Deformation bands in the Etchegoin Formation at Parkfield: Implications for stresses on the San Andreas Fault in Central California

Consideration of the stresses on the San Andreas Fault requires a comprehensive understanding of regional deformation patterns. This study focuses on deformation bands, tabular surficial structures that record relatively recent regional stress orientations, located at adjacent to the fault at Parkfield, Kreyenhagen Hills, and Kettleman Hills. At Parkfield, the primary field site, there are five sets of deformation bands: one EW-striking set, dipping north at an angle slightly steeper than bedding, and mutually cross-cutting two conjugate pairs which consistently cut the EW-set. One conjugate pair strikes at approximately 040 ÌŠ while the other strikes at approximately 350 ÌŠ. Both pairs have conjugate dips ranging from approximately 70 - 80°. Unfolding the in situ data produces a loss of conjugate symmetry, indicating that deformation bands formed syn- or post-folding. We found SHmax by bisecting the acute angle between the two sets of deformation bands. At Parkfield, we estimate that the angle between SHmax and the San Andreas Fault is approximately 55 - 60°. Relative to the San Andreas Fault, SHmax orientations rotate anomalously to more acute angles at greater distances from the fault. Orientations of SHmax demonstrate a high level of consistency relative to the local structural fabric, with values ranging between 50° and 60°. This suggests that the local folds likely have significant influence over the orientations of SHmax demonstrated by deformation bands. In thin section, deformation bands contain stress-induced transgranular microcracks which occur preferentially acutely oriented to the direction of adjacent deformation bands. This may be indicative of shear-compaction deformation. Paired with observed normal faulting accommodated by the deformation bands, this may have implications for the transpressional model of fold behavior. Additional research is necessary to draw any further conclusions