Date of Award:

8-2024

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geosciences

Committee Chair(s)

Dennis L. Newell

Committee

Dennis L. Newell

Committee

Alexis K. Ault

Committee

Donald E. Penman

Abstract

Earthquakes occur along planes of geologic weakness called faults. Thin, light-reflective surfaces on faults are called fault mirrors and often considered signatures of previous earthquakes along these faults. The Cordillera Blanca mountain range in the Peruvian Andes provides a natural laboratory to explore meter scale exposures of fault mirrors. Three naturally formed fault mirror samples were collected from the Cordillera Blanca detachment fault which runs along the edge of the Cordillera Blanca mountain range. Prior work on this fault reveals that it is capable of destructive earthquakes every few thousand years and is a significant earthquake hazard to the surrounding communities.

The goal of our investigation is to discover the textural and chemical properties of silica FMs and use that information to determine the deformation processes involved in their formation. Our results document multiple fault slip events, variations in grain size and a thin zone of no discernable grains directly below the fault mirror surface. Our chemical observations reveal the material the fault mirror has developed on is crystalline (quartz). This result differs from prior observations of silica fault mirrors which show they can develop on a non-crystalline to partially crystalline material. We suggest that if these fault mirrors formed during earthquake slip events, that the accommodation of slip within a small zone plays a role in propagating slip along the Cordillera Blanca detachment fault.

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