Abstract

Shear-related fabric in the middle to Late Cretaceous San Juan thrust system, Washington, is observed in imbricated blocks and in fault zones a few millimeters to tens of meters thick. Foliation and lineation are defined in tectonic blocks by alignment of (1) newly grown muscovite and chlorite and (2) flattened and stretched radiolaria, mineral porphyroclasts, and rock fragments. Thin argillaceous shear zones bear a pronounced lineation defined by lithologic "streaks" and slickenside corrugations. The slickenside and stretching lineations show a well-defined preferred orientation in a horizontal northwest-southeast direction. Widespread S-C fabrics, asymmetric pressure shadows, fault drag features, and asymmetric folds in the X-Z plane yield a sense of tectonic transport at most localities of upper plate to the northwest. Aragonite, prehnite, lawsonite, pumpellyite, and rare Na-amphibolite and Na-pyroxene characterize the metamorphism during the thrusting. The findings of this study indicate north-west, orogen-parallel, transport of the San Juan thrust system. We suggest that this structure is a large step-over zone transferring dextral strike slip from the western Cascades to the oceanward side of Wrangellia.

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