STRUCTURE AND GEOCHEMISTRY OF FAULT ZONES EXHUMED FROM SEISMOGENIC DEPTHS, SIERRA NEVADA, CALIFORNIA: IMPLICATIONS FOR EARTHQUAKE NUCLEATION AND PROPAGATION

The structure and geochemistry of strike-slip fault zones exhumed from 5-12 km in the Sierra Nevada, California, has been examined in order to understand fault slip processes at seismogenic depths. The faults formed along reactivated joints in granite and consist of a narrow (1-4 cm-thick) slip-surface with chlorite and epidote mineralisation. There is little deformation away from the main fault surface, regardless of the fault displacement. Microstructures along small, low displacement (~10 m) faults are essentially identical to those on large faults with displacements up to 10 km. This indicates that these slip-surfaces nucleate early in the history of fault growth, and that they must propagate and link up to form a through-going surface. Subsequent slip is confined to this narrow fault plane. Chlorite/epidote mineralisation and its associated whole-rock geochemical alteration is also confined to within the fault plane. The host rock away from faults is almost completely unaltered. However, quartz mineralisation is localised at steps or bends in the fault surfaces. The main fault surfaces thus represent a barrier to fluid flow, with migration of quartz-rich fluids localized within geometrical discontinuities along, and at the tips of, the faults. The process of slip-surface growth should determine the location of these discontinuities and this may be controlled here by the presence of pre-existing joints. It is likely that zones of high pore fluid pressure form in the thin fault zones, and be relieved at the fault tips and bends; however the total volume of fluid in the faults appears to be small. In our field area there are no explosive breccias, which have previously been cited as evidence for fluid-controlled earthquake rupture. Thus models of earthquake rupture must take into account the narrow slip-localisation surface with restricted volumes of fluid flow, and the network of relatively high volume conduits formed by steps and bends in the fault surface.