Abstract

Permian and Triassic rocks exposed at scattered localities in northern California, northwesternmost Nevada, Oregon, and western Idaho represent oceanic trench and volcanic arc products whose tectonic and stratigraphic relationships are consonant with those of an arc-trench complex. Farther inland, however, along a generally north-trending belt through north-central Nevada, a thick succession of oceanic upper Paleozoic mafic volcanic rocks, bedded chert, and siliceous and calcareous clastic rocks has been carried eastward on the Golconda thrust for some distance over partly correlative shallow-water marine carbonate and clastic rocks of the continental shelf. Thrusting of these oceanic rocks, that crop out farther towards the continental platform than the correlative arc-trench rocks, evidently took place during the Sonoma orogeny in Late Permian and Early Triassic time and was followed by a brief episode of arc-like volcanism along the otherwise non-volcanic orogenic belt. For the sake of speculation, the Sonoma orogeny is attributed to collapse of a marginal basin against the Pacific side of North America, an event possibly triggered by changes in relative lithosphere-plate motions occasioned by the unique configuration of continents and ocean basins in Permian-Triassic time.