases the tradeoff between velocities and discontinuity (e.g. Moho) depth. We will utilize surface-wave data derived from two types of observations: 1) the relative phase variations of Rayleigh and Love waves from regional and teleseismic earthquakes propagating across the array; and 2) the velocities of interstation Love and Rayleigh wavefields produced via cross-correlation of ambient seismic noise. In the first, we will utilize an array analysis to estimate the phase behavior of the observed wavefield as they propagate across the array (Friederich, 1998; Li et al., 2003; Pavlis and Mahdi, 1996; Pedersen et al., 2003). With ~30 broadband stations distributed over a 200-km by 150-km region with a nominal station spacing of 10–40 km, we will be able to resolve the propagation characteristics of surface waves within a period band of approximately 5–40 s, sampling the crust and shallowest mantle. Using a cross-correlation procedure (Chevrot, 2002; Gaherty, 2004), we will extract the phase and propagation direction of these waveforms, providing constraints on smaller scale structure within the region. Longer-period constraints over a broader area will be provided by the stations of the GSN station in Port Moresby, as well as the stations from WoodSeis2000.

Figure 13. Example of scattered-wave imaging from Alaska, showing structure within the crust and upper mantle, for an experiment with similar station density to that proposed. The image shows migrated forward- and back-scattered waves inverted for shear wave speed perturbations (Rondenay et al., 2005). Lines denote subducting crust to 130 km depth, intermediate-depth seismicity, and other discontinuities within the upper mantle.

Figure 14 shows a conservative estimate of the spatial sampling, the “hit count” of intra-array paths on a 0.1°x0.1° grid, using a limited set of ~130 energetic events that occurred during a typical 12-mo period. The surface waves from these events cross the array at a wide range of back-azimuths (inset in Fig. 14). Spatial and azimuthal coverage of the D’Entrecasteaux Islands, the Dayman Dome, and the intervening basin are excellent. With a total array aperture of approximately 150 km, we expect to constrain average velocity and anisotropy to depths of order 120 km, and spatial variations above 60 km depth should be resolved to within few 10’s of km.

This coverage can be further enhanced by exploiting the diffuse, or random, wavefield associated with ambient ground-motion noise. By cross-correlating the ambient noise recorded at any pair of stations and averaging over many correlations, we can obtain an approximate Green’s function between the two stations (Shapiro and Campillo, 2004; Shapiro et al., 2005). The dispersive character (phase and/or group velocities) of these Green’s functions can be measured using standard techniques, generally in the period range between 5 and 25 s, providing highly localized constraints on crustal structure along a path between each station (Shapiro and Campillo, 2004; Shapiro et al., 2005). Application of this method to WoodSeis2000 data shows that the signals clearly resolve group velocities at periods that sample the upper crust (Ferris and

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Abers, 2006). Paths crossing Goodenough Basin show low group velocities consistent with granitic bulk composition and rule out the existence of a thick, high-velocity ophiolite in the upper crust as proposed by Martinez et al. (2001).

Figure 14. A conservative estimate of surface-wave coverage expected for the proposed analysis. Seismic sources considered for this estimate are over 130 events drawn from the CMT catalog. For local and regional events, we used only those events with Mw > 5.5, and for teleseismic events we require Mw > 6. Inset displays the combined back-azimuthal distribution of the 29 local (red sticks), 78 regional (green sticks), and 24 teleseismic (black sticks) events, which is excellent. Spatial sampling of subarray structure is estimated by counting up the number of inter-station "raypaths" that sample each 0.1o x 0.1o cell. Coverage across the entire region is exceptionally good, with most cells sampled by hundreds of raypaths. White cells are sampled by < 20 rays.

Shear-wave Splitting (LDEO). Estimates of seismic anisotropy provide a possible means of detecting crustal and mantle fabric in shear zones associated with crustal extension and the flow paths associated with UHP exhumation and emplacement. Constraints on crustal anisotropy can be derived from shear-wave splitting observed on relatively high-frequency local events, as well as apparent splitting observed in scattered-wave (receiver function) images (e.g., Wilson et al., 2004). It is increasingly recognized that crustal fabric may produce important anisotropy in certain tectonic regimes (e.g., Okaya and Christensen, 2002; Okaya et al., 2004; Shapiro et al., 2004), including extensional regimes with strong crustal deformation fabrics such as MCC shear zones. Estimates of mantle anisotropy are typically derived from longer-period core phases such as SKS. One can envision three likely contributors to mantle fabric: asthenospheric flow, pre-extension geologic fabric within the continental lithosphere, and localized flow associated with extension and uplift. The continental lithosphere fabric is the most difficult to predict from simple models. In contrast, localized fabric associated with rifting might be expected to be extension-parallel (e.g., Ruppel, 1995). The combination of the regional and localized fabric is likely to produce complex splitting observations that are dependent upon back-azimuth or initial polarization (Rumpker and Silver, 1998; Saltzer et al., 2000; Silver and Savage, 1994). We anticipate abundant deep local events from New Britain and Solomon slab, along with a good distribution of SKS/SKKS (85–125°) events.

