A kinematic model for the formation of the Siletz‐Crescent forearc terrane by capture of coherent fragments of the Farallon and Resurrection plates

Abstract

[1] The volcanic basement of the Oregon and Washington Coast ranges has been proposed to represent a pair of tracks of the Yellowstone hotspot formed at a mid-ocean ridge during the early Cenozoic. This interpretation has been questioned on many grounds, especially that the range of ages does not match the offshore spreading rates and that the presence of continental coarse clastic sediments is difficult to reconcile with fast convergence rates between the oceanic plates and North America. Updates to basement geochronology and plate motion history reveal that these objections are much less serious than when they were first raised. Forward plate kinematic modeling reveals that predicted basement ages can be consistent with the observed range of about 55-49 Ma, and that the entire basement terrane can form within about 300 km of continental sources for clastic sediments. This kinematic model indicates that there is no firm reason to reject the near-ridge hotspot hypothesis on the basis of plate motions. A novel element of the model is the Resurrection plate, previously proposed to exist between the Farallon and Kula plates. By including the defunct Resurrection plate in our reconstruction, we are able to model the Farallon hotspot track as docking against the Oregon subduction margin starting about 53 Ma, followed by docking of the Resurrection track to the north starting about 48 Ma. Accretion of the Farallon plate fragment and partial subduction of the Resurrection fragment complicates the three-dimensional structure of the modern Cascadia forearc. We interpret the so-called "E" layer beneath Vancouver Island to be part of the Resurrection fragment. Our new kinematic model of mobile terranes within the Paleogene North American plate boundary allows reinterpretation of the three-dimensional structure of the Cascadia forearc and its relationship to ongoing seismotectonic processes.

Future accreted terranes: a compilation of island arcs, oceanic plateaus, submarine ridges, seamounts, and continental fragments

“…In Central America, various "ophiolitic" units are found with OIB geochemical signatures, which are interpreted as hotspot-related seamounts or submarine ridges (Hoernle et al, 2002;Geldmacher et al, 2008;Buchs et al, 2009). The enigmatic Siletz terrane of northern California and Oregon is composed of volcanics with OIB signatures that have been variously interpreted as a hot spot track, slab window, and mid-ocean ridge (Schmandt and Humphreys, 2011;McCrory and Wilson, 2013). Examples of accreted seamounts, identified primarily by their OIB signature, are the alkali basaltic units found in Japan (Isozaki et al, 1990).…”

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

“…They are composed dominantly of tholeiitic and alkalic submarine and subaerial basalt, with attendant intrusive rocks, submarine breccias, marine sediments, and rare silicic fl ows that formed islands and seamounts built on ocean crust (Snavely et al, 1968). Together, they compose a large oceanic terrane that was accreted to North America in the Eocene (Snavely and MacLeod, 1974;Simpson and Cox, 1977;Duncan, 1982;Heller and Ryberg, 1983;Wells et al, 1984;McCrory and Wilson, 2013). The SRV and onlapping strata of the Oregon Coast Range have undergone large, clockwise paleomagnetic rotation (e.g., Simpson and Cox, 1977).…”

“…McCrory and Wilson (2013) proposed that Siletzia consists of two terranes, the Siletz and Crescent, which were accreted, along with the Yakutat terrane, during breakup and rotation of fragments of the Resurrection and Farallon plates. Schmandt and Humphreys (2011) and Gao et al (2011) revisited the docking of Siletzia.…”

“…We consider all of these units part of Siletzia, after Irving (1979). Siletzia is thus a composite terrane, composed of the Crescent terrane of Washington and British Columbia and the Siletz terrane of Oregon, which are similar in composition, age, and history, but have undergone variable amounts of tectonic rotation (e.g., McCrory and Wilson, 2013).…”

Geophysical and geochemical constraints on the evolution of oceanic lithosphere from formation to subduction

“…To the north of the Oregon Coast Range, the Siletz terrane thickness decreases with a minimum value of 6 km offshore Vancouver Island. South of the Oregon Coast Range the Siletz terrane is absent, with the forearc instead mostly supported by the Klamath block [McCrory and Wilson, 2013]. The regions of thin or absent Siletz terrane are areas of increased forearc seismicity, increased subduction complex width and more silicic and voluminous volcanic activity [Tréhu et al, 1994] .…”