Hindukush & Burma of Gondwana

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GR Letter

When and why the continental crust is subducted: Examples of Hindu Kush

and Burma

Tetsuzo Seno a,, Hafiz Ur Rehman b

a Earthquake Research Institute, University of Tokyo, Bunkyo-ku, Tokyo 113-0032, Japanb Department of Earth and Environmental Sciences, Kagoshima University, Kohrimoto 1-21-35, Kagoshima 890-0065, Japan

a b s t r a c ta r t i c l e i n f o

Article history:

Received 7 April 2010Received in revised form 24 May 2010

Accepted 26 May 2010

Available online 4 June 2010

Keywords:

Continental crust

Subduction

Collision

Hindu Kush

Burma

The Indian subcontinent has been colliding against Asia along the Himalayas. Hindu Kush and Burma in this

collision zone have intermediate-depth seismicities beneath them, with most of the continental crust

subducted into a few hundred km depth. The subduction, not collision, in these regions is an enigma long

time. We show that the continental lithosphere subducted beneath Hindu Kush and Burma traveled over the

Reunion and Kerguelen hotspots from 100 Ma to 126 Ma and is likely to have been metasomatized by

upwelling plumes beneath those hotspots. The devolatilization of the metasomatized lithosphere impinging

on the collision boundary would have provided a high pore fluid pressure ratio at the thrust zones and made

the subduction of the continental lithosphere in these regions possible. The subducted lithosphere could give

intermediate-depth seismicities by devolatilization embrittlement. Such subduction of hotspot-affected

lithosphere without accompanying any oceanic plate would be one candidate for producing ultrahigh-

pressure metamorphic rocks by deep subduction of the continental crust.

2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

When two continental plates converge, smooth subduction is hard

to operate, and collision occurs (McKenzie, 1969; Dewey and Bird,

1970). In such collision zones, offscraping of the upper crust from

underthrusting continental lithosphere at shallow depths (e.g.,

Molnar, 1984; Butler, 1986; Mattauer, 1986) and indentation and

severe deformation of the upper plate (Tapponnier et al., 1982) are

seen. The rest of the lithospheric slab with lower crust is often

recognized as subducting in the mantle by tomographic studies (e.g.,

Van der Voo et al., 1999; Bijwaard and Spackman, 2000; Replumaz

et al., 2004). Capitanio et al. (2010) showed by numerical modeling

that such continental lithosphere, without upper crust, is dense

enough to be subducted into the upper mantle.

More strictly saying, however, offscraping of the upper crust does not

always happen in collision zones. For example, coesite (a high-pressure

polymorph of SiO2) and diamond in eclogite blocks or layers or as

inclusions in zircons in ultrahigh-pressure (UHP) metamorphic rocks are

found in the sialic crust subducted to a more than 100 km depth and

exhumed up to the surface (Coleman and Wang, 1995; Ernst and Liou,

1999; Chopin, 2003; Zhang et al., 2009). Although some of them might be

accompanied with subduction of a continental plate dragged down by a

foregoing oceanic plate, in other cases no oceanic plate is accompanied.

Which extent of the continental crust can be subducted is, then, animportant issue to understand the mechanism of formation of geological

structures. Searle et al. (2001) listed Hindu Kush as a type locality of UHP

formation, not associated with any oceanic plate. He attributed the

subductability of the continental crust beneath Hidu Kush to the

eclogitization of the subducted lower crust of the Indian continental

margin. In this study, we will treat Hindu Kush and Burma and present a

different hypothesis on the subductability of the crust in these regions, by

paying attention to the fact that they have intermediate-depth

seismicities.

Seno (2008) showed that the pore fluid pressure ratio (= Pw/

n, where Pw is the pore fluid pressure and n is the normal stress at

the thrust) is a controlling factor for offscraping/subductability of the

underthrusting upper continental crust in collision zones. The value of

determines the shear stress along the roof of a continental crustal

block embedded in the slab. It is assumed that for small enough ,

offscraping of the upper crustal block occurs by cutting the faults

bounding the block (See also van den Beukel, 1992). He showed that

the buoyancy of the crust is not important, contrary to previous

studies (e.g., van den Beukel, 1992), and offscraping of the upper crust

occurs for less than 0.4. On the contrary, traditional geodynamic

and laboratory models treating subductability of the continental crust

do not pay much attention to the role of the shear resistance at the

subductionzone thrust (Chemendaet al., 2000; Capitanioet al., 2010).

