Mixed-Mode Formation of Amorphous Materials in the Creeping Zone of the Chihshang Fault, Taiwan, and Implications for Deformation Style

Experimental results demonstrate that amorphous materials can be generated by frictional sliding at a wide range of conditions and that once formed the frictional strength of the fault is reduced. Nevertheless, amorphous materials have not been described in many natural faults, and their importance in natural systems is not well understood. We have identified amorphous materials within a 3-mm-thick slip zone (i.e., narrow band) in the Chihshang Fault, Taiwan, as well as within a submicrometer thick band in a distributed network of scaly clays. The scaly clays resemble those developed in other faults that demonstrably accommodated slow creep, and they also impart very low permeability implying that fluid-related alteration (which could result in amorphization) will be inefficient. For this reason, and because of its throughgoing nature and position within a deforming zone, we infer a shear-related origin for the amorphous materials. Therefore, amorphous materials can be observed in near-surface environments because they have been formed during recent deformation under limited fluid-circulation circumstances. The narrow slip zone containing amorphous materials is throughgoing and is interpreted to have acted as a frictional interface that accommodated brittle deformation at the macroscopic scale, whereas the surrounding mélange accommodated distributed “ductile” shear. The formation of amorphous materials therefore typifies the “mixed-mode” style of deformation in this creeping fault zone.