JURASSIC OROGENESIS IN NW NEVADA: TIMING, KINEMATICS AND RELATION (IF ANY?) TO THE SEVIER OROGENY

Recent studies have suggested that the Sevier orogeny initiated with Late Jurassic (ca. 160-150 Ma) crustal shortening in NW Nevada along the Luning-Fencemaker thrust belt (LFTB), and have linked this shortening with initation of flexural basins farther east, including that accomodating the Late Jurassic Morrison Fm. of Utah. Geochronology (U-Pb zircon and 40Ar/39Ar) and structural studies from NW Nevada are in conflict with this model. Our work indicates that shortening in NW Nevada began at ~201 Ma (earliest Jurassic) in the Black Rock arc terrane (BRT). This west-vergent deformation was accompanied by regional metamorphism and magmatism and may have been caused by collision of an outboard arc terrane. The locus of deformation then shifted east, as the BRT arc was thrust over a deep marine back arc basinal terrane, and the basinal terrane was internally shortened by up to 75% within the east-vergent LFTB. Stratigraphic, structural and geochronologic data indicate that basinal terrane shortening propogated eastward over an interval spanning the Early to Middle Jurassic, from ca. 190 Ma to ≥164 Ma. LFTB shortening culminated with emplacement of the basinal terrane onto the continental shelf along the Fencemaker thrust. This occured prior to intrusion of plutons with K-Ar and Rb-Sr ages of ca. 168-153 Ma (new U-Pb dating is in progress). Jurassic orogenesis was followed by a period of uplift and erosion, during which metamorphic and plutonic rocks were exhumed from a depth of 7-14 km prior to deposition of unconformably overlying and undeformed Early Cretaceous (125-123 Ma) alluvial strata. In summary, most or all Jurassic deformation in NW Nevada occurred in the Early to Middle Jurassic, and Jurassic orogenesis was followed by a prolonged period of exhumation and erosion through the Early Cretaceous. These relations are inconsistent with models seeking a kinematic link between the LFTB and the Cretaceous Sevier thrust belt farther east. Flexural basins associated with the LFTB should have formed in the Early to Middle Jurassic, not the Late Jurassic; the Morrison Fm. basin must owe its origin to other causes. Finally, it is probably not valid to include NW Nevada as part of the Sevier orogenic plateau (or Nevadaplano), and shortening within the LFTB should not be included in total shortening estimates for the Sevier belt.