The value of could be related to intermediate-depth seismicity as

described below. Intermediate-depth earthquakes are usually not

observed in collisionzones(Seno, 2007).When theyare seen, they are

Gondwana Research 19 (2011) 327333

Corresponding author. Research Institute, University of Tokyo, Bunkyo-ku, Tokyo

113-0032, Japan. Tel.: +3 5841 5747; fax: +3 5841 8260.

E-mail address: [email protected] (T. Seno).

1342-937X/$ see front matter 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

doi:10.1016/j.gr.2010.05.011

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mostly representing relict subduction of an oceanic plate trailing a

continent prior to the collision (e.g., McCaffrey et al., 1985; Wang

et al., 2006). Hindu Kush and Burma in the Himalayas, although

continental crust is converging there, accompany intermediate-depth

seismicities apparently without any oceanic plate. Why they accom-

pany intermediate-depth seismicities has been an enigma long time.

High lithostatic pressure makes the occurrence of intraslab earth-

quakes impossible even if the rheology is predicted to be in the brittle

regime (Griggs and Handin, 1960). Although Negredo et al. (2007), forexample, showed that the slab beneath Hindu Kush could be in the

brittle regime, it does not suffice to explain the observed seismicity.

Similarly, Lister et al. (2008) explained the intraslab seismicity beneath

Hindu Kush by the deformation associated with boudinage. Even if

earthquake faulting is associated with boudinage, this itself does not

explain why faulting can occur under such a high lithostatic pressure.

In recent years, dehydration embrittlement hypothesis has been

proposed for occurrence of intermediate-depth earthquakes (Raleigh

and Paterson, 1965; Kirby et al., 1996; Seno and Yamanaka, 1996;

Peacock and Wang, 1999; Omori et al., 2002). In this hypothesis,

hydrous metamorphic minerals like amphibole and serpentine within

the oceanic plate dehydrate as the temperature and pressure rise

when it is subducted. This raises the pore fluid pressure and reduces

the effective stress, making earthquakes possible to occur (Raleigh

and Paterson, 1965). It has been shown that the observed intraslab

seismicity are well explained by the dehydration locus expected from

phase diagrams of hydrous minerals (e.g., Peacock and Wang, 1999;

Omori et al., 2002; Hacker et al., 2003; Yamasaki and Seno, 2003).

Because dewatering from the sediments and pore spaces in the crust

of the subducting slab finishes at shallow depths (Fyfe et al., 1978;

Jarrard, 2003; Hacker, 2008), dehydration from the altered crust and/

or serpentinized slab mantle would be a major source of the pore

fluids brought to thethrust. Forexample,the valuesof in subduction

zone thrusts are more than 0.9, as estimated from the force balance

between the shear stressat the thrust and the differential stress in the

wedge (Seno, 2009). These high valuesof must represent porefluids

released from the deeper part of the slab. Therefore intermediate-

depth earthquakes could be a proxy for the level of dehydration, and

thus, for in the thrust. This would be also true for subductingcontinental lithosphere. Dehydration embrittlement can be extended

to devolatilization embrittlement, if we include CO2 and other

volatiles as pore fluids fed from the metasomatized lithosphere

(Kirby, 1995), and we will use this general term.

The intermediate-depth seismicities in Hindu Kush and Burma,

therefore, suggest that devolatilization is occurring in the slab beneath

these regions to a similar extent in subduction zones. Although it is

difficult to determinethe exact value of in these regionsat present, we

guess that it is close to the values in subduction zones. If so, it would

suffice to prevent offscraping and promote subduction of the upper

crust. On the contrary, the general absence of intermediate-depth

seismicity in collision zones implies absence of devolatilization from the

subducting continental lithosphere (See Seno and Yamasaki, 2003;

Seno, 2007). This can be attributed to greater breakdown depths ofwhite mica and biotites contained in the granite or granulite in the

continental crust (Ernst et al., 1998; See also Seno, 2007). This would

cause a low value of and result in decoupling between the upper and

lower crusts, possibly along detachment faults with horst-graven

structures (Guillot et al., 2000), and offscraping of the upper crust in

collision zones.

