Tectonic controls on the origin and segmentation of the Cascade Arc, USA

Abstract The magmatic response above subducting ocean lithosphere can range from weak to vigorous and a narrow zone widely distributed. small young Cascade Arc, riding on the margin of tectonically active North American plate, has expressed nearly this entire volcanic activity. This allows an unusually good examination arc initiation early growth. We review tectonic controls Cascade-related magmatism its inception present, with new considerations influences stress strain Arc was created after accretion Siletzia oceanic plateau at ~ 50 Ma ended period flat-slab subduction. (1) initiated dipping-slab subduction beneath most northern (beneath Washington Oregon) (2) enabled more southerly flat slab Nevada) roll back toward California. As abandoned fragmented foundered Oregon Washington, extension volcanism ensued throughout northwest USA; in Nevada rolled Early signs were evident by 45 ancestral well established 35 Ma. Thus, 55–35 subduction-related evolved amagmatic regional flare-up clearly two different settings. modern Cascades structure 7 when change Pacific plate motion caused partial entrainment Sierra Nevada/Klamath block. block pushes north west Coast Ranges block, breaking into three segments: southern extensional arc, central transitional compressional arc. Extension enhances mafic promoting basalt decompression melts depleted mantle (low-K tholeiites) that are subequal volume fluxed calcalkaline basalts. Compression restricts activity north; is dominantly silicic intra-plate-like basalts cluster close main volcanoes. accommodates dextral shear deformation, resulting wide distributed basaltic vents diverse affinities no clear axis.