CONSTRAINTS ON DEFORMATION OF THE CENTRAL TIBETAN LITHOSPHERE FROM INDEPTH III GEOPHYSICAL SURVEYS

INDEPTH III seismic and MT data offer fresh constraints on the style and degree of crustal and mantle deformation in the vicinity of the Banggong-Nujiang suture, the surface expression of the boundary between the Lhasa and Qiangtang terranes in central Tibet. Although high conductivities and relatively low seismic velocities at depth support the suggestion that partial melt continues to be present beneath both central and northern Tibet, the more prominent mid-crustal anomalies found during INDEPTH II and attributed to specific magma accumulations beneath the southern Lhasa block (e.g. seismic “bright spots”, a distinct mid-crustal low velocity zones, ultra-high conductivity pockets) are absent or less obvious in central Tibet. The depth to the top of high conductivity appears to increase northward beneath the Qiangtang. Crustal seismic refraction results indicate a decrease in crustal thickness from south (ca 62 km) to north (ca 57 km), and receiver functions indicate distinct changes in Moho character, and perhaps depth, beneath the Banggong Suture. Small-scale seismic reflection tests indicate a thick zone of reflective lamination (ductilely thickened crust?) that corresponds with an increase in seismic velocity at depth beneath the Lhasa block, with reflectivity terminating at the Moho. Local seismicity is confined to the uppermost crust, consistent with a weak lower crust. Earthquake clusters in certain graben suggest concentrated deformation and possible magmatic activity. New focal mechanisms are dominated by strike-slip and normal events, but also include the first thrust mechanisms for central Tibet. Shear wave splitting studies indicate a pronounced contrast in anisotropy between southern and north Tibet (lithosphere penetrating faults?), with rapid lateral variations that correlate with surface morphology (crustal component to anisotropy?). New Pn results continue to support the suggestion of relatively fast (cold?) upper mantle beneath the Lhasa terrane in contrast to slower (hot?) mantle beneath the Qiangtang. Teleseismic tomography suggests a zone of high velocity dipping vertically beneath the northernmost Lhasa block, a possible remnant of subducted (delaminated?) Indian lithosphere.

*This abstract is coauthored with Team Indepth III.