STRUCTURE AND COMPOSITION OF THE FAULT-ZONE OF THE SAN ANDREAS FAULT IN TEJON PASS, CALIFORNIA

Major faults in the crust display grain size reduction and gouge formation, and the analysis of gouge processes will contribute to the understanding of earthquake physics. We present our recent investigation of the gouge zone of the San Andreas Fault in the segment that slipped during the 1857 earthquake in Tejon Pass. The preliminary mapping of the fault zone will be presented at scales of 1:10 to 1:40 and micro-structural analysis in thin sections. The mapping covers a strip about 150 m long and 6 m wide along the profile across the San Andreas fault-zone, which is comprised here by distinct zones of (1) pulverized granite, (2) cataclasite to ultracataclasite, (3) dark clay, and (4) faulted young sediments. The pulverized granite appears as a solid rock, but disintegrates into a extremely fine-grained powder. It contains multiple, small localized shear surfaces. The cataclasite-ultracataclasite zone is composed of several colored bands of cohesive fault rocks that are sub-parallel to the San Andreas trend. The young sediments, which partly originated in a lake environment, contain poorly sorted clastic sediments and caliche concretions that are all cut by multiple faults. The many small faults and shear surfaces throughout the outcrop display transport direction which is normal to trend of the San Andreas Fault. Mapping of the microstructures is underway. We will use electron microprobe methods and compositional and image analyses to determine grain morphology and variations in composition. The results of this project will characterize the fault-zone properties in the most active segment of the San Andreas Fault.