Seismicity: crustal faults, intermediate-depth earthquakes (BU). A byproduct of inversion for velocities will be a reliable suite of hypocenters (e.g., Fig. 8). These will provide direct constraints on the geometry of faulting, the likely regions of strain accommodation and their spatial relationships, and the potential existence of intermediate-depth seismicity within a subducting slab beneath the rift. Figure 12 shows the region where we can expect to locate earthquakes well within the crust; it includes virtually the entire MCC zone and most likely faulting regions including the Trobriand Trough. The array design will provide the first useful sampling of the region north of the MCCs and west of Goodenough. It should elucidate the fault patterns associated with exhumation to test whether or not extensional faulting continues on the north flanks of the MCCs as in the east, or is transferred to fault systems farther south. At

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intermediate depths the region of high accuracy extends more widely, so the possibility of intermediate-depth earthquakes associated with the Trobriand Trough can be tested; a handful near 50 km depth were seen by WoodSeis2000 but at the edge of the array (Fig. 8). In regions with sufficient earthquake density, application of double-difference relative location techniques (Waldhauser and Ellsworth, 2000) will further clarify the geometry of faulting. 6. INTEGRATING SEISMOLOGY WITH PETROLOGY (UCSB, BU, LDEO)

A significantly more robust interpretation of geophysical data comes from applying petrological constraints. Specifically, rock physical properties—such as density, elastic moduli, and H2O content—can be calculated from known rock compositions, measured lattice-preferred orientations, known equilibrium phase relations, known and assumed phase transformation rates, and thermal models (Hacker and Abers, 2004). Expansions of mineral physical properties databases now permit rather accurate calculations to pressures of ~8 GPa (Connolly and Petrini, 2002; Hacker et al., 2003a). Combined with increasingly sophisticated thermal models (e.g., Currie et al., 2004; van Keken and King, 2005), this enables more sophisticated modeling of subduction zones. It is now possible, for example, to create models of wavespeeds for comparison with seismic images (Husen et al., 2003) and for forward modeling of guided waves (Martin et al., 2006). Models of slab dehydration have been used to discuss the H2O flux through the subduction factory (Kerrick and Connolly, 2001a, b) and the relationship to intermediate-depth seismicity (Hacker et al., 2001; Hacker et al., 2003b). Mineral elasticity data can be used with measured LPOs to calculate velocity anisotropy (Mainprice, 1990). Of singular importance to this study, the pressure-induced phase transformations expected in subducted continental crust will produce distinctive wavespeed and density signatures (Fig. 15).

Figure 15. P-wave speeds (km/s) and densities (g/cm3), calculated for an eclogite and a quartzofeldspathic gneiss at 600–800°C and 1–3 GPa calculated using Perple_X (http://www.perplex.ethz.ch/). Vp/Vs (not shown) also provides a clear diagnostic.

These efforts form a key part of the integration among components of this proposal. Specifically, seismic observations from body waves and surface waves will place constraints on Vp, Vs, and Q for the upper and lower crust, and any anomalies observed within the mantle that may appear as potential HP/UHP rocks at pressure. The mineralogies and LPOs of HP/UHP rocks at the surface will be quantified, and using previously developed techniques (Hacker et al., 2003a; Hacker and Abers, 2004; Mehl et al, 2003), we will provide predictions, as a function of P and T, of the allowable range of seismic wavespeeds for these rocks (see Facilities section for

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details). The abundance of mantle partial melt and water can be constrained, to some extent, from analysis of tephras from active volcanic rocks, combined with other aspects of source petrology. Comparisons of recently exhumed HP/UHP rocks, mantle-derived volcanic rocks, and seismic imaging provide critical quantitative tests of hypotheses for the origin and distribution of these rocks. Combinations of multiple parameters provide some of the best possibilities for resolving the many-dimensional space of composition, temperature, and fluids that may affect rocks (e.g., Forsyth, 1993; Wiens and Smith, 2003). For example, Q is highly sensitive to temperature, somewhat to the presence of fluids, and insensitive to bulk composition. The Poisson ratio (or Vp/Vs) is higher in the presence of fluids for some pore geometries and for serpentine (only temperatures where serpentines are stable), but lower where quartz is present (Christensen, 1996).

