Lithospheric Flexure in the Sichuan Basin and Longmen Shan at the Eastern Edge of Tibet Estimated from Combined Gravity
Fielding, Eric1; McKenzie, Dan2
1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, UNITED STATES; 2Bullard Laboratories of the Department of Earth Sciences, University of Cambridge, UNITED KINGDOM
The mountain range at the steep eastern edge of the Tibetan Plateau, the Longmen Shan, was deformed by a Mw 7.9 earthquake with oblique thrust and strike-slip motion in 2008. The tectonic processes and structure of the lithosphere beneath this range have been controversial. Gravity measurements reflect the distribution of mass within the Earth, including the large load of rock above the geoid in mountain ranges. We investigate the response of the lithosphere to the load of the Longmen Shan and estimate the flexural rigidity or effective elastic thickness Te using new gravity data acquired by recent satellites, including GOCE and GRACE, and the combined GOCO2S and EIGEN-6c datasets. GOCO2S is a combination of available satellite gravity datasets, while the EIGEN-6C combination also includes ground observations that provide finer spatial resolution in many areas. The free-air gravity anomalies over the Longmen Shan show that its mass is supported by flexure of the adjacent Sichuan Basin lithosphere, similar to the flexural support of the Himalayas. The finer spatial resolution of GOCE reveals the flexural signals along the edges of the Tibetan plateau that were not well resolved by earlier satellites. Analysis of a stacked profile of the free-air anomalies shows that the effective elastic thickness of the basin lithosphere is greater than 10 km, but has a broad minimum misfit function with no upper limit on the thickness. Two-dimensional admittance analysis shows the Te of easternmost Tibet is very low, approximately 7 km. Our results are consistent with tectonic theories for the formation and maintenance of the Longmen Shan by thrusting over the edge of the Sichuan Basin lithosphere.