PALEOZOIC EVLOUTION OF THE CORDILLERAN CONVERGENT MARGIN: A VIEW FROM THE EASTERN KLAMATH MOUNTAINS

The Klamath province records Early Paleozoic-Early Cretaceous subduction-related crustal accretion. The Eastern Klamath terrane (EKt) (Trinity, Redding, and Yreka subterranes) provides the earliest record of convergent-margin tectonism and was the nucleus of subsequent crustal accretion. The polygenetic Trinity ophiolite forms the basement to supracrustal rocks of EKt and was stabilized by extension-related Late Silurian-Early Devonian supra-subduction magmatism (Trinity gabbro; Redding basalt). Yreka mélange and turbidite was deposited on Trinity basement in a trench-proximal position and experienced contractional deformation by Early Devonian. Devonian-Carboniferous turbidites of mixed cratonic and volcanic provenance overlie the Trinity ophiolite in the Redding section and record forearc sedimentation contemporaneous with arc volcanism in the Northern Sierra. The Trinity fault separates Eastern Klamath terrane (EKt) in the hangingwall from Central Metamorphic terrane (CMt) rocks in the footwall. CMt, a large sheet of amphibolite generated from an NMORB protolith, exhibits a diachronous cooling history. Portions of the CMt cooled below Ar closure in hornblende during Silurian (~438 Ma) subduction beneath the Trinity ophiolite. Much of the CMt amphibolite, however, cooled during Early Permian (274-294 Ma). The Permian CMt preserves a rutile (P > 1.3 GPa and T ~ 600ºC) → ilmenite → titanite reaction sequence synchronous with dynamothermal metamorphism produced during an eclogite → amphibolite → epidote-amphibolite retrograde decompression. Thus, the Trinity fault which accommodated Silurian subduction was reactivated as a Permian extensional structure. The exhumation event was contemporaneous with sedimentation in Permian McCloud carbonate platform (Redding section). Late Permian supra-subduction volcanism (> 261 Ma) was initiated in the EKt forearc (Bollibokka Grp) following CMt exhumation. These Permian magmatic rocks recorded an enriched mantle source compared to a highly depleted source recorded by Sil-Devonian magmatism. Thus evidence points to a major Permian thermal event consistent with convection of new fertile mantle beneath the EKt forearc and a reconfiguration of the convergent plate margin, possibly triggered by trench roll back or spreading ridge subduction.