7. GEODYNAMIC MODELING (LDEO, IGNS, SU).

We propose numerical simulations of several processes affecting the evolution of the region. Specifically, we plan to undertake three sets of geodynamics models testing different hypotheses about the geological development of extension, subduction, and exhumation of UHP rocks in the PNG region. The numerical modeling techniques used are based upon work with which we or our colleagues have experience. Below we describe three sets of models that concern different scales and are presented in order of increasing size of the region being treated:

7.1 Upper crustal brittle extension and related crustal thinning. This set of models will investigate the development of the topographic and geologic structure of the core complexes of the D’Entrecasteax Islands and the Dayman Dome and the crustal structure of their immediate surroundings. These models are most similar to the models that the Lamont group has developed in recent years, namely MCC development. This effort will build directly on existing 2-D models of lithospheric extension and the evolution of normal faults into metamorphic core complex-like structures (e.g. Buck, 1993, Lavier and Buck, 2002). These studies can be distinguished from many other excellent rift models in that we will focus mainly on fault development and testing the amount and rate of fault weakening needed to produce faults with sufficient slip to produce MCC features. Because part of the exhumation of Papua New Guinea HP/UHP rocks may occur due to offset along shear zones, this is a critical feature to consider.

This first task will test whether extension of plausible pre-rift crustal and mantle structures can explain the observed characteristics of the MCCs in this region. In these models we will follow the mechanical and thermal evolution of extending lithosphere, including both faulting and lower crustal and mantle viscous flow (e.g. Huismans and Beaumont, 2003; Nagel and Buck, 2004). Two additional factors will be explored: 1) The role of extension induced by vertical density stratification, versus extension driven solely by plate rotation (incorporating results from 3D models of Ellis and Wallace); and 2) The effects of vertical and lateral variations in crustal composition/density, such as the kind of layering proposed by Martinez et al. (2001). We will also explore the rheologic, thermal and buoyancy effects of crustal magmatism. It should be possible to compare the results fairly directly to the seismologic, geologic and petrologic constrains provided by the work proposed.

We plan to apply and compare two numerical approaches. Both methods allow very general cases of rheology (including elastic, brittle plastic and viscous rheologies) as well as following the temperature and compositional structure of an evolving model domain. The first technique is based on the tried-and-tested FLAC finite difference approach (Cundall, 1982) which we have applied to problems of lithospheric deformation and fault formation over the last decade (e.g.,

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Poliakov and Buck, 1998; Buck et al., 2005). The other approach is an alternating Lagrangian and Eulerian (ALE) finite-element approach that has been used to model large-scale lithospheric deformation (e.g. Huismans and Beaumont, 2002). The latter approach is being adapted for general use by the initiative in Computational Infrastructure for Geodynamics (CIG) and we anticipate having access to this code within the next year. We will also have access to a smaller, tailored ALE code developed by Ellis. The importance of comparing codes is critical, to make sure that results are not numerical artifacts of a particular method.

We will first use 2D models to elucidate which processes could account for features of this active region. It should also be possible to simulate crustal scale extension using 3D models. Adaptation of existing codes, and those being developed via the Computational Infrastructure in Geodynamics (CIG) initiative, will result in codes that can be applied to this problem in a timely fashion.

7.2 Exhumation of HP/UHP rocks during subduction of continental material. This second set of models will use the same numerical techniques as the first but focus on a larger-scale problem. These models will examine how continental crustal material might be subducted to great depths, heated, weakened and either scraped off or buoyantly rise to be exposed by extension near the surface. We will test which processes could bring rocks rapidly from great depths to the surface, including the hypothesis that exhumation is facilitated by extension of the near-surface lithosphere. These models will look at general cases of how crust could be brought up from depth and then at specific conditions that may have existed and exist in the Papua New Guinea–Woodlark region. Using the kinds of numerical methods adapted to deal with problems of lithospheric scale extension and contraction (e.g., Buck et al., 2001) we will set up simple geometries of continental subduction. To simplify the models we will force the subduction of oceanic and continental plates into asthenosphere and look at thermal and rheologic evolution of those plates. The intent is to examine the timing and depth of subduction before crustal material detaches from and moves up from the rest of the subducting plate.

As well as effects of near-surface extension, we will also test whether strain heating plays an important role in detachment and exhumation. Since rheology is strongly temperature dependent, it is conjectured that strain heating can lead to weakening and flow in response to buoyancy forces. Given the complexity and domain size of these models we expect to only consider 2D treatments here. Figure 16 illustrates one possible sequence of exhumation and shows preliminary numerical results for part of that sequence.

