CHARACTER OF DEFORMATION ADJACENT TO THE SAN ANDREAS FAULT, DURMID HILL, CA

Rock adjacent to strike-slip faults accrues deformation continually. Some deformation is recoverable and is released during earthquakes. The remaining deformation is permanent deformation manifest as folds, subsidiary faults, or other fabrics developed in the 'borderlands' of the fault. Slip on the southernmost exposed segment of the San Andreas fault (SAF) has been accompanied by the development of folds in Miocene, Pliocene, and Pleistocene strata found on a basement of older metasediments. Previous workers have shown that folds in Pleistocene strata at Durmid Hill have geometries consistent with transpressional kinematics. In particular, they showed that (1) the attitudes of folds hinge lines are consistent with reorientation during fault-parallel shearing, (2) layers were elongated significantly parallel to fold hinges as folds formed; the distinctive Bishop Ash layer is in fact boudined, and (3) late, left-lateral faults cut and offset fold hinges.

I report here results of an on-going analysis of minor structures at Durmid Hill. The Bishop Ash layer possess overprinting sets of fractures. Earlier fracture sets record elongation consistent with elongation at higher angles to the SAF whereas later sets record elongation at lower angles to the SAF. New mapping of boudins in the ash layer indicate that (1) boudinage occurred prior to folding, and (2) elongation decreases monotonically away from the SAF. The early layer extension suggests substantial attachment to the basement, whereas the smooth decrease in extension magnitude suggests that the strain gradient across Durmid Hill is more uniform than earlier workers had proposed. Folding is demonstrably related to localized detachment faults. Detachments higher in the sedimentary sequence are themselves folded, whereas those lower in the sequence are not. Deeper detachments, which extend farther from the trace of the San Andreas fault, must have formed later than earlier detachments. New mapping has also documented a second left-lateral subsidiary fault at Durmid Hill that cuts all earlier structures and elongates rock oblique to the SAF. Left-lateral faults suggest renewed attachment between cover and basement. Overprinting minor structures indicate attachment, detachment, and renewed attachment between basement and cover.