Abstract

The late Cenozoic structural and stratigraphic history of the northwestern San Bernardino Mountains supports two distinct episodes of uplift, in late Miocene to earliest Pliocene and Quaternary time, that we hypothesize are related to movements on low-angle structures beneath the range. In this paper, we document the nature, distribution, and timing of late Cenozoic deformation and deposition in the northwestern San Bernardino Mountains, and we illustrate the neotectonic evolution of the area in a series of interpretive paleotectonic block diagrams.

In the first episode of deformation, late Miocene to earliest Pliocene motion on the south-southwest-directed Squaw Peak thrust system disrupted drainage in pre-existing Miocene nonmarine basins and uplifted the western third of the present range to form the ancestral San Bernardino Mountains. Crystalline rocks of the San Bernardino Mountains were thrust southward across the present site of the San Andreas fault between 9.5 and 4.1 Ma, at a time when the San Gabriel fault was the active strand of the San Andreas transform system. We speculate that the Liebre Mountain crystalline block at the northern margin of the Ridge Basin may be the missing upper plate of the Squaw Peak thrust, now offset along the San Andreas fault.

The second episode of deformation began with uplift of the northern plateau of the modern San Bernardino Mountains on north-directed, range-front thrusts in early Pleistocene time, between 2.0 and 1.5 Ma. Synchronous uplift of the northern plateau, recorded in early Pleistocene fanglomerates on the northwestern margin of the range, is interpreted to be the result of movement of a relatively coherent crustal block northward up a south-dipping detachment ramp beneath the central range. In middle Pleistocene time, activity on the northern range front began to wane, and the locus of uplift shifted to a narrow zone of arching and northward tilting adjacent to the San Andreas fault, which subsequently migrated rapidly northwestward along the San Andreas fault from the western San Bernardino Mountains into the northeastern San Gabriel Mountains. We attribute this pattern of deformation to the passage of a bulge or strike-slip ramp attached to the southwest side of the San Andreas fault at depth.

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