7.3 Regional-scale subduction evolution and a comparison of the two large-scale models for regional evolution. We will examine subduction-zone evolution in models in which the location of the subducting slab or slabs freely evolves. We propose to examine the conditions that might favor each of the two end-member models for regional PNG evolution: the back-arc model and the microplate rotation model. For the microplate rotation cases we will look at the free (unconstrained locally) subduction of oceanic and attached continental lithosphere and try to constrain the effects of this subduction on the stress field of the overlying plate. The intent is to see whether the subduction of buoyant material can help induce extensional “unzipping” of the overlying lithosphere. For the back-arc model cases we will try to determine when the subduction of continental lithosphere might produce a new subduction zone with opposite dip. Ellis and her colleagues in New Zealand have started to examine this type of problem using large-scale, 3D numerical codes. The FLAC-based numerical approach we have been using has been adapted to look at subduction initiation and evolution by Gurnis et al. (2004) and it, along

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with the ALE codes mentioned earlier, will be used for detailed examination of 2D subduction development.

Figure 16. a) Schematic illustration of a two-stage model of exhumation which may explain features of eastern (Papua New Guinea. The crust first detaches from a mantle lithospheric slab driven by buoyancy. The detachment of the crust can only happen as the lower crust heats up and thermally weakens. The crust thickened by the diapiric upwelling is then extended and part of that crust is tectonically exhumed. The extensional exhumation brings material to the surface in the D’Entrecasteaux Islands. b) Numerical model results showing the first phase in a two-stage exhumation model. The starting point of the model is an idealized situation at the time of maximum subduction of crust. Low-density crust is attached to a cold mantle lithospheric slab and overlain by warmer upper mantle. The lower crust is hot and weak enough to allow motion of the upper crust relative to the slab. While the crust rises the dense mantle slab moves deeper. The numerical setup and model technique are similar to those used by Lavier and Buck (2002). 8. TECTONIC AND GEODYNAMIC SYNTHESIS (ALL PIs) The available geologic evidence from eastern Papua New Guinea suggests that the HP/UHP rocks formed as a result of Australian continental subduction beneath Pacific oceanic lithosphere and that subsequent exhumation of the HP/UHP rocks is the result of formation, rotation, and subduction of the Woodlark microplate at the New Britain trench. Our working hypothesis, the microplate rotation model, suggests that eastern Papua New Guinea may represent a new paradigm for plate-boundary zones, never before recognized in an active setting or in the geological record. This transient plate-boundary zone is in the process of transitioning from subduction to rifting to seafloor spreading, a history that may ultimately be obscured by an arc continent collision. Therefore, characterization of this region will lead to an improved understanding of the processes governing the origin, structure, composition, and dynamic evolution of obliquely convergent plate boundaries.

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9. COMPARISON WITH OTHER UHP TERRANES (UCSB, SU)

Is it possible that the HP/UHP rocks of eastern Papua New Guinea were exhumed via a unique process? Or alternatively, if one fast forwards millions of years into the future, will the evidence for HP/UHP metamorphism be obliterated by the rifting process? Or will the HP/UHP rocks be sandwiched in a zone of arc-continent collision that obscures their true affinities? The implication of the latter is that eastern Papua New Guinea is most likely NOT geologically unique except for the fact that it is ‘caught in the act’. Many models for the exhumation of UHP terranes invoke wedge extrusion via buoyant rise of subducted continental crust as thin sheets bound above by a normal fault and below by a thrust fault (Ernst, 2001; Hacker et al., 2000). However, an aspect of any buoyancy-driven model is that the UHP slab may become neutrally buoyant and ponds at the Moho (c.f., Walsh and Hacker, 2004) such that another mechanism must then be responsible for their exhumation to the surface. We note that the overall structural geometry of the D’Entrecasteaux MCCs—an asymmetric dome bounded by a normal-sense shear zone with stretching lineations that record an element of vertical axis rotation (Hill, 1994; Little et al., 2004; in press)—bears striking resemblance to the Dabie UHP terrane (c.f., Hacker et al., 2000). Given that tectonic denudation in eastern Papua New Guinea may ultimately be driven by subduction of the Solomon Sea oceanic lithosphere at the New Britain trench, a 2-D version of the microplate rotation model also bears similarities to the slab extraction model of Froitzheim et al. (2003) for the Adula nappe in the Central Alps. Thus, tectonic studies of eastern Papua New Guinea have potential to shed light on controversies related to the lack of applicability of the wedge extrusion model to all UHP terranes and provide insight on mechanisms for rapid exhumation UHP rocks to the surface.

