Mid‐Paleozoic olistoliths in eastern Hayfork Terrane Mélange, Klamath Mountains: Implications for Late Paleozoic‐Early Mesozoic Cordilleran forearc development

The western Paleozoic and Triassic belt in the southern Klamath Mountains contains several convergent‐margin tectonic assemblages that show great variation in lithology and structural patterns. Many preserve evidence of intermediate‐ to high‐pressure facies‐series metamorphism consistent with a subduction and/or accretionary origin prior to regional mid‐Jurassic magmatism and shortening. Retention of a pre‐Middle Jurassic record is revealed by new ⁴⁰Ar/³⁹Ar data from the eastern Hayfork terrane chert‐argillite mélange, which document unexpectedly old cooling ages of 346–360 Ma for micas and amphiboles in a variety of blocks within mélange matrix. These Early Mississippian cooling ages and petrologic criteria indicate that the blocks probably originated as olistoliths derived from an exhumed Central Metamorphic belt in the late Paleozoic. Minor low‐temperature discordance of the block ages suggests low‐grade reheating between about 240 and 280 Ma, coincident with an age on eastern Hayfork matrix of 263 Ma that reflects Early Permian or younger mélange formation. The mineral spectra indicate regional temperatures have not significantly exceeded ∼300°C since the Mississippian. This Early Permian accretionary history may only be preserved in the southern Klamath Mountains where mid‐Jurassic granitoids are sparse. We suggest that a late Paleozoic eastern Hayfork accretionary complex was constructed proximally to denuded Eastern Klamath forearc basement in an environment analogous to the modern Sumatran convergent margin.