Miocene sedimentary strata exposed in the eastern Chemehuevi Mountains, southeastern California, record development of an evolving low-angle normal fault system. The sequence includes more than 1 km of conglomerate and sandstone with rare interbedded monolithologic breccia and volcanic flows. Clasts of Peach Springs Tuff in basal units indicate that this succession is younger than 18 Ma. These rocks have been displaced by a regionally extensive low-angle normal fault, the Chemehuevi detachment, and are folded and faulted. Structural reconstructions and the character of associated fault rocks indicate that the Chemehuevi fault was initiated at a low angle and that the footwall was progressively unloaded through thinning and displacement of its cover during extensional deformation. The syntectonic sedimentary rocks described here provide evidence that movement continued on the gently dipping (< 15°) fault even after part of the fault was breached and the footwall eroded.

The conglomerates and sandstones were deposited by stream flow and debris flow on alluvial fans. Synsedimentary faulting is suggested by angular discordance below one monolithologic breccia bed and by local coarsening-upward sequences. Clast types reveal progressive unroofing of hanging-wall rocks to expose the Chemehuevi fault zone, from which chloritic, brecciated granite clasts were derived. Clasts were then derived from both the hanging wall and the footwall, footwall debris being dominant high in the section. Distinctive clasts show that late displacement on this evolving fault system was on the order of 1 to 5 km.

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