Abstract In sedimetary basins not currently undergoing primary compaction (e.g., Rocky Mountain Basins), p-wave velocities noticeably vary with azimuth, yet the mechanism(s) controlling the anisotropy remain uncertain. Possible geologic causes for azimuthal anisotropy include but are not limited to sedimentary fabrics, steep bedding, changes in local in-situ or residual stress, and open or mineralized fractures. To test these hypotheses, P-wave velocity azimuths (Vfast) from a proprietary seismic survey of a NNW-trending Laramide Anticline on Casper Arch in central Wyoming were compared to image log data from the seismic coverage area and fracture orientations from nearby analog structures.

The Colorado Proterozoic province is separated from Archean rocks of the Wyoming province by a major structural boundary, the Cheyenne belt. Proterozoic rocks south of the Cheyenne belt are exposed in the Sierra Madre, Medicine Bow Mountains, and Laramie Range of southern Wyoming. They consist of metavolcanic units, metagraywacke, pelitic schist and gneiss, amphibolite, and felsic to mafic intrusive rocks that locally resemble rocks of central Colorado. North of the Cheyenne belt, Archean granite and gneiss of the Wyoming craton are overlain by a Late Archean and Early Proterozoic supracrustal sequence that contains quartzite, metadolomite, phyllite, and subordinate metavolcanic rocks. The eugeoclinal character of the metamorphic rocks south of the Cheyenne belt contrasts sharply with the dominantly siliciclastic supracrustal rocks north of the Cheyenne belt. Although specific sequences south of the belt have not yet been correlated between the Sierra Madre, Medicine Bow Mountains, and Laramie Range, similarities in age, lithology, and major element chemistry suggest that they are part of a single geologic terrane. Macroscopic structure and microscopic kinematic indicators within the Cheyenne belt suggest that accretion of the Proterozoic rocks of northern Colorado to the Archean Wyoming craton was accomplished primarily by large-scale thrusting. Following accretion of individual thrust blocks, the boundary zone was steepened by folding and reactivated locally during a period of strike-slip movement. Presence of similar lithologies and shear zones south of the Cheyenne belt suggests that the southern margin of the Wyoming craton may have been a long-lived zone of crustal accretion.