If we admit devolatilization in theslabto operate in Hindu Kush and

Burma, volatilization in the lithosphere prior to subduction must have

occurred. This would be the most difficult problem to solve on the

existence of the intermediate-depth seismicities in Hindu Kush and

Burma.We will propose in this paper that this could happenbecause the

Indian lithosphere had passedover the Reunion and Kerguelenhotspots

in the Cretaceous and was metasomatized by the upwelling plumes

beneath these hotspots. When the metasomatized lithosphere has

impinged on Hindu Kush andBurma, it would have been devolatilized to

lubricate the mega-thrustand produce intermediate-depth seismicities.

2. Tectonic settings

Wefirst present tectonic settings of Hindu Kush andBurma (Fig. 1),in

orderto show that a continentalplate isconvergingbeneathbothof these

regions, and yet features of collision are weak. Hindu Kush is located

withinthe Asian plate,southwest of Pamirand west of Karakorum.To thesouth, there is the Mesozoic Kohistan-Ladakh arc (Fig. 1A), which was

weldedto Asia at111-75 Ma(Pettersonand Windley, 1985; Mikoshiba

et al., 1999). India later collided with this arc at55 Ma (e.g., de Sigoyer

et al., 2000;Guillotet al., 2003;Leech et al., 2005). Thegeologicalterranes,

the HighHimalaya, theLesserHimalaya, andthe Siwaliks, associated with

the collision between India and Kohistan, are similar to those in other

Himalayan regions where Indiaand Asiacollided. However,the widths of

the High and Lesser Himalayas are less than 1/2 of those in the central

and eastern Himalayas to the east (Coward et al., 1987; Guillot et al.,

2008). This indicates that offscraping and accretion of the upper crust of

the underthrusting Indian continental margin have been in a lesser

extentthan in other Himalayanregionsand a considerable amountof the

crust is subducted south of Kohistan.

South of Kohistan, the plate boundary where the Indian plate is

currently underthrusting is likely to be located from the Main Frontal

Thrust to the Main Boundary Thrust (Fig. 1A, Searle et al., 2001). To the

north, there is an intermediate-depth seismicity dipping steeply

northward to a depth of300 km beneath Hindu Kush and dipping

southward to a depth of150 km beneath Pamir (yellow colored in

Fig. 1A, Billington et al.,1977; See alsoSearleet al., 2001). Studies of these

seismicitiesusing focal mechanisms and seismictomography (e.g., Pegler

and Das, 1998; Van der Voo et al., 1999; Pavlis and Das, 2000; Koulakov

and Sovolev, 2006) show that the seismic zone beneath Hindu Kush is

continuous to that beneath Pamir. Because Kohistancollided against Asia

during the late Mesozoic and India against Kohistan during the early

Tertiary, no oceanic plate is recently subducting in this region as pointed

out by Searle et al. (2001). Negredo et al. (2007) also showed that the

subducting slab is continental lithosphere by comparing the seismicity

withthe reconstruction of the Indian plate geometry. Onethe other hand,the intermediate-depth earthquakes are the phenomena occurring

within only past several Ma, provided with the depth of the seismicity

of 300 km and the convergence rate of4 cm/yr (DeMets et al., 1990)

(Seealso Negredoet al., 2007;Replumaz et al., 2010).Thus itis difficult to

relate the intermediate-depth seismicity beneath Hindu Kush with

subduction of an oceanic plate, contrary to some previous authors

(Billington et al., 1977; Chatelain et al., 1980; Roecker, 1982; Pavlis and

Das, 2000; Seno, 2007) and thus continental lithosphere is subducting

here.

Burma is a microplate (blue-colored in Fig. 1B) decoupled from the

Sunda plate (pink-colored in Fig. 1B; See Stein and Okal, 2007). In

western Burma, there is a belt of high-grade metamorphic rocks with

Mesozoic ophiolites (green colored in Fig. 1B, Maurin and Rangin, 2009).

