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.

[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.

[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.

[13] The Metchosin Igneous Complex at the southern tip of Vancouver Island was described in a series of reports (1910, 1912, 1913, 1917) by Clapp, who recognized it as correlative with the Crescent formation on the other side of the Strait of Juan de Fuca.

[14] 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.

[18] The map shows the exposures (black) and inferred near-surface extent (pink) of Siletzia, the latter being what can be detected in the upper crust by aeromagnetic, gravitational, or seismological studies.

[22] 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 (CRBF) — is largely uncertain.

[37] 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.

[41] At the southern end are sediments derived from the Klamath Mountains,[42] while sand of the overlying Tyee Formation has an isotopic composition corresponding to rock of the Idaho Batholith.

[46] Subsequent high-precision U-Pb dates from northern Siletzia[47] show a narrowly constrained age of 51 Ma for the Metchosin complex on Vancouver Island.

It is now argued[49] 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.

[50] Weaver, reckoning a minimal thickness of only 3,000 feet, still estimated "nearly 10,000 cubic miles of rock";[51] he put the total volume to be as great, if not greater, than the better known Columbia River Basalts.

Measurements of such paleomagnetic fields in the Oregon Coast Range show rotations of 46 to 75°, all of it following the presumed accretion to the continent (alternately, formation) of the Siletz terrane at about 50 Ma.

[58] A key piece of evidence is that the Crescent Formation is laid over sediments (the Blue Mountain unit) derived from the continent, including boulders of quartz diorite some 65 million years old.

[60] Such under-thrusting implies that the northern end of Siletzia was initially further away from the continent, and permits radial motion about a more southerly or more easterly pivot near the Washington-Oregon border, as recently suggested.

[62] Geodetic surveys show that the region continues to rotate, likely as a result of extension of the Basin and Range Province[63] and asthenospheric flow around the southern edge of the subducting Juan de Fuca Plate.

This raises a question of how much of the arcuate shape of the Crescent Formation is due to loss of material from the center after uplift by the Olympic Mountains, and how much reflects oroclinal bending.

[74] Studies of Siletzia's origins have generally focused on accounting for two principal observations: the large paleorotation (described above), and the voluminous output (over 50,000 cubic miles, exceeding the volume of most continental rift zones, and some flood basalt provinces).

[77] Seeking to explain the observed clockwise paleorotation, and noting that Siletzia appeared to have rotated as a rigid block, Simpson & Cox (1977) proposed two models.

[78] In the second model (subsequently refined by Hammond 1979), Siletzia was originally adjacent to the Olympic–Wallowa Lineament, then rifted from the continent and rotated about a northern pivot near the Olympic Peninsula.

An early and widely cited paper by Duncan (1982) (drawing on features of the fairly new theory of plate tectonics) exemplifies the off-shore or "seamount" type of models.

[80] The latter was demonstrated by a recent study that showed, on the basis of geochemistry, that the Grays River volcanics followed the Siletzia eruptions,[81] and thus are not representative of the initial phase of Siletz magmatism.

[82] The range of the original ages was also a problem, as the rate of Kula-Farallon spreading over that time would produce a chain of seamounts much longer than observed, and too far away from the continent to explain the continentally derived sediments.

Wells et al. 1984 proposed that the Siletzia basalts might have "leaked" through transform faults (perpendicular to a spreading ridge) during changes in direction of the tectonic plates.

[87] This idea was further developed by Babcock et al. (1992), who suggested rifting might have been initiated by a change in plate direction, or by kinematic effects as the Kula-Farallon ridge migrated along the continental margin.

[90] Breitsprecher et al. (2003) subsequently identified the fan-shaped wake of volcanics of distinctive geochemistry left by the widening Kula-Farallon slab window across northeastern Washington and into Idaho.

This terminated the Laramide orogeny that had been uplifting the Rocky Mountains, and triggered the ignimbrite sweep, a wave of large-volume silicic magmatism that swept over much of western North America between 20 and 50 Ma.

[102] This undoubtedly affected the enigmatic and controversial Challis Arc (stretching from southeastern British Columbia to the Idaho Batholith, roughly parallel with the Olympic–Wallowa Lineament), but the details of this are unknown.

[107] Several other significant events occurred around 42 Ma, including cessation of metamorphism of the Leech River Schists[108] (resulting from the Metchosin/Crescent Formation being thrust under Vancouver Island) and the end of strike-slip motion on the Straight Creek Fault;[109] these may reflect the last movement of Siletzia relative to North America.

[112] Deposition of sand from the then proximal Idaho Batholith into the Tyee Formation in southern Oregon may have continued as late as 46.5 Ma,[113] but was interrupted when Siletzia rifted from the continent and began rotating away.