Management Plan

The proposed Continental Dynamics (CD) project will run for 5 years with Baldwin (SU) providing overall coordination of research efforts. The group will convene scientific progress meetings on an annual basis during the course of this collaborative study to discuss planning, field operations/logistics, results, and publications. The analytical facilities used in this project are either under the direction of the respective PIs, or their collaborators, so access to these facilities is guaranteed (see letters of support). Field seismic instruments are provided by national facilities (IRIS-PASSCAL; OBSIP). Special sessions at national and international meetings (e.g., Fall AGU meetings) will be organized to enhance scientific exchange and discussion among the investigators. Upon completion of the project a conference (e.g., Penrose) will be convened on the geodynamics of HP/UHP exhumation in obliquely convergent plate boundary zones. Role of Principal Investigators

Geoffrey Abers (BU) has primary responsible for the on-land seismic data collection, routine processing, and archival. The BU and LDEO groups will collaborate on all aspects of seismic analyses, but BU will be primarily responsible for tomography, scattered-wave imaging, and local seismicity. BU and UCSB continue their collaboration in using seismic velocities to constrain composition, temperature and melt.

Suzanne Baldwin (SU) will serve as project manager and ensure coordination of research efforts among all groups. She will lead geologic expeditions in the Milne Bay Province, supervise SU and University of Papua New Guinea students and participate in petrologic,

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thermochronologic, isotope geochemical research and synthesis of results. She directs the Syracuse University Noble Gas Isotope Research Laboratory. Roger Buck (LDEO) has primary responsibility for numerical simulations of MCC development and the exhumation of HP/UHP rocks in eastern Papua New Guinea and will supervise a LDEO student. SU, BU, UCSB will collaborate with him to provide constraints for geodynamic models.

Jim Gaherty (LDEO) has primary responsibility for the Ocean Bottom Seismic data collection and processing, and provide field support for the on-land seismic data collection. The BU and LDEO groups will collaborate on all aspects of seismic analyses, but LDEO will be primarily responsible for surface-wave and anisotropy analyses.

Brad Hacker (UCSB) is a metamorphic petrologist–structural geologist–geochronologist with special interest in theoretical and field studies of the rates of phase changes and tectonics. He will supervise UCSB graduate student Nick Barth. Together with Matthew Salisbury they will measure velocities and calculate velocities and other physical properties of Papua New Guinea rocks that can be used to i) evaluate seismic data and ii) constrain UHP exhumation models. A student of UHP terranes globally, he will contribute to the geodynamic and tectonic synthesis.

Brian Horton (UT Austin) will carry out sedimentological mapping, dating, and magnetostratigraphic work on Neogene clastic units exposed in basins associated with the low-angle Owen Stanley fault along the northern edge of the Dayman dome.

Paul Fitzgerald (SU) heads the fission-track laboratory and has extensive experience in low-temperature thermochronology and its application to tectonics and landscape evolution. He will participate in fieldwork, undertake and supervise fission-track and (U-Th)/He analyses and be involved in determination of the landscape evolution and tectonic synthesis.

Paul Mann (UT Austin) will carry out field-based mapping of the Owen Stanley low-angle, oblique–normal fault in the area west of Goodenough Bay and along the edge of Dayman dome and will supervise a grad student, and an undergraduate field assistant from the University of Papua New Guinea.

Terry Plank (BU) directs the Boston University ICP-MS and laser-ablation facility. She will oversee the geochemical analyses of volcanic rocks, and the study of melt inclusions in recent volcanics. She will participate in sample collection, and supervise undergraduate and graduate students involved.

Laura Webb (SU) will be involved in mapping, structural analyses (kinematics of ductile and brittle deformation, fault-slip analyses, and microstructural analyses), petrography, 40Ar/39Ar thermochronology, and U-Pb and REE ion probe analyses. Her contribution to the tectonic synthesis will include how results from Papua New Guinea relate to worldwide UHP terranes. International and National Collaborators (see letters of support) Hugh Davies (University of Papua New Guinea; Papua New Guinea geologic and tectonics; research visa sponsor); Tim Little (Victoria University of Wellington; structural geology); Laura Wallace (N.Z. Institute of Geologic and Nuclear Sciences; geodesy); Nathan Daczko (Macquarie University; Dayman Dome structural geology/petrology); Simon Kawagle (University of Papua New Guinea; on-land seismic data collection); Ian Smith (Auckland University; geochemistry of volcanic rocks); Marty Grove (UCLA; in situ (U-Th)/Pb, trace and REE ion probe analyses; zircon thermometry); Jeff Vervoort (Washington State University; (U/Th)/Pb detrital/inherited zircon analyses, Sr–Nd–Hf–Pb analyses); Andreas Mulch

