Siletzia

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Siletzia , from Vancouver Island (Van) to the Klamath Mountains in Oregon . The pink area shows the extent near the surface according to magnetic and gravimetric investigations, divided into the crescent (CR) and the siletz terran (SZ); the intersecting broken lines are alternative zones of the border between CR and SZ. Leaks (with names) in black; Age data (in red, Ma = millions of years) on the left come from McCrory & Wilson (2013b), age data on the right come from Duncan (1982). The blue line is the Columbia River (the Washington-Oregon border), the red line is the Corvallis-Waldo Hills Fault, and the broken blue lines are the Olympic-Wallowa Lineament (OWL) and the Klamath Blue Mountains -Lineament (KBML), the red triangles are the main volcanoes of the Cascade Range . Modified from an illustration by Duncan (1982). The Grays River and younger parts of the Tillamook Volcanoes are now considered post-Siletzian.

Siletzia is the massive formation of marine basalts and intermediate sediments from the early to mid Eocene in the Forearc Basin of the Cascadia subduction zone ; it forms the basement of the western parts of the US states of Oregon and Washington and the southern tip of Vancouver Island . Today it is in Siletz - and the Crescent - Terran shared. (The portion of Siletzia under Oregon and southeastern Washington, excluding the Olympic Peninsula and Vancouver Island, is also called the Willamette Plate.)

Siletzia corresponds geographically to the Coast Range Volcanic Province (also called Coast Range Basalts ), but differs from the little younger basalts that were expelled after Siletzia merged with the continent and differs in chemical composition from it. The Siletzia basalts are tholeiites , characteristic of magma originating from the earth's mantle that emerges from a mid-ocean ridge between the plates of the oceanic crust. The younger basalts are alkaline or calcitic , which is characteristic of subduction magma. This change in composition reflects a shift from marine to continental volcanism that became apparent around 48 to 42 million years ago . It is also associated with the merging of Siletzia with the North American continent. (Other authors distinguish the Siletzian formations. For an up-to-date categorization see McCrory & Wilson (2013b)).

Various theories have been proposed regarding the volume and diversity of Siletzian magmatism, as well as the approximately 75 ° rotation, but the evidence is insufficient to determine the origin of Siletzia; this question remains open.

The merger of Siletzia with the North American continent an estimated 50 million years ago (coinciding with the formation of the arch of the Hawaii-Emperor chain ) was a major tectonic event related to the rearrangement of the Earth's tectonic plates . This is believed to have resulted in a shift in the subduction zone that completed the Laramian orogeny that unfolded the Rockies and brought about important changes in tectonics and volcanic activity across much of western North America.

Discovery and Exploration

The Siletzia rocks were formed in various locations by tectonic uplifts (such as on the periphery of the Olympic Mountains ), anticline folds (as in the case of the Black Hills and Willapa Hills in southwest Washington), and thrusts over other formations (along various faults in the central and southern Oregon) brought to the surface. This outcrop was variously as Metchosin formation of Vancouver Iceland as Crescent formation , as the volcanoes of the Black Hills and the Willapa Hills and as Siletz River volcanoes and Roseburg lineup referred Oregon. (See map . Washington's Grays River Volcanoes and Oregon's Tillamook Volcanoes are now considered post-Siletzian.) Elsewhere, Siletzia is covered by younger volcanic and sedimentary deposits.

The exploration of Siletzia began in 1906 with Arnold's description and naming of a small surface formation on the north side of the Olympic Peninsula near Port Crescent. Although this formation is small, he thought it very likely that much more of it was buried under younger sediments. Recognizing that similar rock emerged elsewhere, the name Crescent Formation was generally applied to all basalts of the Early and Middle Eocene of the Olympic Peninsula and the lowlands of the Puget Sound.

The Metchosin formation at the southern tip of Vancouver Iceland was described in a series of studies (1910, 1912, 1913, 1917) Clapp, of the relationship with the Crescent formation on the other Seiter of the Strait of Juan de Fuca recognized . Weaver recognized that these "metchosin volcanoes" comprised various Eocene basalts in western Washington and the Oregon Coast Range as far as the Klamath Mountains to the south . The Siletz River volcanoes were described by Snavely and Baldwin in 1948 on the basis of surface formations near the Siletz River in Oregon (originally named "Siletz River Volcanic Series" by Snavely & Baldwin (1948), renamed by Snavely et al. (1968) ). The Roseburg and other formations in southern Oregon have been described in various studies since the 1960s.

