Crustal and upper mantle structure beneath western Tibet using seismic traveltime tomography and receiver functions

My research focuses on the crustal and upper mantle structure beneath western Tibet. In order to understand the geodynamic conditions beneath this region I performed tomographic inversions to determine 3D velocity structure. Also, I used Ps receiver functions (RFs) to investigate seismic discontinuities. I found laterally uniform low seismic velocity in the crust of Tibet, and a clear change in seismic properties at the India-Asia boundary. Crustal thickness varies from 43 km beneath the Himalayas to 88 km in the northern part of the Lhasa block. The Vp/Vs ratio also varies systematically, with lower values in the Lhasa block and higher values beneath the India - Asia boundary, and south of it. In many locations RFs show evidence for a mid-crustal boundary where speed decreases with depth. However, no evidence of a continuous low velocity zone in the crust is found in tomographic images, suggesting that low velocity bodies detected by RFs are relatively thin. Additional boundaries where speed increases with depth are seen 15 - 25 km above the crust-mantle transition. Again, tomography does not detect a continuous fast layer in the lower crust of western Tibet, suggesting thin discontinuous features. A narrow very fast feature in the uppermost mantle beneath part of the study region is consistent with formerly Indian crustal rock that has converted to eclogite when Indian lithosphere was underthrust beneath the Tibetan Plateau. Patterns of velocity variation in the upper mantle show close similarity for P and S waves, and thus likely represent differences in mantle rock temperature. A key finding is the absence of a continuous fast anomaly expected if Indian lithosphere was subducting into the mantle beneath Asia. Instead, I found two separate fast anomalies beneath western Tibet. One is nearly vertical, ~100 km across, and circular in cross-section, and is positioned beneath the present day India – Asia boundary. It may be interpreted as a viscous drip formed by Indian lithosphere material. Another is thin and slab-like in shape, and is present further north. Coexistence of these features suggests there was a recent change in the mode of emplacement of Indian mantle lithosphere beneath Tibet.Ph.D.Includes bibliographical referencesby Ayda Shokoohi Raz