Genesis and evolution of a Permian‐Jurassic magmatic arc/accretionary wedge, and reevaluation of terranes in the central Klamath Mountains

Abstract

Investigation of key areas of the central Klamath Mountains demonstrates that genetic relationships connect several regionally significant lithologic units that were previously considered to be separate tectonostratigraphic terranes. The rocks embody an intraoceanic tholeiitic to alkaline magmatic arc and contiguous accretionary wedge formed in Permian to Middle Jurassic time. The Eastern Hayfork and St Claire Creek units comprise Permian‐Jurassic oceanic sedimentary rocks deposited nearby and coeval with eruption of the North Fork‐Salmon River volcanoplutonic arc. The more inboard St Claire Creek unit consists of stratigraphically coherent chert and minor argillite probably deposited in a back‐arc setting, whereas the more outboard Eastern Hayfork sedimentary rocks were disrupted during formation of an accretionary wedge. The volcanoplutonic North Fork‐Salmon River arc extends for 200 km in the central Klamath Mountains and was active episodically or continuously from Permian through Middle Jurassic time. Alkalic members of the mafic igneous suite may have erupted following subduction of a spreading center. A varied heating history has produced an amalgam of submarine and regional low‐pressure, moderate‐temperature metamorphism, overprinted locally by contact metamorphism. Geochronology of weakly metamorphosed mafic dikes/sills (10 new 40Ar/39Ar analyses) suggests that significant heat was retained regionally into Early Cretaceous time.