V24C-01
Origin of Siletzia, a Large Igneous Province in the Cascadia Forearc, and the Early History of the Yellowstone Hotspot
Tuesday, 15 December 2015: 16:00
310 (Moscone South)
Ray E Wells, U.S. Geological Survey, Menlo Park, CA, United States, David Bukry, USGS, Menlo Park, CA, United States, Richard M Friedman, University of British Columbia, Vancouver, BC, Canada, Douglas G Pyle, University of Hawaii at Manoa, School of Ocean and Earth Science and Technology, Honolulu, HI, United States, Robert A Duncan, Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, United States and Peter J Haeussler, USGS Alaska Science Center, Anchorage, AK, United States
Abstract:
Siletzia is a Paleogene large igneous province (LIP) forming the oceanic basement of coastal OR, WA and S. BC that was accreted to North America (NAM) in the early Eocene. Crustal thickness from seismic refraction ranges from 10 to 32 km, with 16 km of pillow and subaerial basalt exposed on the Olympic Peninsula. At 1.7-2.4 x 106 km3, Siletzia is at least 10 times the volume of the Columbia River flood basalts. U-Pb and 40Ar/39Ar ages, global coccolith (CP) zones, and magnetostratigraphy allow correlation of Siletzia with the 2012 geomagnetic polarity time scale. Siletzia was erupted 56–49 Ma (Chron 25-22), and accretion was completed between 51 and 49 Ma in Oregon. Siletzia’s composition, 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 accompanied the voluminous tholeiitic to highly alkalic Tillamook magmatic episode in the forearc (41.6 Ma; CP14a; Chron 19r). We examined the origin of Siletzia and the possible role of a long-lived Yellowstone hotspot (YHS) in GPlates. In most reference frames, the YHS is ~ 500km offshore S. OR, near an inferred northeast-striking Kula- Farallon and/or Resurrection-Farallon ridge 60 to 50 Ma. The YHS could have provided the 56–49 Ma source on the Farallon plate for Siletzia, which in the model accretes to NAM by 50 Ma. A sister plateau, the Eocene basalt basement of the Yakutat terrane, now in Alaska, may have formed on the adjacent Kula (or Resurrection) plate and accreted to British Columbia at about the same time. Following accretion, the leading edge of NAM overrode the YHS ca. 42 Ma. The encounter with an active YHS may explain the voluminous 42-34 Ma Tillamook episode and forearc extension. 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 likely hotspot track on NAM.
