TIMING AND BASIN RESPONSE TO LATE PALEOZOIC SUBDUCTION INITIATION ALONG THE SOUTHWESTERN MARGIN OF LAURENTIA: INSIGHTS FROM THE DARWIN HILLS SEQUENCE AND OSBORNE CANYON FORMATION OF THE DARWIN BASIN, INYO COUNTY, CALIFORNIA

The late Pennsylvanian to early Permian Darwin Hills sequence and Osborne Canyon Formation of eastern California record an abrupt transition from carbonate shelf and slope depositional environments to the deep marine Darwin Basin. The timing of this transition varies from the late Pennsylvanian at the westernmost exposure of the basin, where it is marked by an angular unconformity, to the early Permian at the eastern margin of the basin. This spatial and temporal distribution of facies indicates episodic, inboard migration of subsidence through the late Pennsylvanian and early Permian. Darwin Basin strata are overlain by another angular unconformity, above which lies shallow water facies of early Triassic age. We suggest that these stratigraphic relationships may reflect the isostatic response of the late Paleozoic southwestern Laurentian margin to the forced subduction initiation that produced intrusions of calcic to calc-alkaline composition by 275 Ma. The basal angular unconformity and Darwin Hills sequence may represent the earliest down-warping of the plate immediately following subduction initiation, whereas the Osborne Canyon Formation may represent the inboard migration of subsidence away from the plate margin. The angular unconformity that caps the stratigraphy of the Darwin Basin and the early Triassic strata which overlie it may represent surface uplift of the area, which is expected to occur in the later stages of subduction initiation. Here we present detailed sedimentology, regional stratigraphic analysis, and petrographic data from the Darwin Hills sequence and Osborne Canyon Formation to constrain the timing of, and upper-plate response to, late Paleozoic subduction initiation in eastern California.