Crustal active faults can slip either steadily by aseismic creep, or abruptly by earthquake rupture. Creep can continuously relax the stress and reduce the magnitude of earthquakes. Identifying the slip behavior of active faults plays a crucial role in predicting and preventing earthquake disasters. Here we carried out multiscale structural analyses of fault rocks from the Guanxian‐Anxian fault zone (GAF) surface rupture of the 2008 Mw 7.9 Wenchuan earthquake (Longmen Shan thrust belt, China), and from the 3rd pilot borehole of the Wenchuan earthquake Fault Scientific Drilling project (WFSD‐3P). Results revealed that distributed R1 shears, pressure solution seams, partly dissolved clasts and mineral grains (e.g., quartz and albite), and newly formed phyllosilicate minerals all prevail in the clay‐rich fault rocks at different depths. Microstructural observations combined with X‐ray diffraction analysis show that the majority of clay minerals in the fault gouges were newly formed by pressure solution. The growth of new phyllosilicates prevents soluble grains from coalescing, thereby maintaining fast diffusive paths along solution seams, which promotes the formation of pressure solution. Both weak phyllosilicate minerals and pressure solution can control aseismic creep. Therefore, the slip behavior of the GAF is dominated by long‐term aseismic creep due to the interplay between new phyllosilicate minerals and pressure solution. Based on P wave velocity and strata distribution, we propose a model in which the GAF is creeping at shallow depths and locked at greater depths, as a possible explanation for the Wenchuan earthquake rupture mechanism.

We document the geological fault zone geometry and internal architecture of the Yingxiu-Beichuan fault (YBF) in the Longmen Shan, eastern Tibetan Plateau, which hosted the catastrophic 2008 Wenchuan earthquake. The YBF has been repeatedly reactivated during its long-term activity and contains widespread brittle fault rocks. Pseudotachylytes are recognized in the Pengguan Complex both from the Wenchuan earthquake Fault Scientific Drilling (WFSD) cores and at a surface outcrop. Microstructural evidence of embayed and rounded fragments, flow structures, vesicles, and microlites of various morphologies indicate that the pseudotachylytes were generated by frictional melting during earthquakes. Cataclasite and fault gouge with fragments of angular shapes and varied sizes observed within the fault zones are also considered as evidence of paleoearthquakes. These imply that the main fault zones in the WFSD cores record ancient seismic slip events. Geological field mapping at the surface and in WFSD cores shows that the YBF is a listric reverse fault, with a 155-290-m thick fault zone that dips 73°-68°at depths <700 m and 30°at depths >700 m. Three faulted sets of alternating Neoproterozoic Pengguan Complex overthrusting upon the Late Triassic Xujiahe Formation in the WFSD-2 core imply that the main tectonic framework of Longmen Shan consists of a series of imbricated thrust sheets that have accommodated strong crustal shortening. These suggest that the accumulated crustal shortening caused by imbricated thrusting and long-term seismic activity along the YBF and deep concealed faults is an important process responsible for the rapid uplift of the Longmen Shan.