E O S

Eos, Vol. 8 0 , No. 42 , October 1 9 , 1 9 9 9

VOLUME 80 NUMBER 42

OCTOBER 19,1999

PAGES 4 9 3 — 5 0 4

EDITORIAL

Your Vote Counts PAGE 493

The election of the AGU officers who will guide our society into the 21st Century has begun.Your ballot provides you with an op­portunity to make a difference. In the last four elections five offices were decided by less than 10 votes; one by a single vote and two others by just three votes.

AGU can only be as strong as the participa­tion of members makes it. The one activity in which every member should participate is vot­ing! The officers you select set policy for the Union and authorize its programs; they also speak for the community of members on scien­

tific issues of importance to the public. If you fail to vote, you let others pick your spokesmen.

Although the total number of votes has in­creased over time, the percentage of mem­bers voting has decreased. Only 16.4% of the membership voted in the election 2 years ago. This was down from 23% of members casting votes in 1989.

AGU is almost unique among scientific socie t ies in making students full members of the society. In AGU the only difference for students is what they are asked to pay Students may vote and hold office. Howev­er, in the last e lect ion fewer than 6% of the students in the membership voted.The cur­rent e lect ion provides a new opportunity for students to b e c o m e more active in the

Union by exercising their right—responsi­bility—to assist in determining who will lead the Sect ions they are interested in and the Union itself.

Members living outside North America have been, on the whole, somewhat less well repre­sented among the voters than they are in the membership numbers. Of special interest in this election is that several candidates on the ballot reside outside North America.

Biographies and photographs of all of the candidates can be found at http://www.agu. org/elections and were published in the Sep­tember 7,14, and 21 issues of Eos.

AGU's current programs and future growth depend on your participation as a member. Your vote is one way that you participate. Your vote does make a difference to the Union and to you.

Ballots were mailed during the first week in October.The deadline for the receipt of ballots at AGU headquarters is December 13,1999.

Vote please.

Tectonic Evolution in the Rocky Mountain Region: 4-D Imaging of the Continental Lithosphere PAGES 493 ,495 ,498

An interdisciplinary study under way this summer and fall in the southern Rocky Mountains hopes to show that the lithospher-ic structure produced there during the formation of the southwestern United States profoundly influenced both physical and chemical modification of the continental lithosphere during all subsequent tectonism. Known as the Continental Dynamics-Rocky Mountain (CD-ROM) project, the study is fo­cusing on understanding the assembly, growth, stabilization, reactivation, and present structure of the Rocky Mountain lithosphere.

The Rocky Mountain region of the western United States has experienced a complex geo­logic history that can only be unraveled by the integration of a broad range of geosci-ence data. This region is of great fundamental interest to studies of the dynamics and evolu­tion of the North American continent. It is

also an area where a better understanding of the scarce water resources, natural resources, and earthquake hazards depends on knowl­edge of the geologic structures present.

It was with this in mind that the Continental Dynamics Program of the National Science Foundation recently provided major funding for this large collaborative geological and geo­physical effort to study the area of southern Wyoming, Colorado, and northern New Mexico shown in Figure l.The German counterpart to NSF and NASA is also supporting the project.

As suggested by the northeast trending zones of low velocity mantle that seem to parallel Pro-terozoic crustal boundaries, the influence of the Rocky Mountain lithosphere may extend to the ongoing reorganization of small-scale as-thenospheric convection and the lithospheric tectonic and magmatic response [Karlstrom and Humphreys, 1998].These zones are within an overall continental-scale northwest trending velocity transition in the mantle that underlies

the transition from the stable craton to the west­ern, tectonically active part of North America.

The hypothesis, if verified, would place pro­found constraints on developing theories of evolution of continental lithosphere by providing a comparison of Archean versus Proterozoic lithospheres, by delineating the process of growth of first-cycle continental lithosphere in the Proterozoic, and by analysis of the interplay between "active" and "passive" processes during lithosphere "disassembly" The target area is an ideal place to combine crustal and mantle geophysical, geological, and geochemical data to characterize the evolution of lithospheric provinces and boundaries in four dimensions from the time of accretion to the present.

Two Tectonic Regimes

The area represents the juncture of two globally significant tectonic regimes that are of fundamental importance for understanding continental tectonics. Together these regimes are an outstanding field laboratory for studies of the continental lithosphere.

