Collided Paleozoic Microplate in the Western United States

Abstract

A region of the Cordillera is underlain by a Paleozoic microplate, Sonomia, that accreted to the passive margin of sialic North America in Triassic time. Sonomia consists chiefly of sequential Paleozoic island arc terranes and is coupled in part to a deeper structure. Sonomia migrated in the Permian with left-oblique convergence above a consuming boundary toward the continent and propelled before it an accretionary wedge of ocean floor strata. Undersliding of the wedge by the continental slope and outer shelf created the Golconda allochthon, followed by an arrested subduction below Sonomia. Pre-collision motions on Sonomia's outboard margin may have been kinematically related to the truncation of the sialic continent south of the Sonomia suture. Sonomia may be related to other Paleozoic arc fragments that accreted at various times in the Pacific Northwest.