Deep structure

Velocity models and tomographic images derived from studies of Rayleigh surface waves (Section 2.1.3) show that the crust and uppermost mantle of the Indian Shield are characterized by high seismic velocities (Mitra et al., 2006). This characteristic suggests that the subcontinent is composed of relatively cool, strong lithosphere. At the northern edge of the Shield, comparatively low velocities occur beneath the Gangetic plains as a result of the molasse sediments and alluvial cover in the Ganga foredeep. Low velocities also characterize the thick crust beneath the Himalaya and Tibetan Plateau. South of the Himalaya broad-band teleseismic data indicate that crustal thickness ranges from 35 to 44 km (Mitra et al., 2005). This variability partially reflects the flexure of the Indian plate (Fig. 10.18), as it is underthrust to the north beneath Eurasia (Section 10.4.3).

Below the Himalaya, seismic reflection and shear wave velocity profiles (Fig. 10.20b) show a well-defined Moho at 45 km depth that descends as a single smooth surface to depths of 70-80 km beneath southern Tibet (Nelson et al., 1996; Schulte-Pelkum et al., 2005). A crustal décollement surface above the Moho dips northward from 8 km below the Sub-Himalaya to a mid-crustal depth of 20 km beneath the Greater Himalaya. Above the décollement a strongly (20%) anisotropic layer characterized by fast seismic velocities has formed in response to localized shearing. Slightly north of the Greater Himalaya the lower crust of the Indian Shield shows a high velocity region that may contain eclogite. These observations suggest that the upper and middle parts of the Indian crust detach along the base of the shear zone and are incorporated into the Himalaya while the lower crust continues its descent under southern Tibet (Fig. 10.21). This conclusion is consistent with gravity measurements that predict an increase in Moho depth beneath the Greater Himalaya (Cattin et al., 2001).

The deep structure of the Tibetan Plateau has been studied using passive and active source seismic surveys, magnetotelluric measurements, and surface geologic studies as part of an interdisciplinary project called INDEPTH (InterNational DEep Profiling of Tibet and the Himalaya). The geophysical data indicate that the reflection Moho beneath Tibet is rather diffuse (Fig. 10.20b), similar to that observed on seismic reflection profiles across the plateaux of the Central

Songpan-Ganzi terrane "Z-

Qaidam basin

Songpan-Ganzi terrane "Z-

Qaidam basin

Figure 10.21 Representative north-south cross-section of the Himalayan-Tibetan orogen (image provided by C. Beaumont and modified from compilation of Beaumont et al., 2004, by permission of the American Geophysical Union. Copyright © 2004 American Geophysical Union). Section incorporates observations from Owens & Zandt (1997), DeCelles et al. (2002), Johnson (2002), Tilmann et al. (2003), Haines et al. (2003). Triangles denote seven seismic recording stations. (LHASA, SANG, AMDO, WNDO, ERDO, BUDO, TUNL). Fault abbreviations as in Figs 10.19 and 10.20. Bulls-eye symbol denotes lateral movement of material out of the plane of the page.

Figure 10.21 Representative north-south cross-section of the Himalayan-Tibetan orogen (image provided by C. Beaumont and modified from compilation of Beaumont et al., 2004, by permission of the American Geophysical Union. Copyright © 2004 American Geophysical Union). Section incorporates observations from Owens & Zandt (1997), DeCelles et al. (2002), Johnson (2002), Tilmann et al. (2003), Haines et al. (2003). Triangles denote seven seismic recording stations. (LHASA, SANG, AMDO, WNDO, ERDO, BUDO, TUNL). Fault abbreviations as in Figs 10.19 and 10.20. Bulls-eye symbol denotes lateral movement of material out of the plane of the page.

