CRUST AND UPPER-MANTLE SHEAR VELOCITY STRUCTURE BENEATH THE TIBETAN PLATEAU AND SURROUNDING REGIONS FROM INTEREVENT SURFACE-WAVE PHASE-VELOCITY INVERSION

Average a priori shear velocity models are constructed for the Tien Sh an, Tarim basin, Pamir-Hindu Kush, Himalaya and NE India. These models are shown to account for most of the lateral velocity heterogeneity a t wavelengths > 1000 km. Interevent fundamental mode Rayleigh and Love wave phase velocities were measured in the period range 32-200 s and were inverted for path-averaged shear velocity structures. These, in t urn, were regionalized to estimate average, isotropic structures benea th the six regions listed above. Low upper mantle velocities are obser ved beneath the western and eastern Tien Shan. The Tarim basin structu re is poorly constrained but exhibits very low upper crustal velocitie s (probably due to deep sedimentary accumulation), and either very hig h upper mantle. velocities exist (> 4.8 km/s) in a layer 70 km thick o r the lithosphere is very thick (similar to 180 km). Low upper mantle velocities are observed beneath the Chang Thang region, the central, a nd the northeast plateau but not beneath the southern or southeastern plateau. The crustal velocity gradient with depth is zero in the east central plateau consistent with lower crustal basaltic intrusions ther e but is positive to the west and southeast of this zone. Upper mantle structures show that the Indian lithosphere could have subducted bene ath the entire plateau only if either 1 mantle in the depth range 115- 185 km has been altered (or removed) to reduce its average velocity or 2 the Indian lithosphere is only similar to 85 km thick.