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Textural Recognition of Shallow Pulverization of Sandstone in the Damage Zone Along the San Jacinto Fault, Southern California

Rockwell J. Whearty and G. T. Girty

We present new evidence for the production of shallow damage in Quaternary sandstones along the San Jacinto fault (SJF), a right lateral strike-slip fault of the southern San Andreas fault system in southern California. The Bautista Formation, a moderately bedded plutonoclastic unit, bounds the southwest side of the SJF in Rockhouse Canyon, Anza Borrego State Park. We studied exposures located at ~20 m and ~100 m below the top of the Bautista alluvial fill. These depths are inferred to represent total burial depths for studied exposures. The age of the deposits is inferred to be middle Quaternary, and other exposures of the Bautista Formation along the Coyote Creek fault contain the Bishop Tuff, supporting this inference.

Thin sections collected in a traverse extending from near the SJF fault core laterally into the damage zones at lower Rockhouse Canyon show extensive in situ fracturing or pulverization that is similar to that documented along the San Andreas fault in Pliocene sandstones (Dor et al., 2009). In contrast, in upper Rockhouse Canyon, sandstones in a similar setting, exhibit only very minor evidence of fracturing. From these observations, we infer that the onset of strong pulverization likely occurs at depths as shallow as 100 m. At the microscopic level, the combination of abundant mode 1 fractures and the lack of evidence from shearing suggests that the observed pulverization is the result of dilation, probably as a consequence of the sudden decrease of normal stress as ruptures propagated along the fault.

We are conducting additional analysis to better understand the chemical and physical processes in a near surface faulting environment. Quantifying elemental mobility and compositional mass balance throughout the damage zones will give us a better insight into the role of fluid migrations at the time of rupture, while resolving the clay mineralogy within the fault will test whether there has been sufficient heat and pressure to force the conversion of illite-smectite to illite.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013