MULTI-SCALE TEXTURAL OBSERVATIONS OF SILICA FAULT MIRRORS AND THEIR POTENTIAL CONNECTION TO SEISMICITY ALONG THE CORDILLERA BLANCA DETACHMENT FAULT, PERU

Textural characterization of fault mirror (FM) surfaces developed along the Cordillera Blanca Detachment fault (CBDF) provide insight into fault slip localization and deformation processes. FMs are thin, light-reflective surfaces comprising nanoparticles that may form in response to coseismic slip. The CBDF is an ~200-km-long, west-dipping, low-to-moderate-angle normal fault that defines the western margin of the Cordillera Blanca massif, Peru. Although there are no documented historic earthquakes on the CBDF, 3-4 m Quaternary fault scarps along its trace and paleoseismic investigations indicate the CBDF is seismically active and capable of large magnitude (≥M7) earthquakes every few thousand years. At most locations, the CBDF cuts granodiorite in the footwall, but locally, the exhumed, master CBDF cuts quartzite and is defined by m-scale FMs.

We characterized three samples of quartzite FMs from one location along the main trace of CBDF using optical petrography, grain size characterization via ImageJ, and scanning electron microscopy (SEM). The host rock comprises equigranular, non-foliated quartzite and grains have an average long-axis length of ~274 μm. FM samples exhibit varying degrees of brecciation and size reduction of individual grains and quartzite clasts with proximity to the FM surface. For example, with decreasing distance from the FM surfaces, the average grain long-axis decreases from ~117 μm to <20 μm, and grain size distribution curves from each sample are right skewed. FMs also exhibit sub-parallel to oblique shear bands and veins of comminuted host rock in what are interpreted to be granular injection veins that are on average ~100 μm in width. Cross-sectional SEM analyses reveal that the FM surface is defined by an up to 10 μm-thick layer comprising silica with no discernable grain boundaries. On-going Raman spectroscopic analyses are aimed at evaluating if this material is amorphous. Regardless, textures support multiple generations of fault slip culminating in extreme slip localization along these FMs and thus the CBDF they delineate. If these FMs formed during seismic slip and these surfaces reflect the rupture path of past earthquakes, then this suggests that extreme slip localization plays a role in facilitating rupture propagation along the CBDF.