Abstract

Glaucophane and related schists are present as tectonic fragments in ophiolitic suture zones and as discrete lithotectonic belts along the accreted Mesozoic/Tertiary Californian margin. Occurrences include parts of the Klamath Mountains, the western Sierran Foothills, the Coast Ranges, faulted marginal segments of the Mojave Desert, the Transverse Ranges, and the southern California borderland. These high-pressure, low-temperature blueschist assemblages reflect the thermal regime of subduction-zone environments. Considerable underflow accompanied drifting and the assembly of far-traveled tectonostratigraphic terranes, as documented by sea-floor magnetic anomaly patterns and age relationships of the oceanic crust-capped lithosphere: the eastern limbs of paleo-Pacific plates (especially the Farallon-Cocos), have been extensively or completely overridden by the westward-encroaching North American plate—7000 km since Early Cretaceous time and nearly 10 000 km since Jurassic time. Subduction is attested to by remnant high-pressure mineral assemblages scattered throughout California; by construction of related, roughly contemporaneous calc-alkaline volcanic-plutonic belts and forearc basin deposits; and by the stranding of ophiolitic complexes. Although substantial northward drift transported exotic oceanic and continental materials to the growing Californian crust and caused extensive dislocation of the post-Paleozoic continental margin, much of the plate motion evidently involved a large component of convergence and eastward underflow. Terrane shuffling has complicated the picture, but the dominant mechanism of continental growth at the Californian margin during Mesozoic and Paleogene time was subduction.

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