CHARACTERIZING PRE-MIOCENE LOWER PLATE STRUCTURES OF THE NORTHERN SNAKE RANGE METAMORPHIC CORE COMPLEX, EASTERN NEVADA: INSIGHTS INTO THE ROLE OF TECTONIC HEREDITY IN METAMORPHIC CORE COMPLEX EVOLUTION

Published models of the northern Snake Range metamorphic core complex commonly envision a single, large magnitude, progressive extensional event spanning the Oligocene-Miocene. However, emerging geochronologic and thermobarametric data from the lower plate of this and many other Cordilleran core complexes indicate an interval of Late Cretaceous to Paleocene burial, extension, and partial exhumation that distinctly pre-dates Miocene detachment faulting. This study combines detailed mapping of a 17 km² area of the northwestern northern Snake Range with U/Pb geochronology and major element geochemistry to characterize pre-Miocene burial structures and exhumational(?) fabrics. A km-scale, N-S trending, non-cylindrical, east-vergent, nearly recumbent syncline that deforms metamorphosed middle-upper Cambrian stratigraphy of the miogeoclinal succession accommodated ~10 km of burial prior to, or coeval with, Late Cretaceous amphibolite grade metamorphism (80-90 Ma). The fold’s axial planar fabric in the western part of the range is cut by a younger, gently east dipping (~18˚) foliation and a strong mylonitic fabric that structurally underlies the Northern Snake Range Décollement. The east dipping fabric is, in turn, cut by a suite of undeformed early Tertiary rhyolitic to dioritic dikes (≥38 Ma). We believe these structures represent an interval of Late Cretaceous burial, followed by partial exhumation prior to Oligocene-Miocene unroofing via brittle slip along the Northern Snake Range Décollement. These results support a polyphase extensional and contractional history of the Snake Range metamorphic core complex and highlight the importance of tectonic heredity in metamorphic core complex evolution.