Earth-Science Reviews 129 (2014) 85–99

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Earth-Science Reviews

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Formation of plate boundaries: The role of mantle volatilization

Tetsuzo Seno a,⁎, Stephen H. Kirby b

a Earthquake Research Institute, University of Tokyo, Bunkyo-ku, Tokyo, 113-0032, Japanb U.S. Geological Survey, 345 Middlefield, Menlo Park, CA 94025, USA

⁎ Corresponding author: Tel.: +81 3 5841 5747; fax: +E-mail address: seno@eri.u-tokyo.ac.jp (T. Seno).

0012-8252/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.earscirev.2013.10.011

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 August 2013Accepted 18 October 2013Available online 6 December 2013

Keywords:Plate tectonicsInitiation of subductionPlate boundariesHydrationCarbonationSerpentine

In the early Earth, convection occurred with the accumulation of thick crust over a weak boundary layerdownwelling into the mantle (Davies, G.F., 1992. On the emergence of plate tectonics. Geology 20, 963–966.).This would have transitioned to stagnant-lid convection as the mantle cooled (Solomatov, V.S., Moresi, L.-N.,1997. Three regimes of mantle convection with non-Newtonian viscosity and stagnant lid convection on theterrestrial planets. Geophys. Res. Lett. 24, 1907–1910.) or back to a magma ocean as the mantle heated (Sleep,N., 2000. Evolution of the mode of convection within terrestrial planets. J. Geophys. Res. 105(E7): 17563–17578). Because plate tectonics began operating on the Earth, subductionmust have been initiated, thus avoidingthese shifts. Based on an analogy with the continental crust subducted beneath Hindu Kush and Burma, we pro-pose that the lithosphere was hydrated and/or carbonated by H2O–CO2 vapors released frommagmas generatedin upwelling plumes and subsequently volatilized during underthrusting, resulting in lubrication of the thrustabove, and subduction of the lithosphere alongwith the overlying thick crust. Once subduction had been initiat-ed, serpentinized forearc mantle may have formed in a wedge-shaped body above a dehydrating slab. In relictarcs, suture zones, or rifted margins, any agent that warms and dehydrates the wedge would weaken the regionsurrounding it, and formvarious types of plate boundaries depending on the operating tectonic stress. Thus, oncesubduction is initiated, formation of plate boundariesmight be facilitated by amajor fundamental process:weak-ening due to the release of pressurized water from the warming serpentinized forearc mantle.

© 2013 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 852. Subduction initiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

2.1. Subduction of the continental lithosphere beneath Hindu Kush and Burma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 862.2. The Reunion and Kerguelen hotspots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 872.3. Hydration/carbonation of oceanic lithosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 892.4. Collision versus subduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 902.5. Subduction initiation in the Archean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

3. Serpentinized forearc mantle wedge and its role in the formation and mobilization of plate boundaries . . . . . . . . . . . . . . . . . . . . . . 903.1. Divergent boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 913.2. Convergent boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 923.3. The San Andreas fault system (SAF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 933.4. Other continental and oceanic transform faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 935. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

81 3 5802 3391.

ghts reserved.

1. Introduction

At present, plate tectonics operates on the Earth. This was probablynot the case during the magma ocean stage (Warren, 1993; Sleep,

86 T. Seno, S.H. Kirby / Earth-Science Reviews 129 (2014) 85–99

2000). Paleomagnetic, geochemical, and isotope studies suggest thatplate tectonics had started earllier than the Archean/Proterozoic bound-ary (Burke et al., 1976; Campbell and Griffiths, 1992). However, even ifwe accept that it had started by that time, it is not easy to describe howit began. Because of the strongly temperature-dependent viscosity ofthemantlematerial (e.g. Kirby, 1983), the lithosphere is not easy to rup-ture. A number of studies show that subduction initiation requires highlevels of stress to disrupt the lithosphere, which is a few tens of kmsthick (e.g. McKenzie, 1977; Cloetingh et al., 1984; Mueller and Phillips,1991). Meanwhile, the largest available plate tectonic forces are of theorder of 1013 N/m (Molnar and Gray, 1979; Parsons and Richter, 1980;England and Molnar, 1991; Fleitout, 1991); these forces are insufficientto cause top-to-bottom disruption of a plate. Strain-weakening is thusrequired to form new plate boundaries (e.g. Bercovici, 1998; Moresiand Solomatov, 1998; Tackley, 1998; Regenauer-Lieb et al., 2001;Tommasi and Vauchez, 2001; Bercovici, 2003). Because plate tectonicsdoes not operate on Venus or Mars, it is inferred that the weakeningmechanisms should include effects of volatiles like H2O and CO2 (e.g.Bercovici, 1998; Regenauer-Lieb et al., 2001; Solomatov, 2004; O'Neillet al., 2007). Their effects on the thermo-mechanical properties of thelithosphere have been considered in numerical simulations of the initi-ation of plate boundaries (e.g. Bercovici, 1998; Regenauer-Lieb et al.,2001). They are, however, in general, not in the context of the formationof each type of plate boundary during the Earth's tectonic history.Eclogitization of granulites induced byfluid injectionmight have playedan important role in deforming the lithosphere (Austrheim et al., 1997;Bjornerud et al., 2002) and could be an example of the effects ofvolatiles. However, it is not yet clear how eclogitization played a rolein the initiation of plate tectonics.

Another difficulty in initiating plate tectonics is the production of alarge amount of basaltic crust at the surface by upwelling of themantle.This occurred due to the high mantle potential temperature during theArchean (Sleep and Windley, 1982; Bickle, 1986; McKenzie and Bickle,1988; Vlaar et al., 1994). The buoyant crust does not cool enough,which prevents subduction and leads to heating-up of the Earth'smantle and a shift back to a magma ocean (Sleep, 2000).

In this paper, we present a scenariowhereH2O and CO2 play a role inthe initiation of plate boundaries in the context of realistic tectonic sit-uations. We propose that subduction was initiated by volatilization ofthe underthrusting lithosphere that was once hydrated or carbonatedby upwelling plumes. The ultramafic minerals that constitute the litho-sphericmantle are affected by vapors rich in H2O–CO2 released from so-lidifying magmas in a plume head, and are altered to minerals such asamphibole, phlogopite, dolomite, magnesite, chlorite, and serpentine(Menzies et al., 1987; Spera, 1987; Wyllie, 1988). If such hydrated/carbonated lithosphere approaches a convergent zone and is under-thrust, volatilization from it would occur due to increases in tempera-ture and pressure. The released volatiles would lubricate the thrustabove, andmake subduction initiation possible. This inference is obtain-ed from the fact that subduction of the lithospherewith thick continen-tal crust is now occurring beneath Hindu Kush and Burma according tointermediate-depth seismicity observed there (Seno and Rehman,2011), although the nature of the crust is chemically different fromthat in the Archean. The lower portion of the lithosphere in these re-gions is likely to have been hydrated/carbonated by plumes when itpassed over the Reunion and Kerguelen hotspots. This will be describedlater in more details.

