Gravity anomalies and deep structure of the northeastern Tibetan Plateau and adjacent areas

Abstract

The northeastern Tibetan Plateau is a region having experienced subduction, collision, and intracontinental extension. In this study, a gravity model (SGG-UGM-2) was used to investigate the distribution of gravity anomalies, Moho depth, and lithospheric effective elastic thickness in the northeastern Tibetan Plateau and adjacent areas. The results were compared with the regional heat flow, crustal magnetic anomalies, and seismicity. The Bouguer gravity anomalies correlate with the surrounding stable blocks (i.e., the Tarim Basin, Alxa Block, Ordos Basin, and Sichuan Basin) and lateral crustal extrusion of the Tibetan Plateau. The isostatic gravity anomalies, Moho depth, and lithospheric effective elastic thickness are –80 to 80 mGal, 35–70 km, and 5–105 km, respectively. Isostatic disequilibrium occurs mainly near boundaries between blocks and is associated with seismic activity. The distribution of Bouguer gravity anomalies and the Moho surface record the northeastward expansion of the Tibetan Plateau. The lithospheric effective elastic thickness decreases gradually from west to east in the Bayan Har Block, reaching its lowest values (<10 km) in the Longmenshan tectonic belt, which confirms the eastward extrusion along the block. The lithospheric effective elastic thickness in the Qaidam Basin is higher than that in the Bayan Har Block and Qilian orogenic belt, which reflects underthrusting of the East Asian lithosphere beneath the Qilian orogenic belt and the fact that extrusion from the Qiangtang Block is blocked by the Qaidam Basin.