Evolution of a Miocene half-graben basin, Colorado River extensional corridor, southeastern California

The <1-km-thick, middle Miocene, predominantly sedimentary succession exposed within the northern Sacramento Mountains contains many characteristics of sedimentation in an extension-generated half-graben basin. These deposits constitute the uppermost plate of a three-plate detachment-fault system. Three facies associations are recognized and are inferred to represent 1) small, high-gradient, gravity-dominated alluvial fans; 2) lake and lake margin; and 3) large, low-gradient, fluvially influenced alluvial fans. The basin probably opened as an asymmetric half graben prior to 14.6 ± 0.9 Ma and after 18.5 ± 0.2 Ma along a gently northeast-dipping detachment fault. Following deposition, extension segmented the basin into numerous west-south-west tilted blocks, which were then covered by a thin veneer of gravels and finally by the lavas of Flattop Mountain at 14.6 ± 0.2 Ma. -from Authors