Deep crustal deformation of the Longmen Shan, eastern margin of the Tibetan Plateau, from seismic reflection and Finite Element modeling

Rivaling the Himalaya in relief, the Longmen Shan is probably one of the most enigmatic mountain ranges in the world: high mountains reach more than 4000m relief but without adjacent foreland subsidence and with only slow active convergence. What are geological and geodynamic processes that built the Longmen Shan? Coseismic deformation associated with the 2008 Wenchuan earthquake could hold clues to answer these questions. The primary features associated with the 2008 Wenchuan earthquake rupture have been narrowly distributed coseismic deformation and predominantly vertical displacements that could be interpreted as the result of slips on high-angle listric seismogenic faults. Deep sounding seismic reflection profiling across the seismogenic faults indeed reveals high-angle listric reverse faulting in the brittle upper crust and east-dipping reflectors that we interpret as ductile shearing, in the viscous lower crust. In conjunction with a visco-elastic finite element modeling of coseismic displacements associated with the Wenchuan earthquake, we show that the high-angle listric nature of earthquake faults produces insignificant horizontal shortening across the fault and facilitates upward slips along the fault that both explain the localized coseismic deformation and vertical displacement, as well as the presence of high mountains without adjacent foreland flexure. We suggest that the formation of the Longmen Shan may be better understood in terms of partitioned lithospheric pure-shear thickening in which upward high-angle listric faulting of brittle upper crust is linked to thickening of the more viscous lithospheric mantle through downward ductile shearing of rheologically deformable lower crust.