Variations in Crust and Upper Mantle Structure Beneath Diverse Geologic Provinces in Asia

Abstract

Abstract : This report presents results of a two year effort to determine crust and mantle lithospheric structure beneath Eurasia and to explore the effects that structural variations have on regional wave propagation. First, variations in crust and lid structure on Pn and Lg propagation are investigated using regionalized velocity models previously determined under China. While explosion Pn/Lg ratios are higher than earthquake ratios for all of the regionalized Chinese velocity models, this difference is much smaller than the variations in Pn/Lg ratios caused by propagation differences. This emphasizes the importance of resolving crust and upper mantle structure for successful discrimination. The second part of this report describes detailed regional wave modeling studies to determine crustal and mantle lithospheric structure beneath Tibet. We find low average crustal P-wave velocities (5.9-6.1 km/s), thick crust (68-76 km) and fast lithospheric mantle (8.2-8.25 km/s) beneath the Lhasa Terrane in southern Tibet. Crustal and mantle lithospheric structure to the north in the Qiangtang Terrane differs dramatically with average crustal P and S-wave velocities 4% faster and 2% slower, respectively relative to the Lhasa Terrane. These differences are too large to be explained by temperature differences alone and require a partially molten uppermost mantle lithosphere in the Qiangtang Terrane.