Date of Award:

Spring 2017

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

James P. Evans

Co-Advisor/Chair:

Alexis K. Ault

Third Advisor:

Susanne U. Janecke

Abstract

Seismogenically active faults (those that produce earthquakes) are very complex systems that constantly change through time. When an earthquake occurs, the rocks surrounding a fault (the “fault rocks”) become altered or damaged. Studying these fault rocks directly can inform what processes operated in the fault and how the fault evolved in space and time. Examining these key aspects of faults helps us understand the earthquake hazards of active fault systems.

The Mecca Hills, southern California, consist of a set of hills adjacent to the southernmost San Andreas Fault. The topography is related to motion on the San Andreas fault, which poses the largest seismic hazard in the lower forty-eight United States. The southernmost San Andreas fault, and the Mecca Hills study location may be reaching the end of its earthquake cycle and is due for a major, potentially catastrophic earthquake. The seismic hazards of the region, coupled with its proximity to major populated areas (Coachella Valley, Los Angeles Basin) make it a critical research area to understand fault zone evolution and the protracted history of fault development.

The goal of this thesis was to directly examine the fault rocks in the Mecca Hills to understand how San Andreas-related faults in this area have evolved and behaved through time. This study integrates a variety of field and laboratory techniques to characterize the structural, geochemical, and thermal properties of the Mecca Hills fault rocks. The results herein document two distinct phases of deformation in the rocks exposed in the Mecca Hills, one around 24 million years ago and the other in the last one million years. This more recent phase of deformation is characterized by fault block exhumation and fluid flow in the fault zones, likely related to changing dynamics of the southernmost San Andreas Fault system. The older event informs how and when these rocks came close to Earth’s surface before the San Andreas Fault initiated.

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