Once subduction started, formation of serpentinized forearc mantlein a wedge-shaped body above a dehydrating slab followed. Kirbyet al. (2003, 2013) proposed that the San Andreas fault system (SAF)was mobilized by weakening of the warming and dehydratingserpentinized forearc mantle, following the cessation of subduction ofthe Farallon plate. Serpentinized wedge mantle formed in a forearcmay be found in certain tectonic settings, such as relict arcs, suturezones, and rifted margins. If the wedge is warmed by any agent, the re-sultant released pressurized water will weaken the region surrounding

the wedge, and will form various types of plate boundaries, dependingon the stress regime. In the latter half of this paper, we show applicationof this weakening mechanism to the creation of each type of plateboundary during the stage after subduction had been initiated.

2. Subduction initiation

In this section, we treat the case where we consider that the initia-tion of subduction has not occurred. In uniform-viscosity convection,both convergent and divergent motions occur at the surface. This maymimic the convection that occurred during the early stages of theEarth with a weak boundary layer sinking into the mantle (Fig. 1a andb, Davies, 1992). However, in these conditions, crust thicker than themodern one would form over the lithosphere (Sleep and Windley,1982; McKenzie and Bickle, 1988). Such crust results from primordialmelt production of themantle having a basaltic composition and a den-sity of ~2900 kg/m3 (McKenzie and Bickle, 1988). It is still preventedfrom subduction due to the plate being thin during the Archean(Davies, 1992; See also Cloos, 1993). At convergent margins, the crustis offscraped, thrust upward, and juxtaposed over crustal slivers, as inmodern collision zones (Mattauer, 1986), with the mantle boundarylayer downwelling alone, leaving the surface tectonics behaving unlikeplate tectonics (Davies, 1992; Fig. 1a and b). In such a tectonic situation,if the average Earth's surface heat flow is larger than the radioactiveheat production, themantle is cooled. If the boundary layer is too strongto be subducted, there occurs a shift to stagnant-lid convection like thaton Venus (Solomatov andMoresi, 1996, 1997; Sleep, 2000). Conversely,if the average surface heat flow is smaller than the radioactive heatproduction, the mantle becomes hotter, and large-scale melting of themantle (i.e. a magma ocean) resumes (Sleep, 2000). Therefore, forplate tectonics to start on the Earth, the lithosphere must be subductedalong with its thick crust as a whole, avoiding the shift to stagnant-lidconvection or to a magma ocean. However, it seems unlikely to occurin preference to the buoyant crust piling up at the surface.

2.1. Subduction of the continental lithosphere beneath Hindu Kush andBurma

We propose a mechanism that describes the start of plate tectonicson the Earth. The locations where continental lithosphere with thickcrust subducts into the modern Earth give us a clue. Hindu Kush andBurma provide evidence of the mechanism (See Rehman et al., 2011and the references therein for the geologic history of this region, andsee Seno and Rehman, 2011 for the tectonic setting). Hindu Kush islocated within the Asian plate, southwest of Pamir and west ofKarakorum. To the south, there is the Mesozoic Kohistan–Ladakh arc,which was welded to Asia during the late Mesozoic. India later collidedwith this arc in the early Tertiary. The geological terranes associatedwith the collision between India and Kohistan–the high Himalaya, thelesser Himalaya, and the Siwaliks–are similar to those in otherHimalayan regions where India and Asia collided. However, the widthsof the high and lesser Himalayas are only ~1/2 of those in other regions.This indicates that offscraping and accretion of the underthrustingIndian continental margin crust have had a smaller extent south ofKohistan.

The plate boundary where the Indian plate is currently underthrustis located south of Kohistan (Searle et al., 2001). To the north,intermediate-depth seismicity dips steeply northward to a depth of~300 km beneath Hindu Kush and dips southward to a depth of~150 km beneath Pamir (e.g. Billington et al., 1977; Searle et al.,2001). Studies of this seismicity using focal mechanisms and seismic to-mography (e.g. Vander Voo et al., 1999; Pavlis andDas, 2000) show thatthe seismic zone beneath Hindu Kush is contiguous to that beneathPamir. Because India has been colliding against Kohistan since theearly Tertiary, no oceanic plate is recently subducting beneath this re-gion, as pointed out by Searle et al. (2001). It is thus difficult to relate

Convergent zoneThick crust

Weak boundary layer

Divergent zone

Divergent zone

Plume Plume

Hydrated/carbonated lithosphere

Hydrated/carbonated lithosphere

Toroidal field

Toroidal field

Div

erge

nt z

one

Con

verg

ent z

one

Volatilizing lithosphere

Hydrated/carbonated lithosphere

Warming serpentinized mantle

Plume

Dehydration boundary

Supercontinent Toroidal field

Subd

ucti

on z

one

Div

erge

nt z

one

Toroidal field

Toroidal field

Con

verg

ent z

one

Mid-oceanridge

Transforrm fault

Subd

ucti

on z

one

Convergent zone

Volatilizing lithosphere

Subduction

Plume

Hydrated/carbonated lithosphere

(a)

(c)

(e)

(b)

(d)

(f)

Fig. 1. Schematic illustration showing the processes of formation of plate boundaries. All figures are in a plan view except (b) and (d). (a) Flow fields for convectionwith a weak boundarylayer in the early stage of the Earth before plate tectonics began. Divergent and convergent zones are represented by pink and gray ellipses, respectively. The portion of the lithosphere thatis hydrated/carbonated by a plume is shown in green. Features like mid-ocean ridges (MOR) appear in divergent zones, arising due to weakening by melting (Sleep, 2000). Toroidal mo-tions occur in a diffuse way (upper and lower). (b) Convection with a weak boundary layer and thick basaltic crust (a cross-section view, modified from Davies, 1992). A weak boundarylayer is downwelling into the mantle in the convergent zone. Thick crust (brown color) is thrust over each other and piles up over the downwelling because it is difficult to subduct thismaterial (Davies, 1992). The portion of the lithosphere that is hydrated/carbonated by a plume is shown in green. The divergent boundary is marked by a red triangle, which indicates amagma chamber. (c) Lithosphere that was earlier hydrated/carbonated by a plume (green color) is underthrusting the convergent zone. A zone of volatilization from the underthrustinglithosphere is shown in blue. Other symbols are the same as (a). (d) Lithosphere that was earlier hydrated/carbonated by a plume (green color) is underthrust in the convergent zone (across-section view). Volatilization from the underthrusting lithosphere (blue color) lubricates the thrust, making subduction possible. Other symbols are the same as (b). (e) Serpentinizedforearc mantle is embedded in a suture zone of a supercontinent (left) and in a rifted margin or relict arc (upper). Any thermal event like an upwelling plume shrinks the serpentinizedforearcmantle by dehydration,whichweakens its boundaries by the release of pressurizedwater (see text). If the supercontinent overrides a divergent zone, drag forces at the base of thecontinentmight lead to breakup of the continent. If a shearing tectonic stress is operating in the relict arc or riftedmargin, theymight turn into a transform fault. (f) Once a complete set ofthe three types of plate boundaries forms via the weakening processes shown in (c), (d) and (e), plate tectonics starts to operate.

87T. Seno, S.H. Kirby / Earth-Science Reviews 129 (2014) 85–99

the intermediate-depth seismicity, which is a phenomenon that hasonly occurredwithin the past severalMa, with subduction of an oceanicplate. The continental crust is thus unusually subducting in this region.

