EVIDENCE FOR COLLISION OF SILETZIA IN THE CENTRAL AND NORTH CASCADES, WASHINGTON

In the Pacific NW, the Paleogene (~60-50 Ma) fore arc from Oregon to southern Vancouver Island is dominated by the very thick (up to 20 km) Coast Range basalts and interlayered oceanic to continental margin strata referred to as Siletzia. The origin of Siletzia is controversial. It has been attributed to volcanism in rift basins formed where a spreading ridge intersected the continental margin, to a hot-spot chain (possibly Yellowstone) or LIP that formed an oceanic ridge that subsequently accreted to the margin. Our work in the central and North Cascades of Washington, combined with previous work, supports a short-lived collisional scenario, as a significant shortening event bracketed between ~52-47 Ma occurred inboard of Siletzia along the length of the terrane. In SW Oregon and Vancouver Island, Siletzia was overthrust by older inboard units during this interval and the early Eocene Cowichan fold and thrust belt formed NE of the terrane on Vancouver Island. In Washington, after well-documented restoration of ~110 km of dextral slip on the Straight Creek-Fraser fault, the northernmost Siletzia is directly west of the strongly folded Paleocene(?) to Eocene Chuckanut and Swauk basins, and the southern margin of the crystalline core of the North Cascades. These basins are inferred to have formed during regional transtension. Short-lived, open to subisoclinal folding, best documented in the Swauk basin, is attributed to Siletzia collision. This basin is overlain with angular unconformity by ~47 Ma Teanaway basalts and intruded by an associated voluminous dike swarm, compatible with continued transtension. NE of the collision zone in the Skagit Gneiss Complex of the Cascades core, folding occurred at ~50-48 Ma and includes open to tight, upright outcrop to map-scale folds and steeply plunging map-scale structures. The latter folds and associated vertical axis rotation are reminiscent of structures in the Cowichan belt on Vancouver Island. Our ongoing research is designed to better define the role and timing of Siletzia collision, ridge subduction, and slab window formation in the Pacific NW.