Date of Award:
Master of Science (MS)
James P. Evans
The late Miocene Cedar Springs fault system is a high-angle transpressional system in the Silverwood Lake area, western San Bernardino Mountains, southern California. This thesis presents the study of oblique-slip faults with modest amounts of slip, which represent the early stages of fault development by using slip as a proxy for maturity. A structural and geochemical characterization is provided for six fault zones ranging from 39 m of slip to 3.5 km of offset in order to develop a model of fault zone geometry and composition. Basic geometric and kinematic results are provided for an additional 29 small-displacement (cm- to m-scale) faults. The main faults of this study can be divided into the fault core composed of sheared clay gouge and micro breccia, the primary damage zone made up of chemically altered rock with microstructural damage and grain-size reduction, and the secondary damage zone, which is characterized by an increased fracture density relative to the host rock. Although there appears to be a general increase in fault core thickness with increasing slip, the correlation is insignificant when analyzing all faults. Both the primary and secondary damage zones appear to thicken with increased slip on the main fault.
Overall, the structure and composition of the faults studied here are similar to those of larger strike-slip and reverse faults. This indicates that the fault core develops early in a fault's history. Subsequent slip appears to be focused along these narrow zones, with some deformation accumulating in the damage zone. Whole-rock geochemical analyses typically show a reduction in the abundance of Na, Al, K, and Ca in the fault core and primary damage zone relative to the host rock. This indicates enhanced fluid-rock interactions in these zones. Calculations of the energy consumed to produce the chemical alteration in the fault core indicate that a considerable amount of the total earthquake energy may be lost to alteration. This thesis concludes that fault processes are similar throughout the different stages of development, and the study of relatively small-displacement faults can therefore be used to understand fault evolution through time and the processes of larger faults in the brittle crust.
Jacobs, Joseph R., "Examination of Exhumed Faults in the Western San Bernardino Mountains, California: Implications for Fault Growth and Earthquake Rupture" (2005). All Graduate Theses and Dissertations. 5246.