The effect of progressive diagenesis on meso- and microscale fault zone structural development and heterogeneity

Jennie Cook

Department of Geology & Geophysics

University of Wisconsin - Madison

Madison, Wisconsin

[email protected]

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 - 150 km. Within the study area the fault cuts syntectonic mudstones, siltstones, and sandstones of the Purisma Formation. Detailed mapping documents a post-lithification damage zone that overprinted pre-lithification mixed zones that bracket a well-developed, exceptionally wide (>15 m) fault core. Deformation within the mixed zone was distributed and characterized by increasing disorganization and boudinage of relatively competent sedimentary layers. Multiple sandstone dikes crosscut these structures, demonstrating that they formed prior to lithification. Deformation is inferred to have occurred largely through particulate flow. The brittle damage zone, which consists of discrete fractures, minor faults, and veins that crosscut both boudins and sandstone dikes, is less extensive than the mixed zone. The transition in macroscale deformation behavior that these structures record is inferred to reflect a transition in grain-scale deformation mechanisms with progressive consolidation, tectonic compaction, and cementation.

AAPG Search and Discovery Article #90060©2006 AAPG Foundation Grants-in-Aid