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318 CHAPTER 10
1997; Zho et al., 2001) of the upper mantle indicate thick crust beneath the Tien Shan is consistent with
that fast mantle velocities occur beneath southern evidence of crustal shortening in this region (Section
Tibet and slow mantle velocities occur north of the 10.4.3).
Bangong–Nujiang suture (Fig. 10.21). These differences
suggest the presence of cold, strong mantle beneath
southern Tibet and anomalously warm, weak mantle
beneath central and northern Tibet. The pattern may 10.4.6 Mechanisms of
indicate that Indian lithosphere has been underthrust continental collision
to at least a point beneath the center of the Tibetan
Plateau. However, this interpretation is in confl ict
Like all other major zones of continental deformation
with estimates of the total amount of convergence
(e.g. Sections 7.6, 8.6, 10.2.5), the evolution of colli-
and shortening of the lithosphere since the collision
sional orogens is governed by the balance among
began. Estimates of the total convergence (∼2000 km)
regional and local forces, the strength and rheology of
derived from magnetic anomalies, paleomagnetic
the continental lithosphere, and by processes that
studies, and estimates of the minimum amount of
change these parameters over time. To determine how
post-collisional shortening (Johnson, 2002) suggest that
interactions among these factors control the develop-
cold Indian lithosphere also may occur beneath north-
ment of the Himalayan–Tibetan orogen, geoscientists
ern Tibet.
have developed physical and analogue models of conti-
High resolution tomographic images of the upper
nental collision. This section provides a discussion of
mantle may help to resolve this discrepancy. Tilmann
the main results and different approaches used in this
et al. (2003) interpreted the presence of a subvertical,
field of study.
high velocity zone located south of the Bangong–
Nujiang suture between 100 km and 400 km depth (Plate 1 Precollisional history. The strength and
9.4(bottom) between pp. 244 and 245). This subvertical rheology of the continental lithosphere at the
zone may represent downwelling Indian mantle litho- start of continental collision is governed by
sphere. The additional Indian lithosphere helps account the pre-collisional history of the two colliding
for the total amount of shortening in the Himalayas and plates. In the case of the Himalayan–Tibetan
Tibet. The downwelling also may explain the presence orogen, millions of years of subduction, arc
of warm mantle beneath northern and central Tibet, magmatism, terrane accretion, and crustal
which would fl ow upwards to counterbalance a defi cit thickening along the southern margin of
in asthenosphere caused by the downwelling. The Eurasia (Section 10.4.2) weakened the
occurrence of calc-alkaline-type volcanic rocks in south- lithosphere. During the India–Eurasia
ern and central Tibet may support this interpretation collision, the many suture zones, thick fl ysch
by requiring a portion of continental crust to have sequences, and other weak zones that
been underthrust into the mantle beneath Tibet from characterized Eurasia allowed deformation to
the north and south (Yin & Harrison, 2000). Neverthe- extend deep into the interior of the
less, the mechanisms by which Indian lithosphere continent (Yin & Harrison, 2000; Tapponnier
shortens and is underthrust beneath Tibet remain et al., 2001).
controversial. Unlike Eurasia, the relatively cool and deeply
At the northern and northwestern margin of Tibet, rooted Precambrian shield of India resulted
the Moho abruptly shallows to depths of 50–60 km in a relatively strong plate that resisted
across the Altyn Tagh Fault and beneath the Tarim shortening during collision. The generally
Basin (Wittlinger et al., 2004). The Moho also appears high mechanical strength and high elastic
to shallow across the Jinsha suture beneath the Songpan– thickness of the Indian lithosphere led to its
Ganzi terrane (Fig. 10.21). From the receiver functions underthrusting beneath southern Tibet
it is impossible to distinguish whether the Moho is part (Section 10.4.3). An exception to its generally
of Indian or Eurasian lithosphere. Relatively thick high strength is the sediment that was
(60 km) crust occurs beneath the Tien Shan and gradu- deposited on the passive continental margin
ally thins to the north to an average of 42 km beneath of northern India from the Early Proterozoic
the Shield of Eurasia (Bump & Sheehan, 1998). The to Paleocene. During collision, these weak