DIAGENETIC CONTROLS ON THE EVOLUTION OF FAULT-ZONE ARCHITECTURE AND PERMEABILITY STRUCTURE, AN EXAMPLE FROM THE SAN GREGORIO FAULT, CA

Sediments are mechanically distinct from fully lithified sedimentary rock because diagenetic changes such as compaction and cementation modify rock properties and thereby affect deformational processes. Such changes affect the character and development of fault zone structures and the final fault-zone architecture, particularly in upper crustal faults. The San Gregorio fault, part of the San Andreas fault system, provides a structural record of transitions in deformation mechanisms with progressive lithification. The San Gregorio is an active, predominantly dextral strike-slip fault with cumulative offset of 90 - 150km. Within the study area, the fault cuts syntectonic sedimentary rocks of the Purisima Formation. Detailed mapping documents a well-developed fault core surrounded by pre-lithification mixed zones that are overprinted by a post-lithification damage zone. Overprinting relationships indicate that deformation became more localized with progressive diagenesis.

Deformation within the mixed zone was distributed and characterized by increasing destruction of relatively competent sedimentary layers. Multiple sandstone dikes crosscut zones of stratal disruption, demonstrating that such zones formed prior to lithification. Deformation is inferred to have occurred largely through particulate flow with an associated decrease in porosity and permeability resulting from grain reorganization, preferential alignment of anisotropic grains, and destruction of more competent, coarser-grained strata. The overprinting, brittle damage zone consists of discrete fractures, minor faults, and veins that crosscut both disrupted sedimentary layers and sandstone dikes. These structures result in dilatancy and a concomitant increase in permeability within the post-lithification damage zone. The transition in macroscale deformation behavior that these structures record is inferred to reflect a transition in grain-scale mechanics with progressive consolidation, tectonic compaction, and cementation. The recognition and identification of deformational processes active in unlithified sediment is important for developing a better understanding of the characteristics and controls on the development of fault rocks in the near surface.