AN INTEGRATED GEOPHYSICAL AND GEODYNAMIC STUDY OF THE STRUCTURE AND EVOLUTION OF THE LONGMENSHAN REGION, EASTERN TIBET

The mechanism for uplift of the eastern Tibetan Plateau has been a matter of debate. There are two main models that are extrusion and crustal flow. These models have been tested by surface observations, but questions about the uplift remain. In addition, the devastating 2008 Mw 7.9 Wenchuan earthquake along the Longmen Shan Fault Zone (LMSFZ) reminds us that the tectonic activity within eastern Tibet is complex and poses a major natural hazard. This activity is accompanied by dramatic uplift along the LMSFZ, but only minor convergence (<4 mm/yr) against the adjacent Sichuan basin is observed. In order to investigate the mechanism for uplift of Longmen Shan (LMS) area, we explored the lithospheric structure across the Songpan-Ganzi terrane, LMS, and western Sichuan basin by undertaking an integrated analysis of a variety of data including new, logistically-challenging controlled-source seismic profiling (reflection and refraction) results, receiver function estimates of crustal thickness, gravity and magnetic data, GPS data, and geologic constraints. Our analysis of crustal structure indicates that the crust is not thick enough to support its current elevation and that the crust is essentially composed of three layers of similar thickness. Thus, based on our crustal structure model, 2D numerical modeling was conducted to investigate uplift mechanisms. The modeling results indicate that the middle crust beneath the Songpan-Ganzi terrane is the most ductile layer, which is the key factor responsible for the crustal-scale faulting, earthquake behavior, and periods of uplift. In addition, the modeling results indicate that the strong Sichuan block acts as backstop for the thrusting along the LMS and crustal thickening to the west.