THE MID-PALEOZOIC TRINITY THRUST FAULT OF THE EASTERN KLAMATH MOUNTAINS, NORTHERN CALIFORNIA: REACTIVATED IN THE PERMIAN AS AN EXTENSIONAL STRUCTURE

The Klamath province (CA-OR) consists of lithotectonic terranes that record mid-Paleozoic to Jurassic age subduction-related crustal growth. These belts are separated by east-dipping, accurate, low-angle faults that are thought to largely record episodes of contractional deformation. Many modern subduction systems however provide evidence for hinge rollback and upper plate extension (e.g. Hamilton, 1995) and some models of long-lived intra-oceanic subduction margins suggest that alternating periods of extension and contraction play an important roll in crustal growth (e.g. Collins, 2002). Extensional episodes in the Klamath province are largely recorded by two supra-subduction zone ophiolites (Silurian-Ordovician Trinity ophiolite; Jurassic Josephine ophiolite). Recent work, however suggests that the mid-Paleozoic Trinity thrust fault may have been reactivated in Permian time as a low-angle extensional structure.

The Trinity fault separates the Trinity ophiolite from the metabasites and metasediments of the underlying Central Metamorphic terrane (CMT). Long-standing models view the CMT as oceanic lithosphere metamorphosed and accreted to the base of a hot Trinity ophiolite during Devonian thrusting (~380 Ma, Rb-Sr isochron age, Lanphere et al., 1968). Although recent trace element data confirms the MORB nature of the CMT, recent thermochronology and P-T estimates are inconsistent with a thrust separation on the Trinity fault. Petrologic data suggest that at ~400 Ma Trinity gabbro was emplaced into relative cool peridotite (< ~500^oC) at depths of < ~3.3 kbars and perhaps < 2 kbars (Schwindinger and Anderson, 1987). In contrast, petrologic data for the CMT indicates metamorphism at ~ 650^o +/-50^oC and 4-8 kbars adjacent to the Trinity fault (Peacock and Norris, 1989; this study). Hornblende from two samples of CMT metabasites near the Trinity fault, both yielded Early Permian ⁴⁰Ar/³⁹Ar ages of 274 +/-2 Ma (Barrow and Metcalf, 2003). Hornblende from a Trinity gabbro sample collected adjacent to the Trinity fault yielded a Late Silurian ⁴⁰Ar/³⁹Ar age of 422 +/-5 Ma, broadly concordant with reported gabbro crystallization ages (U-Pb zircon 431-398 Ma, Wallin and Metcalf, 1998). These findings suggest that the CMT was later exhumed from depth by Early Permian extension along the Trinity fault.