Structural geology of the northern Galice Formation, western Klamath Mountains, Oregon and California

The Galice Formation is a thick, turbiditic flysch sequence that depositionally overlies the Josephine ophiolite in the western Klamath Mountains of northwestern California and southwestern Oregon. The Josephine ophiolite and Galice have been interpreted as the basement and sedimentary cover of a Late Jurassic back-arc basin that opened proximal to the continental margin. During the Nevadan orogeny (ca. 151-147 Ma), the ophiolite and overlying Galice sediments were thrust eastward beneath the continental margin along the regional Orleans fault. Two distinct Nevadan deformational phases are recognized in the Galice – an initial D1 or main-phase deformation and a later brittle deformation dominated by thrusting. D1 involved development of tight to isoclinal northwest-vergent F1 folds which possess an axial-planar S1 cleavage. S1 ranges from a weak pressure-solution cleavage in the northernmost Galice, to a strong slaty cleavage or schistosity in the southern Galice. S1 is ubiquitously parallel to bedding in the absence of recognizable F1 fold closures. Bedding is commonly transposed into parallelism with cleavage in southern exposures of the Galice. Syntectonic fibers in pressure shadows of pyrite grains reflect a north-south gradient in Nevadan penetrative strain. The fibers are straight and parallel to the trace of slaty cleavage, suggesting that cleavage formed during a stage of pure flattening. Percent shortening normal to Sl determined from fibers in the northern Galice (SW Oregon) is approximately 33%. However, fibers from slates in northern California record an average of 70% shortening normal to S1. A large increase in the intensity of S1 towards the south also suggests a gradient in finite strain. Main-phase Nevadan deformation was followed by an episode of brittle shearing. Nevadan thrusts reveal top-to-northwest sense to displacement, similar to vergence observed from F1 folds. A second generation of folds, F2, appear to be genetically related to the thrusts. Nevadan thrusts clearly overprint main-phase structures (F1, S1). In the northernmost Galice, angular F1 folds are thrusted-out parallel to bedding in one of the fold limbs. Further to the south, F1 folds are more typically truncated by thrusts that are parallel to S1 cleavage planes. Small-scale thrust systems in thinly-bedded rocks have produced peculiar bedding truncations or pseudocross-bedding . These small thrusts locally truncate the S1 cleavage. Fundamental conclusions obtained in this study are: 1) The Galice Formation is tightly folded on both outcrop and regional scales. 2) There is a north-to-south increase in Nevadan regional metamorphic grade and penetrative strain. This gradient may reflect a small southward component of underthrusting along the Orleans fault. 3) Nevadan vergence from folds and thrusts in the Galice is dominantly northwestward. 4) Most Nevadan thrusting in the Galice took place after the main-phase penetrative deformation and peak metamorphism. This ductile-to-brittle transition may have resulted from continued underthrusting during regional uplift