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(Hannover University; stable isotope geochemistry; Matthew Salisbury (Geol. Survey of Canada; seismic velocity measurements). Duration of Project

The duration of the proposed research is five years commencing July 1, 2007. Table 3 outlines the timeline/work plan during the course of this study. The schedule for the seismic deployment depends upon the availability of seismic equipment, from PASSCAL and OBSIP and represents an optimistic current assessment given published instrument availability, to begin late Year 2 or early Year 3. It will be adjusted accordingly once a request is scheduled. Distribution of data and/or samples and publication of results

Rock samples collected will be cataloged and stored in SU’s Heroy Geology Laboratory where storage facilities are available and a database established to track hand samples, mineral separates, thin sections, sample irradiations, thermochronologic analyses, etc. Upon completion of this project, samples will be made available to other scientists wishing to perform analyses not covered under this proposed research. All seismic data will be archived at the IRIS Data Management Center, in accordance with standard IRIS-PASSCAL and OBSIP policies. All volcanic rock geochemical data will be submitted to GEOROC and PetDB databases. The nature of stored data, samples and research results of the project will be described on websites. Results will be presented at national and international meetings, and published in the earth science literature. Broader Impacts Science and society will benefit through international collaboration associated with the proposed activities and programs in eastern Papua New Guinea. Results will contribute to an improved understanding of the processes governing the origin, structure, composition, and dynamic evolution of continental building blocks in obliquely convergent plate boundary zones. These include how deformation localizes within the crust, how UHP rocks are exhumed from mantle depths, and the fundamental processes by which the Earth’s tectonic plates rift and rupture. The proposed research will provide student and post-doc training in the fields of seismology, thermochronology, petrology, structural geology, geochemistry, and geodynamic modeling. Analytical work at SU, BU, UCSB, UCLA, Cornell, WSU, and Stanford will provide opportunities for training of students and international collaborators in the use of cutting-edge technology. Results will be incorporated in courses taught by PIs at all levels. Members of our research team include a significant number of women. Our research plan broadens the participation of underrepresented groups, namely Papua New Guinea collaborators, with University of Papua New Guinea students and faculty participating in geophysical and geologic fieldwork, analytical work, data acquisition, data processing, and data analysis. PIs will contribute to University of Papua New Guinea undergraduate curricula through lectures and short courses. The field area occupies a volcanic and seismically hazardous region; proposed outreach activities include educating the local communities about these natural hazards during the field campaigns. Funding to support University of Papua New Guinea students and staff to participate in field campaigns and also to travel to the US to work in the labs and participate in annual meetings is included in the budget. Funding will facilitate international collaboration with earth scientists at the Department of Geology at the University of Papua New Guinea, the Geological Survey of Papua New Guinea, Victoria University of Wellington, Institute for

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Geological and Nuclear Sciences, Macquarie University, as well as national collaboration with isotope geochemists at Stanford and Washington State University. Table 3. Summary timeline for proposed work. Year 1 Year 2 Year 3 Year 4 Year 5 Fieldwork: D’Entrecasteaux I (DI), Pocklington Rise (PR), Woodlark Rise (WR), Dayman Dome (DD), OBS = Ocean bottom seismometer

DI (SU, VUW, UCSB) Passive seismic site selection DD GPS campaign 1

PR Deployment DI + DD, OBS deployment; DD

WR Service and remove passive seismometers OBS retrieval GPS campaign 2

GPS campaign 3

Fault-slip analyses DI data PR data WR data Petrography & microstructural analyses

DI samples DI samples DD samples

DI, PR samples DD samples

PR, WR samples WR samples

Electron Probe Analyses DI samples DI samples

DI, PR samples DI samples

PR, WR samples DI samples

WR samples

ICP bulk rock / volcanic volatiles analyses

DI samples DI samples Volcanic samples

Volcanic samples

Electron Backscattered Diffraction

DI samples (TL) DI samples

PR samples (TL) DI samples

WR samples (TL) DI samples

Thermochronology AFT = apatite fission track, He = (U-Th)/He

40Ar/39Ar analyses of WR recon samples

DI, DD samples: U-Pb, 40Ar/39Ar, AFT, He

PR, DD samples: U-Pb, 40Ar/39Ar, AFT, He

WR samples: U-Pb, 40Ar/39Ar, AFT, He

Radiogenic isotope geochemistry (Vervoort)

DI samples

PR samples WR samples

U-Pb detrital studies (Vervoort)

DI samples

PR samples WR samples

Stable isotope analyses (Mulch)

DI samples

PR samples WR samples

Processing and Archiving Seismic data

PASSCAL data PASSCAL data PASSCAL data, OBS data

Interpretation of results; integration of data sets; manuscript prepared.

