Structure and Dynamics of the Himalaya and Tibet
Tibet forms the largest and highest plateau on Earth today. The pattern of horizontal strain is increasingly well-known, and suggests that deformation is essentially continuous, with diffuse deformation of the crust and upper mantle over broad areas. The opposed view that discrete tectonic blocks, internally undeformed, are being expelled eastward between lithospheric strike-slip faults, is now largely discredited. The analogous debate about vertical strength and strain profiles in Tibetan lithosphere is still far from resolved. Opposing views are that the entire lithosphere deforms homogeneously ('vertical coherent deformation'), or that deformation is dominated by a more rapid ductile flow in the middle and/or lower crust above a stronger upper mantle ('channel flow'). The existence, and if so, depth extent, of such channelized flow depends critically on the actual strength profile. The strong dependence of rock strength on both composition and temperature as well as hydration state and presence of partial melt means the relative strength of upper crust, lower crust and upper mantle will vary in time and space. An evaluation of this relative strength requires detailed knowledge of the lithosphere, in large part obtained through seismic studies.
Klemperer's research group has been carrying out seismic studies of Tibetan lithosphere since 1992, participating in the INDEPTH project (International Deep Profiling of Tibet and the Himalaya), more recently in analysis of HIMPROBE data from the Indian Himalaya, and prospectively in planned SINOPROBE profiling activities in central and western Tibet. Beneath the Lhasa block, our controlled-source reflection studies directly imaged crustal fluids (probably water-rich melts, with porosity up to 20%), as shallow as 15 km depth, indicating a very weak middle crust; but the controlled-source profiles do not tell us whether large areas of the crust or only tiny patches are similarly weak. More recently we have exploited regional earthquake phases traveling parallel to the Himalaya to exploit the strong dependence of shear-wave velocities on the presence of partial melt. Our velocity models that average along-strike for tens to hundreds of km show a low-velocity layer (LVL) with c. 10% velocity reduction centered at ~30 km depth and apparently continuous from the Tethyan Himalaya to the Tibetan plateau. The image shows this LVL (a trough in the individual velocity-depth functions) shows good spatial correspondence with a low-resistivity layer inferred from companion magnetotelluric studies along the same profile (color background). Comparison of our results with laboratory measurements and theoretical models suggests 3Ð7% melt is present in a channel in the upper-middle crust of the NW Himalaya at the present day, and the physical conditions to enable active channel flow may be present today.