Siletzia

"Siletz terrane" redirects here. For other uses, see Siletz (disambiguation).

Siletzia, from Vancouver Island (Van) to the Klamath Mountains in Oregon. The shaded area shows near-surface extent as inferred from magnetic and gravitational studies (Silberling et al. 1987; Wells, Weaver & Blakely 1998), divided into the Crescent (CR) and Siletz (SZ) terranes; the crossed dashed lines are alternate locations of CR-SZ boundary. Outcrops (with names) in black; ages (in red, Ma = millions of years) on the left are from McCrory & Wilson (2013b, fig. 1), ages on right are from Duncan (1982, fig. 2). The blue line is the Columbia River (Washington—Oregon boundary), the red line is the Corvallis—Waldo Hills fault, the dashed blue lines are the Olympic–Wallowa Lineament (OWL) and Klamath—Blue Mountains Lineament (KBML), and the red triangles are the main Cascades Volcanoes. Modified from figure by Duncan (1982). Grays River Volcanics and later parts of the Tillamook Volcanics are now considered post-Siletzian. (Chan, Tepper & Nelson 2012)

Siletzia is a massive formation of early to middle Eocene epoch marine basalts and interbedded sediments in the forearc of the Cascadia subduction zone, on the west coast of North America. It forms the basement rock under western Oregon and Washington and the southern tip of Vancouver Island.[1] It is now fragmented into the Siletz and Crescent terranes.[2]

Siletzia corresponds geographically to the Coast Range Volcanic Province (or Coast Range basalts),[3] but is distinguished from slightly younger basalts that erupted after Siletzia accreted to the continent and differ in chemical composition.[4] The Siletzia basalts are tholeiitic, a characteristic of mantle-derived magma erupted from a spreading ridge between plates of oceanic crust. The younger basalts are alkalic or calc-alkaline, characteristic of magmas derived from a subduction zone.[5] This change of composition reflects a change from marine to continental volcanism that becomes evident around 48 to 42 Ma (millions of years ago), and is attributed to the accretion of Siletzia against the North American continent.[6]

Various theories have been proposed to account for the volume and diversity of Siletzian magmatism, as well as the approximately 75° of rotation, but the evidence is insufficient to determine Siletzia's origin; the question remains open.[7]

The accretion of Siletzia against the North American continent approximately 50 million years ago (contemporaneous with the initiation of the bend in the Hawaiian-Emperor seamount chain) was a major tectonic event associated with a reorganization of Earth's tectonic plates.[8] This is believed to have a caused a shift in the subduction zone, termination of the Laramide orogeny that was uplifting the Rocky Mountains, and major changes in tectonic and volcanic activity across much of western North America.[9]

  1. ^ Snavely, MacLeod & Wagner 1968, p. 454; Phillips, Walsh & Hagen 1989, p. 209; Brandon & Vance 1992, p. 571; Trehu et al. 1994, p. 237.
  2. ^ Silberling et al. 1987. The portion of Siletzia under Oregon and southeastern Washington, leaving out the Olympic Peninsula and Vancouver Island, has also been called the Willamette Plate. Magill et al. 1982, p. 3771, and see fig. 11, p. 3772.
  3. ^ Brandon & Vance 1992, p. 571.
  4. ^ Brandon & Vance 1992, p. 571.
  5. ^ Phillips, Walsh & Hagen 1989, pp. 200, 205; Cady 1975, p. 573.
  6. ^ Phillips, Walsh & Hagen 1989. Authors have differed as to which formations are Siletzian. For a recent categorization see McCrory & Wilson 2013b, Table 1.
  7. ^ Bromley 2011, p. 9; McCrory & Wilson 2013b, para. 2.
  8. ^ Sharp & Clague 2006, p. 1283.
  9. ^ Gao 2011, pp. 44, 48.