ABSTRACT

We evaluate fault kinematics and the influence of Precambrian basement during Laramide shortening in the Black Hills (Wyoming and South Dakota)—the most distal Laramide uplift with respect to the Late Cretaceous–Paleogene plate boundary. Minor conjugate thrust and strike-slip faults in Paleozoic–Mesozoic strata across the Black Hills accommodated overall ENE–WSW subhorizontal shortening with a mean shortening azimuth of 068–248°, similar to regional patterns of Laramide deformation. Oblique-slip faults are rare, and acute bisectors of conjugate faults subparallel bedding, suggesting that most faults formed in an Andersonian stress regime before significant tilting of strata. Thrust faults are more common near map-scale monoclines, and steeply inclined monocline limbs record multiple phases of minor thrust faulting and late-stage shortening at a high angle to bedding. Approximately 90% of measured faults in Paleozoic–Mesozoic strata are compatible with subhorizontal Laramide shortening. Spatially, shortening directions in these strata smoothly vary from NNE–SSW to ESE–WNW along the eastern margin of the uplift, refracting toward perpendicular to the Precambrian basement core. Most faults in the basement cut steeply dipping metamorphic fabrics and are kinematically incompatible with Laramide deformation. Less common fabric-parallel faults record predominantly strike-slip motion, and 74% of measured faults in Precambrian rocks have slickenline rakes < 45°. Frictional-sliding analysis suggests that < 7% of Precambrian basement fabrics were favorably oriented for reactivation as reverse faults in the Laramide stress field. We propose that the basement influenced the trend of σ1 across the uplift, but that reactivation of Precambrian structures did not play an important role in controlling the geometry/style of deformation in the Black Hills. The systematic variation of Laramide shortening directions suggests that stress-strain trajectories were largely guided by basement anisotropy/fabrics.

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