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

Abstract

Siletzia is a basaltic Paleocene and Eocene large igneous province in coastal Oregon, Washington, and southern Vancouver Island that was accreted to North America in the early Eocene. New U-Pb magmatic, detrital zircon, and 40 Ar/ 39 Ar ages constrained by detailed fi eld mapping, global nannoplankton zones, and magnetic polarities allow correlation of the volcanics with the 2012 geologic time scale. The data show that Siletzia was rapidly erupted 56-49 Ma, during the Chron 25-22 plate reorganization in the northeast Pacifi c basin. Accretion was completed between 51 and 49 Ma in Oregon, based on CP11 (CP-Coccolith Paleogene zone) coccoliths in strata overlying onlapping continental sediments. Magmatism continued in the northern Oregon Coast Range until ca. 46 Ma with the emplacement of a regional sill complex during or shortly after accretion. Isotopic signatures similar to early Columbia River basalts, the great crustal thickness of Siletzia in Oregon, rapid eruption, and timing of accretion are consistent with offshore formation as an oceanic plateau. Approximately 8 m.y. after accretion, margin parallel extension of the forearc, emplacement of regional dike swarms, and renewed magmatism of the Tillamook episode peaked at 41.6 Ma (CP zone 14a; Chron 19r). We examine the origin of Siletzia and consider the possible role of a long-lived Yellowstone hotspot using the reconstruction in GPlates, an open source plate model. In most hotspot reference frames, the Yellowstone hotspot (YHS) is on or near an inferred northeast-striking KulaFarallon and/or Resurrection-Farallon ridge between 60 and 50 Ma. In this confi guration, the YHS 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 contemporaneously 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 voluminous high-Ti basaltic to alkalic magmatism of the 42-35 Ma Tillamook episode and extension in the forearc may be related to the encounter with an active YHS. 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. In the reference frames we examined, the YHS arrives in the backarc ~5 m.y. too early to match the 17 Ma magmatic fl are-up commonly attributed to the YHS. We suggest that interaction with the subducting slab may have delayed arrival of the plume beneath the backarc.

Detrital zircon U–Pb reconnaissance of the Franciscan subduction complex in northwestern California

“…The Franciscan Coastal Belt may have been translated north a moderate distance after accretion. Based mainly on Dickinson et al (1979Dickinson et al ( , 1988Dickinson et al ( , 2012, Heller et al (1985Heller et al ( , 1987, Underwood and Bachman (1986), Renne et al (1990), Wells et al (2000Wells et al ( , 2014, Wyld (2000), Stewart et al (2003), Reed (2004) Figure 6H shows the distinctive data pattern exhibited by mid-Maastrichtian to Palaeocene strata from one specific portion of the unnamed forearc basin of southernmost California (Jacobson et al 2011; see also Sharman et al 2014). Key populations are ca.…”

Tracing chlorine sources of thermal and mineral springs along and across the Cascade Range using halogen concentrations and chlorine isotope compositions

“…The Cascade Range comprises volcanic deposits, mainly basalt to dacite in composition, of Eocene-Quaternary age with cumulate thickness up to several kilometers across the older western and more recent High Cascades sectors (Hildreth, 2007). The underlying basement includes marine shales, limestones and sandstones, Paleozoic accreted oceanic terranes of OIB-and MORB-like basalt, and tectonic slivers of serpentinized peridotites (Wells et al, 2014).…”

Holocene earthquakes and right-lateral slip on the left-lateral Darrington–Devils Mountain fault zone, northern Puget Sound, Washington

“…The E-W to NW-SE strikes of most of these faults and the coincidence of some of these faults with the margins of similarly trending Tertiary basins and uplifts are thought to refl ect the N-S compression caused by northward transport of the Oregon block against a backstop of quasi-stable crust in southern British Columbia ( Fig. 1A; Wells et al, 1998Wells et al, , 2014Wells and Simpson, 2001;McCaffrey et al, 2007). From south to north, paleo seismic and geologic mapping studies have identifi ed surface deformation indicating Holocene activity on the Olympia and Tacoma faults in the Tacoma Basin (Bucknam et al, 1992;Sherrod, 2001;Sherrod et al, 2004aSherrod et al, , 2004bNelson et al, 2008;Barnett et al, 2010;Sherrod and Gomberg, 2014), the Seattle fault in the Seattle Basin (Bucknam et al, 1999;Johnson et al, 1999;Sherrod et al, 2000;Nelson et al, 2003aNelson et al, , 2003bNelson et al, , 2014Kelsey et al, 2008), the Southern Whidbey Island, Utsalady Point, and Darrington-Devils Mountain faults in the Everett Basin (Johnson et al, 1996(Johnson et al, , 2001(Johnson et al, , 2004Dragovich and DeOme, 2006;Sherrod et al, 2008), and the Boulder Creek, Birch Bay, Sandy Point, and Drayton Harbor faults in the Bellingham Basin (Kelsey et al, 2012;Sherrod et al, 2013;Sherrod and Gomberg, 2014).…”