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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
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