STRUCTURAL EXPRESSION OF LOW-ANGLE NORMAL FAULTS DEVELOPED IN WRENCH SETTINGS: AN EXAMPLE FROM THE WEST SALTON DETACHMENT SYSTEM

The west Salton detachment system (WSDS) records complex, evolving 3D strain in a wrench-dominated rift and differs from "typical" detachments. Other detachments that formed in oblique rift settings share similar attributes, suggesting that detachments formed in wrench settings and orthogonal rifts may be end members of a spectrum.

The Salton trough is the north end of the Gulf of California rift. The WSDS forms the west boundary for ≥70 km and controlled sedimentation in the western trough. The WSDS and south San Andreas both formed in latest Miocene or earliest Pliocene time. Most WSDS slip probably ended by ~1-2 Ma but parts appear active within the Elsinore fault system.

Atypical characteristics of the WSDS include the following. (1) Folds of the detachment trend E to ESE and are not perpendicular to the regional detachment trend (NNW after restoring San Jacinto offset). (2) Wavelength of folds in the WSDS (<10 km) is smaller than "typical" detachments (>10 km). One (Whale Peak) may have been an original corrugation but another to the north (Yaqui Ridge) likely grew more recently. (3) South of Borrego Springs, only single detachment strands are present on antiformal noses but 2 or 3 strands are present in synforms. Relative ages of strands in synforms are inconsistent: some high strands appear older (back rotated by lower strands) but some lower strands are more strongly folded, suggesting that they are older. Some strands were probably abandoned in synforms while slip persisted on the noses, where striae trend mainly E-ESE (see next). (4) Fault striae are highly variable: most trend 070 to 110, generally parallel to detachment folds, but striae also are common from 180-240, 280-310, and 350-070. Multiple directions at single outcrops and systematic changes along and across folds suggest that slip direction has rotated CW from ~NE to ~E or ESE, consistent with increasing NW-SE dextral shear and with CW rotation of extension to the south. We think that folding reoriented some early NE striae to SW trend. Flex slip may have caused N and S striae. Late slip explains the maxima from 070-110 with some back-rotated planes showing WNW striae. (5) The upper plate is commonly folded but is not strongly extended by faults; many folds do not mimic folds of the WSDS, and large tracts (e.g., S. Fish Creek basin) are homoclinal.