Siletzia
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. It is now fragmented into the Siletz and Crescent terranes.
Siletzia corresponds geographically to the Coast Range Volcanic Province, but is distinguished from slightly younger basalts that erupted after Siletzia accreted to the continent and differ in chemical composition. 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. This change of composition reflects a change from marine to continental volcanism that becomes evident around 48 to 42 Ma, and is attributed to the accretion of Siletzia against the North American continent.
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.
The accretion of Siletzia against the North American continent approximately 50 million years ago was a major tectonic event associated with a reorganization of Earth's tectonic plates. 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.
Exposures and discovery
The rock of Siletzia has been exposed in various places by tectonic uplift, anticlinal folding, and overthrusting onto other formations. These exposures have been variously named the Metchosin Formation of Vancouver Island, the Crescent Formation, Black Hills Formation, and Willapa Hills volcanics of Washington, and the Siletz River Volcanics and Roseburg Formation of Oregon. Elsewhere Siletzia is covered by younger volcanic and sedimentary deposits.The discovery of Siletzia began in 1906 with Arnold's description and naming of a small exposure on the north side of the Olympic Peninsula near Port Crescent, Washington. Though this exposure is small, he recognized as very likely that much more of it was buried under younger deposits. With recognition that similar rock exposed at other outcrops is part of the same formation, the name Crescent Formation is now generally applied to all early and middle Eocene basalts on the Olympic Peninsula and Puget lowlands.
The Metchosin Igneous Complex at the southern tip of Vancouver Island was described in a series of reports by Clapp, who recognized it as correlative with the Crescent formation on the other side of the Strait of Juan de Fuca. Weaver recognized that these "Metchosin volcanics" included various Eocene basalts in western Washington and the Oregon Coast Range as far south as the Klamath Mountains.
The Siletz River Volcanics was described in 1948 by Snavely and Baldwin after exposures near the Siletz River, Oregon, and the Roseburg and related formations in southern Oregon described in various reports from the 1960s on.
Etymology
"Siletzia" was coined in 1979 by Irving,, to describe the full extent of these Eocene basalts and interbedded sedimentary formations.Extent
The [|map] shows the exposures and inferred near-surface extent of Siletzia, the latter being what can be detected in the upper crust by aeromagnetic, gravitational, or seismological studies.There are only two exposed contacts of Siletzia with the older North American basement. One is near Roseburg, Oregon, where it is thrust against formations of the Klamath Mountains. Everywhere else the contact between Siletzia and the rest of the continent is concealed under younger deposits, especially the Cascade Range volcanics. The contact around the Olympic Mountains is actually the bottom contact with the underlying oceanic sediments, tilted up by the uplift of the Olympics and exposed by erosion of about 10 to 12 km of overlying deposits.
The location of the near-surface contact between the Crescent Formation and the pre-Cenozoic metamorphic basement of the continent — what has been the termed the Coast Range Boundary Fault — is largely uncertain. The Leech River Fault extends southeast past Victoria, B.C. to cross the Strait of Juan de Fuca, possibly connecting with the southeast striking Southern Whidbey Island Fault. This extends to the Rattlesnake Mountain Fault Zone, some 25 kilometers east of Seattle, which is believed to be the western edge of the pre-Cenozoic basement. However, gravity data indicates that at this latitude the Crescent Formation extends no further east than Seattle.
Further south, near Mount St. Helens, is a similar situation, where the St. Helens Fault Zone is believed to be the eastern edge of the Crescent Formation, but the pre-Cenozoic continental basement is near Mount Rainier. Separating these is the marine sedimentary formation known as the Southern Washington Cascades Conductor ; it is possible that it was deposited over a fragment of Siletzia. Or not: the oldest parts of the SWCC likely predate Siletzia, and the nature and location of the contact between these two formations is unknown.
In central Oregon, Siletzia forms a platform on which the older, now defunct volcanoes of the Western Cascades rest. The younger High Cascades to the east are believed to rest on sediments that accumulated in the basin between Siletzia and the continent.
In southern Oregon, Siletzia has been thrust against the Mesozoic Klamath Mountains of southern Oregon along the Klamath—Blue Mountain Lineament. Near Roseburg this contact is exposed at the Wildlife Safari Fault where the Late Jurassic Dothan Formation has been thrust over the Roseburg Formation.
Off the coast of southern Oregon, the western edge of Siletzia is the Eocene Fulmar Fault. This is a strike-slip fault, where part of Siletzia has been split off; the missing piece may be the Yakutat terrane now at the head of the Gulf of Alaska. Further north, the terrane boundary is believed to come ashore near the Columbia River.
The way the Crescent Formation wraps around the Olympic Mountains may reflect oroclinal bending as a result of being crushed against Vancouver Island. It has also been attributed to loss of the deposits originally overlying the Olympics prior to their uplift, resembling a dome where top and western end has been removed.
Siletzia's actual thickness, and the estimates of that thickness, vary. Under Oregon, the Siletz terrane appears to extend 25 to possibly 35 km into the trough between the subducting Juan de Fuca Plate and the edge of the continent, where it is slipping over sediments accumulated in the bottom of the trough. The Crescent terrane is believed to be thinner, from as little as 12 and 22 km under the western and eastern ends of the Strait of Juan de Fuca, but possibly as much as 20 and 35 km thick.
Composition
The various formations of Siletzia are characterized as marine tholeiitic pillow basalts and volcanic breccia, often interbedded with sedimentary layers of continental origin, lying on oceanic crust. These are usually capped by a layer of alkalic volcanics deposited subaerially. All this suggests these formations were initially deposited in an oceanic environment, possibly as seamounts or an island arc.On the Olympic Peninsula the Blue Mountain unit at the base of the Crescent Formation includes sediments of continental origin, suggesting that the continent was close by; other sediments were eroded from the pre-Cenozoic rock of Vancouver Island and the northern Cascade Range. At the southern end are sediments derived from the Klamath Mountains, while sand of the overlying Tyee Formation has an isotopic composition corresponding to rock of the Idaho Batholith.
Age
Eruption of the Siletzia basalts has been placed roughly in the late Paleocene through the mid Eocene; more specific dates have been difficult to obtain and somewhat variable. Early K-Ar and 40Ar-39Ar radiometric dating by Duncan gave dates of 57 and 62 Ma to the northern and southern ends, and a date of 49 Ma for the Grays River volcanics near the center of Siletzia. This is suggestive of a spreading ridge, and has been a strong constraint on models of how Siletzia formed. Other researchers have since found younger dates for the Crescent basalts, removing much of the age symmetry.Dating from 2010 based on 40Ar-39Ar, U-Pb, and coccoliths shows a narrower range of ages from 56 Ma in the south to 50 or 49 Ma in the north. Subsequent high-precision U-Pb dates from northern Siletzia show a narrowly constrained age of 51 Ma for the Metchosin complex on Vancouver Island. Of particular interest is the somewhat broader range of approximately 53 to 48 Ma for Crescent Formation basalts on the east side of the Olympic Peninsula that overlie the Blue Mountain unit, reliably dated at 48 Ma or younger. This structural relationship was previously understood as showing that Siletzia – or at least its northern end – was built on the continental margin. It is now argued that the unconformity of ages can be explained by the Blue Mountain unit being thrust under Siletzia some time after 44.5 Ma, and shows that Siletiza was not necessarily emplaced along the continental margin.