Mesozoic-cenozoic tectonic history of the southwestern Oregon coast in relation to cordilleran orogenesis

Abstract

Mesozoic rocks of the Cordilleran mobile belt extend to the continental margin on the southwestern Oregon coast. The region has been eugeosynclinal since at least the early Paleozoic, but dominance of continental-type andesites suggests the presence of essentially continental crust throughout its known history. Difficulties in understanding the history of the central Pacific coast have centered around problems of the subdivision, dating, and correlation of seemingly homogeneous graywacke terranes. Detailed mapping, sedimentary petrography, and isotopic dating provide considerable clarification in Oregon. The Jurassic Dothan formation (about 150 m.y. old) now appears clearly older than the Rogue and Galice formations. All three units were involved in the late Jurassic Nevadan orogeny which occurred here 135 to 145 m.y. ago. None of the three is equivalent to any paleontologically dated part of the largely post-Nevadan Franciscan complex of California (latest Jurassic to late Cretaceous) some unfossiliferous older strata, however, are almost certainly included in the Franciscan. It appears that during the Cenozoic the Franciscan and the older Oregon rocks were structurally juxtaposed by shearing in northwest California where the California Coast Ranges abut against the Klamath province. Diastrophism has been nearly continuous in southwestern Oregon. Plutonic and metamorphic activity culminated in the late Jurassic between 135 and 145 m.y. ago (Nevadan), but more volcanism and severe orogenic disturbances soon followed. Deformation in the westwardly-convex Klamath structural arc continued, though diminishing, into the early Tertiary. Many specific, short orogenic events in the Cordillera have been named, but it is suggested that Cordilleran orogeny or revolution be adopted for the entire sequence of late Mesozoic through early Cenozoic orogenesis in western North America. The most striking subsequent Cordilleran structural discontinuity occurred in the mid-Cenozoic. Cascadan orogeny is suggested for a late Cenozoic diastrophic sequence during which the entire continental margin has been increasingly fragmented by intense faulting accompanied by widespread volcanism. Great northwest-trending shear zones were superimposed upon the coastal edge of the Klamath arc during this youngest orogenesis.