This ophiolite belt was regarded as an eastward continuation of theIndus-Zangpo suture zone by Mitchell (1984). In Tibet, south of the

ophiolite belt, the Tethys and High Himalayas, i.e., the Mesozoic and

Paleozoic rocks offscraped from the Indian continental margin, charac-

terize collision. However, they are absent in Burma. Instead, west of the

ophiolite belt, there is the Indo-Burman Wedge representing a Tertiary

accretionary complex growing to the southwest (dark-blue colored in

Fig. 1B, Maurin and Rangin, 2009). East of the ophiolite belt, there are

Miocene-Quaternary calcalkaline volcanics (v-marks in Fig. 1B, Mitchell,

1984). These suggest that, in the Burman arc, collision did not occur,

unlike the Himalayas, but subduction of the crust has been occurring

during the late Cenozoic (Mitchell, 1984; Maurin and Rangin, 2009).

There is an intermediate-depth seismicity beneath Burma down to

a depth of 200 km (yellow-colored in Fig. 1B, Le Dain et al., 1984;

Biswas and Gupta, 1986; Ni et al., 1989; Vanek et al., 1990; Satyabala,

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1998). Although some workers (Ni et al., 1989; Guzman-Speziale and

Ni, 1996; Rao and Kumar, 1999) argued that subduction has stopped

and the intermediate-depth seismicity represents relict subduction or

that theIndian-Burman relative motionis along the arc, the geological

structures off and on the Burman coast (Mitchell, 1984; Maurin and

Rangin, 2009; Nielsen et al., 2004) and the reconstruction of the

relative motion on the basis of the global and local plate motion data

(Nielsen et al., 2004; Stein and Okal, 2007) demonstrate that the Bay

of Bengal has a significant component of convergence to the Burma

plate (Fig. 1B). Seismic tomography studies show that a high P-

velocity anomaly dips from the Bengal Basin eastward to a depth of

400 km along with the shallow - intermediate-depth seismicity (Li

et al., 2008; Replumaz et al., 2010), suggesting that the Indianlithosphere is subducting eastward beneath the Burma plate. The Bay

of Bengal and the Bengal Basin on land have a crustal thickness of

N25 km (Brune andSingh, 1986; Kaila et al., 1992). These indicate that

Burma is a continent-continent convergence zone, where most of the

upper crust is subducting with an intermediate-depth seismicity,

similarly to Hindu Kush.

3. Reunion and Kerguelen hotspot traces

The intermediate-depth seismicities to depths of a few 100 km

beneath Hindu Kush and Burma and the tectonic settings described

above suggest that the subducted lithosphere is continental and yet

devolatilized to produce the intraslab seismicities. The subducted

lithosphere is different from that in ordinary collision zone in a sense

that it has much of the upper crust. However, dehydration is not

expected from the upper crust as mentioned before. The problem now

concerns why and where the continental lithosphere in these regions

was volatilized. This would not seem an easy problem to solve.

We invoke a volatilizationmechanism in relation to metasomatization

of the lithosphere by plumes beneath hotspots. We first show that this

part of the lithosphere could be located over hotspots during the

Cretaceous. Western India had traveled over the Reunion hotspot, and

eastern India over the Kerguelen hotspot. In Fig. 2, we plot the volcanics

that are related to these hotspots, and hotspot traces. For the Reunion

hotspot, we plotthe DeccanTraps, the alkali intrusions3 m.y. older than

the Deccan (Basu et al., 1993), and the South Tethyan suture zone

volcanics (80 Ma, Mahoney et al., 2002), by the dark brown colors.Geochemical and isotope data (Mahoney et al., 2002) of these volcanics

suggest that their source is the Reunion hotspot. For the Kerguelen

hotspot, we plot the Ninetyeast Ridge, its northern extension buried

beneath the sediments in the Bengal Fan (Curray et al., 1979), the

Rajmahal Traps and the lamprophyres in NE. India (118-115 Ma, Baksi,

1995;Kentet al., 2002),by theyelowcolors. Geochemical andisotope data

(Mahoney et al., 1983; Storey et al., 1989; Kent et al., 2002 ) suggest that

their source is the Kerguelen hotspot. The age distributions of these

volcanics indicate that the Reunionand Kerguelenhotspotswere active at

least back to 80 Ma and120 Ma, respectively.

