PENNSYLVANIAN-PERMIAN EVOLUTION OF THE DARWIN BASIN: A BASIN DEVELOPED OVER THE NASCENT CORDILLERAN SUBDUCTION ZONE

The late Paleozoic Darwin Basin of Eastern California represents abrupt subsidence of the southwestern margin of Laurentia beginning at the Pennsylvanian-Permian Boundary. Subsidence is recorded by an abrupt, east-southeast propagating stratigraphic transition from shallow-water facies of the Bird Spring Formation, to a retrogradational sequence of base-of-slope carbonate apron and basinal facies of the Owens Valley Group. Previous models for the evolution of the Darwin Basin rely on flexural loading by the Last Chance Allochthon or transtension associated with the California-Coahuila Transform Fault; however, none of these models can adequately explain the spatial and temporal distribution of subsidence observed in the Late Paleozoic of California and southern Nevada. In this report we present 15 new measured sections and associated facies analysis, conodont biostratigraphy and point and clast counts from Pennsylvanian-Permian strata of the Darwin Basin that track the progressive retrogradation of base-of-slope facies on to the former shelf. We also report on XRD data, whole-rock geochemistry and Ar/Ar geochronology results from 5 clay-rich beds from within the Darwin Basin that we interpret as bentonites. We propose that subsidence of the Bird Spring carbonate shelf and formation of the Darwin Basin was caused by negative dynamic topography associated with initiation of the Cordilleran Subduction Zone. The nascent subduction zone propagated southeast along the trace of the California-Coahuila Transform fault producing bentonites within the Darwin Basin by the early Permian and calc-alkaline plutons in southern California and Sonora by the middle Permian.