Seismic data analysis Scattered wave inversion; tomography; Surface waves & splitting

Scattered wave inversion; tomography; Surface waves & splitting

Local earthquakes; Surface waves & splitting

Integrating seismic results with petrology

Perple-X modeling

Modeling with geophysicists and geodynamicists

Geodynamic modeling Brittle extension Exhumation MRM vs BAM endmembers Synthesis Integrate P-T-t-D studies with geophysics &

geodynamic modeling Annual Meetings SU BU SU LDEO SU

Guide to colors: SU, BU, LDEO, UCSB, UTA, GPS collaborators, all PIs. BAM=Backarc model; DD=Dayman Dome; DI=D’Entrecasteaux Islands; He=(U-Th)/He; OBS=Ocean bottom seismometer; PR=Pocklington Rise; MRM=Microplate rotation model; WR=Woodlark Rise. RESULTS OF PRIOR NSF SUPPORT – LEAD INSTITUTION (All publications resulting from prior support are indicated in the reference list by an asterisk. See supplementary documents for PIs from all other institutions).

Suzanne Baldwin (SU), NSF EAR-0208334, $312,009, 06/01/02–05/31/06, “Continental Extension in the Western Woodlark Basin: P-T-t-D Constraints from Normanby and Misima Islands” (co-PI, P.G. Fitzgerald). The age of eclogite facies metamorphism in the D’Entrecasteaux Islands was determined and showed that the youngest exhumed eclogites known on Earth occur in the lower plates of MCCs west of the active seafloor spreading rift tip (Baldwin et al., 2004a; Monteleone et al., in press). Coesite was discovered and confirmed in an 8 Ma eclogite indicating that some rocks were subducted to mantle depths (Baldwin et al., 2005). Two new metamorphic core complexes on Normanby Island (Little et al., 2006, Tectonics, in press) and Misima Island (Peters et al., 2005) were documented. A microplate rotation model

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that invokes extensional eversion of an Australian continental slab previously subducted beneath the Papuan Ultramafic Belt was proposed to explain cm/yr HP/UHP exhumation in the MCC lower plates (Webb et al., 2005). Manuscripts in progress are based on work presented at national and international meetings (20 abstracts). A session was co-convened at the Fall AGU 2004 meeting. Funding supported Brian Monteleone’s Ph.D. dissertation (SU, now at ASU), Katie Peters’ M.Sc. thesis (VUW), and partially supported three graduate and five undergraduate research assistants (SU). Results of this study are discussed in the main body of this proposal.

Paul Fitzgerald (SU), NSF EAR-0002008. $106,996, 4/1/00–3/31/05. "Collaborative Research": Four-dimensional evaluation of a major continental detachment fault: Structural, paleomagnetic and thermochronologic constraints” (co-PIs J.E. Faulds, E.M. Duebendorfer). This study examined the along-strike variation of the South-Virgin-White Hills detachment that extends ~60 km from SE Nevada into NW Arizona along which displacement decreases north to south from ~17 to ~5 km. Apatite fission track thermochronologic data from footwall samples tested end-member hypotheses, concluding that timing of extension is synchronous along strike, but that fault slip is faster (~8 km/my) in the north than the south (1–2 km/my) (Fitzgerald et al. 2003, 2006). Differential movement can be geometrically envisaged as a result of rotation about a pole lying south of the White Hills, yet combining structural observations (Duebendorfer et al., 2003) with paleomagnetic data (Faulds et al., 2004) and thermochronologic data indicates differential motion is also accommodated on transverse structures (Fitzgerald et al., in prep.). A new vector approach using low-temperature thermochronology to constrain fault dip during slip was also proposed (Fitzgerald, 2006).

L.E. Webb (SU), NSF-EAR 0537165, Tectonics, $267,223, 3/06–2/09, "Collaborative Research: Strike-Slip History of the East Gobi Fault Zone, Mongolia: Modes of Intraplate Deformation, Sedimentary Basin Evolution, and Regional Fault Linkages" (co-PI C.L. Johnson, University of Utah). This newly funded project employs a multidisciplinary approach to constrain the timing, kinematics, and amount of offset associated with two sinistral strike-slip phases along the East Gobi Fault Zone: early Mesozoic ductile shear and Cenozoic brittle faulting. The PIs, two PhD students, and one undergraduate student completed a five week field season in collaboration with Mongolian researchers and students in June–July 2006. We tracked the fault zone along strike toward the Chinese border and assessed and sampled potential offset markers. Field data and samples are currently being processed and microstructural analyses are underway. Three conference presentations have included findings from the 2006 field season.

