DETERMINING THE 3-D KINEMATIC HISTORY OF THE WYOMING SALIENT, SEVIER FOLD-THRUST BELT: PRELIMINARY FRACTURE ANALYSIS AND IMPLICATIONS FOR MULTIPLE GENERATIONS OF ELASTICO-FRICTIONAL DEFORMATION

By incorporating fracture analysis with strain studies and paleomagnetic techniques, we seek to resolve the distribution of displacement pathways within the Wyoming salient of the Sevier fold-thrust belt. Because the displacement field is comprised of translation, rotation, and strain, with manifestation of such components changing throughout the temporal and spatial evolution of orogenic systems, the main objective of our integrated approach is to develop a comprehensive three-dimensional kinematic model. Preliminary results from fracture analyses within the Triassic Ankareh and Jurassic Twin Creek Formations suggest that deformation in the elastico-frictional regime occurred at several stages within the development of the Wyoming salient. Information gathered from the fractures include: surface morphology, architecture, abutment relationships, density, spacing, offset and orientation.

Generally, our observations indicate that the filled/ornamented fractures with the most complex abutment relationships, varied architectures, highest densities, and range of orientations appear to coincide with the oldest phases of deformation. On the other hand, most of the open fractures, often without any perceptible offset, and most typically found to be oriented in an E-W direction, seem to post-date much of the thrust-related deformation. The style of fracturing is somewhat different in the Ankareh and Twin Creek Formations and also seems to vary spatially within the thrust sheets (Absaroka, Prospect and Hogsback), and in comparing the nature of fracturing among thrust sheets. In addition, the characteristics of the fractures are highly affected by local structures. Thus, our analysis is conducted at different scales: from the salient as a whole to individual folds. While fracture density increases towards the fault zones of the major thrust sheets, local populations also increase near smaller wedge faults. In terms of the relationships fractures may have with the other elements we are analyzing (strain and rotations), we predict that regions with the highest rotations will spatially correspond to the most complicated fracture patterns. In addition, in many regions, high strains may be accommodated by cataclastic flow and displacement along minor faults.