Crustal structure of the Qiangtang and Songpan-Ganzi terranes (eastern Tibet) from the 2-D normalized full gradient of gravity anomaly

Abstract

Numerous geophysical studies have revealed the lithospheric structure of the Qiangtang and the Songpan-Ganzi terranes in the eastern Tibetan Plateau. However, crust–mantle evolution and crustal response to the Indian lithospheric subduction are still controversial. Answering these questions requires additional information regarding crustal structure. In this study, the 2-D normalized full gradient (NFG) of the Bouguer gravity anomaly was used to investigate anomalous sources and interpret the crustal structure underneath the Qiangtang and Songpan-Ganzi terranes. The NFG-derived structures with low-order harmonic numbers (N = 33 and N = 43) showed that an anomalous source beneath the southern Qiangtang terrane had a characteristic northeastward-dipping shape, suggesting the northeastward motion of the crustal material induced by underthrusting Indian lithospheric mantle. The NFG images with harmonic number N = 53 showed a large-scale anomalous source in the lower crust of the transformational zone from the Qiangtang terrane to the Songpan-Ganzi terrane, consistent with thickening crust and resistance of lower crustal flow. The anomalous source demonstrated by the NFG results with harmonic number N = 71, located in the upper crust underneath the Ganzi-Yushu fault, suggested a seismogenic body of the 2010 M_W6.9 Yushu event.