The name "Siletzia" was coined by Irving in 1979 to describe these Eocene basalts and the sediment formations embedded in them in their entirety.

expansion

The map shows the surface formations (black) and the near-surface formations (pink) of Siletzia derived from them. The latter can be discovered in the outer crust of the earth through aeromagnetic, gravitational and seismological studies.

There are only two superficial zones of contact between Siletzia and the older (pre- Cenozoic ) basement mountains in North America. One is near Roseburg, Oregon , where it was pushed against formations of the Klamath Mountains (for discussion see below), the other is along the Leech River Fault at the southern end of Vancouver Island, where they encountered the pre-Cenozoic, Wrangellian Terran has moved neighboring Pacific Rim Formation . All other points of contact between Siletzia and the rest of the continent are hidden under younger debris, especially under the Cascade Volcanoes . The contact zone around the Olympic Mountains is actually the base contact with the oceanic sediments below, which was inclined upwards by the uplift of the mountain range and came to the surface through the erosion of 10 to 12 kilometers of overlying deposits.

The location of the near-surface contact between the Crescent Formation and the pre-Cenozoic metamorphic base of the continent, known as the Coast Range Boundary Fault (CRBF), is largely uncertain. The Leech River Fault extends southeast beyond Victoria across the Juan de Fuca Strait, possibly connected to the southeast striking Southern Whidbey Island Fault (SWIF). (The Leech River Fault / CRBF have also been associated with possible faults in Discovery Bay and Puget Sound - see Puget Sound faults - but the evidence tends to speak against it; see e.g. Babcock et al. (1992) and Babcock et al. (1994)) This extends to the Rattlesnake Mountain Fault Zone (RMFZ), some 25 kilometers east of Seattle , which is believed to be the western limit of the pre-Cenozoic basement. However, gravitational data indicate that the Crescent Formation (at least on the surface) does not extend beyond east Seattle at this longitude.

Further south, near Mount St. Helens , there is a similar situation; the St. Helens Fault Zone (SHZ) is considered the eastern limit of the Crescent Formation, but the pre-Cenozoic basement is found at Mount Rainier . The separation of these forms the formation of marine sediments known as the Southern Washington Cascades Conductor (SWCC); it is possibly stored over a fragment of Siletzia. Or maybe not: the oldest parts of the SWCC may be older than Siletzia, and the nature and location of contact between these two formations is unknown.

In the central part of Oregon, Siletzia forms a platform on which the older, now extinct volcanoes of the Western Cascades lie. It is believed that the younger High Cascades (the high elevations of the Cascade Range) rest in the east on sediments that have accumulated in the basin between Siletzia and the continent.

In southern Oregon, Siletzia has been pressed against the Mesozoic Klamath Mountains along the Klamath — Blue Mountain Lineament (KBML) . This contact zone came to light near Roseburg (Oregon) at the Wild Safari Fault , where the late Jurassic Dothan Formation was pushed over the Roseburg Formation.

Beyond the coast of southern Oregon, the Eocene Fulmar Fault forms the western boundary of Siletzia. This is a leaf shift in which parts of Siletzia were split off; the missing piece could be the Yakutat Terran , which now forms the tip of the Gulf of Alaska . Further north, the terran boundary is believed to be part of the coastline on the Columbia River . (Parsons et al. (1999) used seismic data to generate a three-dimensional image of Siletzia near Washington, which includes the western border.)

The way the Crescent Formation winds around the Olympic Mountains (“Oly” on the map ) may reflect an oroclinal turn as a result of the clash with Vancouver Island. It has also been traced back to the loss of the deposits originally covering the Olympic Mountains prior to their uplift, similar to a dome with the top and western end removed.

Siletzia's current thickness and estimates vary. Under Oregon, the Siletzian Terran appears to extend to a depth of 25 or possibly 35 km into the channel between the submerging Juan de Fuca Plate and the continent's border, where it slides over sediments accumulated at the bottom of the channel. (McCrory & Wilson (2013b) assume 27 ± 5 kilometers.) The Crescent Terran (below Washington) is thought to be thinner, from a minimum of 12 ... 22 km below the western and eastern ends of the Juan de Fuca Strait, but is possibly also up to 20 ... 35 km strong.

composition

The various formations of Siletzia are characterized as marine tholeiitic "pillow" basalts and volcanic breccia , often interrupted by sedimentary layers of continental origin and lying on an oceanic crust. These are usually covered by calcitic volcanic submarine deposits. All of this suggests that these formations were originally deposited in an oceanic setting, possibly as submarine mountains or an island arch. A more detailed description of the Siletz River volcanoes can be found in Snavely et al. (1965), and one of the crescent formations at Lyttle & Clarke (1975).

