INTEGRATING STRUCTURAL, PALEOMAGNETIC, AND GEOCHRONOLOGIC DATASETS TO BETTER UNDERSTAND TECTONIC SYSTEMS: AN EXAMPLE FROM THE NORTH AMERICAN SEVIER FOLD-THRUST BELT

Understanding the evolution of tectonic systems, recorded in their progressive 3-D displacement fields, over a range of spatial and temporal scales is challenging, and has been central to Steven Wojtal’s contributions. Herein we explore progressive deformation within the Wyoming salient of the Sevier fold-thrust belt, integrating regional minor fault and vein, finite strain, paleomagnetic, and geochronologic data sets. The Sevier belt developed during protracted Cretaceous to Paleogene subduction, and within a westward thickening sedimentary wedge deposited along the previously rifted Laurentian margin. Thrusts and folds developed above a regional decollement and display systematic regional curvature. Thrust sheets were emplaced progressively eastward toward the foreland as part of a topographically tapered wedge. Widespread layer parallel shortening (LPS) preceded large-scale faulting in each thrust sheet, with varying development of cleavage, veins, minor faults, and grain-scale fabrics, depending on lithology and structural level. Thrust-parallel shear increased toward the bases of thrust sheets, with a switch to layer-parallel extension at the deepest, weakest levels of internal thrust sheets where T exceeded 300 °C. Shortening directions estimated from cleavage, finite strain, and minor fault data display a radial pattern around the salient, and veins record widespread tangential extension. Paleomagnetic data indicate systematic counterclockwise/ clockwise vertical-axis rotations in northern/ southern parts of the salient, which allow restoration of the early strain field and slightly curved initial structural trends. LPS magnitudes ranged from ~5% in frontal thrust sheets to ~20% in more western sheets, which must be accounted for in palinspastic restorations. Vein, isotopic, and fluid inclusion data record importance of fluid flow and episodically elevated fluid pressures, especially along fault zones. Grain size reduction and alteration, along with hydrolytic weakening at the deepest levels, also contributed to weakening. A model for tectonic evolution of the salient has been developed that incorporates primary and synorogenic sedimentary architecture, weak fault zones, and progressive displacement and stress fields tied to hinterland growth and evolving topography.