Abstract

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. Our hypothesis is that this region has been an important zone of weakness from the Paleoproterozoic through to the present, and that Miocene extension and uplift was significantly influenced by inherited structures and tectonic boundaries. Two processes: 1) reactivation of basement structures, and 2) utilization of lower Paleozoic rheologic heterogeneities, influenced were the Colorado Plateau margin was originally defined and how Miocene extension was made manifest at this margin from ~20 to 5 Ma. Intense 1.7 Ga fabrics and the presence of exotic lithologies (ultramafics, pillow volcanics, chert) suggest that this structure may have been part of a Paleoproterozoic province boundary. NE-trending upper greenschist grade dextral-transpressional mylonites are ubiquitous throughout the VMA, suggesting that this area was a high strain zone during Mesoproterozoic (~1.4 Ga) transpressive deformation. 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 was generally exhibited by brittle oblique dip-slip fault reactivations of strongly fissile mylonitic foliations in the basement, steep and shallow normal faulting in the Tapeats sandstone, 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. Highly angular (>60°) relationships are observed in the Lower Paleozoic section on the west limb of the VMA. This structural-rheological partitioning and deflection of strain may give insights into the complex Miocene strain field observed at the Colorado Plateau - Basin and Range margin.