First, a 1500-km-wide juvenile Proterozoic orogenic belt records an episode of rapid accretion of continental material from man­tle sources and the assembly of related ter-

T R A N S A C T I O N S , A M E R I C A N G E O P H Y S I C A L U N I O N

Eos, Vol. 80 , No. 42 , October 1 9 , 1 9 9 9

110 W 106 W 44 N

40 N

32 N

102 W Geologic Targets © Archean -

Proterozoic Suture

~ \£ ) Fish Creek-Soda Creek Shear Zone

©

Laramide thrust fault

Xenolith localities

Colorado mineral belt and Proterozoic

shear zones North boundary

Yavapai-Mazatzal province transition

Frontal Laramide thrust fault

© © Jemez lineament

and Proterozoic shear zones

© Basement reflectors seen on industry seismic data

Fig. 1. Index map of the Continental Dynamics-Rocky Mountain study area with exposures of Precambrian rocks outlined and filled with light gray. Solid lines represent the two reflection profiles. Dashed line represents the refraction/wide-angle reflection profile. Wavy lines represent shear zones in Precambrian rocks. Stars represent shotpoints for the Deep Probe project. Dashed boxes along the reflection profiles indicate locations of seismic arrays being deployed for teleseismic studies.

ranes (Yavapai, Mazatzal) to southern Laurentia between 1.8 and 1.6 Ga. Second, the present high elevations of the regional orogenic plateau in the southern Rocky Mountain/Rio Grande rift region in particu­lar are the manifestation of Phanerozoic and still ongoing modification and disassembly of Proterozoic lithosphere.

This area lies on a profound lateral velocity gradient in the mantle that marks the transition from high velocities under the core of North America to low velocities under the western United States. Clues to the nature and origin of this transition and hence of mantle dynamics will come from better images of the complex transition in the Rocky Mountain region.

CD-ROM involves a series of seismic experi­ments during the summer and fall of 1999 to image the structure of the lithosphere along a

transect that obliquely crosses the Rocky Mountain region from central Wyoming to central New Mexico (Figure l ) .The project is collecting about 400 km of deep seismic reflection data. Because both surface and deep structures are being studied, additional active and passive source seismic experiments are being undertaken. After being analyzed and coordinated, the diverse data will be inte­grated with the broad spectrum of geological, geochemical ,and geophysical data being gathered along the transect.

For geologic control, the transect was cho­sen to coincide with exposures of Proterozoic basement.Thus, by combining geophysical data with geological studies of Proterozoic shear zones, crustal xenoliths, Laramide struc­tures, thermochronology and denudation his­tory, CD-ROM will provide new images of the

structure and new insights into the tectonic evolution of the Rocky Mountain region.

Emulating Canadian Model

This project emulates and builds upon the highly successful interdisciplinary approach of the Canadian Lithoprobe project and complements the cooperative Canadian-U. S. Deep Probe experiment (Figure 1) that tar­geted the mantle of western North America [Deep Probe Working Group, 1998; Snelson et al., 1998] .Our 3-year investigation began in 1997 and focuses on lithospheric bound­aries within the CD-ROM transect (Figure 1) . Following the Canadian model, CD-ROM brings together a critical mass of investiga­tors—19 in all from 14 different institutions —to document processes of assembly and modification of both crustal and mantle provinces along a transect that is more than 1000 km in length.

B e c a u s e of the chal lenge of simultane­ously studying li thosphere assembly along northeast trending boundaries and its dis­assembly along north to northwest trending boundaries, the approach also involves an array of geophysical studies linked at strate­gic tie points and integrated with diverse geologic investigations. In particular, an integrated set of large-scale seismic experi­ments will be undertaken using state-of-the-art data acquisi t ion techniques . Satellite data, image processing, digital terrain mod­els, Geographic Inform-ation System (GIS) technology, and modern visualization s chemes will be employed to create a data­base suitable for integrated analysis and for viewing in creative ways.

Boundaries Targeted

In the first of five major components of the project, active source seismic experiments will span the entire transect and target two key boundaries: the Archean-Proterozoic boundary exposed at the Cheyenne Belt near the Colorado/Wyoming border and the inter­preted boundary between Proterozoic terranes along the Jemez lineament, an align­ment of Neogene volcanic centers in north­ern New Mexico (Figure l ) .The Cheyenne belt is the Proterozoic suture zone between the Archean Wyoming province and accreted Proterozoic arc terranes [Chamberlain, 1998]. The recent Canadian-US. Deep Probe experi­ment showed that this general area of south­ern Wyoming represents a dramatic transition in crustal and mantle structure. However, the experiment was not detailed enough to pin­point the location of this transition and char­acterize it in any detail.

