EOCENE RIDGE SUBDUCTION AND TERMINATION OF THE NORTH CASCADES MAGMATIC ARC, WASHINGTON

Major modification of a subduction regime results from arrival of an oceanic ridge at a trench, which is an inevitable process at convergent plate boundaries. Effects of ridge subduction are most clearly seen in the fore arc and perhaps least understood in the arc. Paleogene ridge subduction in the Pacific NW is widely accepted and resulted in mafic and felsic fore-arc magmatism, including adakitic and peraluminous rocks, but its influence on the North Cascades arc (Cascades core) has rarely been considered. The Cascades core displays mainly tonalitic, arc magmatism from 96-60 Ma and associated mid-Cretaceous contraction and subsequent transpression. A 60-50 Ma magmatic lull was followed by ~50-45 Ma plutonism and localized high-grade metamorphism. In contrast to earlier plutonism, 50–45 Ma magmatism was marked by abundant granodiorite and granite, including alkaline and peraluminous rocks, and mafic bodies with transitional arc to extensional compositions. Eocene plutons are coeval with felsic to mafic dikes that intruded the arc and adjacent back arc and fore arc. Bimodal initial εNd isotopic values for the plutons include amongst the lowest values in the Cascades core, and magmas were derived from two very different sources (Matzel et al., 2008). The 50-45 Ma episode has long been viewed as the waning of a 96–45 Ma arc, but given the magmatic gap, change from calc-alkaline to much more diverse magmatism, and spatial distribution of igneous bodies we suggest that it represents slab window magmatism in the site of the former arc, or a hybrid situation where uprising asthenosphere and older arc lithosphere are both involved in magma genesis. We also propose that ridge subduction led to the regional change to Eocene transtension, which was marked by large dextral faults in the fore arc and arc and synchronous ductile orogen-parallel stretching in the Cascades core, rapid exhumation of crystalline rocks, and formation of thick terrestrial sedimentary basins directly adjacent to the arc at ca. 52-46 Ma. In one basin, dike swarms also record orogen-parallel extension, and there were rapid changes from shortening to extension to shortening, compatible with temporal changes expected from ridge-trench interaction. The Cascades did not resume as a “typical” arc until the modern arc initiated at ~40 Ma.