Age and volcanic stratigraphy of the Eocene Siletzia oceanic plateau in Washington and on Vancouver Island

Abstract

Geophysical, geochemical, geochronologic, and stratigraphic observations all suggest that the basalts that underlie western Oregon and Washington (USA), and southern Vancouver Island (Canada) form a coherent terrane of Eocene age, named Siletzia. The total volume of basalt within Siletzia is comparable to that observed in large igneous provinces and several lines of evidence point toward the terrane's origin as an accreted oceanic plateau. However, a thick sequence of continentally derived turbidites, named the Blue Mountain unit, has long been considered to floor the northern part of the terrane and its presence has led to alternative hypotheses in which Siletzia was built on the continental margin. We present new high-precision U-Pb zircon dates from silicic tuffs and intrusive rocks throughout the basaltic basement of northern Siletzia, as well as detrital zircon age spectra and maximum depositional ages for the Blue Mountain unit to help clarify the volcanic stratigraphy of this part of the terrane. These dates show that northern Siletzia was emplaced between 53.18 ± 0.17 Ma and 48.364 ± 0.036 Ma, similar to the age and duration of magmatism in the central and southern parts of the terrane. Turbidites in the basal Blue Mountain unit have maximum depositional ages as young as 44.72 ± 0.21 Ma and are distinctly younger than the basaltic basement that forms Siletzia. This age relationship implies that they were thrust under the terrane after 44.72 ± 0.21 Ma along one or more enigmatic faults. The younger age for these sedimentary rocks no longer requires construction of Siletzia on the continental margin, and we consider our revised stratigraphy to provide further support for the origin of the terrane as an accreted oceanic plateau.

Eocene initiation of the Cascadia subduction zone: A second example of plume-induced subduction initiation?

“…This interlayering relationship was taken as evidence that Siletzia erupted very near the continent (e.g., Cady, 1975;Brandon et al, 2014). However, new high-resolution ages of detrital zircons from four Blue Mountain sandstone samples indicate maximum depositional ages from 47.77 ± 0.57 to 44.72 ± 0.21 Ma, indicating that Blue Mountain strata are younger than Siletzia volcanic rocks (Eddy et al, 2017; see also Wells et al, 2014, their sample JV440). The volcanic rocks that are interlayered with Blue Mountain strata thus postdate the main episode of Siletzia construction; along with the Blue Mountain strata, they probably compose part of the younger Olympic subduction complex that was thrust beneath Siletzia after the latter was emplaced (Eddy et al, 2017).…”

“…However, new high-resolution ages of detrital zircons from four Blue Mountain sandstone samples indicate maximum depositional ages from 47.77 ± 0.57 to 44.72 ± 0.21 Ma, indicating that Blue Mountain strata are younger than Siletzia volcanic rocks (Eddy et al, 2017; see also Wells et al, 2014, their sample JV440). The volcanic rocks that are interlayered with Blue Mountain strata thus postdate the main episode of Siletzia construction; along with the Blue Mountain strata, they probably compose part of the younger Olympic subduction complex that was thrust beneath Siletzia after the latter was emplaced (Eddy et al, 2017). Deformation associated with Siletzia emplacement ("docking") is bracketed between 51.309 ± 0.024 and 49.933 ± 0.059 Ma, as inferred from the timing of shortening in the nonmarine Swauk forearc basin to the east ( Fig.…”

“…Siletzia has a preserved area of ~100,000 Stern and Dumitru | Eocene Cascadia plume-induced subduction initiation GEOSPHERE | Volume 15 | Number 3 km 2 and an estimated volume of 1,700,000-2,600,000 km 3 , qualifying it as a LIP (Wells et al, 2014). Documented volcanism occurred from 53.1 to 48.4 Ma near the northern end of Siletzia (Eddy et al, 2017) and from 56 to 52 Ma near the southern end (Wells et al, 2014, their figure 9). Deeper, unexposed parts of Siletzia are presumably mostly older, although given that LIPs may form very rapidly, they may be only slightly older and may also contain substantial volumes of post-56 Ma sills, intrusions, and/or underplated rocks.…”

“…Cowichan foldand-thrust belt England and Calon, 1991 Umpqua foldand-thrust belt Wells et al, 2014 Klamath detritus inter Cross sections (Fig. 4A) bedded with Siletzia volcanic rocks after 55 Ma Wells et al, 2014 Swauk basin (schematic) Eddy et al, 2016bEddy et al, , 2017 Docking 51.3-49.9 Ma Eddy et al, 2016bEddy et al, , 2017 complex deformation, and volcanism. This probably created a new Tyee River to Oregon, rather than diverting the existing Princeton River from a mouth in California to a new mouth in Oregon (see also Table S1 [footnote 1]).…”

“…5). Siletzia volcanic rocks on the island and on the Olympic Peninsula of northwestern Washington were emplaced between 53.18 ± 0.17 Ma and 48.364 ± 0.036 Ma, based on eleven new high-resolution zircon U-Pb chemical abrasion-thermal ionization mass spectrometry ages (Eddy et al, 2017). On the peninsula, the base of Siletzia structurally overlies younger subducted sedimentary and volcanic rocks of the Olympic subduction complex, tilted up by Miocene uplift and exposed by erosion of ~11 km of overlying deposits (Brandon and Calderwood, 1990;Stewart et al, 2003).…”