CD-ROM reflection studies over the exposed Cheyenne Belt shear zone (Figure 1) will test whether the lithospheric scale boundary

Eos, Vol. 80 , No. 42 , October 1 9 , 1 9 9 9

coincides with the inferred ancient suture. Proposed sutures between Proterozoic arcs in Colorado and New Mexico are more difficult to define at the surface [Shaw and Karlstrom, 1999],although numerous northeast trending shear zones and geophysical anomalies are candidates that can be evaluated by our new seismic data. In particular, an ~220-km-long deep seismic reflection line in New Mexico (Figure 1) will cross the Jemez lineament.

The acquisition of the seismic reflection da­ta is scheduled for fall 1999.The entire core of the transect will be traversed by a 1000-km-long seismic refraction/wide-angle reflection profile that was recorded during August 1999. This element of the project has been greatly enhanced by the German contribution, and its main purpose is to produce a refined velocity structure for the crust and uppermost mantle.

In a second major component, teleseismic studies are being conducted along the north­ern and southern portions of the transect (Figure 1) and will be combined with existing data for Colorado [Lerner-Lam et al., 1998] to target mantle structure, velocities, and aniso-tropy.The deployment began during spring 1999. Receiver functions derived from these data will be integrated into a regional analysis of crustal structure. Initial results indicate irregular Moho "topography" in Colorado that does not correlate well with surface topogra­phy [Sheehan et ai, 1995; Keller et al., 1998] . These data will extend the coverage of new mantle data that are emerging from the Col­orado Plateau, Snake River Plain, and Deep Probe experiments.

Exposed Rocks

Geologic and geochronologic studies of ex­posed rocks along the transect are the third major component of the project.The studies are essential for tying reflections to surface structures with known histories. Our three ma­jor targets are the Proterozoic shear zones, Laramide thrusts, and the present architecture of the Rocky Mountains.The shear zones date back to the 1.8-1.6 Ga time of assembly of the crust but also were reactivated at 1.4 Ga. Most shear zones are subvertical at the surface, but these vertical structures are inferred to repre­sent rotated and folded boundaries that were initially low angle [Shaw and Karlstrom, 1999].

Based on experience from the Lithoprobe experiments as well as geologic studies along the transect, reflection data may reveal a crustal-scale horizontal thrust geometry that has been overprinted and obscured by vertical foli­ations. Laramide structures have long been enigmatic in terms of their regional dip (steep versus shallow) and depth of penetration. Com­bined with regional balanced cross sections, we hope to image the depth to which these struc­tures penetrate (midcrust, Moho, or possibly even deeper). Geologic studies of denudation and uplift history are also under way to help

evaluate the timing and mechanism of esta­blishment of buoyant mantle that seems to underlie the highest parts of the Rocky Moun­tains [Pazzaglia and Kelley, 1998].

Xenolith studies, the fourth major compo­nent, have been undertaken to directly investi­gate the compositional and physical proper­ties of deep-seated portions of the Precam­brian crust [Lester and Farmer, 1998] .The studies so far have emphasized xenoliths en­trained in Devonian kimberlite diatremes from the State Line district in northern Colo­rado and in Late Cenozoic ultrapotassic lavas at Leucite Hills, south and north, respectively, of the Cheyenne Belt (Figure 1).

Midcrustal intermediate to felsic composi­tion xenoliths from the State Line kimberlites have U-Pb zircon ages clustering at 1.7 Ga and 1.45 Ga [Abbasi et ai, 1998] .These ages are broadly consistent with the ages of rocks ex­posed near the surface and so bear no evi­dence of Archean age crust dipping south­ward beneath the exposed Proterozoic crust in northern Colorado.

Lower crustal mafic granulite xenoliths en­trained in the kimberlites have yet to be directly dated by U-Pb methods but display a range of Nd model ages from 1.0 Ga to 1.7 Ga. These xenoliths have compressional wave velocities, at lower crust pressures, that vary from 6.8 km/s to 7.0 km/s and show evidence of one main metamorphic reaction, involving growth of gar­net and plagioclase ± clinopyroxene from earli­er orthopyroxene and plagioclase.Textures suggest that the latter reaction occurred under progressively lower temperature conditions and is consistent with a history involving isobaric cooling of orthopyroxene-bearing igneous rocks in the deep crust.

