Description
In the San Felipe Hills, Imperial Valley, California, a thick section of tightly folded and faulted supracrustal rocks overlies crystalline basement. The sedimentary section is composed of fluvial and lacustrine rocks which correlate to the Pleistocene Borrego and Brawley (?) formations. Two deformational events have affected the San Felipe Hills. The dominant E-W trending folds, NW trending dextral faults, and NE trending sinistral faults were produced by the primary deformational event (Dl). A second deformational event (D2) produced N-NE trending folds. Angular unconformities observed in the mapped area indicate that D1 was contemporaneous with deposition of the Borrego Formation. The second deformational event, D2, postdates D1, as E-W trending folds are refolded by N-NE trending folds. The San Felipe Hills lie approximately along strike between the Clark and Superstition Hills faults at the southern end of the San Jacinto fault zone, but no through-going strike-slip faults are observed at the surface in the area. Experimental wrench faulting in clay models produced folds and faults at specific orientations to the underlying wrench fault, prior to producing a through-going strike-slip fault. The deformation observed in these models implies that strike-slip movement in basement rocks is accommodated by ductile folding and faulting in overlying sedimentary rocks. Analysis of the structural fabric in the San Felipe Hills indicates that D1 deformation probably resulted from slip on a buried NW-SE trending dextral wrench fault in the underlying crystalline rocks. Empirical and experimental modeling of deformation at the depth of the basement in the San Felipe Hills indicates that the sedimentary section is detached from the basement. Balanced cross-sections constructed to the depth of the basement imply that deformation above this decollement may be accommodated by fault-propagation folding. Recent studies along the central San Jacinto fault zone yield minimum Holocene dextral-slip rates of 9 mm/yr. However, in the Imperial Valley, only about 2-3 mm/yr is accounted for by slip at the surface on the Coyote Creek fault. The results of this study suggest that a significant portion of strain across the San Jacinto fault zone in the Imperial Valley is accommodated at the surface by ductile folding of sedimentary rocks in the San Felipe Hills. The orientation and location of the proposed buried wrench fault indicates that it is most likely the continuation of the Clark fault, which appears to die out at the surface in folded sedimentary rocks NW of the San Felipe Hills.
