Structural evolution of a Mesozoic backarc fold-and-thrust belt in the U.S. Cordillera: New evidence from northern Nevada

Stratigraphic and structural analysis in Blue Mountain and nearby areas of northern Nevada yields new insight into the evolution of the Luning-Fencemaker fold-and-thrust belt, a Mesozoic orogenic belt that formed during tectonic closure of a Triassic backarc basin. Three stratigraphic units are defined in Blue Mountain and correlated with Upper Triassic (Norian) units in nearby areas. These units have a combined structural thickness of >6 km and consist mostly of shale (now slate and phyllite); quartzose sandstone is much less common, and carbonate is rare. Two phases of deformation affected these strata. D1 deformation is pervasive and was associated with tight to isoclinal folding at various scales, widespread cleavage development under low-grade metamorphic conditions, and reverse faulting. This deformation accommodated a minimum of 55%-75% northwest-southeast shortening, with southeast-directed tectonic transport. D2 deformation is only locally developed and is manifested by relatively open folds, crenulations, and a spaced cleavage. This phase of deformation accommodated a minor amount of northeast-southwest shortening and was not accompanied by metamorphism. Structures and deformation phases in Blue Mountain can be correlated with those found in nearby areas. The D1 phase of deformation is pronounced across the entire northern part of the Luning-Fencemaker fold-and-thrust belt and resulted in structural closure of the backarc basin via development of a ductile fold-and-thrust belt in the Jurassic. D2 is unrelated to development of the fold-and-thrust belt as a structural province and occurred much later, in the middle to Late Cretaceous. The structural evolution of this backarc fold-and-thrust belt appears to be quite simple and did not evidently involve widespread polyphase deformation. Data from Blue Mountain and nearby areas suggest that it is now possible, for the first time, to map out megascopic structures (folds and faults) within the interior of the province.