Amorphous materials and clay mineral formation in the coseismic gouge from a surface rupture of the Ms 8.0 Wenchuan earthquake

Microstructures from a fault gouge formed during the Ms. 8.0 Wenchuan earthquake in China were investigated in detail. In samples collected near the highest vertical surface offset, two types of amorphous materials were found in the high-strain shear zone: 1) material formed by creeping during the inter-seismic period, which is characterized by ground sandstone clasts; and 2) coseismic material, characterized by the comminution of the sandstone clasts of the former type. Interaction of amorphous material with fluids during creeping resulted in Fe-rich nano-sized grains in the pores, Mg- and Al-rich stratified zones near the pores, amorphous silica next to the stratified zones, as well as new clay minerals in the stratified zones. Feldspar and phyllosilicates were transformed into clay minerals and amorphous silica. We propose that amorphous materials and clay minerals contributed to the dynamic weakening of the Yingxiu-Beichuan fault.