Most mountain belts display varying degrees of curvature over a range of scales, recording complex, three-dimensional deformation histories. However, despite their ubiquitous occurrence, few examples exist where deformation histories and processes responsible for orogenic curvature have been adequately quantified from geologic evidence. These histories are embedded in regional structural trends, paleomagnetic directions, mesoscopic structures, and grain-scale fabrics of deformed rocks. The Utah-Idaho-Wyoming salient of the Sevier orogenic belt is an ideal location to study processes that produce curvature over a range of scales. Regional trends of major thrust faults and folds curve through 90 degrees from the north to south end of the salient, with additional local curvature near cross-strike transfer zones. Rocks in the salient also carry multiple paleomagnetic components and display systematic suites of mesoscopic and microscopic structures (including cleavage, minor folds, minor fault networks, and deformed fossils), providing a record of rotations and internal strain within individual thrust sheets.
Initial studies in the Wyoming salient reveal both primary pre-thrusting and secondary syn-folding paleomagnetic components that record complex rotation histories related to thrust curvature. Initial studies also reveal broadly consistent regional strain patterns, with widespread layer-parallel shortening at high angles to thrust trends, minor tangential extension parallel to thrust trends, and minor wrench shear. Calcite-twin strains, which accumulated during early layer-parallel shortening prior to large-scale thrusting, provide another reference frame that records locally significant rotations. Ongoing work is focused on combined paleomagnetic and structural studies of three 'swaths' located in the southern, central, and northern frontal parts of the salient, and on two stratigraphic levels (Jurassic Twin Creek Limestone and redbeds of the Triassic Ankareh Formation) that are well exposed, carry multiple magnetizations, and contain multiple strain markers.
Analysis of slip lineation data from major thrust faults and construction of two-dimensional cross sections indicate that thrust transport directions change overall from ENE in the northern part to ESE in the southern part of the salient, and fault slip magnitude decreases toward the north and south ends of the salient. Three-dimensional cross sections, constrained by abundant seismic data, and incorporating new strain and rotation data from this study, are being constructed to develop a robust kinematic model of the salient. The model will be used to identify controlling mechanisms responsible for mountain belt curvature, including variations in stratigraphic thicknesses, strength changes along faults, motion of indentors, and foreland buttress effects.
This investigation addresses several fundamental questions concerning the evolution of mountain belts. (1) What are the relations between rotations, strain, changes in thrust fault slip, and salient curvature? (2) What are the consequences of incorporating strain and rotation data into three-dimensional models of orogenic belts? (3) What factors are most important in developing curved mountain belts and how can these factors be best identified? Ultimately, findings from the Wyoming salient will be compared with similar ongoing studies of other curved mountain belts, providing an array of viable models for the development of curved orogens. Lastly, inclusion of strain and rotation data into three-dimensional restorable deformation models should improve our general understanding of hydrocarbon accumulations in fold-thrust belts.
