Seismically constrained thermo-rheological structure of the eastern Tibetan margin: Implication for lithospheric delamination

Abstract

article i nfo The eastern Tibetan margin bordered by the Longmen Shan range exhibits significant lateral differences in the lithospheric structure and thermal state. To investigate the roles of these differences in mountain building, we construct a thermo-rheological model along a wide-angle seismic profile across the eastern Tibetan margin based on recent seismic and thermal observations. The thermal modeling is constrained by the surface heat flow data and crustal P wave velocity model. The construction of the rheological envelopes is based on rock me- chanics results, and involves two types of rheology: a weak case where the lower crust is felsic granulite and the lithospheric mantle is wet peridotite, and a strong case where the lower crust is mafic granulite and the litho- spheric mantle is dry peridotite. The results demonstrate: (1) one high-temperature anomaly exists within the uppermost mantle beneath eastern Tibet, indicating that the crust in eastern Tibet is remarkably warmer than thatinthe Sichuanbasin,and(2)therheologicalstrengthofthe lithosphericmantlebeneatheasternTibetiscon- siderably weaker than that beneath the Sichuan basin. The rheological profiles are in accord with seismicity dis- tribution. By combining theseresults with the observed crustal/lithospheric architecture, Pn velocity distribution and magmatism in the eastern Tibetan margin, we suggest that the delamination of a thickened lithospheric mantle root beneath eastern Tibet is responsible for the growth of the eastern Tibetan margin.