NEVADAN OROGENESIS REDEFINED: MID-CRUSTAL MIGMATIZATION, CONSTRICTIONAL DEFORMATION, AND SOURCE REGION DE-FOOTITATION ABOVE AN EAST-DIPPING SUBDUCTING SLAB, CA/OR

The Nevadan orogeny is one of the oldest recognized and most controversial orogenies in the North American Cordillera. Since first recognized in 1906 and placed in a plate tectonic context in the 1970’s, workers have ascribed the Late Jurassic orogeny to a) collision of an outboard far travelled exotic terrane; b) intra-arc deformation localized in the warm framework rocks of Jurassic magmatism above an east-dipping subduction zone. While previous work suggests a relatively low grade of metamorphism and deformation for the Nevadan orogeny, we present new CA-ID-TIMS U/Pb ages of zircons in leucosomes and structural data that indicate amphibolite to locally granulate facies metamorphism and deformation, including partial melting of amphibolitic basement rocks, during regional doming at ~ 154-157 Ma.

Restoration of the 162-164 Ma Josephine ophiolite directly west of the soon-to-be high-grade rocks indicates that the marginal basin spreading center comprised a suite of E-W trending (current coordinates) ridge segments linked by N-NW trending transform faults reflecting broad transtension affecting the arc and forearc of the Cordillera at this latitude. Voluminous arc magmatism (>25,278 km³) of the Wooley Creek suite began around 161 Ma, east of the infilling Josephine marginal basin and accompanied regional extension and rupturing of the margin at this time. By 157 Ma, initiation of closure of the Josephine basin began the “de-footatating” (a Jason Saleeby colloquialism) of the hot Wooley Creek source region from its mid to upper crustal plutons along the low angle Orleans thrust, resulting in underthrusting of “cold” marginal basin rocks beneath the Wooley Creek suite magmas. This initiated Nevadan orogenesis which continued into the Early Cretaceous.

Doming and migmatization in the central KMP may be explained by intracrustal duplexing and juxtaposition of hot Wooley Creek suite hangingwall rocks with underthrusted, cool marginal basin terranes. The geometry and orientation of intra-arc deformation coupled with detrital zircons studies that demonstrate widespread input from the Wooley Creek arc and proximal Cordilleran hinterlands suggests to us that Nevadan orogenesis is best explained by changes in plate kinematics over an east-dipping subducting plate throughout the Late Jurassic and Early Cretaceous.