In western Burma, there is a belt of high-grade metamorphic rockswith Mesozoic ophiolites (e.g. Maurin and Rangin, 2009). This ophiolitebelt was regarded as an eastward continuation of the Indus-Zangpo su-ture by Mitchell (1984). In Tibet, south of the ophiolite belt, the Tethysand high Himalayas, i.e., the Mesozoic and Paleozoic rocks offscrapedfrom the Indian continental margin, display collision characteristics.However, these characteristics are absent in Burma. Instead, to thewest of the ophiolite belt, there is the Indo-BurmanWedge, i.e. a Tertia-ry accretionary complex growing to the southwest (Maurin and Rangin,2009). The Miocene-Quaternary calcalkaline volcanics are found to theeast of the ophiolite belt (Mitchell, 1984). The Quaternary volcanicrocks show isotope and trace element patterns that indicate subduction

of the Indian continental lithosphere (Zhou et al., 2012). There isintermediate-depth seismicity beneath Burma down to a depth of200 km (e.g. Le Dain et al., 1984). These characteristics suggest that, inthe Burman arc, subduction, rather than collision, has been occurringduring the late Cenozoic. Because the Bay of Bengal and the BengalBasin to the north have a crustal thickness N25 km (Brune and Singh,1986; Kaila et al., 1992), Burma is an anomalous place where continen-tal crust is subducting with intermediate-depth seismicity, similar toHindu Kush.

2.2. The Reunion and Kerguelen hotspots

The unusual subduction of the Indian continental lithosphere be-neath Hindu Kush and Burma is in marked contrast to the collision inother parts of the Himalayas. Volcanics produced by the Reunion and

88 T. Seno, S.H. Kirby / Earth-Science Reviews 129 (2014) 85–99

Kerguelen hotspots during the Mesozoic-Tertiary are distributedforming the traces of these hotspots on the Indian plate (Fig. 2). In ac-cordance with the peculiarity mentioned above, Seno and Rehman(2011) found that the traces of the Reunion and Kerguelen hotspotson the Indian plate, formed at the time of ~100–126 Ma (Storey et al.,1989; Muller et al., 1993; Kent et al., 2002), are located in the vicinityof the intermediate-depth seismicity beneath Hindu Kush and Burma(Fig. 2). The Indian continental lithosphere in these regions is thus likelyto have been affected by plumes upwelling beneath these hotspots. Fol-lowing Wyllie (1988), we expect that uprising plumes with partialmelts involving H–O–C release these volatiles as vapors when magmas

Rajmaha

Lamp

Deccan Traps

Alkali intrusions

S. Tethyan suture zone volcanics

58.6

73.6

80.2

84

90

100

68.5

Ma

118

83.5

72-73

Ma

Hotspot-related volcanics

68.5

Reunion

Kerguelen

N

E

Reunion

Fig. 2. Spatial relation between the Reunion and Kerguelen hotspot traces and the intermpossible hydration/carbonation of the continental lithosphere by upwelling plumes (Seno and(depth N 80 km, 1983–2004,M N 4.0)) are shown by green dots. Volcanics related to the Reuni(triangles, Basu et al., 1993), and the ~80 Ma South Tethyan suture zone volcanics in Pakistan (Mi.e. the Ninety-east Ridge, its northern extension that is buried beneath the sediments in the BeIndia (120–115 Ma), are shown in orange. The hotspot traces that are located using Muller etyounger than 100 Ma are shown by the blue lines with ages. The 90–100 Ma trace for the ReunKush. The 115–126 Ma (Kent et al., 2002) and 110 Ma traces (Storey et al., 1989) for the Kerguewithin the belt of the intermediate-depth seismicity in Burma.

solidify near the base of the lithosphere (Fig. 3a, see also Figs. 45 and 46of Wyllie, 1988). This would alter mantle minerals at depths b 100 kmabove the plate base by permeation through pores (Mibe et al., 1999),producing amphibole at T b 1050 °C, chlorite at T b 700 ~ 800 °C, andserpentine/talc at T b 600–700 °C (e.g. Poli and Schmidt, 1995; Ulmerand Trommsdorff, 1995; Wunder and Schreyer, 1997; Bose andNavrotsky, 1998; Iwamori, 1998; Bromiley and Pawley, 2003;Fumagalli and Poli, 2005; Till et al., 2012), in addition to otherperidotite–CO2 minerals at similar temperature–depth ranges (Breyet al., 1983; Wallace and Green, 1988; Falloon and Green, 1989; Caniland Scarfe, 1990; Dasgupta et al., 2007). As noted in the introduction

l Traps

rophyres

Ma

84

90

100

115-126

115

110

Muller et al.(1993)

Kent et al.(2002)

Storey et al.(1989)

80.2

Predicted hotspot traces

Ma

80150300km

Depth

Intermediate-depthseismicity

Kerguelen

ediate-depth seismicity beneath Hindu Kush and Burma. This provides an example ofRehman, 2011). The epicenters of intermediate-depth earthquakes (Inter. Seism. Centreon hotspot, i.e. the Deccanflood basalts, the alkali intrusions ~3 M.y. older than the Deccanahoney et al., 2002), are shown in dark-brown. Volcanics related to theKerguelen hotspot,ngal Fan (Curray et al., 1979), the Rajmahal Traps and the lamprophyres (triangles) in NE.al.'s (1993) solution for the Indian plate motion over the Reunion and Kerguelen hotspotsion hotspot is located at the southern edge of the intermediate-depth seismicity in Hindulen hotspot are shown by large andmedium-size blue stars, respectively. These are located

Lower crust

Upper crust

Volatilization Subductioninitiation

Lubricated megathrust

Continentallithosphere

Hydrated/carbonated zone

Hydrated/carbonated zone

Partial melt

Plume

(a) Continental lithosphere with a plume below

(b) Continent-continent convergent zone

Fig. 3. (a) Hydration/carbonation of continental lithosphere by an upwelling plume (modified from Fig. 45 ofWyllie, 1988). Lines 2 and 3 represent crossing points between the geothermand the solidus for peridotite–C–H–O.Within the plume,melting occurs at greater depths due to its higher temperature. Line 1 represents the lithosphere–asthenosphere boundary, abovewhich partial melts accumulate and form magmas. Vapors of CO2 and H2O are released from the cooling magmas and hydrate or carbonate the lower portion of the lithosphere (greencolor) by permutation or hydrofracturing (see text). (b) Volatilization of hydrated/carbonated continental lithosphere in a continent–continent convergent zone (after Seno and Rehman,2011). Generally, when continental lithosphere converges, most of the crust is offscraped, and the delaminated boundary layer sinks. If the lower portion of the lithosphere is hydrated/carbonated by a plume as in (a) (green color,Wyllie, 1988), it heats up and volatilizeswhilst being underthrust to intermediate-depths. Released volatiles (blue color)migrate upward andreduce frictional resistance to underthrusting. This would allow the continental lithosphere to be subducted as a whole (black arrow). Intermediate-depth seismicity is produced in thesubducted lithosphere by devolatilization embrittlement of the previously hydrated/carbonated mantle. This stage corresponds to Fig. 1d.

89T. Seno, S.H. Kirby / Earth-Science Reviews 129 (2014) 85–99

section, we call these chemical alternation processes of lithosphericmantle minerals “hydration and/or carbonation” in this paper. Ifthe lithosphere is in tensional tectonic stress, magmas, and vaporsmight further migrate upward by magma-hydrofracturing, inducinghydration/carbonation in the shallower portion (see Fig. 46 of Wyllie,1988; Spera, 1987;Wilshire andKirby, 1989). Fig. 3a schematically illus-trates this hydration/carbonation, adapting Fig. 45 ofWyllie (1988). Be-cause the solidus at level 3 in Fig. 3a is assumed to be ~1300 °C (seeFig. 20 of Wyllie, 1988), the altered minerals are stable in the litho-sphere when they are more than a few tens of kms above the solidus(green shaded in Fig. 3a). The lower portion of the Indian continentallithosphere, once located above the hotspots, is thus likely to havebeen hydrated/carbonated by plumes.

