ASSESSING THE LONG-TERM DISPLACEMENT FIELD ASSOCIATED WITH DEFORMATION OF THE BORDERLANDS OF THE SAN ANDREAS FAULT, DURMID HILL, CA

The surface trace of the San Andreas fault (SAF) intersects the NNW-trending Brawley seismic zone in the east-central portion of the Salton Trough. Immediately W of the SAF and NW of the Brawley seismic zone, shortening and thickening of Pleistocene Borrego Formation strata produced Durmid Hill, a topographic feature approximately 10 km long, 2 km wide, and 100 m high. Exposures there provide a unique view of the permanent deformation of the SAF borderlands over the last ~750 ka. The most striking structures here are folds with hinges oblique to the SAF. Within 1 km of the fault, fold hinge traces make ~ 30° angles with the SAF trace. These folds are tight to isoclinal and attenuated limbs indicate significant intra-layer deformation. The ratio of final to original layer length (l'/lo) measured normal to fold hinges is < 0.5; l'/lo measured parallel to fold hinges is > 3. Between 1 and 4 km from the SAF, fold hinge traces make 35-45° angles with the fault trace. These folds are close to tight, overturned to the southeast, and with evidence of moderate intra-layer deformation. Measured normal to fold hinges, l'/lo is ≈ 0.5, and l'/lo parallel to fold hinges is ≤ 2. More than 4 km from the SAF, fold hinge traces again are oriented 30-35° to the fault trace, but the folds are open to close and generally lack evidence of intra-layer deformation. Measured normal to fold hinges l'/lo is > 0.5, and l'/lo parallel to fold hinges is ≈ 1. All folds are associated with detachments. Within 2 km of the SAF, detachments are tightly folded; farther from the fault, detachments are gently folded or not folded. A series of left-lateral faults with traces ~30-35° to the SAF trace cut across folds within 2 km of the fault. Overprinting patterns indicate (1) formation of detachment or fault-related folds that are elongated parallel to their hinges, followed by (2) fold tightening and fold distortion accompanied by strong intra-layer deformation, and (3) the development faults oblique to the SAF. Sequential reconstruction yields a three-dimensional displacement field with the directions of maximum shortening and elongation consistently oblique to the SAF. The shapes of folds and boudins and the intra-layer deformation are consistent with interseismic displacement; fold growth and faulting may occur during seismic slip.