Downloaded from geosphere.gsapubs.org on May 12, 2015 Geologic history of Siletzia, a large igneous province in the Oregon and Washington Coast Range: Correlation to the geomagnetic polarity time scale and implications for a long-lived Yellowstone hotspot Ray Wells1, David Bukry1, Richard Friedman2, Doug Pyle3, Robert Duncan4, Peter Haeussler5, and Joe Wooden6 1U.S. Geological Survey, 345 Middlefi eld Road, Menlo Park, California 94025-3561, USA 2Pacifi c Centre for Isotopic and Geochemical Research, Department of Earth, Ocean and Atmospheric Sciences, 6339 Stores Road, University of British Columbia, Vancouver, BC V6T 1Z4, Canada 3Department of Geology and Geophysics, University of Hawaii at Manoa, 1680 East West Road, Honolulu, Hawaii 96822, USA 4College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, 104 CEOAS Administration Building, Corvallis, Oregon 97331-5503, USA 5U.S. Geological Survey, 4210 University Drive, Anchorage, Alaska 99508-4626, USA 6School of Earth Sciences, Stanford University, 397 Panama Mall Mitchell Building 101, Stanford, California 94305-2210, USA ABSTRACT frames, the Yellowstone hotspot (YHS) is on southern Vancouver Island (Canada) to Rose- or near an inferred northeast-striking Kula- burg, Oregon (Fig. 1). These volcanic complexes Siletzia is a basaltic Paleocene and Eocene Farallon and/or Resurrection-Farallon ridge include the Siletz River Volcanics (SRV) of Ore- large igneous province in coastal Oregon, between 60 and 50 Ma. In this confi guration, gon, the Crescent Formation of Washington, and Washington, and southern Vancouver Island the YHS could have provided a 56–49 Ma the Metchosin igneous complex of southern Van- that was accreted to North America in the source on the Farallon plate for Siletzia, couver Island. They are composed dominantly early Eocene. New U-Pb magmatic, detrital which accreted to North America by 50 Ma. of tholeiitic and alkalic submarine and subaerial zircon, and 40Ar/39Ar ages constrained by A sister plateau, the Eocene basalt base- basalt, with attendant intrusive rocks, submarine detailed fi eld mapping, global nannoplankton ment of the Yakutat terrane, now in Alaska, breccias, marine sediments, and rare silicic fl ows zones, and magnetic polarities allow correla- formed contemporaneously on the adjacent that formed islands and seamounts built on ocean tion of the volcanics with the 2012 geologic Kula (or Resurrection) plate and accreted to crust (Snavely et al., 1968). Together, they com- time scale. The data show that Siletzia was coastal British Columbia at about the same pose a large oceanic terrane that was accreted rapidly erupted 56–49 Ma, during the Chron time. Following accretion of Siletzia, the lead- to North America in the Eocene (Snavely and 25–22 plate reorganization in the north- ing edge of North America overrode the YHS MacLeod, 1974; Simpson and Cox, 1977; Dun- east Pacifi c basin. Accretion was completed ca. 42 Ma. The voluminous high-Ti basaltic can, 1982; Heller and Ryberg, 1983; Wells et al., between 51 and 49 Ma in Oregon, based on to alkalic magmatism of the 42–35 Ma Tilla- 1984; McCrory and Wilson, 2013). The SRV and CP11 (CP—Coccolith Paleogene zone) coc- mook episode and extension in the forearc onlapping strata of the Oregon Coast Range have coliths in strata overlying onlapping conti- may be related to the encounter with an active undergone large, clockwise paleomagnetic rota- nental sediments. Magmatism continued in YHS. Clockwise rotation of western Oregon tion (e.g., Simpson and Cox, 1977). The SRV is the northern Oregon Coast Range until ca. about a pole in the backarc has since moved thought to be an allochthonous terrane, although 46 Ma with the emplacement of a regional the Tillamook center and underlying Silet- latitudinal transport is small, probably no more sill complex during or shortly after accretion. zia northward ~250 km from the probable than a few hundred kilometers (Beck, 1984). Isotopic signatures similar to early Columbia hotspot track on North America. In the ref- We consider all of these units part of Siletzia, River basalts, the great crustal thickness of erence frames we examined, the YHS arrives after Irving (1979). Siletzia is thus a compos- Siletzia in Oregon, rapid eruption, and tim- in the backarc ~5 m.y. too early to match the ite terrane, composed of the Crescent terrane of ing of accretion are consistent with offshore 17 Ma magmatic fl are-up commonly attrib- Washington and British Columbia and the Siletz formation as an oceanic plateau. Approxi- uted to the YHS. We suggest that interaction terrane of Oregon, which are similar in composi- mately 8 m.y. after accretion, margin paral- with