Abstract

The kinetics of continental rifting are recorded in the late Neogene sediments of the western Imperial Valley, California. The Gulf of California opened in its present form about 4 m.y. ago, capturing the Colorado River. Some 5 km of deltaic and fluvial sediments accumulated in the Vallecito–Fish Creek area during the interval 4.3−0.9 m.y. ago. Initial sedimentation rates were 5.5 mm yr−1, which diminished in an exponential fashion to 0.5 mm yr−1. When isostatic adjustments are accounted for, this implies tectonic subsidence rates of 1.5 mm yr−1, decreasing systematically to 0.1 mm yr−1. After 0.9 m.y. ago, right-lateral shearing along the Elsinore fault zone cut across the basin, causing it to be tilted and uplifted at a mean rate of 5.9 mm yr−1. The basin was concurrently rotated as a unit 35° clockwise. The dissected pediments and badlands that now characterize the area were formed and eroded in the past 0.9 m.y.

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