2.3. Hydration/carbonation of oceanic lithosphere

This kind of hydration/carbonation by plumes, albeit for oceanic lith-osphere, has been invoked by Kirby (1995) and Seno and Yamanaka

(1996) to explain the occurrence of double-seismic zones in thecircum-Pacific subduction zones. Based on the devolatilization embrit-tlement mechanism for intraslab intermediate-depth earthquakes(Raleigh and Paterson, 1965; Nishiyama, 1992; Kirby et al., 1996; Senoand Yamanaka, 1996; Jung et al., 2004; Takahashi et al., 2009),they interpreted double seismic zones to represent volatilization ofslabs due to increasing temperature and pressure during subduction.The upper plane of double seismic zones can be explained bydevolatilization embrittlement of altered basalts in the subductingoceanic crust (Kirby et al., 1996; Peacock and Wang, 1999; Hackeret al., 2003; Yamasaki and Seno, 2003; Kita et al., 2006; Nakajimaet al., 2009). The lower plane requires the deeper half of the oceaniclithosphere to be hydrated/carbonated prior to subduction. Kirby(1995) and Seno and Yamanaka (1996) attributed this to be thecarbonatization and hydration, respectively, similar to that depicted inFig. 3a, by H2O–CO2 vapors released from magmas in plumes rising upin the Pacific Ocean. Although direct evidence for this metamorphismis scarce, Seno and Yamanaka (1996), Hacker et al. (2003), and

90 T. Seno, S.H. Kirby / Earth-Science Reviews 129 (2014) 85–99

Yamasaki and Seno (2003) showed that the dehydration locus ofserpentinites formed in the lower portion of subducting lithosphere isconsistent with the observed geometry of the double seismic zone; onthe contrary, the locus formed by infiltration of seawater in the upperhalf of oceanic lithosphere by normal faulting (Peacock, 2001; Raneroet al., 2005; Faccenda et al., 2009) is not consistent.

2.4. Collision versus subduction

It is generally believed that, when continental lithosphere ap-proaches a convergent boundary, subduction is hindered due to thethick buoyant crust, and therefore, collision generally occurs (e.g.McKenzie, 1969; Molnar and Gray, 1979; van den Beukel, 1992; Cloos,1993). However, Seno (2008) showed that buoyancy is not an essentialfactor in preventing subduction, but absence of dehydration from a slabis rather important. Continental crust does not contain metamorphicminerals that dehydrate at intermediate-depths (Ernst et al., 1998),which makes both pore fluid pressure ratio in the thrust andintermediate-depth seismicity low (Seno and Yamasaki, 2003; Seno,2007), resulting in offscraping and piling up of the crust in collisionzones (Seno, 2008). Conversely, in subduction zones, dehydration ofthe slab crust and mantle (Anderson et al., 1976; Peacock, 1993; Poliand Schmidt, 1995; Okamoto and Maruyama, 1999; Peacock andWang, 1999; Seno et al., 2001; Hacker et al., 2003; Fumagalli and Poli,2005; Kameda et al., 2011; Kuwatani et al., 2011; Till et al., 2012) resultsin pore fluid pressure ratios as high as 0.90–0.98 (Wang and He, 1999;Wang and Suyehiro, 1999; Seno, 2009; Kimura et al., 2012), which ren-der smooth subduction with very low friction at the thrust.

The unusual subduction of the continental crust beneathHinduKushand Burma should be closely related to the existence of intermediate-depth seismicity in these regions. As mentioned before, the lower por-tion of the Indian lithosphere could be hydrated/carbonated by plumesuprising beneath the Reunion and Kerguelen hotspots, similar tooceanic lithosphere with a double seismic zone. After traveling overthousands of kms to the north, the hydrated/carbonated lithosphere ap-proaches and is underthrust beneath Asia (Fig. 2), and volatilization oc-curs from the lithosphere due to increases in temperature and pressure.This would result in intermediate-depth seismicity by devolatilizationembrittlement and the released volatiles would lubricate the thrustabove, enabling subduction under Asia (Fig. 3b). The intermediate-depth seismicity beneath these regions is essentially the same as thatin the western Pacific (e.g. Billington et al., 1977; see other referencescited in Seno and Rehman, 2011), which implies that the extent of hy-dration/carbonation of the slab is similar to that of subducting slabs inthe circum-Pacific. Because the diameter of the temperature anomalyand related volcanics of an uprising plume is generally of the order of1000 km (White and McKenzie, 1989; see also the spatial dimensionof the volcanics related to the Reunion and Kerguelen hotspots shownin Fig. 2), the induced subduction is not spot-like, but can be in a zoneextending over several hundreds of km. Although direct evidence ofsubduction of hydrated/carbonated continental lithosphere in these re-gions is not yet available, Sumino et al. (2011) noted OIB signatures inthe 3He/4He ratio of diamond exhumed in the ultra-high pressuremeta-morphic belt of northern Kazakhstan, as an example of such subduction.The Quaternary volcanic rocks in Burma also show isotope and traceelement patterns indicating subduction of the Indian continental litho-sphere there (Zhou et al., 2012), as mentioned before.

2.5. Subduction initiation in the Archean

We propose that subduction initiation in the Archean occurred in asimilar manner, although, in this case, the thick crust that preventedsubduction is basaltic. Hydration/carbonation of the lithosphere byplumes, as shown in Fig. 3a, would have occurred more frequently dur-ing the Archean, because plume activity was much higher at that time(Campbell and Griffiths, 1992; Kroner and Layer, 1992). Conversely,

the thickness and thermal state of the lithospherewere notmuchdiffer-ent from those at present (Bickle, 1978; Burke and Kidd, 1978; Jarvisand Campbell, 1983; England and Bickle, 1984; Bickle, 1986), due tothe efficient removal of heat from the Earth's interior by the high platecreation rate at that time. When such lithosphere approached and wasunderthrust in the Archean convergent zones, as in Hindu Kush andBurma, increases in temperature and pressure caused by underthrust-ing would have induced volatilization from the hydrated/carbonatedlithosphere, and the volatiles thus released would have lubricated thethrust (Fig. 3b). If the subducted crust was transformed to densereclogite as shown by Austrheim et al. (1997), it would have aided fur-ther underthrusting of the lithosphere (Ringwood and Green, 1966;Ahrens and Schubert, 1975). Eclogitization of the lower crust in the vi-cinity of the thrust zone also enhanced itsweakness by the interplay be-tween fluid injection, chemical reaction, and deformation (Austrheimet al., 1997; Bjornerud et al., 2002) andmade subduction easier by over-coming the energy dissipation associated with plate bending (Conradand Hager, 1999). These together would have made such lithospherewith thick crust enter the deeper mantle, resulting in subductioninitiation (Figs. 1d and 3b).

