PROJECT INDEPTH: PROBING THE BASEMENT ON THE ROOF OF THE WORLD

Project INDEPTH is a multinational, multidisciplinary initiative that has now collected an extensive suite of geophysical data extending from the high Himalayas to the central portion of the Tibetan plateau. INDEPTH I detailed the geometry of the Main Himalayan detachment beneath which Indian continental crust is subducting beneath the deforming leading edge of Asia, providing an important new constraint on the amount of plate convergence that could be attributed to crustal shortening in the Himalaya. INDEPTH II seismic and magnetotelluric indications of partial melt in southern Tibet have lent support to tectonic models involving warm, weak crust and attendant material flow at depth. INDEPTH III results that are consistent with such flow beneath the central plateau include a highly conductive crust, restriction of local seismicity to the uppermost crust, reflective lamination in the lower crust, and coherent crustal anisotropy. Mantle tomography of INDEPTH III teleseismic recordings indicate a steeply dipping zone of anomalously fast (cold?) material in the mantle beneath central Tibet that likely marks subducted Indian lithosphere, an interpretation consistent with the gravity field over Tibet. Receiver functions computed beneath INDEPTH stations indicate a segmentation of the Moho that may reflect post-collisional reactivation of older accreted terranes. INDEPTH IV is now poised to complete its megatransect with new surveys across the northeast boundary of the Tibet Plateau as represented by the Kunlun Mountains and Qaidam Basins. INDEPTH IV will not only address outstanding crustal issues such as the role of Moho faults and extent of lower crustal flow, it will test tectonic models that postulate subduction of Asian lithosphere beneath the northern plateau.