Metamorphic evolution of the luning-fencemaker fold-Thrust Belt, Nevada: illite crystallinity, metamorphic petrology, and 40Ar/39Ar geochronology

The Luning-Fencemaker fold-thrust belt (LFTB) of Nevada accommodated substantial Mesozoic shortening in the back-arc region of the U. S. Cordillera. Metamorphic grade in this belt has not been studied, and timing of deformation is not well defined. In this study, we present new data on metamorphic grade and age of deformation in the northern LFTB. We focus on the D-1 phase of deformation, which accommodated most shortening in the belt. Metamorphic grade was established by petrographic study and microtextural analysis of deformed metasedimentary rocks, bulk x-ray diffraction and illite crystallinity analyses of slates and phyllites, and microprobe analyses of metamorphic mica. These techniques indicate that metamorphism during D-1 deformation occurred under anchizone- to epizone-grade conditions at temperatures probably near 300degreesC. Timing of D-1 deformation was investigated by 40Ar/39Ar whole-rock dating of slates and phyllites. Age spectra indicate that D-1 deformation occurred before 142-144 Ma (Late Jurassic), and probably significantly before this time, potentially in the Early Jurassic. The timing of D-1 deformation defined here provides a basis for linking back-arc shortening with Jurassic orogenesis in arc provinces to the west and sheds light on space-time patterns of Mesozoic orogenesis within the Cordillera. Metamorphic data indicate that exposed rocks of the northern LFTB were buried under similar to7-14 km of overburden during D-1 deformation. Burial was caused by structural thickening during fold-thrust belt development. Erosion and exhumation of the LFTB may be linked to deposition of Middle to Upper Jurassic sedimentary strata in the foreland basin of the Utah-Idaho trough.