3. Serpentinized forearc mantle wedge and its role in the formationand mobilization of plate boundaries

In uniform-viscosity convection, divergence can occur by ductile ex-tension of a boundary layer, without any mid-ocean ridge (MOR) likefeature (Davies, 1992). Toroidal motions, if any, can be accommodatedby diffuse deformation between the loci of convergence and divergence(Fig. 1a and c; see also Fig. 1 of Bercovici, 1996), rather than by trans-form faults (Fig. 1f). As an exception, partial melting in the divergencezone may weaken the boundary layer and form a linear divergentboundary with a MOR-like feature (Fig. 1a and c, Sleep, 2000). Othertypes of plate boundaries, however, do not formeasily in the lithosphereas it cools and becomes strong. In this section, we propose the followingmechanism to cause strain localization, which leads to the formation ofnew plate boundaries in such strong lithosphere (Fig. 1e and f).

Initiation of subduction discussed in the previous section may formserpentinized forearc mantle in subduction zones. In the modernEarth, recent geophysical evidence shows that serpentinized mantleoccurs in cold, shallow, wedge-shaped regions of forearcs abovedehydrating slabs in several well-studied subduction zones (Kamiyaand Kobayashi, 2000; Bostok et al., 2002; Brocher et al., 2003;Hyndman and Peacock, 2003; Blakely et al., 2005; Matsubara et al.,2005; Seno, 2005; Wang et al., 2006). Shallow serpentinized forearcmantle forms because water is released into the forearc fromdowngoing, warming, and thus dehydrating slab crust and mantle,and also because the descending slab cools the forearc mantle nearestthe trench and slab, resulting in a large regionwhere serpentinite is sta-ble (e.g. Hyndman and Peacock, 2003). The volume of water potentiallystored in the forearc is huge, about 200 km3/km of margin or more,depending on the degree of serpentinization of the forearc mantle(Kirby et al., 2013). The inboard and outboard wedge boundaries ofthe serpentinized mantle are controlled by the thermal structure ofthe forearc, the plate interface, and the forearc crust–mantle boundary.

If the subducting slab gets progressively warmer (e.g. as a MOR ap-proaches the trench, or the rate of subduction slows) or subductionstops, the forearc mantle will begin to warm and the region of serpen-tine stability will shrink, releasing water along the wedge boundaries.Kirby et al. (2013) calculated the release rate of water for the post-subduction case pertinent to the California Coast Ranges. Most of the~1014 kg/km water stored in the wedge mantle is released as freewater by 10–25 M.y. after subduction ceases; its longevity depends onspecific model assumptions on the wedge size and the warming mech-anism. In particular, these calculations were conducted for the subduc-tion zone where the warm Farallon slab was subducting. In coldsubduction zones, a volume of serpentinized forearc mantle could be

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much larger, and consequently the water release time could be longer.During the period of dehydration of such serpentinized forearc mantle,the released pressurized water weakens its boundary by the mecha-nisms described below in more detail. If a regional shearing tectonicstress operates, the dehydrating boundary could become a transformfault (Fig. 1e and f), as suggested for the Coast Ranges by Kirby et al.(2013).

Such formation of plate boundariesmay not be restricted to active orrelict forearcs. In some cases, serpentinized forearc mantle that over-rides active margins may be amalgamated into a collision zone (Burkeet al., 1977; Dewey, 1977). Utilizing the result that water release occursover a time scale of tens of M.y., some of this serpentinized mantle maysurvive and be stabilized into suture zones, if the period of orogeny isshort enough. Furthermore, since the Proterozoic, rifts disrupting a su-percontinent often occurred along suture zones (e.g. Burke et al.,2003). Disrupted smaller continents amalgamated again to form anew supercontinent. This is called a Wilson cycle or a supercontinentcycle, after Wilson (1966) pointed out that a proto-Atlantic (IapetusOcean) closed during the Middle–Upper Paleozoic and the Atlantic re-opened in the earlyMesozoic. It is now known that such cycles have oc-curred a few times since the Proterozoic (McKerrow and Ziegler, 1972;Dalziel, 1995; Rogers, 1996). For example, West Gondwana collidedwith Laurasia at ~750, 490, and 370 Ma since the dispersal of Rodinia(Dalziel, 1995). Following breakup of a supercontinent, rifted marginssurrounded the ocean that had opened. Some of the serpentinizedforearc mantle may survive and remain in such rifted margins. Relictarcs, suture zones, and rifted margins are thus possible loci for storageof serpentinized mantle. Any agents warming them would releasewater and weaken the dehydration boundaries. This strain localization,with sufficient tectonic stress, would create new plate boundaries atthese localities.

Possible mechanisms for the released water to weaken the litho-sphere are (1) reduction of fault strength by increased pore fluidpressure (Hubbert and Rubey, 1959), (2) hydrofracturing, and(3) reaction-enhanced ductility. The effects of increased pore fluidpressure are similar to those for active faults in the crust. Fluid produc-tion rate arising from the interplay between dehydration, permeabilityof the host rock, pore creation by faulting, pore compaction by ambientstresses, and shear heating controls the temporal change in thepore pressure and thus, the strength of the dehydrating boundary(e.g. Sleep and Blanpied, 1992; Segall and Rice, 1995; Sibson, 1996).However, application of this weakening mechanism to realistic ser-pentinized forearcmantle awaits future studies. Hydrofracturing occursunder the following four conditions for the dehydrating material: posi-tive volume change (ΔV N 0) through the dehydration reaction, lowpermeability, low strain rate (Nishiyama, 1989), and small differentialstress with σHmin b 0 (Secor, 1965; Etheridge, 1983). For antigorite,ΔV N 0 is satisfied by dehydration that occurs at depths b 75–125 km(Ulmer and Trommsdorff, 1995; Wunder and Schreyer, 1997;Bromiley and Pawley, 2003; see also Fig. 4b of Yamasaki and Seno,2003), which is the depth range of the serpentinized mantle of our in-terest. Enhancement of ductility of the lower crust by transformationto eclogite from granulites through fluid injection (Austrheim et al.,1997; Bjornerud et al., 2002) may be in the category of reaction-enhanced ductility, although there is evidence for the existence of brit-tle deformation because pseudotachylites are observed simultaneouslyfor this transformation (Austrheim and Boundy, 1994). In the followingsubsections, we depict the processes of formation for each type of plateboundary in terms of these weakening mechanisms.

3.1. Divergent boundaries

In this subsection, we consider the formation mechanism of a diver-gent boundary within thick lithosphere. Generally, it is difficult for slabpull or suction forces to disrupt continental lithosphere. Previous stud-ies suggested that plumes play important roles in the breakup of a

continent or a supercontinent. Morgan (1971) pointed out temporal co-incidence between flood basalts and continental breakup and suggesteda connection between plume activity and the onset of seafloor spread-ing. Duncan (1984) also noted that hotspots have been overridden byspreading ridges in the central Atlantic. White and McKenzie (1989)showed that huge volcanic provinces are located in rift zones, whichmay be associated with uprising plumes (i.e. volcanic rifted margins).Courtillot et al. (1999) further advanced the Morgan's (1971) idea onthe temporal coincidence between plume activities and breakup.

