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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 low-angle segments which occur at different structural levels separated by a north northeast-trending high-angle segment. Meisling and Weldon (1989) suggested that the low-angle segments are thrust faults and that the high angle segment is a dextral lateral ramp connecting them. Mapping of the north end of the high-angle segment, at the scale of 1:500, revealed eastand westdirected contractional structures and evidence for later dextral slip on the high-angle segment. Contraction occurred in three stages: 1) east-directed folding and reverse faulting; 2) west-directed backthrusting; 3) northeast-directed thrust faulting and refolding of stage 1 folds. The high-angle fault cuts these structures. The curved surface of the main fault plane and numerous smaller fault-bend axes within the fault zone plunge gently southeast, suggesting predominant lateral slip; "drag" of rocks adjacent to the fault plus an obliquely oriented thrust fault and fold pair suggest the slip was right-lateral. 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 occurred between 4.2 to 9.5 Ma, possibly from 4.2 to 6.2 Ma; 3) mobilized gouge fills faults of all stages, suggesting similar conditions during slip. Also, the contractional structures trend obliquely to the Squaw Peak fault and display an apparent clockwise rotation through time, suggestive of Formation over a wrench fault. The structural evolution of this study area supports the thrust and ramp model for the Squaw Peak fault. Thrust motion on the two gently north-dipping segments of the Squaw Peak fault, before they were connected by the high-angle fault, set up a rightlateral shear couple. The resultant contraction in the stepover region, represented by stages 1 through 3, preceded propagation of the high-angle fault between the two thrust faults. Further slip within the stepover region occurred on the high-angle fault, effectively shutting off the earlier contractional structures.

Structural evolution of part of the north-trending segment of the Squaw Peak Fault, Cajon Pass, California