| Abstrakt: |
The Sichuan basin, located adjacent to the eastern margin of the Tibetan Plateau, serves as an ideal marker for testing the extrusion process of the plateau. The basin is seismically active, with the strongest earthquake, the 2008 Mw 7.9 Wenchuan earthquake, occurring in the Longmen Shan range along its northwestern edge. A new regional compilation of focal mechanism solutions of earthquakes in and surrounding the basin reveals that a large fraction of the events have focal depths ranging between 8 and 25 km, corresponding to the crystalline basement of the basin. Seismic deformation involves right‐lateral oblique reverse faults, mostly trending northeast–southwest, similar to the kinematics of the mainshock of the Wenchuan earthquake. Shallow earthquakes (3–8 km) suggest that some of the seismic faults rupturing the crystalline basement are growing toward the surface. To the southwest, the seismicity transitions to activity along the left‐lateral Xianshuihe–Xiaojiang fault zone. The spatial relationship between these two sets of fault zones is consistent with a model in which the Sichuan basin responds to the southeast extrusion of the Chuan‐Dian block at the southeast margin of the plateau by a counterclockwise bookshelf rotation of the crystalline basement. This deformation pattern initiated ∼4–2 Ma, as shown by the age of the Xiaojiang fault segment. The history of left‐lateral movement along the Xianshuihe‐Xiaojiang fault can be traced back to ∼12 Ma when the eastward growth of the Tibetan Plateau resulted in the shortening of both the Longmen Shan thrust belt and the sedimentary over the Sichuan basin. Plain Language Summary: This Sichuan basin bounds the Tibetan Plateau on the east. A catastrophic earthquake (Mw 7.9) occurred along its boundary in 2008. However, the driving mechanism for this earthquake remains debated. We combine geologic and seismic studies to elucidate the origin of tectonic and seismic activity in the basin. By interpreting the focal mechanism solutions of basin‐internal earthquakes and the surface deformation, and considering the timing of the plateau's extrusion, we propose that the Sichuan basin experienced two stages of counterclockwise rotation in response to the southeastward extrusion of the plateau. The early stage, occurring at ∼12 Ma, involved deformation of the sedimentary cover, while the late stage, initiated ∼4–2 Ma, involved bookshelf rotation of the crystalline basement of the basin. Key Points: The seismicity of the Sichuan basin is restricted to a depth range of 8–25 km, dominated by the counterclockwise rotation of the basementThe internal deformation is attributed to the southeastward extrusion of the Chuan‐Dian block along the Xianshuihe‐Xiaojiang faultThe rotation history of the Sichuan basin initiated 12 and 4–2 Ma, involving the sedimentary cover and bookshelf rotation of the basement [ABSTRACT FROM AUTHOR] |
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