Uppermost mantle structure of the eastern margin of the Tibetan plateau from interstation Pn traveltime difference tomography

Abstract

Tomographic inversion with interstation Pn differential traveltime data provides a reliable velocity image of the uppermost mantle across the Longmenshan Fault Belt, which borders the eastern Tibetan plateau and Sichuan Basin. Low Pn velocities (<7.8km/s) are revealed beneath the eastern Tibetan plateau to the west of the Longmenshan Fault Belt, while high Pn velocities (>8.1km/s) appear beneath the Sichuan Basin to the east, suggesting a relatively weak upper mantle beneath eastern Tibetan plateau and a strong upper mantle beneath the Sichuan Basin. The Pn velocities are consistent with the proposition that the rigid and stable lithosphere of the Sichuan Basin may act as a barrier to the eastward escape flow of the ductile crust of the eastern Tibetan plateau. Consistency of Pn velocities with the smoothed topography, crustal thickness and Bouguer gravity implies that the dynamic processes of the lithospheric mantle should be responsible for the crustal thickening and relief of uplift in the eastern Tibetan plateau. However, the Longmeshan Fault Belt appears to be separated into two segments by the Huya and Leidong faults as manifested by significant discrepancies in topography gradient, seismic activity, rupture process and aftershock distribution of Wenchuan earthquake. This segmentation is also supported by difference in Pn velocities along the strike of the fault belt, suggesting different tectonic environments in the crust and lithospheric mantle. Understanding of the mechanism for the segmentation requires further investigation.