COMBINED ZHE AND AR-AR MDD THERMOCHRONOMETERS FOR UNDERSTANDING THE DEEP-TIME (>1 GA) THERMAL HISTORY OF THE GRAND CANYON

Deep-time zircon (U-Th)/He (ZHe) thermochronology is a promising emerging method for constraining Precambrian basement thermal histories. Because in some areas Grand Canyon Precambrian basement exhumation has known constraints based on preserved Precambrian sediments, this area provides a well-constrained opportunity to explore the potential and limitations of the ZHe deep-time approach and address important questions regarding the spatial variability of erosion beneath multiple unconformity surfaces. We present new ZHe dates (n=26) from eastern and western Grand Canyon, as well as a synthesis of all previously published ZHe data from Grand Canyon Precambrian rocks. New ZHe dates range from ca. 135 to 945Ma across eU values of 29-566ppm. These data are jointly modeled with K-feldspar MDD and examined against geologic constraints in order to reconcile conflicting interpretations for basement thermal histories that are based on ZHe only. The model results broadly suggest thermal history differences between eastern and western Grand Canyon, but more specifically across major shear zones. Discrete thermal histories are determined for segmented basement blocks defined by major shear zones and indicate the importance of a more granular approach for combining samples for thermal history analysis. We show that models need to consider the presence or absence of Supergroup sediments, as well as the spatial position with respect to crustal boundaries. Discrete models are bounded by the Crystal shear zone (River Mile 98), the presence of the Grand Canyon Supergroup (River Mile 77-138), and the Gneiss Canyon shear zone (River Mile 235) and suggest that discrete blocks experienced different thermal histories related to pulses of burial and exhumation. Jointly modeled K-feldspar MDD data with ZHe data, coupled with a more discrete sampling protocol, appears to provide more robust thermal histories and improves the resolution of the thermal evolution of the Grand Canyon basement and importance of shear zone controls.