SILETZIA'S ROLE IN CORDILLERAN OROGENESIS, NORTHWEST USA AND ADJACENT CANADA (Invited Presentation)

What do we really know about Siletzia, the Paleogene submarine basalt basement of the Cascadia forearc? Field relations, isotopic and paleontologic ages, geochemistry, and geophysical imaging summarized in Schmandt and Humphreys, 2011; Wells et al., 2014; Eddy et al., 2017; Phillips et al., 2017; Miller et al., 2023; and Tepper and Clark, 2024 document that Siletzia is composed of oceanic tholeiitic basalts with OIB, MORB, and plume signatures. Its age is 56-48 Ma, which overlaps in age with a major reorganization in Pacific Basin plate motions and near trench volcanism on Vancouver Island. Siletzia fills the opening of the Columbia Embayment in the Mesozoic orogen, surely not a coincidence. Siletzia is 10–30 km-thick, a possible oceanic plateau with LIP volumes, and is similar in age, structure and composition to the oceanic Yakutat terrane now accreting in S. Alaska. Paleomagnetism documents modest northward translation (~350 km, a constraint on emplacement of Baja B.C.) and large CW rotation, up to 79°, consistent with the current GNSS velocity field. Siletzia was thrust beneath the continent and accreted by 50 Ma, producing a fold and thrust belt along the suture, with substantial oroclinal bending, uplift, and erosion of the ancestral continental margin. Siletzia’s accretion coincides with termination of a magmatic lull in the N. Cascades 60-52 Ma; the peak of dextral faulting in the N. Cascades 60-35 Ma; initiation of the Cascade arc in WA 46-42 Ma, and initiation of southward migration of Cordilleran magmatism at 50 Ma. These observations are consistent with creation of the Siletzia LIP on or near a spreading ridge adjacent to the continent, its subsequent collision, indentation of the margin, northward escape of crustal slivers, and an outboard jump in subduction. Ridge-trench interactions over ~80 m.yr. in the NE Pacific basin inferred from plate theory are consistent with Siletzia and Yakutat’s origin and transport histories on adjacent plates. Prolonged ridge subduction could have thinned the continent and promoted N. motion of B.C. by rifting in the Columbia Embayment (Humphreys, 2021). The Yellowstone hotspot, offshore in the Eocene, may have produced a hotspot track and contributed to the great volume of Siletzia and the Yakutat terrane. Subduction of older parts of the Siletz island chain may have caused flat-slab subduction prior to 52 Ma.