Superposed compressional and extensional strain in Lower Paleozic rocks of the northwestern Grant Range, Nevada

Graduation date: 1989The Grant Range, in east-central Nevada, is a\ud north-east trending range bounded on the west by a\ud west-dipping normal fault system. Rocks within the range\ud record a complex polyphase Mesozoic ductile\ud compressional and Cenozoic brittle extensional\ud deformational history. The northwestern Grant Range\ud exposes deformed, regionally metamorphosed and\ud unmetamorphosed, Cambrian to Mississippian carbonate and\ud clastic strata, and minor Tertiary granitic and\ud andesitic dikes. Cambrian and Ordovician rocks are\ud ductilely strained and metamorphosed. Metamorphic grade\ud decreases stratigraphically upwards, generally\ud commensurate with the degree of ductile strain.\ud Two Mesozoic compressional events are recorded in\ud the rocks of the northwestern Grant Range. The first\ud event produced mesoscopic, east-vergent folds with\ud spaced axial-planar cleavage. These folds were\ud overprinted by small-scale, west-vergent thrust faults\ud and folds of the second event. Regional metamorphism\ud began during the first folding event, but outlasted\ud deformation. Static metamorphism was followed by\ud west-vergent deformation, which marked the end of\ud metamorphism. The compressional structures may have\ud been part of an east-vergent anticline or the hanging\ud wall of an east-vergent thrust fault.\ud Ductile Mesozoic compressional structures and\ud fabrics are cut by an arched, imbricate stack of\ud Cenozoic low-angle normal faults of a more brittle\ud character. The low-angle normal faults omit\ud stratigraphic section, and each successively\ud structurally higher fault is generally younger than the\ud one below it. Most faults appear to be\ud west-directed. The age of the low-angle normal fault\ud system is constrained only as late Oligocene to\ud Pleistocene, but could be largely Miocene to\ud Pleistocene. Some granitic and andesitic dikes cross-cut\ud or are cut by low-angle normal faults, indicating that\ud igneous activity is at least in part synchronous with\ud extension.\ud The geometry of the low-angle normal faults suggest\ud that these faults could be rotated, extinct fault\ud segments formed as a result of arching of the upper\ud reaches of the high- to moderate-angle west-dipping\ud normal fault system responsible for the uplift of the\ud Grant Range. This would suggest that the low-angle\ud normal faults are products of progressive Basin and\ud Range extension and do not represent a distinct\ud extensional event