Zones of Alpine Tectonism and Metamorphism Klamath Mountains, Southwestern Oregon

Abstract

Mesozoic, eugeosynclinal, metasedimentary, and metavolcanic strata are intruded by elongate and arcuate northeast-trending alpine ultramafic belts in the Klamath Mountains of southwestern Oregon. The ultramafic belts conform to the regional patterns of alternating antiforms, synforms, and conformable high-angle faults. Throughout the region, the belts of alpine ultramafic rocks occupy axes of maximum meta-morphic intensity. The highly deformed, higher-grade metavolcanic-metasedimentary strata and centralized ultramafic-mafic intrusions form distinctive associations which define zones of tectonism. In several zones (herein termed "zones of alpine tectonism") where the intensity of tectonism varies from brittle to plastic, the grade of metamorphism varies from chlorite zone to staurolite zone. Patterns of mesoscopic structures in all zones of alpine tectonism are consistent with deformation during Late Jurassic Nevadan orogeny and suggest a chronological sequence. Isoclinal folding of ultramafic-mafic sheets and intense plastic deformation of adjacent eugeosynclinal strata are recognized in the cores of certain tectonic zones and are considered representative of the first phase of Nevadan orogeny. The ultramafic-mafic sheets were folded by diapiric intrusion and served as loci for continued movement during a later stage of Nevadan orogeny. Later movement was responsible for the occurrence of the more widespread, cataclasized chlorite-zone assemblages which occur within and surrounding the zones of tectonism.