EOCENE DEFORMATION IN THE NORTHERN COLORADO RIVER EXTENSIONAL CORRIDOR ASSOCIATED WITH LARAMIDE CRUSTAL DUPLEXING (Invited Presentation)

Saddle Island is a metamorphic core complex located along Lake Mead, NV in the northern Colorado River Extensional Corridor near the boundary between Mesozoic foreland structures of the Sevier thrust front and the postulated early Cenozoic Laramide Kingman uplift. Saddle Island is composed of a metamorphosed, variably mylonitized Precambrian lower plate and a brittlely deformed upper plate that are separated by the Saddle Island Detachment. The kinematic linkages and temporal evolution among the Saddle Island structural components have been debated by previous work that has attributed the lower plate mylonites to either Sevier aged top-NE thrusting (Choukroune and Smith, 1985) or Miocene top-SW extensional faulting as the downdip continuation of the Saddle Island detachment (Duebendorfer et al., 1990). We conducted new field mapping, structural analysis, and zircon U-Pb geochronology in the upper and lower plates to contextualize samples and resolve the structural evolution of Saddle Island. Mylonitic samples in the lower plate were targeted for novel crystallographic vorticity axis (CVA)-enhanced apatite U-Pb petrochronology to date mylonitic shearing. Apatite U-Pb ages from the lower plate mylonites are Eocene and CVA analysis shows that apatite has the same kinematic deformation geometry as the fabric forming quartz and feldspar, supporting that apatite U-Pb petrochronology dates shearing. Eocene mylonite ages are consistent with previous regional low temperature thermochronologic evidence of regional uplift and denudation in the Paleocene-Eocene. We propose a tectonic model involving basement-involved crustal duplexing and ductile deformation that provides a mechanism consistent with geologic and chronologic data for a large Laramide uplift in southern NV and AZ during the late Cretaceous–Paleogene. The region was overprinted by Miocene brittle extensional and transtensional deformation during formation of the northern Colorado Extensional Corridor.