Date of Award:

5-1998

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geosciences

Department name when degree awarded

Geology

Committee Chair(s)

James P. Evans

Committee

James P. Evans

Committee

Kevin Hestir

Committee

Susanne U. Janecke

Abstract

Three small strike-slip fault zones exposed in granitic rock in the central Sierra Nevada, California, provide field-based data to construct three-dimensional representations of each fault zone in order to compare with the geometries predicted by existing fault-growth models. All three fault zones are nearly vertical, strike -N60°E, and have left-lateral slip. The fault zones range from 60 to 140 m in length and 1 to 12 m wide. Each fault zone consists primarily of parallel to subparallel fracture and fault traces 2 to 56 m long and is separated 25 cm to 7 m by intact rock. One fault zone contains two simple fault zones that consist of fractured rock separated from relatively unfractured rock by two nearly parallel boundary faults. Fracture and fault trace characteristics are a function of fault zone development and complexity. Traces interconnect primarily by way of junctions and steps, with traces branching away from each other at junctions having angles between 10° to 80° whereas steps branch away at angles between 10° to 40°. Faults terminating as a splay or horsetail splay are rare. Splay fractures strike away from the fault traces at angles of 10° to 60°.

Individual faults and the fault zones have irregular displacement-length profiles. Episodic brittle fracturing, hydrothermal mineralization, and alteration are pervasive along fractures and faults. Thickness, composition, and location of hydrothermal mineralization and alteration along fault traces show no consistent pattern and indicate a brittle strain softening process occurred. The widespread distribution of chlorite-epidote mineralization suggests that each fault zone acts as a through-going passageway for fluids.

Fault-growth models involving the in-plane propagation of shear displacement along faults and having strain as the boundary condition match the field data the best. All three fault zones resemble those fault-growth models in which fault zone development is a nonuniform process with the growth of individual fractures and faults affecting the nucleation, propagation, and geometry of subsequent fractures and faults. Three-dimensional representation of these fault zones will constrain spatial statistical and stochastic modeling of fault zone nucleation and propagation.

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