INDEPTH III seismic data: From surface observations to deep crustal processes in Tibet

Abstract

[1] During the summer of 1998, active-source seismic data were collected along a transect running 400 km NNW-SSE across the central Tibetan Plateau as the third phase of project INDEPTH (International Deep Profiling of Tibet and the Himalaya). The transect extends northward from the central Lhasa block, across the Jurassic Bangong-Nujiang Suture (BNS) at 89.5°E, to the central Qiangtang block. A seismic velocity model for the transect to $25 km depth produced by inversion of P wave first arrivals on $3000 traces shows (1) a $50-km-wide region of low velocity (at least 5% less than surrounding velocities) extending to the base of the model at the BNS; (2) sedimentary cover for the southern Qiangtang block that is $3.5 km thick; (3) a distinct interface between sedimentary cover and Qiangtang basement or underplated Jurassic melange in the central Qiangtang block; and (4) evidence that the Bangoin granite extends to a depth of at least 15 km. The BNS has little geophysical signature, and appears unrelated to the $5 km northward shallowing of the Moho which is associated with the BNS in central Tibet. Geophysical data along the main INDEPTH III transect show little evidence for widespread crustal fluids, in contrast to the seismic ''bright spots'' found in southern Tibet and to magnetotelluric evidence of fluid accumulations in eastern Tibet. A comparison between the global average and Tibetan velocity-depth functions offers constraints for models of plateau uplift and crustal thickening. Taken together with the weak geophysical signature of the BNS, these velocitydepth functions suggest that convergence has been accommodated largely through pure-shear thickening accompanied by removal of lower crustal material by lateral escape, likely via ductile flow. Although we cannot resolve the details, we believe lateral lower crustal flow has overprinted or destroyed evidence in the deep crust for the earlier assembly of Tibet as a series of accreted terranes.

Crustal–lithospheric structure and continental extrusion of Tibet

“…The INDEPTH seismic profiles (Zhao et al 1993;Nelson et al 1996;Haines et al 2003;Tilmann et al 2003) have also revealed major differences in lithospheric structure between northern and southern Tibet (Fig. 8b).…”

“…LVZ, low-velocity zone. Haines et al 2003). The INDEPTH IV line extends across northern Tibet to the Qaidam basin, along which a 15 km offset in the Moho has been detected across the northern boundary of the Tibetan Plateau from c. 65 km crustal thickness beneath Tibet to c. 48 km beneath the Qaidam basin (Vergne et al 2002).…”

“…Clark & Royden (2000) used the regional topographic gradients to model a Newtonian fluid flowing through a 15 km thick lower crust, estimating a viscosity of 10 16 Pa s for beneath the plateau and 10 18 Pa s for low-gradient margins and 10 21 Pa s for a steep margin. Haines et al (2003) also presented evidence from INDEPTH III seismic data that lower crustal ductile flow had destroyed evidence in the deep crust for earlier accretion of distinct terranes.…”

“…Therefore we are reliant on deep crustal seismic experiments (tomography, receiver functions and seismic anisotropy), notably the INDEPTH (Zhao et al 1993;Nelson et al 1996;Haines et al 2003;Tilmann et al 2003) and Hi-CLIMB (Chen & Tseng 2007;Hetényi et al 2007;Nábelek et al 2009) profiles, to interpret the structure of the lower crust. The composition of the lower crust and upper mantle can directly be known only from xenoliths entrained in ultrapotassic or potassic (shoshonitic) volcanic rocks that have sampled the lower crust on their way up to the surface (e.g.…”

“…Lines of section of the two crustal profiles in Figure 15 are also shown. Clark & Royden 2000;Haines et al 2003) and (b) the Himalayan mid-crustal 'channel flow' model (e.g. Beaumont et al 2001Beaumont et al , 2004Grujic et al 2002;Searle et al 2003Searle et al , 2006Searle et al , 2010bLaw et al 2004Godin et al 2006).…”