PERMIAN EXHUMATION OF A LARGE COHERENT SHEET OF METABASITE AT A PACIFIC-TYPE CONVERGENT MARGIN: CENTRAL METAMORPHIC TERRANE, KLAMATH MOUNTAINS

The Central Metamorphic terrane consists of the Salmon schist, a large sheet of amphibolite generated from an NMORB protolith (oceanic crust) and Grouse Ridge Formation, an amphibolite-grade semipelitic unit. The Trinity thrust fault separates Eastern Klamath terrane in the hangingwall from CMt rocks in the footwall. Long-standing paleotectonic models interpret the CMt as oceanic crust metamorphosed during mid-Paleozoic subduction-related underthrusting and accretion beneath the eastern Klamath terrane. Recent work, however, has revealed a diachronous cooling history for the CMt. Portions of CMt cooled below 40Ar/39Ar hornblende closure during the Silurian (~438 Ma), however much of the CMt amphibolite cooled below 40Ar/39Ar closure during an Early Permian (274-294Ma) event. The Permian CMt preserves a rutile (P > 1.3 GPa and T ~ 600¨¬C) °ć ilmenite °ć titanite reaction sequence synchronous with dynamothermal metamorphism and records an eclogite °ć amphibolite °ć epidote-amphibolite retrograde decompression P-T-time path. The coplanar relationship between amphibolite deformation fabrics and the Trinity fault suggests decompression during exhumation via extension on the Trinity fault. Thus, the history of a major portion of the CMt is one of syn-extensional retrograde metamorphism rather than syn-thrusting prograde metamorphism as previously thought. Thus, the Trinity fault, which appears to have accommodated Silurian subduction, appears to have been reactivated as a Permian extensional structure. The Permian CMt with relict high-P rutile represents a large coherent sheet of metabasite (dimensions ~1-5 km thick, up to 15 km wide and ~35 km long). This is significant because the exhumation of such a large coherent sheet of metabasite from eclogite facies conditions has not been previously reported and presents a conundrum for buoyancy driven exhumation models. Thus, these high-P rocks might signify a yet unrecognized crustal architecture in Pacific-type convergent margins.