Formation, evolution, and inversion of the middle Tertiary Diligencia basin, Orocopia Mountains, southern California

Abstract

The Diligencia basin, located in the eastern Orocopia Mountains of southern California, contains 1500–2000 m of Oligocene−Miocene continental, siliciclastic sedimentary rocks and subordinate limestone and evaporite deposits, intercalated with basaltic lavas. These rocks are locally intensely folded and faulted, defining in present-day coordinates an elongate east-west−trending basin, and are unconformably overlain by flat-lying late Pliocene(?) and Pleistocene alluvial deposits. The sedimentological, stratigraphic, and structural history of the basin is compatible with late Oligocene−early Miocene formation as a half-graben basin produced by orthogonal, Basin and Range−type extension, and latest Miocene to Pliocene basin inversion in either a localized transpressional Transverse Range regime, or a more diffuse compressional regime associated with north-south shortening of the entire Mojave block. Facies associations in the lower part of the Diligencia basin display a distinctly asymmetric distribution across the basin, indicating deposition in a half-graben with a steep, possibly fault controlled, south-facing northern escarpment and a more gentle north-facing southern slope. Paleocurrent data, particularly from high-energy deposits on the northern basin margin, indicate stream flow toward the southeast to south-southeast, oblique to the deformed basin margins, and suggest an approximate northeast to east-northeast strike for the basin before deformation. Previously published paleomagnetic data, however, indicate that the elongate, currently east-west−trending, fault block containing the Diligencia basin has rotated clockwise by as much as 110° about a vertical axis (angle depends on data and model used) since deposition ceased. If correct, this would suggest that the basin may have originally opened to between the northeast and east-northeast, subparallel to well-documented extension directions in Miocene age basins in the Mojave Desert to the east. Clockwise rotation on east-west fault blocks exposed in this region has previously been bracketed as between ca. 10 and 4.5 Ma. Structural and paleomagnetic data indicate that inversion of the Diligencia basin occurred after block rotation, implying a latest Miocene to Pliocene age for inversion. We speculate that basin inversion within a north-south (and still active) compressive stress field resulted from the locking, and subsequent internal deformation, of this previously passively rotating elongate crustal block.