IS IT POSSIBLE TO CREAT A SINGLE KINEMATIC MODEL FOR THE EVOLUTION OF LARAMIDE ARCHES? A LOOK FROM THE SWEETWATER ARCH, WYOMING

The Sweetwater arch of south-central Wyoming defines a series of WNW-trending basement cored uplifts whose axes are transverse to the overall NW trend of most Laramide foreland structures in the North American Cordillera. Structural, anisotropy of magnetic susceptibility (AMS), and paleomagnetic data from over 100 sites in Triassic Chugwater Group red beds and a thin limestone interval constrain patterns of early layer-parallel shortening (LPS). Early LPS in limestone was accommodated by minor faults with conjugate wedge and strike-slip geometries that show consistent relationships with respect to bedding around large-scale folds. AMS records subtle grain fabrics related to sedimentary compaction and minor LPS in red beds. Regionally, LPS directions trend WSW-ENE across much of the Laramide foreland, but are deflected more SSW-NNE along the central sector of the Sweetwater arch. This observed deflection reflects a combination of stress refraction and localized vertical-axis rotation and wrench shear along the arch. The Shirley Mountains and Freezeout Hills areas near the eastern terminus of the arch define a complex structural region with multiple trending folds and variably striking basement faults, which coincides with intersection of the Casper and Laramie arches and with variably trending geophysical anomalies that likely reflect Precambrian crustal structures. Within this zone, LPS data are more complex and record local constrictional strain with a temporal evolution from early WSW-ENE directed LPS, to shortening trends that are more orthogonal to locally evolving structures. This change is interpreted to reflect an increasingly heterogeneous stress field as reactivated basement faults propagated and interacted during Laramide shortening. When combined with data from more typical NW-trending arches, it is clear that complex Laramide structures developed via a variety of kinematic histories, partly related to local basement heterogeneities.