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1 References * - denotes references related to results from prior support for PI's from Syracuse University Abbott, L. D., Silver, E. A., Thompson, P. R., Filewicz, M. V., Schneider, C., and Abdoerrias, A., 1994,

Stratigraphic constraints on the development and timing of arc-continent collision in northern Papua New Guinea: Journal of Sedimentary Research, v. B64, p. 169-183.

Abers, G.A., 1991, Possible seismogenic shallow-dipping normal faults in the Woodlark-D'Entrecasteaux extensional province, Papua New Guinea: Geology, v. 19, p. 1205-1208.

Abers, G.A., 2001, Evidence for seismogenic normal faults at shallow dips in continental rifts, in Wilson, R.C.L., Whitmarsh, R.B., Taylor, B., and Froitzheim, N., eds., Non-volcanic rifting of volcanic margins: a comparison of evidence from land and sea, Volume 187: London, The Geological Society of London, p. 305-318.

Abers, G.A., Mutter, C.Z., and Fang, J., 1997, Shallow dips of normal faults during rapid extension: Earthquakes in the Woodlark-D'Entrecasteaux rift system, Papua New Guinea: Journal of Geophysical Research, v. 102, p. 15,301-315,317.

Abers, G.A., A. Ferris, M. Craig, H. Davies, A.L. Lerner-Lam, Mutter, J.C., and Taylor, B., 2002, Mantle compensation of a region of active metamorphic core complexes, Woodlark Rift, Papua New Guinea: Nature, v. 418, p. 862-865.

Abers, G.A., and Roecker, S., 1991, Deep structure of an arc-continent collision: earthquake relocation and inversion for upper mantle P and S wave velocities beneath Papua New Guinea: J. Geophys. Res., v. 96, p. 6379-6401.

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*Baldwin, S.L., Monteleone, B., Webb, L.E., Fitzgerald, P.G., Grove, M., and Hill, E.J., 2004a, Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea: Nature, v. 431, p. 263-267.

*Baldwin, S.L., Finn, C.A., Webb, L.E., Fitzgerald, P.G., Little, T., and Anderson, E., 2004b, Microplate rotation leads to rapid exhumation of a subduction complex in eastern Papua New Guinea: EOS Transactions of the American Geophysical Union, Fall Annual Meeting, v. 85, p. F1738.

*Baldwin, S. L., 2005, Thermochronology of active plate boundaries: Geological Society of America Abstracts with Programs, v. 37, no. 7.

*Baldwin, S.L., E., W.L., and Monteleone, B., 2005a, Late Miocene–Pliocene eclogites of eastern Papua New Guinea: the youngest known HP/UHP terrane on Earth, Seventh International Eclogite Conference, Volume 150: Seggau, Austria, Mitt. Österr. Miner. Ges.

*Baldwin, S. L., Webb, L. E., Monteleone, B. D., Little, T. A., Fitzgerald, P. G., and Chappell, J. L., 2005b, Metamorphism and exhumation of the youngest known HP/UHP terrane on Earth, eastern Papua New Guinea: EOS, Transactions, American Geophysical Union, v. 86, no. 52, p. V54B-01.

*Baldwin S.L., and Fitzgerald, P. G., 2006, Using thermochronology to determine the timing and rates of tectonic processes, in 16th Annual V.M. Goldschmidt Conference, Melbourne, Australia.

*Baldwin, S. L., Monteleone, B. D., Little, T. A., Webb, L. E., and Fitzgerald, P. G., 2006, Subduction to rifting evolution of the Australian-Woodlark plate boundary zone of eastern Papua New

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2 Guinea: insights into the 4-D nature of continental subduction and exhumation processes: Geological Society of America Abstracts with Programs, v. 38, no. 7, p. 274.

*Baldwin S.L., Webb, L. E., Monteleone, B., Little, T. A., Fitzgerald, P. G., Peters, K., and Chappell, J. L., 2006, Continental Crust Subduction and Exhumation: insights from eastern Papua New Guinea, in 16th Annual V.M. Goldschmidt Conference, Melbourne, Australia.

Brooks, K., and Tegner, C., 2001, Affinity of the Leg 180 dolerites of the Woodlark Basin: geochemistry and age, in Huchon, P., Taylor, B., and Klaus, A., eds., Proceeding of the Ocean Drilling Project Scientific Results, Volume 180, http://www-odp.tamu.edu/publications/180_SR/180TOC.HTM, p. 1-18.

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mantle and crust: Bulletin of the Bureau of Mineral Resources, Australia, v. 128, p. 1-48. Davies, H.L., 1980, Crustal structure and emplacement of ophiolite in southeastern Papua New Guinea:

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