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Description
Mid-Tertiary detachment-related sedimentation and deformation have produced a complexly faulted and tilted alluvial-fan complex exposed in the Baker Peaks region of southern Yuma County, Arizona. Arkosic sandstone and boulder conglomerate units of debris-flow, sheet-flood, channel-fill and sieve deposits characterize the alluvial-fan complex, which fines distally to the north. Alluvial-fan sediments were shed northward off the topographically high source terrane of the Copper Mountains antiform into the downwarped Gila Trough. The modern Gila Trough appears to have been present as a large-scale mid-Tertiary basin, formed during this regional extensional deformation, rather than formed simply as a late-Tertiary down-faulted basin. The thick non-progradational sequence of strata appears to have been coevally deposited during formation of a complex system of fractures and growth faults, suggesting that alluvial-fan development was syntectonic and in equilibrium with regional crustal warping and extension. Upper-plate strata dip moderately to steeply to the southwest and are interpreted to have been rotated along growth faults, which merge into the exquisitely developed detachment fault exposed south of the Baker Peaks. The orientation of fractures, small-scale folds and mullion structures displayed within the patina-colored microbreccia ledge of the detachment fault and the rotation of the upper-plate strata support a northeast-southwest oriented extension direction. The arcuate exposure of the detachment fault as it shallowly plunges to the north and west under the steeply tilted strata provides a three-dimensional view of the folded fault surface. This fault geometry and the northeast-trending Copper Mountains antiform and anomalous Gila Trough, appear to be genetically related to the development of regional topographic, as well as structural, highs and lows. These regional trends appear to have fundamentally controlled the distribution and facies patterns of mid-Tertiary elastic sequences. Sedimentation patterns within the Baker Peaks and nearby ranges strongly support the idea that coeval deposition and deformation are genetically related to detachment-related crustal extension. Correlation within southern Yuma County suggests that syntectonic uplift, sedimentation and detachment fault deformation appear to have been initiated prior to the mid Tertiary volcanic outpourings, indicating that detachment tectonics commenced earlier in this region of the Mojave-Sonoran detachment terrane than in regions to the north along the Colorado River.
