DEVELOPMENT OF SILICA FAULT MIRRORS VIA SLIP LOCALIZATION ALONG THE CORDILLERA BLANCA DETACHMENT FAULT, PERU

Silica fault mirrors (FMs) exposed on the main trace of the Cordillera Blanca detachment fault (CBDF), Peru, show evidence for extreme slip localization during fault development. FMs are thin, light-reflective surfaces comprising nanoparticles that may form at seismic slip rates. The CBDF is a ~200-km long, NW-SE striking, shallow to moderately west-dipping normal fault that delineates the western edge of the ca. 13-5 Ma, granodioritic Cordillera Blanca batholith. Holocene fault scarps and paleoseismic investigations indicate the CBDF is seismically active, capable of large magnitude earthquakes (≥M 7) on a frequency of every few thousand years and represents a significant regional seismic hazard.

Three silica FMs that cut non-foliated quartzite breccias were investigated using optical petrography, grain size characterization via ImageJ, scanning electron microscopy (SEM), Raman spectroscopy, and electron backscatter diffraction (EBSD). Intact breccia clasts of quartzite are on average 373 +/- 64 µm in diameter and comprise quartz grains with a mean long-axis length of ~274 µm. Clasts are supported by a moderate- to poorly sorted matrix comprising quartz grains with average long-axis lengths ranging from 117 µm down to <20 µm, with right-skewed distributions. SEM imaging of FM surfaces in plan view reveals multiple slickenline orientations expressed as nm-deep grooves that are parallel and oblique to the fault dip direction. In cross-section, we observe sub-parallel to oblique 101 +/- 35 µm-wide shear bands, evidence of multiple brecciation events, veins of comminuted quartzite (interpreted to be granular injection veins), and extreme grain size reduction within ~10-20 µm of the FM interface. Locally, material at the FM surface lacks discernable grain boundaries at the SEM scale. Raman spectroscopy and EBSD data from the naturally “polished” slip surface indicate this material is quartz (i.e., crystalline). 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 developed along the rupture path of past earthquakes, then this suggests that extreme slip localization plays a role in facilitating rupture propagation along the CBDF.