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Chemical and physical changes within the damage zone of the Mojave segment of the San Andreas fault: Implications for the origin of pink potassium feldspar grains
Rockwell, Thomas K.
Girty, Gary H.
We mapped a tabular zone of pink potassium feldspar alteration in granitic rocks within the damage zone of the San Andreas fault from Arrowhead Springs to Gorman in southern California. Alteration of K-feldspar to a pink color is spatially associated with the active trace of the fault. In the literature, such coloration is commonly attributed to metasomatism. To better understand the processes leading to this alteration along the San Andreas, we studied two transects perpendicular to the trace of the fault with different structural histories near Littlerock and Lake Hughes, California. Lake Hughes is on a portion of the San Andreas that has been active throughout much of the faults history, while Littlerock is on a relatively young strand that was one a part of an extensional step over. Furthermore, Littlerock exhibits a broad zone of pulverization within the inner damage zone whereas Lake Hughes contains a much narrower zone of pulverization within this part of the fault zone, in spite of its larger displacement. To study the origin of the pink coloration and to better understand the deformation processes active with the damage zone, we collected samples from all three distinct zones along each transect; parent rock, outer damage zone, and pulverized rocks within the inner damage zone. In order to calculate porosity, the collected samples were analyzed for bulk and grain density. XRF was used to determine their chemical composition. Thin sections were made in order to study textures and microstructures, and selected samples were analyzed for particle size distribution. The results of our study indicate that mechanical fragmentation dominates in both transects, and that pink K-feldspar is the result of an increase in volumetric strain due to an increase in fracture density and porosity, which then acted as a conduit for fluid migration within the fault zone. Fluids within intra-granular fractures oxidize the iron inclusions that are commonly found within K-feldspar crystal structure, resulting in the pink coloration. Fracturing and fluid migration within the fault zone appears to be cyclic as indicated by numerous cross cutting open, healed, and in filled cracks. The two transects differ slightly in their chemical alteration signal. Littlerock experienced a bulk mass increase of about 10% in the outer damage zone and 12% in the pulverized rocks of the inner damage zone due to elemental mass changes, while Lake Hughes experienced no bulk mass change and relatively little elemental mass change. Both sites contain evidence for crystal-plastic deformation within feldspars and quartz. However, at Littlerock these features increase within the damage zone towards the fault core, whereas at Lake Hughes, they are present within the wall rock and only increase slightly within the damage zone. Both sites contain plagioclase which has been altered to sericite, and at Littlerock, some has been altered to laumontite. However, at Lake Hughes, sericite is also present within the wall rock samples. At Littlerock biotite is altered to chlorite, while at Lake Hughes the wall rock contains chlorite and biotite partially altered to clay. Some of the observed mineral alteration products are generally associated with elevated temperatures. The pink alteration of K-feldspar at both sites is a clear indicator of micro-scale fracturing and fluid migration associated with the San Andreas fault. It appears that at Littlerock, migration of hydrothermal fluids occurred within an extensional step-over whereas at Lake Hughes fluid migration occurred in a more simple setting.
San Diego State University
Master of Science (M.S.) San Diego State University, 2010
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