Collaborator:  Dr. Fred Chester (Texas A & M)

Research Abstract:
Many models have been proposed to account for the weak-fault behavior of large strike-slip faults in the San Andreas system.  Fundamental to many of these models are the physical and/or chemical role(s) of fluids.  In addition, several models have been proposed that invoke chemical and mechanical effects of fluids in the nucleation, propagation, and arrest of seismic ruptures.  These models envision significantly different sources, quantities, compartmentalization, and residence times for fluids in seismogenic fault zones.  On the basis of existing data, it is difficult to discern which model(s) most accurately describes the fluid-rock interaction in faults and the role fluids played in the seismogenic cycle.  This difficulty is due, in part, to the very limited amount of geochemical data that is presently available for faults of the San Andreas system.  Study of the mineral phases, the major and trace element chemistry, and in particular, the stable isotope geochemistry of rocks in fault zones is one of the most effective ways of documenting fluid-rock interaction.  At this time, however, only a few geochemical studies have been conducted on fault rocks in the San Andreas system.

Our research will help determine the extent of fluid involvement in the seismogenic cycle and to discern which model(s) most accurately describes fluid-rock interaction in large strike-slip faults of the San Andreas system.  This will be accomplished through an integrated structural-geochemical study of fault rocks exposed in and along the two most deeply exhumed, large-displacement faults in the San Andreas system --  the Punchbowl and San Gabriel faults.  Detailed mapping and structural analysis of the fault cores will provide the requisite control that is so vital for collecting samples for geochemical analyses and interpreting the geochemical data in relation to the seismogenic cycle.  The geochemical portion of the study will characterize the mineral phases, the major and trace element chemistry, and the stable isotope geochemistry of the fault rocks and adjacent host rocks.  Results from the analyses, especially the stable isotope analyses, will better constrain the fluid-rock interaction in fault zones than is possible with the current data.

This study will provide new and much needed data.  It will be possible by completion of this study to determine which model(s) most accurately describes fluid-rock interaction in large strike-slip faults and to eliminate from future consideration those model(s) that are inconsistent with the geochemical data.  Ultimately, this study will help improve our understanding of earthquake processes in the San Andreas fault system and elsewhere.