Tectonic and stratigraphic evolution of the late Paleozoic Darwin Basin, eastern California, USA, and implications for the onset of subduction along the southwestern Cordilleran margin of Laurentia

The Darwin Basin, eastern California, USA, represents a key sedimentary record of tectonics along the southwestern margin of Laurentia for the late Pennsylvanian and Cisuralian (Early Permian). The basin formed when the subsidence rate of the southwestern margin of the Bird Spring Shelf abruptly increased, resulting in the deposition of deep-water facies in areas that had been shelf or littoral environments since the Neoproterozoic. We present new sedimentologic, stratigraphic, conodont biostratigraphic, and subsidence analyses of Darwin Basin strata that document the Pennsylvanian-Permian evolution of the Darwin Basin. We argue that the Darwin Basin evolved in two phases: (1) a Pennsylvanian to mid-Cisuralian foreland basin phase that co-occurred with transpressional deformation of the southwestern margin of Laurentia and was characterized by unconformity development, deposition of coarse-grained carbonate slope-apron facies, syndepositional deformation, and the development of complex basin-floor bathymetry; and (2) a mid-Cisuralian to Guadalupian (middle Permian) phase that preceded and co-occurred with the onset of arc magmatism outboard of the Darwin Basin and is characterized by rapid, uniform subsidence of the entire basin, burial of paleobathymetric highs, and the end of syndepositional deformation. We propose that the transition between these phases may represent a local response to the initiation of the North American Cordilleran subduction zone to the west of the Darwin Basin. We argue that the onset of rapid mid-Cisuralian subsidence documented within the Darwin Basin provides an estimated date of subduction zone initiation that is consistent with previous estimates of the timing of this event, the arc magmatic record, and Cenozoic subduction zone analogs.