Structural studies in the Cecilville quadrangle of the Klamath Mountains, northwestern California, indicate a tectonic imbrication of three of the four Klamath geologic subprovinces. The structural sequence is interpreted as follows (bottom to top): (l) rocks of the western Paleozoic and Triassic subprovince; (2) the Siskiyou thrust zone; (3) the Siskiyou thrust plate composed of rocks of the central metamorphic subprovince; (4) the Trinity thrust zone and the sheetlike Trinity ultramafic pluton present along it; and (5) the Trinity thrust plate composed of rocks of the eastern Klamath subprovince. Minimum westward displacements of the Siskiyou and Trinity thrust plates are estimated at 15 to 20 miles and 20 miles, respectively, based on the positions of fensters and klippen. All rocks previously assigned to the Stuart Fork Formation are now recognized as parts of the western Paleozoic and Triassic subprovince, some exposed within fensters in the Siskiyou thrust plate.

The relationship of the Trinity ultramafic pluton to the Trinity thrust zone has not been resolved. The peridotitic pluton may have been injected along the thrust contact, presumably during Mesozoic:(?) thrusting. An alternative is that the pluton was intruded into the Grouse Ridge Formation of the central metamorphic subprovince during Paleozoic regional metamorphism, and that later thrust faulting was localized across it. Regional relationships favor the first alternative; whereas, field relationships of the pluton with rocks of the Grouse Ridge Formation favor the second. The problem can be resolved if there has been more than one episode of ultramafic emplacement in the south-central Klamath region.

Regional metamorphism of moderate grade of the central metamorphic subprovince has been isotopically dated by Lanphere and Irwin (1967) as Early to Middle Devonian. Thrust faulting of probable Jurassic age was accompanied in some areas by low-grade regional metamorphism, responsible for partial retrogressive metamorphism of central subprovince rocks. It is unlikely that any Paleozoic formation now exposed in the Klamath Mountains can be considered as the parental unit for the Salmon Hornblende Schist of the central metamorphic subprovince, and the same may be true for the overlying Grouse Ridge Formation. The two formations possibly represent basaltic oceanic crust (sima) and Ordovician or pre-Ordovician eugeosynclinal rocks deposited on it.

The presence of westward-directed thrust plates in the Klamath Mountains is in keeping with recent discoveries of westward thrusting elsewhere in Cordilleran eugeosynclinal areas. Apparent westward migration in northern California of tectonic episodes characterized by thrust faulting and accompanying injection of mantle peridotites along the faults can best be explained by periodic underthrusting of the continental margin by an active oceanic block.

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