Multi-scale Structural Characterization of the Mecca Hills Fault System in the NE block of the Southern San Andreas Fault System, California

Kelly K. Bradbury, Amy C. Moser, Sarah A. Schulthies, & James P. Evans

Published August 22, 2016, SCEC Contribution #7021, 2016 SCEC Annual Meeting Poster #110

We examine the structural architecture, mineralogy, and alteration of fault zones across a range of scales in the Mecca Hills in southern California. Our research group is focusing on outcrop-scale field and laboratory measurements of 4 well-exposed NNE trending subsidiary SSAF faults (Painted Canyon, Platform, Eagle Canyon, and the Hidden Springs Fault) that exhibit a range of lengths and displacements, and that formed in the same tectonic regime, at nearly the same time, east of the main SSAF.

Faults exhibit a bimodal distribution of strike-slip and dip-slip slip vectors. NE-SW striking strike-slip faults are at high angles to the main traces of the major dextral faults, whereas dip-slip faults strike E-W. These data imply separate structures accommodate strike-slip and dip-slip motion and transfer slip.

We determine the nature and distribution of deformation features within the damage zones of each fault, the micro- to nano-scale composition and texture of the protolith and fault-related rocks, and the alteration signatures present within each fault zone. Complex fault geometries with multiple, anastomosing strands that dip <90° and asymmetric damage zones are observed. Several m’s thick gouge zones and 10’s of meters wide damage zones occur. Numerous highly reflective silica-rich slip surfaces are identified and exhibit textures suggestive of vapor phase-liquid partitioning during slip. Matrix-supported brecciation and cataclastic flow textures imply mineralization associated with rapid fault slip at these localities. Iron oxide mineralization is also associated with cataclasis at several localities.

Two predominate phases of veins are consistently detected within fault damage zones and suggest hydrothermal alteration in the Mecca Hills region: 1) a N-trending quartz ± calcite ± zeolite system, typically oriented parallel to faulting &/or foliation; and 2) an EW-trending system composed of quartz ± calcite ± laumontite ± palygorskite ± trace sulfides & metals. Quartz vein deformation microstructures display evidence for multiple stages of brittle deformation.

Petrographic, SEM, and geochemical results imply that fault rocks in the Mecca Hills damage zones did not form from simple mechanical grinding of the protolith. For example, a relative decrease in SiO2, increase in CaO, and increase in FeO occurs between the protolith and the fault-related rocks. Barium concentrations also decrease significantly from the protolith to the fault-related rocks, suggesting hydrothermal fluids preferentially stripped Ba from the system.

These data, coupled with our pre-existing work on the San Andreas fault address: 1) the differences in the physical properties between a major plate boundary fault and smaller scale subsidiary faults; and 2) provides critical information on the physical and chemical processes active during fault zone deformation in a region of triggered slip and creep.

Key Words
Southern San Andreas Fault, Mecca Hills, Structural Characterization

Citation
Bradbury, K. K., Moser, A. C., Schulthies, S. A., & Evans, J. P. (2016, 08). Multi-scale Structural Characterization of the Mecca Hills Fault System in the NE block of the Southern San Andreas Fault System, California. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology