UNFOLDING THE STORY OF LATE CRETACEOUS TECTONIC INHERITANCE IN THE RUBY-EAST HUMBOLDT-WOOD HILLS METAMORPHIC CORE COMPLEX, NEVADA: PHANTOM THRUST BELTS, THINNING FOLD-NAPPES, AND OTHER ENIGMAS

Relationships in the Ruby Mountains-East Humboldt Range (EHR)-Wood Hills metamorphic core complex and environs offer crucial, even if perplexing clues to the role of tectonic inheritance during the orogenic evolution of the Sevier hinterland. Despite the regional position of the basal Tertiary unconformity at Upper Paleozoic or higher levels, metamorphism in the core complex records pressures up to 10 kb in strata as young as Mississippian, implying deep tectonic burial. How were the rocks buried? Evidence in the core and cover of the complex points to burial by thrust faulting. Fold nappes in the core of the complex, such as the Winchell Lake nappe, fold major thrust faults, and the cover of the core complex preserves vestiges of lower grade and nonmetamorphic rocks correlating with the highest grade rocks of the core. These observations in tandem with metamorphic map relations suggest excisement of a SE-tapering thrust wedge, the “Windermere allochthon,” resulting in exhumation of the core complex. The west-rooted Cenozoic extensional system that exhumed the metamorphic core probably closely followed, largely excised and transported this thrust wedge to the WNW. Restoration of an estimated 50+ km of slip along the Cenozoic extensional system restores the Adobe Range and the leading edge of the Roberts Mountains thrust (RMT) directly above the EHR. But the RMT predates late Mesozoic tectonism, does it not? The “RMT” in the Adobe Range cuts Triassic strata indicating that the Antler belt was at least partly reactivated during Mesozoic time. The bulk of shortening needed to bury the RM-EHR-WH was probably focused along the paleocontinental margin, rooting beneath the RMT and transporting it over tectonically shuffled eastern facies rocks. The fold-nappes of the RM-EHR appear to correspond with the axis of maximum tectonic burial, but surprisingly did not form synchronously with the burial phase. Rather, they accompanied heating, widespread partial melting, decompression, partial exhumation, and profound thinning and lateral flow of the deeper crust. Although relations in the Pequop Mts and Wood Hills record extension at this time, there does not appear to be enough extension to explain the full 5 kb (~18 km) of exhumation. Thus deep-crustal flow must have helped to transport the deepest-seated rocks to mid-crustal levels.