EXTENSIONAL SHEAR SENSE INDICATORS ASSOCIATED WITH THE TRINITY FAULT, EASTERN KLAMATH MOUNTAINS PROVINCE, CA

Interpretation of the Central Metamorphic terrane (CMt) as the lower-plate of a Devonian intraoceanic subduction zone is a basic tenet of paleotectonic models for the Klamath province. Specifically, such models hold that the Trinity and Redding subterranes represent mantle and magmatic-arc crust of the overriding lithospheric plate and the east-dipping CMt represents subducted oceanic crust (geochemistry supports NMORB protolith) that was metamorphosed and accreted during thrusting beneath Trinity subterrane along the Trinity fault. In this model the Trinity fault separating the CMt and Trinity subterrane represents a Devonian plate boundary.

Permian 40/39Ar cooling ages (274-292 Ma, hbld, musc) CMt from just below the Trinity coupled with rutile->ilmenite->titanite decompression reaction textures in CMt metabasite lead Barrow and Metcalf (2006) to hypothesize that the Trinity fault represents a Permian extension structure. As a test of this hypothesis we conducted a meso- and microstructure study of oriented samples collected from a traverse perpendicular to the Trinity fault. Samples from metasedimentary rocks and metabasite in the footwall yielded the best interpretable results and show consistent normal shear sense. Metabasite samples exhibit extensional asymmetric folds, poorly developed extensional S-C fabrics, and extensional microfaults. Micaceous quartzite samples exhibit mica fish and sheared plagioclase porphyroclasts (quarter structures) consistent with normal shear sense. Lattice preferred orientation data from quartz (quartzite) and calcite (marble) collected by electron backscatter diffraction returned weak but definite extensional asymmetries. The micaceous quartzite yielded a Permian 40/39Ar muscovite cooling age (277 Ma, Barrow 2007) in contrast to Devonian cooling ages from the upper plate. While the Trinity fault may have been active as a Devonian subduction thrust fault, its most recent motion was extension that exhumed NMORB crust from depth.