Bott (1982) further proposed that a rising plume warms and thinsthe lithosphere, which may help disrupt it under slab suction forces.Even if, however, the mechanical thickness of the lithosphere is halved,its strength is still an order larger than the magnitude of plate tectonicforces. Therefore, pre-existing heterogeneities in the lithospheric struc-ture are necessary for breakup, in addition to plate tectonic forces andthinning by plumes (Dunbar and Sawyer, 1989; Courtillot et al., 1999;Tommasi and Vauchez, 2001). In fact, breakup often occurs along su-tures (e.g. Burke et al., 2003), and Tommasi and Vauchez (2001) attrib-uted the weakness of suture zones to lattice preferred orientation ofolivine. However, reduction in strength produced by this mechanismis less than two thirds of magnitude (see Fig. 5 of Tommasi andVauchez, 2001) and is still short of that needed for plate tectonic forcesto cause breakup.

In Fig. 4a, we assume that 200-km-thick cratonic lithosphere has atemperature of ~1200 °C at the base (adapting values of Figs. 7 and 8of Wyllie, 1988), serpentinized forearc mantle is embedded in the su-ture zone, and subduction occurs from both sides of the continent(Bott, 1982). Antigorite is stable at depths b 150 km in such lithosphere(see Fig. 4b of Yamasaki and Seno, 2003). Even if the temperature at thebase is by ~100 °C higher during the Archean (Jarvis and Campbell,1983), antigorite is still stable at a similar depth range. We now assumethat plumes rise beneath some locations in the suture. Thermal anoma-lies in the regions adjacent to the plumeswould significantly reduce thestrength of the lithosphere by dehydration (Fig. 4b). This mechanism,together with thinning of the lithosphere (Bott, 1982) and weakeningdue to lattice preferred orientation (Tommasi and Vauchez, 2001),might lead to breakup of a supercontinent, which is then succeededby oceanfloor spreading (Fig. 4c). In Fig. 1e, we assume a differenttectonic situation where a supercontinent in a divergent zone, withoutsubduction from both sides, overrides an upwelling plume. In thiscase, basal drag forces, with a dehydration weakening mechanismsimilar to that shown in Fig. 4b, may lead to breakup of a continent.

Supporting this inference, serpentinized mantle is often found innon-volcanic rifted margins in the Atlantic (Fig. 5, Billot et al., 1989;Whitmarsh et al., 1993; Reid, 1994; Chian et al., 1995; O'Reilly et al.,1996). This may be comprised of relics of serpentinized forearc mantleembedded in suture zones, whose dehydration boundaries acted asweak zones for continental breakup. In volcanic margins, we do not ob-serve such serpentinized mantle, but high velocity layers in the thick-ened lower crust, which are interpreted as igneous material added atthe time of rifting (White et al., 1987). We speculate that serpentine isdried out in such volcanic margins beneath which plumes of hightemperature rose up (White and McKenzie, 1989).

This explanation for the serpentinized mantle found in non-volcanicrifted margins is an alternative to downward infiltration of seawaterthrough brittle fractures of the entire crust proposed by others(O'Reilly et al., 1996; Perez-Gussinye and Reston, 2001). Note thatsuch infiltration is only possible after extension of the rifted crust islargely accomplished and already thinned (Perez-Gussinye and Reston,2001). The fact that serpentinized mantle is observed at continent–ocean boundaries, not beneath centers of abyssal plains (Billot et al.,1989; White et al., 1990; Pinheiro et al., 1992; Whitmarsh et al., 1993;Reid, 1994; Chian et al., 1995) seems to rule out such infiltration. Isotopestudies by Skelton and Valley (2000) in the Iberia Abyssal Plain alsoshow that a massive volume of the mantle was serpentinized before itwas exhumed. The mass of water stored in the serpentine beneath the

(a)

(b)

(c)

Suture zone

Serpentinized mantle

Plume

Dehydration

Initiation of spreading

Plume

Continental litosphere in tension

Non-volcanic margin

Suction force

Rifting

200 km, 1200 °C

Fig. 4. Possible mechanism of continental breakup (modified from Fig. 1 of Bott, 1982). Subduction occurs from both sides of a supercontinent. Fsu = trench suction, Frp = ridge push.These forces are not enough large to cause continental breakup. (a) In this case, we assume that a suture zone was formed in the middle of the supercontinent by the closure of an inter-vening ocean basin, and that serpentinized forearcmantle, formed during this earlier phase of subduction, is embedded in the suture zone. Note that antigorite is stable in the upper 3/4 ofthe lithosphere if the temperature at the base is ~1200 °C (Wyllie, 1988). (b) In the succeeding stage, we assume that a plume begins rising beneath the suture (White and McKenzie,1989). Dehydration of the serpentinized forearcmantle occurs as it is affected by such a plume,making the dehydration boundary veryweak. (c) Alongwith non-volcanicmargins heatedby the plume, this would make it easier for the continent to rift and break up. The above tectonic situation is different from Fig. 1e (left), but the proposed mechanism of formation of adivergent boundary is the same.

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Rockall Trough amounts to 0.6 × 1014 kg/km (O'Reilly et al., 1996),which is on the same order of the mass of water estimated for theCalifornia Coast Ranges (Kirby et al., 2013), suggesting that our interpre-tation is plausible.

3.2. Convergent boundaries

Since the initiation of plate tectonics, a number of subduction zonesmust have been newly created. Stern (2004) classified these into in-duced nucleation subduction zones (INSZ) and spontaneous nucleationsubduction zones (SNSZ). In INSZ, transference and polarity reversaltypes are distinguished. Transference INSZ move the new subductionzone outboard of the failed one. The on-going development of a plateboundary in the Indian Ocean, southeast of India, in response to theIndo–Asian collision (Wiens et al., 1985; Neprochnov et al., 1988;Stein et al., 1989) is a typical example of transference INSZ processes.Polarity reversal INSZ also follow collision, but in these cases, a new sub-duction zone forms behind the magmatic arc. As discussed in theIntroduction, a force one order larger than plate tectonic forces is re-quired to initiate a new subduction zone. This is the same for INSZ. Alow pore fluid pressure ratio at a collision zone thrust due to lesser

dehydration from underthrusting continental crust might producesuch a large tectonic force (Seno, 2007, 2008). The fact that no featurelike a subduction zone has yet developed in the Indian Ocean (e.g. Bulland Scrutton, 1990; Van Orman et al., 1995) indicates, however, thatthe magnitude of such a collision force would not be overwhelming.

SNSZ results from gravitational instability of oceanic lithosphere,either at a passive margin or at a transform fault/fracture zone.Cloetingh et al. (1984), however, showed that spontaneous collapse ofold lithosphere at a passive margin is unlikely due to its strength. Infact, there is no example of SNSZ at passivemargins during the Cenozoic(Stern, 2004). Therefore, collapse under a tensional or strike–slip tec-tonic setting has been invoked (Turcotte et al., 1977; Erickson, 1993;Kemp and Stevenson, 1996). Accordingly, numerical simulations showthat lithosphere in contact with another one along a transform fault/fracture zone could be unstable if there is enough difference in age(e.g. Fujimoto and Tomoda, 1985; Hall et al., 2003), resulting in SNSZ.Not many such examples are, however, known in geologic history. TheIzu–Bonin–Mariana arc is often referred to as a typical example ofSNSZ formed by such collapse along a transform fault (Hilde et al.,1977; Hawkins et al., 1984; Wells, 1989; Stern and Bloomer, 1992).Seno and Maruyama (1984), Seno (1988), Hall et al. (1995),

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Deschamps and Lallemand (2002), and Taylor and Goodlife (2004),however, discounted such an origin, because the Bonin arc has been ac-tive at least since 48 Ma, with an arc trend oblique to the hypothesizedtransform fault.

