EARLY JURASSIC DEFORMATION IN THE PINE FOREST RANGE, NORTHWEST NEVADA, AND IMPLICATIONS FOR CORDILLERAN TECTONICS

Triassic and Jurassic rocks in the Black Rock Desert of northwest Neva da comprise part of the Mesozoic volcanic are successions found throug hout the westernmost U.S. Cordillera. Detailed structural studies in t he fine Forest Range, located in the western Black Rock: Desert, provi de new insight into the Jurassic structural history of this part of th e Mesozoic are system. Regional Jurassic deformation and metamorphism in the Fine Forest Range affected a thick (similar to 10.5 km) section of lower(?) Paleozoic through Triassic strata that were tilted after deformation and are now exposed in approximately cross-sectional view, At high stratigraphic levels, deformation produced a slaty cleavage a nd relatively open folds, both of which are concentrated in rheologica lly incompetent rock types, and was accompanied by metamorphism at low grade. At deeper stratigraphic levels, the regional foliation intensi fies and becomes more pervasive, folds tighten, and metamorphic grade increases through greenschist into amphibolite grade. At the deepest s tratigraphic levels, the rocks are amphibolite grade L-S tectonites wi th a pronounced mylonitic fabric. The similar to 2 km-thick-zone of my lonites is interpreted as a ductile shear zone that formed when the st ratigraphic section was still upright, whereas the less deformed, youn ger rocks are interpreted to occupy the upper plate of the shear zone and to record a strain gradient away from the shear zone. A variety of structural and regional relations indicate that the shear zone is a d uctile thrust fault along which younger rocks moved to the northwest ( in present-day coordinates) with respect to older rocks. The timing of regional Jurassic deformation and metamorphism is constrained by rela tions in and around two plutons dated by the U-Pb zircon method. The 2 01+/-1 Ma Big Creek pluton was intruded as a sill-like body along the upper levels of the shear zone and is associated with a variety of fea tures indicating syntectonic intrusion, including internal textural fe atures in plutonic rocks and metamorphic changes in wall rocks with pr oximity to the pluton. The 185+/-1 Ma Theodore pluton which intrudes a cross part of the shear zone has a thermal aureole which statically ov erprints the shear zone fabric. Deformation was thus ongoing at 201 Ma and over by 185 Ma. The Early Jurassic age of regional shortening def ormation in the Pine Forest Range contrasts with the Middle to Late Ju rassic (similar to 170 similar to 150 Ma) age of regional shortening d ocumented in many other parts of the Mesozoic are system, including th e Klamath Mountains, Sierra Nevada, and Mojave Desert regions of Calif ornia. It also contrasts with the record of Early Jurassic extensional tectonism documented in the Sierra Nevada, Mojave Desert, and western Arizona segments of the Mesozoic are system. These conflicting relati ons indicate a complex tectonic evolution for the Jurassic U.S. Cordil lera plate margin and lend support to the idea that disparate parts of this plate margin were juxtaposed along younger strike-slip faults.