DETERMINING THE 3-D KINEMATIC HISTORY OF THE WYOMING SALIENT OF THE SEVIER FOLD-THRUST BELT: PRELIMINARY RESULTS FROM A PALEOMAGNETIC INVESTIGATION OF THE TRIASSIC ANKAREH FORMATION

Most orogenic belts display curvature at a variety of scales, reflecting potentially complex 3D displacement fields comprised of varying translation, internal strain, and rotation. In an effort to understand origins of orogenic curvature, a collaborative effort is underway to determine the 3D kinematic history of the Wyoming salient of the Sevier fold thrust belt, combining paleomagnetic analysis, fracture and strain studies, and construction of balanced cross sections. Preliminary data are reported here on 60 paleomagnetic sites collected from the Triassic Ankareh Formation from throughout the salient. Two components of magnetization are present: 1) a viscous magnetization acquired in a recent field, and 2) a characteristic remanence magnetization that has dual polarity and a linear decay toward the origin of demagnetization diagrams. Preliminary fold tests indicate pre-folding magnetization for most Ankareh sites. The estimated regional pre-folding inclination of about +15 degrees agrees well with the expected Late Triassic local inclination value for Wyoming, suggesting a primary age of magnetization. In situ declination values of the characteristic remanent magnetization show considerable regional variations (up to 90 degrees), recording significant relative vertical-axis rotations around the salient. Moreover, systematic and differentiable thrust sheet rotation patterns are emerging that are consistent with overall structural trends around the salient. Pronounced counter-clockwise rotations are widespread in northern sections of the salient near the intersection of the fold-thrust front with the Teton/Gros Ventre foreland uplifts. Smaller clockwise rotations occur in south-central sections where the fold-thrust belt bows outward north of the Uinta foreland uplift. Additionally, local rotations are associated with transfer zones within individual thrust sheets. Overall rotation patterns in the Wyoming salient suggest that at least part of the observed curvature is secondary and care should be taken when constructing classic cross-sections perpendicular to structural trends, which may have been progressively rotated during deformation.