Tectonic relations between the Galice Formation and the Condrey Mountain Schist, Klamath Mountains, Northern California

Abstract

The Upper Jurassic Galice Formation and parts of its 164 to 162 Ma Josephine ophiolite basement, both of the western Klamath terrane (WKT) have been suggested to constitute lower grade equivalents of the Condrey Mountain Schist (CMS). The high P/T CMS is exposed through a window within the Rattlesnake Creek terrane (RCT) which has been thrust westward over the WKT. The Ga lice-CMS correlation was based on lithologic similarities and structural position. U/Pb zircon, Ar/Ar and biostratigraphic data, however, preclude a simple stratigraphic correlation. These data may be construed as evidence for no relationships between the two assemblages with their sequential accretion as unrelated terranes. A more imaginative analysis may be constructed from the available data, however, which views the assemblages as integral parts of a rifted arc-interarc basin system which formed and subsequently closed by rapid subduction over a time interval of ~20 m.y. This model is based on a Cordilleran-scale view of the problem and on the well-documented origin of the Josephine ophiolite and its structurally underlying Rogue arc sequence as coeval interarc basin and fringing arc complexes which together formed the substrate for the Galice formation; and on a more speculative analysis of the petrogenetic setting of the CMS protolith. The age data indicate that the CMS protolith formed at ~170 Ma and earlier, and was thrust eastward beneath the RCT probably between 160 and 155 Ma. The RCT was at amphibolite grade near the thrust, and an inverted metamorphic gradient developed within the CMS. The WKT was thrust beneath the CMS and its RCT upper plate between 155 and 150 Ma. The RCT plates above and to the west of the CMS contain mafic extensional complexes of ~170 Ma (China Peak) and ≥164 Ma (Preston Peak) age. These complexes as well as the CMS protolith are interpreted as early manifestations of Josephine interarc basin formation. This petrogenetic and thrusting history spanned in time the Siskiyou and Nevadan regional compressional events. Within this time interval, however, major lithosphere-scale extension is indicated by the Josephine ophiolite and other extensional assemblages. This apparent contradiction in regional kinematic patterns is reconciled by recognition of important tangential displacement patterns along the Cordilleran plate edge during Jurassic time.

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