INFLUENCE OF PROTEROZOIC AND LARAMIDE STRUCTURES ON THE MIOCENE EXTENSIONAL STRAIN FIELD, SE NEVADA

The Virgin Mountain anticline (VMA) is a NE-trending basement-cored uplift that straddles the Colorado Plateau – Basin and Range margin in southeast Nevada and northwest Arizona, approximately 15 km south of the Utah border. This region has been an important zone of crustal weakness from the Paleoproterozoic through to the present, with the current structure a result of dramatic Miocene extension (~15-10 Ma) superimposed and partitioned within older structures and tectonic boundaries. Two processes; 1) reactivation of basement structures formed during Proterozoic assembly and contraction and Laramide contraction, and 2) strain partitioning within lower Paleozoic rheologic heterogeneities, influenced where the Colorado Plateau margin was originally defined and how extensional deformation was manifest at this margin from ~15 Ma to present. Intense ~ 1700 Ma fabrics and the presence of exotic lithologies (ultramafics, pillow volcanics, chert) suggest that the basement rocks may have been part of a Paleoproterozoic province boundary. The NE-trending, upper-greenschist grade, dextral-transpressional Virgin Mountain shear zone was active between 1600-1500 Ma, based on preliminary U-Pb ages of monazite rims linked to shear sense and metamorphic textures. This suggests that the Proterozoic rocks of the VMA experienced a long-lived contractional history of up to 200 Ma during a time interval previously recognized in the southwest U.S.A. for its tectonic quiescence. N to NW-trending “monoclinal-type” geometries in the Paleozoic and Mesozoic sections, and the position of the VMA between the thin-skinned Sevier thrust belt and thick-skinned monoclines of the Colorado Plateau suggest a Laramide (~65 Ma) component of deformation. Miocene extension occurred primarily through brittle oblique-sense reactivations of strongly fissile mylonitic foliations in the basement, sub-horizontal detachment faulting along the Great Unconformity, basal-glide and westward tectonic thinning in the Bright Angel shale, and steep to moderate normal and antithetic normal faulting in the overlying Paleozoic carbonates. Significant strike-slip deformation post-dated initial uplift and exhumation of the Proterozoic rocks, as determined from stratigraphic, geochronologic, and structural relationships along the west limb of the VMA.