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

Abstract

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 4000 m 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.

Three‐Dimensional Model of the Lithospheric Structure Under the Eastern Tibetan Plateau: Implications for the Active Tectonics and Seismic Hazards

“…Despite the fact that the 3‐D image of the Moho shows a continuous surface at first order (Figure ), recent studies based on high‐resolution seismic reflection profiles have demonstrated that the Moho is discontinuous with prominent offsets (Guo et al, ; Robert et al, ; Z. J. Zhang et al, ). It is possible that some large, crustal‐scale thrust faults exist there (Feng et al, ; Lu et al, ). Therefore, we suggest that the “lower crustal flow” and the “brittle crustal shortening” models are not exclusive in building the eastern TP margin.…”

“…Following the 2008 Wenchuan earthquake, numerous geophysical and geological surveys have provided constraints on the crustal and lithospheric structures of the eastern TP (Bai et al, ; Feng et al, ; Guo et al, ; Lei, Li, et al, ; Z. Liu et al, ; Z. Wang et al, , ; X. Wang, Li, et al, ; Wei et al, ; Z. J. Zhang et al, ). Most studies show that the Moho depth changes from ~60 km beneath the eastern TP to ~40 km beneath the SCB (Lei et al, ; H. Y. Li et al, ; Q. Y. Liu, van der Hilst, et al, ; C. Y. Wang et al, ; Xu & Song, ; P. Z. Zhang, ; Y. Q. Zhang et al, ).…”

Evolution and Distribution of the Early Aftershocks Following the 2008 Mw 7.9 Wenchuan Earthquake in Sichuan, China

“…Comparison of the early aftershocks in the 24 hr following the mainshock with the seismic reflection profile along the CC' cross‐section region (Feng et al, ). Black and red lines are inferred dipping faults by Feng et al () and from profile CC' in this paper, respectively. Aftershocks (bold circles) are color‐coded by the occurrence times following the mainshock.…”

“…Many studies over the last decade focused on subsurface structure, source process, and postseismic response of the 2008 Wenchuan earthquake by geological survey (Hubbard & Shaw, ; Ran et al, ; Xu et al, ; P. Z. Zhang et al, , ), seismology (Chen et al, , ; Deng et al, ; Feng et al, ; S. Li et al, ; Q. Liu et al, , , ; P. P. Zhao et al, ), geodesy (M. Huang et al, ; Shen et al, ; Q. Wang et al, ), and magnetotellurics (G. Z. M. Zhao et al, ). Different tectonic models have been proposed to explain the occurrence of the Wenchuan earthquake (Burchfiel et al, ; Feng et al, ; Hubbard & Shaw, ; Jia et al, ; Y. Q. Li et al, ; Shen et al, ; Q. Wang et al, ; Xu et al, ; P. Z. Zhang et al, ), but the geometry of seismogenic faults ruptured during the 2008 Wenchuan earthquake is still under debate (e.g., Xu et al, ; P. Z. Zhang et al, ).…”

Quaternary activity of the range front thrust system in the Longmen Shan piedmont, China, revealed by seismic imaging and growth strata

“…This multilayer velocity model illustrates the nature of the subsurface velocity architecture, similar to the seismic velocity model of Wang et al [], and it enables the quantification of structural geometries more accurately than by simply using a constant velocity of 4600 m/s [ Jia et al ., ; Li et al ., ] (Figure ). In our model, the converted base of the shallow detachment layer in the LMS piedmont reaches an average depth of about 6 to 8 km, which is consistent with previous estimates from a deep seismic reflection profile using the common depth point stacking method [ Feng et al ., ] and depth conversions using a 2‐D velocity model [ Hubbard and Shaw , ; Hubbard et al ., ].…”