KINEMATICS OF LARAMIDE DEFORMATION AND THE INFLUENCE OF BASEMENT FABRICS IN THE BLACK HILLS UPLIFT, SOUTH DAKOTA AND WYOMING

NE-SW to E-W horizontal shortening directions during Laramide deformation are well documented throughout Colorado and Wyoming. In the Black Hills (the NE-most Laramide uplift), fault kinematics and shortening directions have not previously been assessed across the range, and relationships between Precambrian structures and Laramide faults have not been described in detail. Here we evaluate shortening directions of brittle faults in Phanerozoic strata and compare these data to faults and metamorphic fabrics exposed in the Precambrian core of the uplift. Faults in Phanerozoic strata are partitioned into conjugate thrust and strike-slip regimes and lack oblique-slip, with some evidence that conjugate thrusts are more common near map-scale monoclines. Inclined bedding bisects thrust fault conjugates, and bedding is parallel to slickenlines of strike-slip faults, indicating that most faults formed before significant tilting of strata. Steeply-inclined monocline limbs record progressive brittle faulting and late-stage shortening at a high angle to bedding. Compiled data from published maps indicate that 12 of 14 major folds are W-vergent. Faults in Phanerozoic strata measured in this study indicate overall ENE-WSW layer-parallel shortening across the uplift. Approximately 90% of measured faults in Phanerozoic strata are compatible with subhorizontal Laramide shortening. Spatially, shortening directions are perpendicular to the gently curved axis of the uplift, locally varying from NE-SW to ESE-WNW. These results are consistent with findings from brittle fault kinematics in other Laramide uplifts. Most faults in the Precambrian core cut steeply dipping metamorphic fabrics and record shortening and extension axes that are inconsistent with Laramide deformation. Less common fabric-parallel faults show predominantly strike-slip slickenlines, and 70% of measured faults in Precambrian rocks have slickenline rakes <45°. Frictional sliding analysis suggests that <8% of Precambrian planar structures were favorably oriented for reactivation by the subhorizontal Laramide σ₁ direction. We posit that Precambrian structures influenced the trend of σ₁ across the uplift but that reactivation of structures did not play a major role in controlling the geometry or style of deformation in the Black Hills.