RECONSTRUCTING THE DRIVERS OF EXHUMATION WITHIN THE CANADIAN ROCKY MOUNTAIN FOLD-THRUST BELT USING LOW-TEMPERATURE THERMOCHRONOLOGY

Abstract

The Rocky Mountain fold-thrust belt (RMFTB) in Canada has been a key locality for understanding geometries and kinematics of thin-skinned deformation. Despite its role as a global analogue for fold-thrust belts, questions persist regarding the timing and duration of deformation. In part, this is because the exhumation associated with RMFTB deformation has been subsequently overprinted by Eocene –Miocene unroofing and more recent glaciation. To resolve the timing of shortening in the RMFTB, we applied low-temperature thermochronology to major faults across the Rocky Mountain fold-thrust belt within Jasper National Park and the surrounding areas. We report zircon U-Pb, zircon fission-track (ZFT), and zircon (U-Th)/He (ZHe) data from the hanging wall of nine faults which cut Proterozoic to Cretaceous strata across a ~145 km east-west transect. One sample from the Omineca belt yielded a ZFT age ~100 Ma and a ZHe age ~20 Ma, which we interpret to record mid-Cretaceous ductile deformation and early Miocene extensional exhumation of the Omineca belt. Samples from the hanging walls of thrust faults in the Main and Front Ranges yielded ZFT ages ~230 – 100 Ma which we suggest record Triassic subduction initiation and mid-Cretaceous RMFTB related shortening. The Main and Front Ranges yielded associated ZHe ages of ~ 60 – 30 Ma which we interpret to record the final early Eocene stages of thrusting and/or onset of extensional exhumation. Within the Foothills, thrust faults yielded ZFT ages ~360 – 100 Ma with associated ZHe ages ~110 – 60 Ma. We suggest these represent Cretaceous RMFTB related shortening, as well as the presence of unreset zircons derived from the arc. Our data suggest that Triassic subduction initiation along the margin of the Canadian Cordillera may have played a larger role in the evolution of the RMFTB than previously recognized. In addition, we find that the single-grain ZFT ages from the shallowly buried samples yield distinct age-rare earth element trends, suggesting a potential geochemical influence on ZFT annealing kinetics. Low temperature thermochronology and associated thermal history modelling from hanging-walls of major faults supports continuous rock cooling from the early Triassic to the Paleocene in the southern RMFTB, which we interpret to reflect long-lived shortening. We find evidence for Cenozoic exhumation in the RMFTB, consistent with regional studies that document large-scale erosion that post-dates shortening related deformation. We suggest that RMFTB propagation was directly linked to orogen-scale development and records protracted shortening as well as extensive post-orogenic erosional unroofing.