The Guanxian–Anxian fault zone in the Longmen Shan, Sichuan, China, exhibits long-term creep-slip but ruptured during the 2008 Wenchuan earthquake, challenging the view that creeping faults rarely generate strong earthquakes. To investigate the transition from creep-slip to stick-slip, we analyzed fault rocks from the WFSD-3, using microstructural observations, XRD, μXRF, Raman spectroscopy, and quartz grain size statistics. Fault rocks show intense foliation, pressure-solution structures, and abundant clay minerals, reflecting long-term aseismic creep. At the interface between black and gray fault gouges at ~1249.98 m, microstructures indicate stick-slip behavior, including truncated grains, angular fragments, and finer grain sizes. Here, clay content drops sharply while strong minerals (quartz, feldspar, calcite, dolomite) increase. Elemental mapping shows Al and K enriched in black gouge, whereas Ca and Si in gray gouge; Raman spectroscopy indicates possible graphitization; the finest quartz grains occur in black gouge. These features mark co-seismic principal slip zone of the Wenchuan earthquake. We propose that fluid-driven transformation of strong minerals into clays facilitates creep-slip, whereas localized precipitation of strong minerals strengthens the fault, causing stress accumulation and controlling the creep-slip to stick-slip transition. This mechanism has implications for reassessing seismic hazards of creeping faults.