Timing of Jurassic orogeny in the Klamath Mountains province: A view from the middle crust

Jurassic orogenesis has had profound effects on the crustal evolution of the North American Cordillera from Alaska through Baja California, Mexico. While often contractional in nature, the orogenic evolution included components of strike-slip, convergent, and extensional tectonic regimes, some of which occurred over dramatically short and punctuated intervals. Within the central part of the cordillera, two Jurassic orogenic events, the Siskiyou and Nevadan orogenies, have been identified in the Klamath Mountains province of Oregon and California. These two orogenic events are thought to have resulted from amalgamation (contractional ± transcurrent motion) of distinct but spatially related terranes along major shear zones. Regional high-grade metamorphism has been variously assigned to one or the other event, and magmatism has been considered to have occurred before, between, and after these events. Precise timing of the tectonic and thermal history of the province has been uncertain due to a paucity of precise age data. New U-Pb (zircon) ages from pinning plutons indicate that the Middle Jurassic terrane amalgamation (Siskiyou orogeny) ended by ca. 172 Ma. However, regional metamorphism originally thought to be related to Siskiyou orogenesis is, instead, Late Jurassic in age, based on new single-crystal U-Pb (zircon) dates from leucosomes and Sm-Nd dating of prograde garnet, plus U-Pb (rutile) and published 40Ar/39Ar (hornblende) cooling ages. Near-peak metamorphic conditions were reached by ca. 157 Ma. A second terrane amalgamation event (Nevadan orogeny) was coeval with this peak metamorphism, during which low-temperature metavolcanic rocks were thrust beneath high-temperature mélange of the Rattlesnake Creek terrane, effectively quenching the high-grade metamorphism. Moreover, rather than being punctuated by tectonic activity, plutonism continued with little or no break from pre-Siskiyou time to post-Nevadan time, reaching a volumetric peak in the Late Jurassic, ca. 158 Ma. The close timing of peak plutonism with peak metamorphic conditions suggests that heat for high-grade metamorphism was related to transcrustal magma transport and differentiation, driven and enhanced by mantle upwelling associated with Middle Jurassic slab breakoff and a westward shift in subduction. Our new geochronologic data, combined with our interpretation of post-Siskiyou slab breakoff, indicate that the Siskiyou and Nevadan orogenies, although closely related in time, were distinct in their thermal, and perhaps, tectonic evolution.