INSIGHTS INTO THE PROPERTIES AND STRUCTURE OF THE UPPER PORTION OF THE SAN ANDREAS FAULT AS SEEN FROM CORE

The seismic to aseismic transition in the upper parts of active faults may be the result of the development of stable slide in clay minerals, or due to the effects of pore fluid pressures at shallow levels. We investigate the composition and structure in core from the upper part of the Mojave segment of the San Andreas Fault (SAF), southern California. Seven northern-inclined cores intersect the ~ 100 m thick steep south-dipping SAF zone. The central fault zone, which is up to 30 m thick, contains cataclasite, ultracataclasites, zones of mineralization and alteration that we have sample to a depth of 140 m. Optical microscopy reveals a range of brittle and semi-brittle deformation processes and evidence for syntectonic alteration to clays and other minerals associated with the high temperatures in the fault zone. Narrow slip surfaces lie within cataclasite bands and gouge zones which consist of fractured, altered and sheared granitic rocks. Alteration mineralogy of the deformed rocks includes epidote, chlorite, zeolites (laumontite, nontronite), clay minerals, and palygorskite. Brecciated epidote in veins indicates shallow, brittle overprinting of higher temperature fault-related mineralization. We examine a 60 cm section of the core with macroscopic X-ray Fluorescence Spectrometry mapping to document fine-scale alteration. Rapid elemental imaging of the core documents the mobility of alteration-related elements K, Na, Mg, the concentration of less mobile Cr, Ni, and Mn, and creating of mm- to cm-scale foliated cataclasites. The patterns observed in the core support a model of syntectonic hydrothermal alteration within the fault zone that represent either, 1) the fault –related rocks we observe formed at depths > than 1.5 km and the fault zone was subsequently exhumed, or 2) elevated thermal gradients produce alteration in the shallow portion of the faults. The distributed deformation and alteration in these shallow levels may help explain how slip and energy is diffused in the upper several km of a fault zone, and in part explains the shallow coseismic slip deficits observed in seismogenic faults and subsequent accommodation by creep.