Andes (Fig. 10.6). Like the Central Andes, the southern part of Tibet is characterized by low velocity zones in the crust and bands of bright intra-crustal reflections at 15-20 km depth that result from either a concentration of aqueous fluids or the presence of partial melt (Nelson et al., 1996; Makovsky & Klemperer, 1999). Low values of Q (~90) in this region are consistent with abnormally high temperatures as well as partially molten crust (Xie et al., 2004). Magnetotelluric data, which measure subsurface electrical resistivity (see also Section 8.6.3), are particular sensitive to the presence of interconnected fluids in a rock matrix. Unsworth et al. (2005) found low resistivity along at least 1000 km of the southern margin of the Tibetan Plateau, suggesting that it is characterized by weak, low viscosity crust. This weak zone is confined on its southern side by the faulted Indian crust of the Greater Himalaya and is underlain by stiff Indian mantle (Rapine et al., 2003).

In central Tibet, teleseismic data and receiver functions provide information on the crustal structure and mechanisms of deformation below the Bangong-Nujiang suture (Ozacar & Zandt, 2004). Strong (>10%)

seismic anisotropy in the upper crust shows a fabric that trends WNW-ESE parallel to both the suture and younger strike-slip faults. Seismic anisotropy (18%) also occurs at 24-32 km depth in the middle crust. This latter zone shows a near horizontal and gently dipping fabric that suggests mid-crustal flow in a north-south direction. The seismic properties of the lower crust and upper mantle also change across the suture (McNamara et al., 1997; Huang W. et al., 2000), although the suture itself has little geophysical expression (Haines et al., 2003). In the lower crust, some north-dipping reflections may represent ductile thrust slices or wedges (Fig. 10.20c) and active-source seismic data show that the Moho shallows by up to 5 km on the northern side of the boundary (Haines et al., 2003). These observations, and the relatively small amount of shortening recorded in the upper crust of Tibet, suggest that the upper crust is mechanically decoupled from the underlying layers across a weak ductilely flowing middle crust.

Surface wave studies (Curtis & Woodhouse, 1997) and Pn and Sn wave observations (McNamara et al.,

1997; Zho et al., 2001) of the upper mantle indicate that fast mantle velocities occur beneath southern Tibet and slow mantle velocities occur north of the 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 indicate that Indian lithosphere has been underthrust to at least a point beneath the center of the Tibetan Plateau. However, this interpretation is in conflict with estimates of the total amount of convergence and shortening of the lithosphere since the collision began. Estimates of the total convergence (~2000 km) derived from magnetic anomalies, paleomagnetic studies, and estimates of the minimum amount of post-collisional shortening (Johnson, 2002) suggest that cold Indian lithosphere also may occur beneath northern Tibet.

High resolution tomographic images of the upper mantle may help to resolve this discrepancy. Tilmann et al. (2003) interpreted the presence of a subvertical, high velocity zone located south of the Bangong-Nujiang suture between 100 km and 400 km depth (Plate 9.4(bottom) between pp. 244 and 245). This subvertical zone may represent downwelling Indian mantle lithosphere. The additional Indian lithosphere helps account for the total amount of shortening in the Himalayas and Tibet. The downwelling also may explain the presence of warm mantle beneath northern and central Tibet, which would flow upwards to counterbalance a deficit in asthenosphere caused by the downwelling. The occurrence of calc-alkaline-type volcanic rocks in southern and central Tibet may support this interpretation by requiring a portion of continental crust to have been underthrust into the mantle beneath Tibet from the north and south (Yin & Harrison, 2000). Nevertheless, the mechanisms by which Indian lithosphere shortens and is underthrust beneath Tibet remain controversial.

At the northern and northwestern margin of Tibet, the Moho abruptly shallows to depths of 50-60 km across the Altyn Tagh Fault and beneath the Tarim Basin (Wittlinger et al., 2004). The Moho also appears to shallow across theJinsha suture beneath the Songpan-Ganzi terrane (Fig. 10.21). From the receiver functions it is impossible to distinguish whether the Moho is part of Indian or Eurasian lithosphere. Relatively thick (60 km) crust occurs beneath the Tien Shan and gradually thins to the north to an average of 42 km beneath the Shield of Eurasia (Bump & Sheehan, 1998). The thick crust beneath the Tien Shan is consistent with evidence of crustal shortening in this region (Section 10.4.3).

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