A SYNTHESIS OF THE PALEOGENE TECTONIC EVOLUTION OF WASHINGTON STATE

A wealth of recent geochronologic, stratigraphic, structural, and geochemical data document profound tectonic transitions in the Pacific Northwest during the Paleogene. In the crystalline core of the North Cascades (southern end of the Coast Mountains continental arc), a magmatic lull began at ~60 Ma, and only minor plutonism occurred from ~60 - 53 Ma in NE Washington, arguably reflecting the onset of low-angle subduction in the region and migration of magmatism farther inboard (southern British Columbia and Idaho). At ~59 Ma, non-marine deposition initiated in the Swauk basin of central and western Washington and continued to ~50 Ma. Offshore between ca. 56-48 Ma, hot-spot magmatism near a spreading ridge formed the Siletzia large igneous province, which was subsequently accreted, causing slab rollback and a SW sweep of 53-49 Ma calc-alkaline magmatism across NE Washington. Crustal shortening from SW Oregon to Vancouver Island, inversion of the Swauk basin, and folding of part of the Cascades core, record the collision of Siletzia with the continental margin between ca. 53 - 49 Ma. At ~51 - 50 Ma, a triple junction marked by near-trench magmatism was located on Vancouver Island. Regional transtension after collision was associated with bimodal volcanism in central Washington, renewed, dominantly granodioritic plutonism at ca. 50 Ma in the Cascades core, and intrusion of mafic and felsic dike swarms into the Swauk basin and core, respectively, from 49 - 45 Ma. We attribute the 50 - 45 Ma magmatism to slab breakoff and foundering. Transtension from 49 – 45 Ma also caused dextral strike-slip faulting with >300 km total offset, the subsidence and rapid deposition of the Chumstick strike-slip basin, and rapid exhumation of 8-10 kb metamorphic rocks in the Cascades core. Exhumation, ductile flow, and magmatism in the core are broadly synchronous with extension and magmatism in metamorphic core complexes in NE Washington, Idaho, and southern British Columbia. The final major event, following slab breakoff at ca. 50 - 48 Ma, was establishment of a new N-S-trending subduction zone at ca. 45 - 43 Ma.