Download When and why the continental crust is subducted: Examples of

Gondwana Research 19 (2011) 327–333
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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
b
Earthquake Research Institute, University of Tokyo, Bunkyo-ku, Tokyo 113-0032, Japan
Department of Earth and Environmental Sciences, Kagoshima University, Kohrimoto 1-21-35, Kagoshima 890-0065, Japan
a r t i c l e
i n f o
Article history:
Received 7 April 2010
Received in revised form 24 May 2010
Accepted 26 May 2010
Available online 4 June 2010
Keywords:
Continental crust
Subduction
Collision
Hindu Kush
Burma
a b s t r a c t
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 ultrahighpressure 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.
⁎ 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).
Which extent of the continental crust can be subducted is, then, an
important 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 λ (= Pw/
σn, where Pw is the pore fluid pressure and σn 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
λ determines the shear stress along the roof of a continental crustal
block embedded in the slab. It is assumed that for small enough λ,
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 λ 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
subduction zone thrust (Chemenda et al., 2000; Capitanio et al., 2010).
The value of λ could be related to intermediate-depth seismicity as
described below. Intermediate-depth earthquakes are usually not
observed in collision zones (Seno, 2007). When they are seen, they are
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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T. Seno, H.U. Rehman / Gondwana Research 19 (2011) 327–333
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 accompany intermediate-depth seismicities has been an enigma long time.
High lithostatic pressure makes the occurrence of intraslab earthquakes impossible even if the rheology is predicted to be in the brittle
regime (Griggs and Handin, 1960). Although Negredo et al. (2007), for
example, 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 the thrust. For example, the values of λ in subduction
zone thrusts are more than 0.9, as estimated from the force balance
between the shear stress at the thrust and the differential stress in the
wedge (Seno, 2009). These high values of λ must represent pore fluids
released from the deeper part of the slab. Therefore intermediatedepth earthquakes could be a proxy for the level of dehydration, and
thus, for λ in the thrust. This would be also true for subducting
continental 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 determine the exact value of λ in these regions at 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 of
white 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 λ 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 the slab to 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 happen because the
Indian lithosphere had passed over the Reunion and Kerguelen hotspots
in the Cretaceous and was metasomatized by the upwelling plumes
beneath these hotspots. When the metasomatized lithosphere has
impinged on Hindu Kush and Burma, it would have been devolatilized to
lubricate the mega-thrust and produce intermediate-depth seismicities.
2. Tectonic settings
We first present tectonic settings of Hindu Kush and Burma (Fig. 1), in
order to show that a continental plate is converging beneath both of these
regions, and yet features of collision are weak. Hindu Kush is located
within the Asian plate, southwest of Pamir and west of Karakorum. To the
south, there is the Mesozoic Kohistan-Ladakh arc (Fig. 1A), which was
welded to Asia at ∼111-75 Ma (Petterson and Windley, 1985; Mikoshiba
et al., 1999). India later collided with this arc at ∼55 Ma (e.g., de Sigoyer
et al., 2000; Guillot et al., 2003; Leech et al., 2005). The geological terranes,
the High Himalaya, the Lesser Himalaya, and the Siwaliks, associated with
the collision between India and Kohistan, are similar to those in other
Himalayan regions where India and Asia collided. 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
extent than in other Himalayan regions and a considerable amount of 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 of ∼300 km beneath Hindu Kush and dipping
southward to a depth of ∼150 km beneath Pamir (yellow colored in
Fig. 1A, Billington et al., 1977; See also Searle et al., 2001). Studies of these
seismicities using focal mechanisms and seismic tomography (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 Kohistan collided 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
with the reconstruction of the Indian plate geometry. One the 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 of ∼4 cm/yr (DeMets et al., 1990)
(See also Negredo et al., 2007; Replumaz et al., 2010). Thus it is 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 the
Indus-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, characterize 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,
T. Seno, H.U. Rehman / Gondwana Research 19 (2011) 327–333
329
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 (bluecolored), 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).
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 the Indian-Burman relative motion is 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 Pvelocity 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 Indian
lithosphere 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 and Singh, 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 volatilization mechanism 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 plot the Deccan Traps, the alkali intrusions ∼3 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; Kent et al., 2002), by the yelow colors. Geochemical and isotope 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 Reunion and Kerguelen hotspots were active at
least back to ∼80 Ma and ∼120 Ma, respectively.
In Fig. 2, we also plot the traces of the Reunion and Kerguelen hotspots
on the Indian 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 been fixed at their present locations.
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T. Seno, H.U. Rehman / Gondwana Research 19 (2011) 327–333
Fig. 2. Relation between hotspot trace and intermediate-depth seismicity around India. The epicenters of intermediate-depth earthquakes (Inter. Seism. Centre Catalogue:
depth N 80 km, 1983-2004, M N 4.0) 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 intermediatedepth 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.
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 that by Storey et al. (1989) (110 Ma, the secondary large
blue star in Fig. 2) overlap with the intermediate-depth seismicity beneath
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
upper plane is explained by the dehydration 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
contain partial melts dissolving C-H-O, and release its vapor from magmas
accumulated below the solidus in the lithosphere. This hydrofractures and
metasomatizes the lithosphere (Fig. 3A, See also Wilshire and Kirby,
1989).
The minerals in the lower portion of the lithosphere metasomatized by H2O and CO2 may enter a collision zone and underthrust to a
depth of a few tens of km along the interplate megathrust (Fig. 3B). As
the temperature and pressure rise, fluid is released from the slab and
migrates along the thrusts, lubricating them and making 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
lithosphere by hotspots and its succeeding devolatilization beneath Hindu
T. Seno, H.U. Rehman / Gondwana Research 19 (2011) 327–333
331
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 CO2
and 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 metasomatized by 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.
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 to
understand. 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 Himalayas around 5547 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
recent several Ma, with the upper crust pealed off making the Pakistan
Himalaya. However, their model requires complex deformation of the
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T. Seno, H.U. Rehman / Gondwana Research 19 (2011) 327–333
subducted lower crust to mimic the shape of the intermediate-depth
seismicity. Furthermore the predicted volume of the offscraped upper
crust amounting to 300 km × 15 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 in
the 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 plate tectonics (Davies, 1992). As the mantle
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 temperaturedependent 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 tectonics is 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 possible way 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 and Burma, located at the two syntaxes of the Himalayan
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 depths in collision zones, these two regions
are anomalous. We try to solve this enigma, on the basis of
devolatilization 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 intermediatedepth 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 metasomatized by upwelling plumes beneath those hotspots. We propose that the
intermediate-depth seismicities beneath these regions occur due to
devolatilization embrittlement of the metasomatized Indian continental
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 provide one important case for UHP rock formation in
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 for useful comments, Ichiro Kaneoka for information of hotspots, and
Hirochika Sumino, Yoko Sakamoto, Bunichiro Shibazaki, and Tomoya
Harada for providing their work in preparation.
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