In Fig. 2, we also plot the traces of the Reunion and Kerguelen hotspots

on theIndian plate younger than 100 Ma (blue lines with ages), based on

the Muller et al.'s (1993)finite rotations of India with respect to hotspots

by assuming that these hotspots have beenfixedat their present locations.

Fig. 1. Index map for Hindu Kush and Burma, where intermediate-depth seismicities are observed in the Himalayan collision zone. (A) Hindu Kush: Offscraped Indian continental

margin (blue, the High and Lesser Himalayas and the Siwaliks), the Kohistan-Ladakh island arc (pink), and the deformed Asia (white) (modified from Searle et al., 2001). The suture

zones between these geological units are indicated by the bold lines. MKT, Main Karakorum Thrust; MMT, Main Mantle Thrust; MCT, Main Central Thrust; MBT, Main Boundary

Thrust; MFT, Main Frontal Thrust. The epicentral area of the intermediate-depth seismicity is taken from Fig. 2 and colored by yellow. (B) Burma: The Burma microplate (blue-

colored), the accretionary Indo-Burman Wedge (dark-blue, Maurin and Rangin, 2009), the ophiolite belt (green, Mitchell, 1984), Miocene-Quaternary calcalkaline volcanics

(v-marks, Mitchell, 1984), the epicentral area of the intermediate-depth seismicity (yellow) taken from Fig. 2, and the Sunda plate (pink).

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The trace of the Reunion hotspot indicates that, if it were active back to

100 Ma, the Indian continental lithosphere over the hotspot at 100 Ma

reaches now the southwestern edge of the intermediate-depth seismicity

beneath Hindu Kush (Fig. 2). The lithosphere over the Kerguelen hotspot

at 100 Ma is north of the Andaman Islands and does not reach the

intermediate-depth seismicity beneath Burma. However, the older

location estimated by Kent et al. (2002) (115-126 Ma, the large blue

star in Fig. 2) and thatby Storeyet al. (1989) (110 Ma, the secondarylarge

bluestar inFig.2) overlap withthe intermediate-depthseismicitybeneath

Burma (Fig. 2).

4. Metasomatization and devolatilization

The above reconstruction of the hotspot traces suggests a possibility

that the Indian lithosphere in the vicinity of the intermediate-depth

seismicities beneath Hindu Kush and Burma had traveled over the

Reunion and Kerguelen hotspots during the mid-Cretaceous. We note

here that a double-planed structure is seen in the intermediate-depth

seismicity in some of the slabs in circum-Pacific subduction zones (e.g.,

Hasegawa et al., 2009; See also Seno and Yamanaka, 1996). Although the

upperplane is explained by thedehydration of altered basalt (Kirby et al.,

1996; Peacock and Wang, 1999; Hacker et al., 2003; Yamasaki and Seno,

2003), it is difficult to explain the lower plane because it is located in the

lower part of the oceanic lithosphere. Kirby (1995) and Seno and

Yamanaka (1996) proposed that it is caused by the devolatilization

embrittlement in the subducting oceanic lithosphere, of which lower part

is metasomatized by hotpots in the Pacific Ocean.

We imagine that the Indian continental lithosphere was similarly

metasomatized in its lower portion by the plume activities beneath the

Reunion and Kerguelen hotspots when it traveled over them. Wyllie

(1988) proposed the processes of metasomatization of the continental

lithosphere by plumes as follows; uprising plumes beneath hotspots

containpartialmeltsdissolving C-H-O,andrelease its vapor from magmas

accumulated below the solidusin thelithosphere.This hydrofracturesand

metasomatizes the lithosphere (Fig. 3A, See also Wilshire and Kirby,

1989).The minerals in the lower portion of the lithosphere metasoma-

tized by H2O and CO2 may enter a collision zone and underthrust to a

depth of a few tensof km along the interplate megathrust (Fig. 3B). As

the temperature and pressure rise, fluid is released from the slab and

migrates along thethrusts, lubricating them andmaking it possible for

the continental lithosphere to be subducted with the upper crust,

producing feeble features of collision. As the slab subducts deeper,

devolatilization embrittlement occurs in the slab, producing an

intermediate-depth seismicity.

