Deformation Bands in the Etchegoin Formation: Implications for Stresses on the San Andreas Fault in Central California

Sarah Titus, Carleton College
Alice Newman, University of Vermont
Lucy Livesay, Carleton College
Amanda Yourd, Carleton College

Deformation bands are well developed in poorly-cemented blue sandstones of the Pliocene Etchegoin Formation in central California. These tabular structures occur both as individual bands and as sets of multiple, parallel, closely-spaced bands. All types of deformation bands seem to act as small faults, accommodating mm- to cm-scale offsets. We examine the orientation of deformation bands from three locations at varying distances northeast of the San Andreas fault: Parkfield (1-3 km), Kreyenhagen Hills (23 km), and Kettleman Hills (32 km). Each location is part of a relatively young fold developed in the fault borderlands. At each site, we find two conjugate sets of deformation bands. These bands tend to be steeply dipping (>75°) and mutually cross-cutting, with an angle between sets that varies from 55° to 85°. When appropriate markers are present on vertical faces, the apparent sense of motion is normal. At Parkfield, we also identify apparent strike-slip motion on horizontal outcrop surfaces. Right-lateral motion is associated with one conjugate set, and left-lateral motion is associated with the other. Restoration of each fold limb to horizontal produces more asymmetry in the orientations of deformation bands, suggesting that these structures formed during or late in the folding process.

By bisecting the acute angle between conjugate sets of deformation bands, we estimate the orientation of the local stress directions including S_Hmax. We find that S_Hmax is highest at Parkfield, with values of ~55°, comparable to results from the SAFOD pilot hole and earthquake focal mechanisms. The value of S_Hmax is ~45° at Kreyenhagen Hills and ~40˚ at Kettleman Hills. This pattern of decreasing S_Hmax with distance from the fault is not consistent with other estimates of stress directions from the region, such as those from the World Stress Map. However, the angle between S_Hmax and the local fold strike for each site is consistently 50-60°. Therefore, we suggest that estimates for S_Hmax far from the San Andreas fault may be controlled not by the fault but instead by anisotropies caused by local structures such as folds.