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

The Diligencia basin, located in the eastern Orocopia Mountains of southern California, contains 1500-2000 m of Oligocene-Miocene continental, silicic lastic 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 eastwest-trend ing basin, and are unconformably overlain by flat-lying late Pliocene(?) an d Pleistocene alluvial deposits. The sedimentological, stratigraphic, and s tructural history of the basin is compatible with late Oligocene-early Mioc ene formation as a half-graben basin produced by orthogonal, Basin and Rang e-type extension, and latest Miocene to Pliocene basin inversion in either a localized transpressional Transverse Range regime, or a more diffuse comp ressional regime associated with north-south shortening of the entire Mojav e block. Facies associations in the lower part of the Diligencia basin display a dis tinctly asymmetric distribution across the basin, indicating deposition in a half-graben with a steep, possibly fault controlled, south-facing norther n escarpment and a more gentle north-facing southern slope. Paleocurrent da ta, particularly from high-energy deposits on the northern basin margin, in dicate stream how toward the southeast to south-southeast, oblique to the d eformed basin margins, and suggest an approximate northeast to east-northea st strike for the basin before deformation. Previously published paleomagne tic data, however, indicate that the elongate, currently east-west-trending , fault block containing the Diligencia basin has rotated clockwise by as m uch as 110 degrees about a vertical axis (angle depends on data and model u sed) since deposition ceased. If correct, this would suggest that the basin may have originally opened to between the northeast and east-northeast, su bparallel 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 D iligencia basin occurred after block rotation, implying a latest Miocene to Pliocene age for inversion. We speculate that basin inversion within a nor th-south (and still active) compressive stress field resulted from the lock ing, and subsequent internal deformation, of this previously passively rota ting elongate crustal block.