Supra‐Subduction Zone Ophiolite Formed in an Extensional Forearc: Trinity Terrane, Klamath Mountains, California

Abstract

The Paleozoic Trinity terrane is a massif of ultramafic rock and mafic intrusive complexes that forms the basement of eugeoclinal rocks in the eastern Klamath Mountains. The origin of the Trinity terrane has been particularly enigmatic and several mutually exclusive origin models have been proposed: (1) mantle diapir; (2) slice of obducted monogenetic oceanic lithosphere; (3) monogenetic backarc‐basin lithosphere; (4) forearc lithosphere; and (5) volcanic arc basement. We have compiled new and existing field, petrographic, geochronologic, isotopic, and whole‐rock chemical data for the Trinity terrane to evaluate these models and conclude that the Trinity terrane is an supra‐subduction zone ophiolite whose mafic intrusive complexes record subduction‐related magmatism and forearc extension that occurred during the inception of intraoceanic subduction. According to this extensional forearc model, all four terranes in the eastern Klamath Mountains evolved principally during the inception and evolution of a si...