STRUCTURAL FABRIC OF THE PALISADES MONOCLINE: A STUDY OF POSITIVE INVERSION

This study investigates the structural fabric of the Precambrian Dox Formation across the underlying fault of the Palisades Monocline, Grand Canyon, AZ. Previous studies of the Grand Canyon region, and specifically the Palisades Monocline, document stratigraphic evidence for fault inversion in the Precambrian Grand Canyon Supergroup. These studies indicate normal faulting occurred during and after Precambrian deposition, followed by Paleozoic deposition and subsequent Laramide reverse-reactivation. Orientations of faults, fractures, joints, veins and slip lineations in the Dox Formation across the fault indicate multiple deformation phases, however, comparison of these measurements with previous study by Reches (1978) of the overlying Paleozoic units allows Precambrian structural fabrics to be distinguished from Laramide deformation. Preliminary interpretation indicates a strong Precambrian extensional fabric, including vertical joint and vein sets striking N55W, and conjugate normal faults striking N58W and dipping 60 degrees NE and SW respectively. The structural fabric suggests a subhorizontal, least compressive stress trending N35E during Precambrian deformation. Previous study by Reches (1978) reports a similar axis for the greatest compressive stress during Laramide deformation, which supports coaxial, positive stress inversion. Positive inversion or fault reactivation of a pre-existing structural weakness is commonly proposed to explain the high angle reverse faults common in Laramide structures, however, poor structural control, incomplete stratigraphic exposure, and structural overprinting obscure evidence of inversion. This natural example of coaxial inversion provides for more controlled comparison with physical models that suggest preservation of structural and microfracture fabrics through inversion processes.