Mesoscale and microscale structure analysis of phase I and II core of the SAFOD drilling project, Parkfield, California

Rafael Almeida

Texas A & M University, Department of Geology and Geophysics

College Station, TX

[email protected]

To further understand the properties of damage zones of large displacement faults in the continental crust, this project will characterize the orientation, distribution, and timing of mesoscopic and microscopic fracturing, and syn-faulting alteration of the damage zone of the San Andreas Fault, Parkfield, California. These data will be acquired from 3 sections of core recovered during Phase I and II of the SAFOD project. The data will be analyzed in terms of the stress, strain and fluid flow history. Detailed wrap-around maps of the lithology and mesoscopic deformation have been prepared. In these the orientation, timing and mineral fill of different fracture sets have been identified. FMI logs were used to orient the core. Oriented samples of representative deformation features and lithologies have been taken for characterization of the microscopic fracture fabric, timing, and fluid history during faulting. Mode I microfracture densities and orientations will be determined as a function of lithology and distance from the main fault. The mesoscale and microscale fabric will be used to determine the paleostress field; by determining vein mineralogy, we will determine distinct events of fluid flow. We will try to establish if these rocks are within the damage zone of the San Andreas Fault, and how the stress state adjacent to the fault has changed over time as well as the possible influence of fluid flow within the damage zone. It will be critical to establish the role these structures play in the energy balance during the passage of a large earthquake.

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