5. Discussion

Although direct evidence for the metasomatization of the Indian

lithosphereby hotspotsand itssucceeding devolatilization beneath Hindu

Fig. 2. Relation between hotspot trace and intermediate-depth seismicity around India. The epicenters of intermediate-depth earthquakes (Inter. Seism. Centre Catalogue:

depthN80 km, 1983-2004, MN4.0) are shown by green. Volcanics related to the Reunion hotspot (dark-brown colored) are the Deccan flood basalts, the alkali intrusions 3 m.y.

older than the Deccan (triangles, Basu et al., 1993), and the 80 Ma South Tethyan suture zone volcanics in Pakistan ( Mahoney et al., 2002). Volcanics related to the Kerguelen

hotspot (yellow colored) are the Ninetyeast Ridge, its northern extension beneath the Bengal Fan (Curray et al., 1979), the Rajmahal Traps and the lamprophyres in NE. India

(triangles). The hotspot traces on the Indian plate over the Reunion and Kerguelen hotspots younger than 100 Ma are plotted using the rotations of the Indian plate with respect to

hotspots by Muller et al. (1993) and are shown by the blue lines with ages. The trace of 90-100 Ma age for the Reunion hotspot is located at the southern edge of the intermediate-depth seismicity in Hindu Kush. For the Kerguelen hotspot, the traces of 115-126 Ma age (large blue star, Kent et al., 2002) and 110 Ma age (medium-size blue star, Storey et al.,

1989) are located on the intermediate-depth seismicity in Burma.

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Kush and Burma presented in this paper is lacking, the coincidence of the

hotspot traces with the intermediate-depth seismicities suggests that it is

likely, and, without such a causal link, it remains difficult to understand

the intermediate-depth seismicities beneath these places. Because the

fundamental ideas presented in this paper are simple, geological,geophysical, and geochemical facts should be more quantitatively

examined in details in the future to test them. However, we feel that

the insights that we have obtained have already important implications

for interpreting geological histories of the Earth, some of which we show

below.

Vrancea in southeast Carpathians in middle eastern Europe is a

unique place with an intermediate-depth seismicity down to a depth of

220 km. A high seismic velocity slab is associated with this seismicity

(Fan et al., 1998). Under Europe, seismic tomography and geochemical

studies (Granet et al., 1995; Wilson and Patterson, 2001) suggest a few

to several plumes upwelling from the 670 km discontinuity. Because

there has been convergence between Europe and Alps since the early

Tertiary, it is possible for a portion of the lithosphere affected by the

upwelling plumes to have been subducted beneath E. Europe and to

produce an intermediate-depth seismicity beneath Vrancea like Hindu

Kush.

Although there are more than a dozen of locations where UHP

rocks are exhumed (Chopin, 2003), mechanisms how continental

crust can be brought to such a great depth have not been easy tounderstand. In some cases, a leading oceanic plate drags down the

continental plate and the continental crust might be subducted to a

such a great depth. The formation of UHP in the Himalayasaround 55-

47 Ma (Kaneko et al., 2003; Guillot et al., 2008) might be such a case

when the Tethys ocean dragged down the Indian continent.

Formation of UHP in W. Alps may also be due to intermittent

subduction of oceanic plates, succeeded by continental plates (Platt,

1986; Seno, 2008).

Searle et al. (2001) listed Hindu Kush as one of type localities of

formation of UHP, not accompanied by oceanic plates, as we already

mentioned. They proposed that the leading edge of the lower

continental crust of the Indian plate has been subducted during

recentseveral Ma, with theuppercrustpealed offmaking thePakistan

Himalaya. However, their model requires complex deformation of the

Fig. 3. (A) Metasomatization of the continental lithosphere (adapted from Wyllie, 1988). Lines 2 and 3 represent crossing points between the geotherm and the solidus for

peridotite-CO2-H2O. Line 1 represents the lithosphere-asthenosphere boundary, above which partial melts accumulate and form magmas. From the cooling magmas, vapors of CO2and H2O are released and metasomatize the lower portion of the lithosphere by hydrofracturing (green-colored). (B) Devolatilization of the metasomatized continental lithosphere

in a continent convergent zone. When the continental lithosphere converges and the lower portion of the lithosphere is metasomatizedby a plume(green-colored), devolatilization

from the lithosphere makes volatiles (blue-colored) to migrate upward and lubricate the thrust zone above. The whole continental lithosphere could be subducted (black arrow)

and produce intermediate-depth seismicity by devolatilization embrittlement.

