A lateral ramp origin for the north trending segment of the Squaw Peak Fault, Cajon Pass, California

Abstract

The Squaw Peak fault places the Miocene Crowder Formation and pre‐Cenozoic basement of the San Bernardino Mountains against the Miocene Cajon (Punchbowl) Formation. The fault consists of two west‐northwest trending, north‐northeast dipping, low‐angle thrust segments that occur at different structural levels connected by a north‐northeast trending high‐angle segment. Meisling and Weldon (1989) proposed that the high‐angle segment is a dextral lateral ramp which links the two thrust segments and is a key element in a latest Miocene contractile event. Our study of the high‐angle segment, reported here, supports the lateral ramp interpretation and further elucidates the development of the Squaw Peak fault as a whole. Scale 1:500 mapping of a section of excellent railroad cut exposures at the north end of the high‐angle segment shows numerous low‐angle contractional structures that predate the high‐angle segment. Contraction occurred in three stages: (1) east directed folding and reverse faulting, (2) west directed backthrusting, and (3) northeast directed thrust faulting and refolding of stage 1 folds. These contractional structures trend obliquely to the trace of the high‐angle segment consistent with right‐lateral simple shear. They also show an apparent clockwise rotation through time consistent with paleomagnetic studies (Liu et al., 1988) that indicate clockwise rotation of the Cajon Formation adjacent to the Squaw Peak fault. Furthermore, stratigraphic contacts and structures south of the study area show apparent right‐lateral “drag” adjacent to the fault. We therefore infer that the contractional structures formed in a zone of progressive right‐lateral simple shear that was parallel to the trace of the future high‐angle segment. The high‐angle segment of the Squaw Peak fault, which is well‐exposed in the study area, cuts these structures. Its surface is markedly nonplanar as are numerous adjacent minor faults. Axes of bends in the main fault plane as well as in the minor faults plunge gently to the southeast. These axes probably approximate the slip lines of the faults; their gentle plunges therefore indicate predominant lateral slip. “Drag” of rocks adjacent to the fault plus an obliquely oriented thrust fault and fold pair further suggest the slip was right‐lateral. Although slip on the high‐angle segment postdated contraction, three observations suggest that the contraction and strike‐slip were part of the same protracted episode: (1) contractional features are spatially related to the Squaw Peak fault; (2) motion on the Squaw Peak fault and spatially related contractile structures occurred in a relatively narrow interval of time, between 4.2 and 9.5 Ma, and possibly between 4.2 and 6.2 Ma; and (3) mobilized gouge fills faults of all stages, which suggests similar conditions during slip. The high‐angle segment therefore represents the last stage in a progression from distributed right‐lateral simple shear to dextral slip on a throughgoing fault surface. Thrust motion on the two gently north dipping segments of the Squaw Peak fault, before they were connected by the high‐angle fault, formed the initial right‐lateral shear couple. When the high‐angle fault linked the thrust segments as a lateral ramp, it shut off the earlier contractional structures.