LATE PALEOZOIC EXHUMATION OF SUBDUCTED OCEANIC CRUST BY EXTENSIONAL REACTIVATION OF A MID-PALEOZOIC SUBDUCTION MARGIN: CENTRAL METAMORPHIC TERRANE, KLAMATH MOUNTAINS, CALIFORNIA

The Central Metamorphic terrane (CMt) is a fault-bounded package of metabasites that underlies the Eastern Klamath terrane along the Trinity fault. Interpretation of the CMt as the lower oceanic plate of a Silurian-Devonian intraoceanic subduction zone is a basic tenet of paleotectonic models for the Eastern Klamath province. Trace element data confirm the NMORB-like composition of CMt metabasite protoliths. Newly discovered relict textures and amphibole compositions, however, suggest a more recent decompression exhumation history for CMt metabasites.

A reaction sequence consisting of rutile cores within ilmenite crystals mantled by titanite is observed in CMt amphibolite samples. Zr-in-rutile thermometry (Watson et al., 2006) combined with experimental data for rutile stability in metabasites (Ernst and Lui, 1998) suggests that relict rutile crystals preserve early P-T conditions of ~ 600^oC and >1.3GPa consistent with eclogite facies. Transition from eclogite facies is further supported by ilmenite-plagioclase symplectites suggesting replacement of garnet (Bhowmik and Roy, 2003) during decompression. Amphibole compositions vary significantly and reflect lower grade (low Na, Al, Ti actinolite) overprint of earlier amphibolite facies compositions (high Na, Al, Ti magnesio-hornblende). Application of the Al-Ti hornblende thermobarometer (Ernst and Lui, 1998) yields upper P-T estimates of ~600^oC and >1.5GPa, consistent with hornblende eclogite facies. Predominate mineral assemblages and metamorphic fabrics indicate dynamic recrystallization of metabasites during declining P-T conditions through amphibolite > epidote amphibolite facies (ilmenite > titanite transition). Exhumation via extensional on the Trinity fault is suggested by the coplanar relationship between metabasite decompression-related deformation fabrics and the Trinity fault.

We suggest that the Mid-Paleozoic (438-400 Ma, ⁴⁰Ar/³⁹Ar, U-Pb zircon) subduction (contractional) margin was subsequently reactivated as an extensional structure that exhumed previously subducted oceanic crust from depths of >40 km. New ⁴⁰Ar/³⁹Ar thermochronology supports an Early Permian (~274-294 Ma) exhumation age. The CMt appears to record an alternating history of contraction and extension at a long-lived subduction margin.