The Nd isotopic compositions of the mafic granulites eNd at 50 Ma of -5.7 to -12.6 overlap the range of initial Nd isotopic compositions for Late Cretaceous/Early Tertiary mafic mon-zonites exposed in the Front Range of north­ern Colorado and are at least consistent with models that involve the mafic lower crust in the formation of the magmas parental to these rocks. Ongoing studies of the mafic xenoliths involve electron microprobe and laser ablation inductively coupled plasma-mass spectrometry studies of the major and trace element compositions of constituent mineral phases, which, along with mineral separate isotopic studies, will be used to fur­ther assess the age and metamorphic history of the lower crust represented by the mafic granulite xenoliths.

The fifth major component is integrated analysis of all available data, employing mod­ern GIS, image processing, and visualization schemes. For example, gravity data coverage is good for the CD-ROM area, and analysis of these data constrained by seismic results can extend the seismic results both laterally and vertically [Snelson et al, 1998]. In addition, the act of simply overlaying different types of geo­

logical and geophysical information on images of remote sensing data can often reveal anomalous relationships that warrant further investigation.

One major goal of the project is to catalyze the next generation of collaborative studies of the Rocky Mountain lithosphere. In that regard, we welcome piggyback experiments, collaboration, and other interactions.Two topical sessions—one on orogenic plateaus and another on Cenozoic tectonics of the southern Rocky Mountains—at the Geologi­cal Society of America annual meeting in October 1999 will have CD-ROM contribu­tions and also focus on related issues. A ses­sion at AGU's Fall Meeting will also take up the lithospheric structure and evolution of the Rocky Mountain region. Up-to-date infor­mation on the project and associated activi­ties can be accessed at Web site: www.geo. utep.edu/CD-ROM.

Acknowledgments

In addition to the authors, the Continental Dynamics-Rocky Mountain working group includes Sam Bowring, Nik Christensen, Ken Dueker, Eric Erslev, Matt Heizler, Gene Humphreys, Roy Johnson, Shari Kelley Alan Levander, Kate Miller, Frank Pazzaglia, Claus Prodehl,Anne Sheehan, Scott Smithson,Sean Willett,and Mike Williams.

We thank Leonard Johnson of the Conti­nental Dynamics program of NSF for sup­port. We also thank the German Research Society for its support.

Authors

G Randy Keller, Karl E. Karlstrom, and G. Lang Farmer For more information, contact G. Randy Keller, Department of Geological Sciences, University of Texas at El Paso, El Paso,TX 79968 USA; E-mail: keller@geo.utep.edu

References

Abbasi, K., S. A. Bowring, and G. L. Farmer, U-Pb zircon dates from lower crustal xenoliths, State Line kim­berlite district, Colorado (abstrj, Geol. Soc. Am. Abstr.Prog.,p.A-352,1998.

Chamberlain, K. R., Medicine Bow orogeny:Timing of deformation and model of crustal structure produced during continent-arc collision, ca . 1.78 Ga, southwestern Wyoming, Rocky Mt. Geol, 33, 259-277,1998.

Deep Probe Working Group, Probing the Archean and Proterozoic lithosphere of western North America, GSA Today, 8,1-5,1998.

Karlstrom, K.E.,and E. D. Humphreys, Persistent influ­ence of Proterozoic accretionary boundaries in the tectonic evolution of southwestern North America: Interaction of cratonic grain and mantle modifica­tion events,Rocky Mt. Geol, 33,161-180,1998.

Keller, G. R., C. M. Snelson, A. FSheehan, and K. G. Dueker, Geophysical studies of crustal structure in the Rocky Mountain region: A review, Rocky Mt. Geol., 33 ,217-228 ,1998 .

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Lerner-Lam, A., A. Sheehan, S. Grand, E. Humphreys, K. Dueker, E. Hessler, H. Guo, D. K. Lee, and M. Savage, Deep structure beneath the Southern Rocky Moun­tains from the Rocky Mountain Front Broadband Seis­mic Experiment,Rocky Mt. Geol., 33,199-216,1998.

Lester, A., and G. L. Farmer, Lower crustal and upper mantle xenoliths along the Cheyenne belt and vicinity,Rocky Mt. Geol., 33 ,293-304 ,1998 .

Pazzaglia, FJ . , and S. A. Kelley, Large-scale geomor-phology and fission-track thermochronology in topographic and exhumation reconstructions of the Southern Rocky Mountains,Rocky Mt. Geol., 33 ,229-257 ,1998 .

Shaw, C. A., and K. E. Karlstrom,The Yavapai Mazatzal crustal boundary in the Southern Rocky Mountains,Rocky Mt. Geol., 34 ,37 -52 ,1999 .