the subducting slab may have delayed tion, age, and history, but have undergone vari- lel extension of the forearc, emplacement of arrival of the plume beneath the backarc. able amounts of tectonic rotation (e.g., McCrory regional dike swarms, and renewed mag- and Wilson, 2013). matism of the Tillamook episode peaked at INTRODUCTION Beneath western Oregon, in the present 41.6 Ma (CP zone 14a; Chron 19r). We exam- forearc, Trehu et al. (1994) documented high- ine the origin of Siletzia and consider the pos- Basement rocks of the Oregon and Washing- velocity mafi c crust 22–32 km thick; they sug- sible role of a long-lived Yellowstone hotspot ton Coast Ranges (northwest USA) consist of gested that it represented an accreted oceanic using the reconstruction in GPlates, an open thick basaltic sequences of Paleocene and Eocene plateau (Fig. 2). The eastern extent of the oceanic source plate model. In most hotspot reference age that are exposed in anticlinal uplifts from crust beneath the volcanic cover of the Columbia Geosphere; August 2014; v. 10; no. 4; p. 692–719; doi:10.1130/GES01018.1; 16 fi gures; 1 supplemental fi le. Received 24 December 2013 ♦ Revision received 7 May 2014 ♦ Accepted 25 June 2014 ♦ Published online 14 July 2014 692 For permission to copy, contact [email protected] © 2014 Geological Society of America Downloaded from geosphere.gsapubs.org on May 12, 2015 Geologic history of Siletzia 56° 60° 64° 68° A B Vancouver B.C. N 0 156° Metchosin Alaska 68° igneous WA Sanak Island complex SeattleS 52° Crescent Formation Fig. 3 156° 144° Nu Fig. 4 ik Pluton ka-Aial GraysGr River G u l f o Border Tillamook f Ranges A Portland l fault a s k Fig. 6 a TF Cascade Head 64° Yakutat terrane - partly Siletz River Newport subducted (dotted) Transition fault Volcanics FF 132° Yachats Explanation 144° 42 Ma Tillamook Volcanics RRoseburg 45-30 Ma AK arc magmatism s 43-37 Ma PNW arc magmatism n i Figs. 9. 12 ≤ 50 Ma extensional a basins t 53–47 Ma Eocene ‘backarc’ n Coast u shear OR magmatism o QCF zone > 50 Ma Paleocene- M British Eocene magmatic arc Columbia C Yakutat terrane (onshore) Paleocene-Eocene near- 120° trench intrusions, volcanics Vancouver Late Cretaceous-Paleocene Island t accretionary complexes s a fault o C 56° 132° 112° 44° Area of Fig. 1B, C e g Pacific Ocean ng 52° R WashingtonWashingtonto post 50 Ma ad Eocene c Idahoaho basins batholithl Cascade Range California Oregon 0 500 km Montana Idaho 48° 120° 44° 112° Figure 1. Tectonic setting of Siletzia. (A) Regional setting showing Paleocene–Eocene continental margin magmatism and location of oceanic Siletzia and Yakutat terranes (from Haeussler et al., 2003). FF—Fairweather Fault; QCF—Queen Charlotte Fault; TF—Tintina fault. (B) Formations composing Siletzia (red) and postaccretion basaltic magmatism (blue, from McCrory and Wilson, 2013). OR—Oregon; WA—Washington; B.C.—British Columbia. (C) Extent of Siletzia in the subsurface from regional aeromagnetic data and deep exploration wells (red bottoms in basalt, blue in sediments; from Wells et al., 1998). Geosphere, August 2014 693 694 Geosphere, August 2014 August 694 Geosphere, Epoch Polarity CP CP Polarity Epoch Chron Zone BRITISH COLUMBIA WASHINGTON OREGON Zone Chron CP18 CP18 30 C11 Vancouver Island Olympic PeninsulaWillapa Hills Tillamook Highlands Newport Roseburg C11 30 E CP17 Roman Nose R CP17 C12 Vancouver Is.-derived marine strata Cascade-derived marine strata C12 E c c CP16 b Carmanah Twin River Lincoln Creek Fm. Keasey Fm.34.6 Alsea Fm. Western Cascades CP16 b Oligocene 34.2 Oligocene C13 a Group neph. syenite, camptonite R a C13 N 35 C15 Cascade Head volcanics C15 35 CP15 Y Y YY Y Y YY Y Y YY Y Y YY Y L 36.9 35.6 L C16 CP15 b 36.8 N CP15 b C16 Yachats Basalt R C17 a Tukwila-Northcraft vol. N N Goble Vol. a C17 Grays River volcanics Fisher Fm. b 39.4 b R 40.0 Ordway 40.1 C18 40.0 YY Y Y YY Y Y YY Y Y C18 40 CP14 N pluton Cowlitz Fm. CP14 40 R CP14a Spencer Fm. a R coal a C19 41.4 Tillamook Volcanics C19 41.6 R CP14a M c Aldwell Fm. McIntosh Fm. Yamhill Fm. Elkton Fm. c M C20 b slope mudstone b C20 CP13 exhumation CP13 CP13 45 45 a 46.0 46.5 a Eocene Eocene Y Y Y Y coalcoalcoal Downloaded from C21 b NNN N b C21 Ma CP12 basalt of Hembre Ridge CP12b R CP12 Ma Mz 48.0 CP12a Tyee Fm.48.0 a 48.7 N a CP11 N N N C22 49.0 coal C22 CP11 R R 50.1 CP11 Umpqua Group CP11 50 50.5 50.5 50.6 CP11 50 N CP10 51.0 C23 CP10-11 51.4 CP10 C23 52.0 52.0 YY Y Y YY E CP10 R RR CP10 Mz CP10 E 53.9 53.3 53.6 53.4 b CP 9b b CP9 a CP9 a 55 C24 10 km R C24 55 55.7? R CP 8b b 56.0 b geosphere.gsapubs.org CP8 R CP8 a oceanic plateau a L CP7 CP7 L C25 C25 CP6 Metchosin Crescent Siletz River CP “late Paleocene” CP6 etal.