EARLY LPS PATTERNS ALONG THE SWEETWATER ARCH- SHIRLEY MOUNTAIN SYSTEM OF THE LARAMIDE FORELAND: REGIONAL REFRACTION OF STRESS FIELD AND DEVELOPMENT OF MULTIPLE STRUCTURAL TRENDS

Analysis of fracture and minor fault data from Triassic red beds and thin limestone intervals, anisotropy of magnetic susceptibility (AMS) measurements, and paleomagnetic data constrain patterns of early layer-parallel shortening (LPS) along a major ESE-trending Laramide arch (Sweetwater Arch, WY). Minor faults with conjugate wedge and strike slip geometries accommodated early LPS and show consistent relationships with respect to bedding around large-scale fold limbs. AMS lineations defined by kinked, rotated, and intersecting phyllosilicate fabrics are consistently perpendicular to LPS directions estimated from minor fault data. Regionally, LPS directions trend WSW-ENE across much of the Laramide foreland, but are deflected more northerly to SSW-NNE trends along the front of the Sweetwater arch (from west of the Ferris Mountains ESE ~85 km to the Seminoe Mountains). The observed deflection reflects a combination of stress refraction and minor localized vertical-axis rotation along the length of the arch. The regional stress refraction pattern along the arch correlates with previously published crustal-scale geophysical magnetic and gravity anomalies. To the east, the arch transitions into a complex region of multiple trending folds and basement faults in the Shirley Mountains. This complex zone is located near the intersection between the ESE-trending geophysical anomaly and a NE-trending anomaly that possibly reflects the Cheyenne Belt boundary. Within the intersection zone, minor fault and AMS data are more complex, and record local constrictional strain with a temporal evolution from an early SSW-NNE directed LPS, to LPS trends that are more orthogonal to locally evolved structures. This change is interpreted to reflect an increasingly heterogeneous stress field as basement faults propagated and interacted during Laramide shortening.