Origin of Siletzia, an Accreted Large Igneous Province in the Cascadia Forearc, and the Early History of the Yellowstone Hotspot
Thursday, 18 December 2014: 11:50 AM
Siletzia as named by Irving (1979) is a Paleogene large igneous province forming the oceanic basalt basement of coastal OR, WA and S. BC that was accreted to North America in the early Eocene. U-Pb (magmatic, detrital zircon) and 40Ar/39Ar ages constrained by mapping, global coccolith (CP) zones, and magnetic polarities permit correlation of basalts with the geomagnetic polarity time scale of Gradstein et al. (2012). Siletzia was rapidly erupted 56–49 Ma (Chron 25-22), and accretion was completed between 51 and 49 Ma in Oregon. Magmatism continued until ca. 46 Ma with emplacement of a basalt sill complex during or shortly after accretion. Siletzia’s great crustal thickness, rapid eruption, and timing of accretion are consistent with formation as an oceanic plateau. Eight m.y. after accretion, margin-parallel extension and regional dike swarms mark the Tillamook magmatic episode in the forearc (41.6 Ma; CP zone 14a; Chron 19r). We examined the origin of Siletzia and the possible role of a long-lived Yellowstone hotspot (YHS) in an open source plate modeling program. In most reference frames, the YHS is on or near an inferred northeast-striking Kula- Farallon and/or Resurrection-Farallon ridge 60 to 50 Ma. The YHS thus could have provided a 56–49 Ma source on the Farallon plate for Siletzia, which accreted to North America by 50 Ma. A sister plateau, the Eocene basalt basement of the Yakutat terrane, now in Alaska, formed on the adjacent Kula (or Resurrection) plate and accreted to coastal British Columbia at about the same time. Following accretion of Siletzia, the leading edge of North America overrode the YHS ca. 42 Ma. The encounter with an active YHS may explain the voluminous high-Ti tholeiitic to alkalic magmatism of the 42–34 Ma Tillamook episode and extension in the forearc. Clockwise rotation of western Oregon about a pole in the backarc has since moved the Tillamook center and underlying Siletzia northward ~250 km from the probable hotspot track on North America.