Therefore, although it has been generally conceived that a passivemargin or a transform fault/fracture zone turns into SNSZ, the mecha-nism is still disputed. Similarly, the amount of tectonic force requiredfor nucleation of INSZ is yet unresolved. Even if thermal crackingweakens oceanic lithosphere (Korenaga, 2007), the lower half of thelithosphere might remain strong, as seen by the dominant reversefault-type earthquakes beneath mid-oceans, outer-rises, and passivemargins (Stein et al., 1979; Forsyth, 1982; Wiens and Stein, 1985;Seno and Yamanaka, 1996).

We propose, similar to the formation of divergent boundaries, thatdehydration weakening of serpentinized forearc mantle may haveplayed an important role in initiating subduction zones in the Earth'shistory after plate tectonics had started. As stated before, serpentinizedforearc mantle may be embedded in non-volcanic rifted margins or inrelict arcs where subduction has stopped (Fig. 1e). If any thermalevent occurs in its vicinity under a regional compressional stress, the re-leased pressurized water will weaken the dehydration boundaries ofthe mantle, and possibly induce INSZ or SNSZ. Upwelling plumes maybe responsible for such thermal events, andmaybehave as a control fac-tor of subduction zone nucleation. This would bemuchmore efficient inweakening lithosphere along a riftedmargin or a relict arc than a simpleinteraction between lithosphere and plumes (Burov and Cloetingh,2010) or sediment loading (Cloetingh et al., 1984; Erickson, 1993). Anextraordinary amount of collision forces is not required for INSZ inthis case.

3.3. The San Andreas fault system (SAF)

The SAF was formed by disappearance of the Farallon plate andsucceeding contact of the Pacific plate with the North American plate(Atwater, 1970). Because the fault system developed a few hundredkm east of the relict trench axis (Fig. 6a, McCulloch, 1989), we cannotapply, in a straightforward manner, Atwater's (1970) idea (Fig. 6b) forthe initiation of the SAF. As already mentioned, Kirby et al. (2003,2013) proposed amodel suggesting that themobility of the SAFwas en-abled by dehydration weakening of the serpentinized forearc mantlebeneath the Coast Ranges. As amargin-parallel shear stress was appliedalong the mantle wedge by the Pacific-North American relative motion(Fig. 6c), the weak zones in the dehydratingmantle would have faultedaseismically, and the crust located above would have deformed bystrike–slip faulting (See Kirby et al. (2013) for more details). Warmingoccurred in this case by a slab gap (Dickinson and Snyder, 1979;Furlong et al., 1989) or by a stalled slab (Bohannon and Parsons, 1995;ten Brink et al., 1999) after the cessation of subduction of the Farallonplate. Fulton and Saffer (2009) supported this idea with a numericalsimulation that showed that such a release of water could maintain apore fluid pressure high enough to mobilize the SAF. In other regions,similar release of pressurized water could occur along a relict arc or arifted margin, if they contain serpentinized forearc mantle warmed bysome agents. If a shearing tectonic stress operates as in the CoastRanges, it will induce transcurrent motion along the dehydratingboundary, which might lead to initiation of a transform fault (Fig. 1eand f).

3.4. Other continental and oceanic transform faults

A spectacular example of formation ofmajor strike–slip faults in col-lision zones is the North Anatolian fault (NAF) of northern Turkey. TheNAF, whose activity was initiated by the collision of Arabia with Eurasia(Sengor, 1979), is located along a suture zone formed by the earlier col-lision between Laurasia and Gondwana (the Pontide–Sakarya collisionduring the Late Cretaceous) (e.g. Sengor, 1979; Yilmaz et al., 1995).

Three metamorphosed units are observed in the western NAF, betweenthe two branches (north and south); their protoliths are the Sakaryacontinent, ophiolites, and the Pontide fragment from south to north(Yilmaz et al., 1995). Yilmaz et al. (1995) regarded themetamorphosedophiolites as the fragments of the Neotethys. We suggest an alternativeinterpretation that these rocks represent the serpentinized forearcmantle formed by the subduction of the Neotethys beneath the Sakaryacontinent, because the serpentinizedperidotites occurwithin the Sakar-ya continent (Figs. 2, 3, 6, and 7 of Yilmaz et al., 1995). If this is the case,its dehydration due to warming in recent times after the cessation ofsubduction of the Neotethys might have induced the strike–slip motionof the NAF. This, however, remains speculative at present, and evidencefor this should be sought from geological data in this region.

The abovemechanismmay also allow strike–slip faulting to occur atlow stresses far into continental interiors. There aremany geological ex-amples of multiple suture zones in East Asia and North America, wherefragments of continents and island-arcs were amalgamated during theMesozoic–early Tertiary (e.g. Ben-Avraham et al., 1981; Klimetz, 1983;Silver and Smith, 1983; Davis and Plafker, 1986; Scholl et al., 1986).These terrane collisions and accretions, in association with the seawardstepping of trenches, suggest the possible existence of serpentinizedforearc mantle in such suture zones. Some of them may have survivedand mobilized huge continental strike–slip faults by later thermalevents. The strike–slip Denali fault north of the Alaska Trench and theAltyn Tagh, Kunlun, and Red River faults in interior Asia might be suchexamples, although further investigations on their origins are necessary.

Ridge–ridge transform faults developing in mid-oceans followingcontinental breakup or trench–trench transform faults along fracturezones may not be relevant to the dehydration weakening proposed inthis study. In these transform faults, infiltration of seawater along thefaults may form serpentine (Calvert and Potts, 1985; Macdonald andFyfe, 1985; Potts et al., 1986), whose low coefficient of friction(Reinen et al., 1991; Moore et al., 1997) may play an important role inreducing the shear strength of these faults (e.g. Escartin et al., 1997).Even for ridge–ridge transform faults in their initiation stage, however,theremay be embryoswithin the serpentinized forearcmantle, becausecontinental breakup often occurs along suture zones (e.g. Burke et al.,2003). Trench–trench transform faults may also originate from arcsthatmight contain serpentinized forearcmantle. In such cases, dehydra-tion of the warming serpentinized forearc mantle may provide an im-portant clue to elucidate the evolution of this type of transform fault.

4. Discussion

Initiation of plate tectonics by the formation of plate boundaries is adifficult, but nevertheless important, problem in Earth sciences. The dif-ficulty arises from two contradictory aspects of plate tectonics: a plate(lithosphere) must be strong and, at the same time, it must be mobilein order to extract heat efficiently from the Earth's interior. Movingplates require weak plate boundaries, but the strength of plates makesthe formation of these boundaries difficult. The thick crust formed atthe Earth's surface also prevents subduction. The problem is diverseand vast, covering chemical alteration, rheology of the surface and inte-rior of the Earth, and its tectonic history. As means to address this di-verse problem, we propose that volatilization of an underthrustinghydrated/carbonated slab and dehydration of serpentinized forearcmantle are major agents for strain localization and weakening of litho-sphere to initiate plate boundary formation.