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subducted lower crust to mimic the shape of the intermediate-depth

seismicity. Furthermore the predicted volume of the offscraped upper

crust amounting to 300 km15 km is not observed south of the Main

Mantle Thrust (Coward et al., 1987).

We propose that subduction of hotspot-affected lithosphere would be

a likely candidate for the formation of UHP rocks without accompanying

leading oceanic plates or thin marginal lower crust. In this case, intraslab

earthquakes are located in the slab mantle. The geometry of intraslab

seismicity need not be simple because the area of metasomatization inthe lithosphere might not be simple. Complex deformation of the

lithosphere to explain the geometry of the intraslab seismicity is thus

not required. Other areas of UHP exhumation in the world deserve to be

examined from this viewpoint. Recently, Sumino and Dobrzhinetskaya

(2009) analyzed noble gases contained in the metamorphic diamonds

exhumed from the UHP belt in Kokchetav massif, Kazakhstan, and found

that their 3He/4He ratios are close to that observed in OIBs, significantly

higher than that of MORB. This indicates a possibility that metasomatism

by plumes might be related to the formation of the UHP in Kokchetav and

subduction of the continental crust to a large depth might have occurred

there similarly to Hindu Kush and Burma. This kind of geochemical

analyses would become important to elucidate the causes of the

subductability of continental crust.

A thick crust covered the surface of the early Earth (Sleep and

Windley, 1982; Bickle, 1986), which has made subduction of the

lithosphere to be difficult. The crust piles up at the surface, and the

surface tectonics is unlike platetectonics (Davies, 1992). As themantle

part of the lithosphere became thicker and the crustal part became

thinner along with the cooling of the Earth's mantle, the lithosphere

became favorable to be subducted. However, due to the temperature-

dependent viscosity of the mantle, the strength of the lithosphere

would have been too large to allow the lithosphere to be subducted

and the convection regime would shift to the stagnant lid-convection

(Solomatov and Moresi, 1997). This implies that plate tectonicsis hard

to operate on the Earth, as on the Venus. The fact that it had started

tells us that there was a way to avoid the stagnant-lid convection. The

mechanism of initiation of continental lithosphere convergence in

Hindu Kush and Burma proposed in this study (Fig. 3B), i.. e.,

devolatilization of the lithosphere metasomatized by hotspots,suggests a possibleway to initiate subduction during the pre-Archaean

or Archaean, while plume activities were dominant (Campbell and

Griffiths, 1992; Kroner and Layer, 1992).

6. Conclusions

Hindu Kush andBurma, located at thetwo syntaxes of theHimalayan

collision zone, have intermediate-depth seismicities beneath them.

Features of collision are feeble because most of the continental crust is

likely to be subducted to a depth of few hundred km in these regions.

Because intermediate-depth seismicity is generally absent and upper

crust is offscraped at shallow depthsin collisionzones, these tworegions

are anomalous. We try to solve this enigma, on the basis ofdevolatilization embrittlement hypothesis for an intermediate-depth

seismicity. We assume that the pore fluid pressure at the megathrust is

raised enough by the devolatilization in the slab with an intermediate-

depth seismicity. We show that the continental lithosphere subducted

beneath these regions traveled over the Reunion and Kerguelen

hotspots during the middle Cretaceous and were possibly metasoma-

tized by upwelling plumesbeneath those hotspots. We propose thatthe

intermediate-depth seismicities beneath these regions occur due to

devolatilization embrittlement of the metasomatized Indiancontinental

lithosphere. The devolatilization would also have lubricated the thrust

zones and made the subduction of the continental lithosphere with the

upper crust in these regions possible. This type of the continental crust

subduction would provideone important case forUHP rock formationin

the geological past.

Acknowledgements

We thank F. A. Capitanio and anonymous referee for constructive

review. We also thank M. Santosh and the participants of the

Metamorphic rocks etc symposium held in Hiroshima, March 20-22,

2010 forusefulcomments, IchiroKaneoka forinformation of hotspots,and

Hirochika Sumino, Yoko Sakamoto, Bunichiro Shibazaki, and Tomoya

Harada for providing their work in preparation.

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