RECONSTRUCTION OF THE SEVIER THRUST WEDGE AND WEDGE TOP: IMPLICATIONS FOR THE MAGNITUDE OF EXTENSION ACROSS THE SEVIER VALLEY AND SEVIER DESERT DETACHMENTS

Reconstructions of Mesozoic thrust sheets provide estimates of large magnitude late Paleogene-Neogene extension across central Utah. Equally important is the constraint that exposures of the Paleocene-Eocene Flagstaff Formation provide for reconstructing the top of the thrust wedge before extension. This paper describes a structural transect near the latitude of Fillmore, Utah for which the deep structure is constrained by seven of the deepest industry boreholes in the region, each within 7.5 km of the profile. When correlated to seismic lines Pan Canadian 2-3 across the southern Sevier Desert, GSI-1 across the Pavant Range, and COCORP 3 across the Valley Mountains and Wasatch Plateau, these data delineate the depth to basement, important hanging wall and footwall geometries for the principal Sevier faults, and the distribution of Cenozoic basin fill sequences. The Flagstaff Formation forms the base of the Cenozoic in most places, with exposures in the Cricket Mountains, Pavant Range, Valley Mountains, and Wasatch Plateau. The sequential reconstruction indicates that the Sevier wedge was underlain by a planar basement surface dipping ~3 degrees westward, with retrodeformed Paleocene-Eocene strata defining a ~2 degree east slope for the wedge top. Restoration of the Flagstaff Formation in the hanging walls of principle extensional faults indicates that extension was partitioned between two detachment systems: the Sevier Valley (SVD) and the Sevier Desert detachments (SDD). The SVD underlies normal faults and basins of the southern Sevier Valley, and lies beneath a zone of active seismicity in the Valley Mountains where it is rooted in a basement fault zone penetrated by the Williams Monroe and Placid WXC wells. The SVA accommodated ~10 km of extension based on deep well and seismic constraints for basement offset. Wedge-top restoration of the Flagstaff Formation of the Cricket Mountains corroborates earlier estimates for 45 to 50 km of post-thrust extension across the better studied SDD. These estimates are bolstered by results from the 1996 Chevron Black Rock well through the Cricket Mountains that penetrated a normal fault placing Jurassic over Cambrian strata near total depth. Palinspastic considerations limit the footwall cutoffs for these Jurassic strata to a breakaway zone in the western Pavant Range.