Although previous studies have discussed variousweakeningmech-anisms of lithosphere, including the effects of volatiles (Bercovici, 1998;Moresi and Solomatov, 1998; Tackley, 1998; Regenauer-Lieb et al.,2001; Bercovici, 2003; O'Neill et al., 2007), their treatments are all gen-eral. In this paper, we provide a practical hypothesis on the roles of vol-atiles in the formation processes of new plate boundaries such asinitiation of subduction, rifting/spreading, and transform faults. Becausethemechanismswepropose are put in the context of the Earth's history,

(a)

(b)

Atlantic

Atlantic

Iber

ia

Greenland

Labrador

94 T. Seno, S.H. Kirby / Earth-Science Reviews 129 (2014) 85–99

PA PA

NANA

NA

Dehydrating boundary

New segment of SAF

New segment of SAF Serpentinized forearc wedge

NA

FA FA

PA

(b) (c)Trench axis Trench axis

Relict trench

W

N

125° 124° 123° 122° 121° 120°

125° 124° 123° 122° 121° 120°

40°

39°

38°

37°

36°

35°

40°

39°

38°

37°

36°

35°

(a)

NA

PA

SAF

Fig. 6. (a) Tectonic setting of the California Coast Ranges: the San Andreas fault system (SAF) and various terranes (stippled area) (after McCulloch, 1989). The location of the SAF is100–200 km landward of the relict trench in which the Farallon plate was subducting during the Tertiary. (b) The Farallon plate (FA) vanished beneath the North American plate(NA), and the contact of the Pacific plate (PA) to NA became the SAF along the relict trench, according to the theory of Atwater (1970). (c) Dehydration from the warming serpentinizedforearcmantle occurs (Kirby et al., 2003, 2013) at the boundaries of thewedge. The SAF could initiate 100–200 km landward of the relict trench along such a boundary. The forearc wedgebehaves as part of PA after the formation of the SAF.

95T. Seno, S.H. Kirby / Earth-Science Reviews 129 (2014) 85–99

they are more easily tested than the mechanisms proposed in previousstudies. Obviously, however, we present only a rough sketch of themechanisms in this paper, and do not claim to have proven them tobe correct. A lot of work remains to be done to examine these mecha-nisms quantitatively based on geological or geophysical data in eachregion.

Sleep (2000) presented a scheme of transition between a magmaocean, plate tectonics, and stagnant-lid convection for terrestrialplanets. In some planets, transitions between thesemodesmayhave oc-curred several times. In this scheme, however, mechanisms of the tran-sition to plate tectonics, except for the role of partial melting in thedivergent zone, were not presented. In the future, the Earth will

Fig. 5. Examples of serpentinite bodies in non-volcanic riftedmargins. (a) Map of the Labradorwhich is inferred to be serpentinized peridotite (Chian et al., 1995). The grey lines are fracture zof the western continental margin of Iberia (contours of ocean depth inmeters), showing zonesA.P. denotes abyssal plain. The solid lines are the lines of seismic experiments. These maps shonon-volcanic rifted margins, not at the center of the abyssal plains.

transition to stagnant-lid convection as it is cooled efficiently by platetectonics. After this, it is expected that radioactive heat productionwill heat up the interior, and plate tectonics will resume (Sleep, 2000).For this to occur, however,mechanisms that enable the initiation of sub-duction (similar to those presented in this study) are necessary; other-wise the Earth will return to a magma ocean. It is noted that both theconvective vigor and volatile extent of the Earth's interior change tem-porally, interacting with each other in a complicated manner. There isno guarantee that the condition for resumption of plate tectonics willbe satisfied in the future. The weakening mechanisms we proposewill, nevertheless, provide a useful key to conjecture about shifts be-tween the modes of convection for terrestrial planets.

Sea, showing the zones of high-velocity (7.2–7.7 km/s) material on eachmargin (shaded),ones and the solid lines are the lines of seismic experiments. (b)Map of the region offshoreof thin crust underlain by serpentinized peridotite (shaded area, Whitmarsh et al., 1993).w that the serpentinized peridotites are located at the ocean–continent boundaries of the

96 T. Seno, S.H. Kirby / Earth-Science Reviews 129 (2014) 85–99

Our hypothesis also has implications for the formation and exhuma-tion of high pressure (HP)/ultrahigh pressure (UHP) metamorphicrocks and emplacement of some ophiolites and ultramafic rocks in sub-duction zones. Belts of HP or UHP metamorphic rocks are found inforearcs of active subduction zones and collision zones. Exposure onthe Earth's surface of such coesite- and diamond-bearing HP or UHProcks implies exhumation from a source in the mantle deeper than100–150 km (Coleman and Wang, 1995; Ernst and Liou, 1999;Chopin, 2003). Although the origin of these rocks is still disputed(Brueckner, 1998; Terry and Robinson, 1999; Guillot et al., 2001), it isvery likely that their exhumation involves tectonic movements notonly at the mid-lower crustal level but also deep in the mantle. It isalso episodic, i.e., not a steady-state process. In many cases, HP/UHPbelts are sandwiched between lower metamorphic-grade units in faultcontact (Worrall, 1981; Burg et al., 1984; Wheeler, 1991; Kanekoet al., 2003),which implies that theywere brought to the surface largelyby faulting or strain localization (Burchfiel and Royden, 1985;Maruyama et al., 1996). These features, i.e., episodicity, strain localiza-tion, and mantle-scale processes, are common to those of plate bound-ary formation discussed in this study, suggesting that a mechanismsimilar to dehydration weakening and mobilization of huge thrusts inrelict subduction or collision zones might be operating in exhumationof UHP/HP rocks. Subduction of continental materials into the deepmantle may provide sources for UHP rocks via the process we proposefor subduction of lithosphere with thick crust. This subject, however,will be treated in a separate paper.

5. Conclusions

In the early Earth, convection would have occurred with thick basal-tic crust accumulating over a soft boundary layer downwelling into themantle (Davies, 1992). This might have transitioned to stagnant-lidconvection as the mantle cooled (Solomatov and Moresi, 1997) orback to a magma ocean as the mantle heated (Sleep, 2000). For platetectonics to operate on the Earth, subduction of lithosphere with thickcrust must have been initiated, thus avoiding these shifts. Based on ananalogy with the continental plate subduction that is currently occur-ring in Hindu Kush and Burma, we propose that volatilization from un-derthrusting lithosphere, once hydrated/carbonated by H–O–C richvapors released from magmas in upwelling plumes, would have madeit possible. Once subduction has been initiated, serpentinized forearcmantle may form in a wedge-shaped body above a dehydrating slab.Therefore, in relict arcs, suture zones, or rifted margins, any agent thatwarms and dehydrates the serpentinized mantle would weaken theregion surrounding it, and form various types of plate boundariesdepending on the operating tectonic stress.

Thus, once subduction initiates, formation of plate boundariesmightbe facilitated by a single fundamental process (except for the MORformed by partial melting in the divergent zone): weakening due tothe release of pressurized water from warming serpentinized forearcmantle. The fundamental ideas presented in this paper are simple, anddetails of geological and geophysical facts in each appropriate regionshould be examined more quantitatively. However, we feel that the in-sights that we have obtained from a preliminary application of theseconcepts will have important implications for interpreting geologicalhistories in mobile belts, and are sufficiently encouraging to reportand put them into other people's discussion.

Acknowledgments

We thank H. Austrheim, an anonymous reviewer, and ManfredStrecker, whose critical comments are very useful to improve the man-uscript, and Satoru Honda and Tadashi Yamasaki for their commentsand discussion on the early version of the manuscript.

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