STRAIN PATTERNS FROM THE WYOMING SALIENT OF THE SEVIER FOLD-THRUST BELT: EVOLUTION FROM EARLY LAYER PARALLEL SHORTENING TO FINAL OROGENIC CURVATURE

To better understand the nature of internal deformation and vertical-axis rotations in the Wyoming salient, samples were collected from redbeds of the Triassic Ankareh Formation and limestones of the Jurassic Twin Creek Formation. Redbeds contain intervals with reduction spots that display a range of ellipsoid shapes, which are geometrically correlated with anisotropy of magnetic susceptibility (AMS) fabrics. Limestones contain variably deformed fossils and display tectonic stylolites and cleavage that are generally ~perpendicular to bedding and formed during early layer parallel shortening (LPS). In the least deformed parts of more external thrust sheets, redbeds have oblate reduction spot (and AMS) fabrics, with Z~0.9 perpendicular to bedding and Y~X~1.0, reflecting primary compaction. Nearby limestones display rare tectonic stylolites that produced <5% LPS. In moderately deformed parts of thrust sheets, redbeds have triaxial reduction spot (and AMS) fabrics, with Z~0.8 parallel to bedding and X~1.1 parallel to local fold axis trends. This represents a composite fabric related to LPS and minor tangential extension parallel to fold axis trends, superimposed on the primary fabric. Nearby limestones display overall moderate intensity cleavage, and extension veins perpendicular to local fold axes. Deformed fossils record ~ 5 to 20% LPS. In strongly deformed parts of more internal thrust sheets, redbeds have reduction spot fabrics with Z<0.8 and X>1.1 parallel to local fold axis trends. Limestones display strong cleavage and distinctly deformed fossils, recording >20% LPS. Shortening (Z) directions for most sites define a radial pattern, remaining subperpendicular to regional fold axis trends around the salient. Furthermore, paleomagnetic data record systematic vertical-axis rotations within thrust sheets subsequent to early LPS. A strike test reveals correlation between Z directions and paleomagnetic vector trends, with observed final curvature of structural trends reflecting about 1/3 primary curvature and 2/3 tectonic curvature. Widespread, minor tangential extension parallel to fold axis trends appears related to development of tectonic curvature. Additional complications occur near oblique ramps and cross folds.