NEW INSIGHTS INTO THE TIMING OF BASEMENT-INVOLVED DEFORMATION IN SOUTHWEST MONTANA DURING THE DEVELOPMENT OF THE NORTH AMERICA CORDILLERA

The timing of “Laramide-style”, thick-skinned shortening involving Archean and Proterozoic basement rocks is essential for understanding how deformation of the upper plate evolved during development of the North America Cordillera. In southwest Montana, a variety of techniques, including dating synorogenic strata, reconstructing basin isopachs, application of low-temperature thermochronology to basement-involved uplifts, and charting spatial and temporal sweeps of magmatism, generally converge on ~ 80 Ma for the onset of Laramide deformation. However, several recent studies suggest exhumation of basement-cored uplifts was active as early as ~90-120 Ma. We present new integrative stratigraphic, geochronologic and low-temperature thermochronologic data from the Beartooth Range and Highland Mountains, Montana to constrain the timing of Laramide-style deformation at these localities. In the Beartooth Range, forward and inverse thermal history models of apatite and zircon (U-Th)/He dates (AHe and ZHe) coupled with decompacted sediment accumulation curves record peak burial temperatures during latest Pennsylvanian, followed by a prolonged phase of cooling to near-surface temperatures during Late Pennsylvanian–Paleocene time. The AHe system records rapid cooling at ~70-55 Ma, but is not sensitive to the onset of basement-cored exhumation. By contrast, the ZHe system records a protracted history of cooling initiating prior to the Late Cretaceous. In the Highland Mountains, we report a U-Pb date of ~75 Ma from a pluton cross-cutting a thin-skinned (i.e. “Sevier style”) fault system, indicating slip prior to this time. AHe and ZHe dates from across the Camp Creek Fault and newly mapped Melrose Fault, coupled with inverse thermal history models, indicate initiation of basement-involved deformation pre-80 Ma, with the majority of rapid cooling during Late Cretaceous-Paleogene time. Together, this indicates coeval thin-skinned and thick-skinned style deformation pre-75 to 80 Ma. Our work highlights the integration of multiple, independent techniques to differentiate between the timing of onset of Laramide-style deformation versus periods of rapid exhumation, as well as the interplay between thin- and thick-skinned deformation during the development of the North American Cordillera.