page_title: Siletzia
section_title: Paleorotation
hierarchical_section_title: Siletzia / Paleorotation
caption_attribution_description: English: Illustration of paleo-magnetic rotation of Oregon.
context_page_description: 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.
context_section_description: When lava solidifies and cools it retains an imprint of the earth's magnetic field, thus recording how it was oriented. 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. These rotations are all clockwise, and show a strong correlation with the age of the rock: about one and a half degrees of rotation per million years. These paleomagnetic rotations and other evidence show that Siletzia — or the part of it constituting the Siletz terrane ("SZ" on map, above), from the Klamath Mountains to the Columbia River — has rotated clockwise as a single, coherent block.
  Did Siletzia pivot about its northern end or southern end? This question has attracted considerable attention, with the evidence long suggesting a northern pivot. 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. This was previously interpreted as requiring the Crescent Formation to have formed close to the continent. However, new high-precision U-Pb dating shows that the overlying basalts are older, and therefore the Blue Mountain unit was not overlaid by the basalts, but thrust under them at a later date. 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.
  This model has Siletzia forming on the continental margin along what is now the Olympic-Wallowa Lineament (OWL; a zone of topographical features of unknown age and tectonic significance), with the southern end of Siletzia and the Klamath Mountains (joined to Siletzia) near the Idaho Batholith in central Idaho. Further evidence for this comes from the sand of the Tyee Formation that overlie the Roseburg Formation. Not only does this sand have the same isotopic composition of rock in the Idaho Batholith (and of sand now coursing down the Snake and Columbia Rivers), but it appears to have not been transported very far from its source. This implies that the Tyee Formation was much closer to the Idaho Batholith when it was deposited, and subsequently rotated away. Geodetic surveys show that the region continues to rotate, likely as a result of extension of the Basin and Range Province and asthenospheric flow around the southern edge of the subducting Juan de Fuca plate.
  North of the Columbia River, matters are more complicated. First, in southwestern Washington there is only half as much rotation as seen in rocks of similar age in Oregon. This is the basis for believing the Crescent terrane has broken from the Siletz terrane (perhaps because they formed on different oceanic plates), and undergone a different rotational history. Second, in Washington there is more variation in the amount of rotation and more faulting, which has led to a speculation that the Crescent terrane has broken up into eight or nine crustal blocks.
  At Bremerton, on the east side of the Olympics, the measured rotations are less, and within the statistical error bounds of being zero; while further north, near Port Townsend, the rotation is slightly counter-clockwise. On Vancouver Island the paleorotations are counter-clockwise, and other evidence shows that the tip of the island has been bent, presumably as a result of the collision of Siletzia. The northwestern tip of the Olympic Peninsula also shows counter-clockwise rotation, of around 45 degrees. 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.
caption: Oregon Rotation: Rotation of Siletzia (green) about a northern pivot point. The Klamath Mountains (blue) rotated with Siletzia, having been formerly adjacent to the Blue Mountains (also blue, and also since rotated) near the Idaho Batholith (right edge). Red dashed line is the Olympic-Wallowa Lineament. Original image courtesy of William R. Dickinson.
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