Sheehan, A. E, G. A. Abers, A. L. Lerner-Lam, and C. H. Jones , Crustal thickness variations across the Col­orado Rocky Mountains from teleseismic receiver functions,./ Geophys. Res., / #0 ,20 ,391-20 ,404 ,1995 .

Snelson, C. M.,T. J . Henstock, G. R. Keller, K. C. Miller, and A. Levander, Crustal and uppermost mantle structure along the Deep Probe seismic profile, Rocky Mt. Geol., 33,181-198,1998.

Ice Thickness Derived From High-Resolution Radar Imagery PAGES 4 9 5 , 4 9 7

A new analysis makes it possible to produce basin-scale estimates of sea ice age and thick­ness from observations of ice motion in radar images of the Arctic Ocean. Anticipated from sequential synthetic aperture radar (SAR) imagery over a series of winters are Arctic-wide fields of ice motion and estimates of ice age and thickness.This analysis technique is implemented in the RADARSAT Geophysical Processor System (RGPS),a data processing system dedicated to analyzing large volumes of SAR imagery of sea ice [Kwok, 1998].

Currently, we have only crude estimates on how much ice there is, how it varies in space and time, where ice is produced and where it melts, and how rapidly it is transported from place to place. SAR provides an amazingly detailed look at sea ice, but the level of detail is so great that it is not at first apparent how to utilize the data for improving sea ice data sets and models.

RGPS is offered as a solution to this conun­drum. It takes over 400 wide-swath radar ima­ges every 24-day cycle and produces gridded fields of ice motion, deformation, and thick­ness. The sea ice thickness distribution is an essential descriptor of the Arctic Ocean sea ice mass and heat balance and is a record of the interplay between dynamics (lead forma­tion, ridging, and advection) and thermody­namics ( ice growth and melt).

These geophysical products can be put to a variety of uses, such as analyzing new ice climatologies and testing ice models or new ideas about sea ice rheology.They also can be assimilated into sea ice models.

Our present knowledge of Arctic ice thick­ness distribution is derived largely from analy­ses of sonar data from submarine cruises. Moored upward-looking sonars have also been used to sample the thickness distribu­tion at fixed locations. However, these observa­tions do not provide a complete spatial pic­ture or continual updates of the distribution.

The new analysis technique allows us to monitor the spatial and temporal variability of the thin ice distribution (< 2 m) in the Arc­tic Ocean using high-resolution SAR imagery This new method estimates ice age and thickness from repeated observations of

Lagrangian elements or cells of sea ice in sequential SAR imagery.

Figure 1 shows the time series of observa­tions of one cell within a matrix of such cells, graphs of the history of cell area, and the thin ice coverage within that cell at the end of a 41-day period. Line segments con­necting the four vertices of a cell define its boundaries.The drift and deformation of a cell over time are obtained by tracking the displacement of its vertices in the SAR imagery. The motion tracking procedure is described by Kwok et al. [1995] .

The age histogram of the ice in a cell is computed from the temporal record of area changes. An age histogram of sea ice specifies the fractional area covered by ice of different chronological ages. Every time a new observa-

a)

tion is available, we interpret a positive area change as the creation of an area of open water. New ice is assumed to grow over this area immediately after opening.The uncer­tainty of the ice age occupying this area is dependent on the time interval between ob­servations. This age range is recorded as a new age category in the histogram. At the same time a new category is introduced, exist­ing age categories are "aged" by the same time interval. In Figure 1, this procedure created five ice age categories from the sequence of positive area changes since Day 335.

Youngest Ice in Cell

A negative change is assumed to have ridged the youngest ice in the cell, reducing its area.The assumption is that once ridging starts, deformation tends to be localized in the recently formed thinner and weaker ice in leads.This area of ridged ice is tracked as a separate category in the age histogram. Ice age is converted to ice thickness using an

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Fig. 1. Time series observations of one Lagrangian element (initially enclosed by a 10 x 10 km square) in RADARSAT imagery, a) The deformation of a cell over a period of 41 days. The area of the cell stays fairly constant until Day 338 when a lead opens. The same lead continued to open between Days 341 and 345, contributing new ice area and ice categories to the cell. At the end of the 41-day period, the area of the thin ice occupies more than 50% of the total cell area, b) The history of the area of the cell, c) The thin ice categories occupying the new areas created since Day 312. The largest thin ice category was created between Days 341 and 345 (RADARSAT imagery, Copyright CSA, 1999).