The unity of the Blue Mountains at the base of the Crescent Formation on the Olympic Peninsula includes sediments (including large boulders of quartz diorite ) of continental origin, suggesting that the continent was once not far away; other sediments were removed from pre-Cenozoic rocks on Vancouver Island and the northern Cascade Range . At the southern end there are sediments from the Klamath Mountains, while the sand overlying the Tyee Formation has an isotopic composition similar to the rocks of the Idaho batholith .

Age

The ejection of the Siletzia basalts took place roughly in the late Paleocene to the middle Eocene; more accurate data is difficult to determine and therefore varies slightly. Early radiometric dating of K-Ar (Potassium Argon) and 40 Ar- 39 Ar (Argon-Argon) by Duncan indicated an age of 57… 62 million years for the northern and southern ends and an age of 49 million Years ago for the Grays River volcanoes near the center of Siletzia. This suggests the origin in a mid-ocean ridge (as previously noted by McWilliams (1980)) and had a strong influence on models that depicted the formation of Siletzia. Since then, other researchers have found more recent data (50-48 mya ) for the formation of the crescent basalts, resulting in a strong age asymmetry. (Variations in geochemical variation could also have shifted the results.)

Dates from 2010 on the basis of 40 Ar- 39 Ar, U-Pb (uranium-lead) and lime flagellates show a narrower range of age from 56 million years in the south to 50 or 49 million years in the north. High-precision U-Pb dating from northern Siletzia, collected later, showed a narrowly defined age of 51 million years for the metchosin complex on Vancouver Island. Of particular interest is the slightly wider span, estimated to be 53 to 48 million years old, for the basalts of the Crescent Formation on the east side of the Olympic Peninsula, overlying the Blue Mountain Unit and reliably dated 48 million years or less. (Wells et al. (2014) calculated a maximum age of the deposits of around 48.7 million years, while Eddy et al. (2017) found four ages between 44.7 and 47.8 million years.) This structural relationship was previously interpreted to the effect that Siletzia - or at least its northern part - was built on the continental margin. It is currently being discussed that the age inequality can be explained by the fact that the Blue Mountain Unit was pushed under Siletzia about 44.5 million years ago, and that Siletzia was not necessarily deposited along the continental margin.

size

Siletzia is quite large: more than 400 mi (644 km) long, almost half as wide (and probably quite deep). The original deposits were between 16 and 35 kilometers thick. Weaver estimates a minimum thickness of only 3,000 ft (914 m) and also "nearly 10,000 cubic miles [approx. 40,000 km³] rock "; he assumed a total volume that was at least as large as the better-known Columbia River basalts . (Quoted in Henriksen (1956)) Snavely et al. determined at least 10,000 ft (3,048 m) thickness and up to 20,000 ft (6,096 m) below the eruptive centers and a volume of up to 50,000 cubic miles (more than 200,000 km³). Duncan (1982) estimated the volume to be around 250,000 km³ (about 60,000 cubic miles), which is the volume of most continental rift zones and some basalt flood regions. The latest estimate puts the volume at 2 million km³.

Paleorotation

Rotation of Siletzia (green) around a northern pivot point. The Klamath Mountains (blue) rotated with Siletzia, were once adjacent to the Blue Mountains (also blue, also rotated since then) near the Idaho Batholith (right border). The red dashed line is the Olympic Wallowa lineament. Original image provided by William R. Dickinson.

As lava cools and solidifies, an imprint of the earth's magnetic field remains, so that the original orientation of the rock is maintained. Measurements of such paleomagnetic fields in the Oregon Coast Range show rotations of 46… 75 °, all as a result of the assumed collision of the Siletzian Terran with the continent about 50 million years ago. These rotations were all clockwise and show a close correlation with the age of the rock: about 1.5 ° per million years. These paleomagnetic rotations and other evidence show that Siletzia - or the portion that forms the Siletzian Terran ("SZ" on the first map ), from the Klamath Mountains to the Columbia River - rotated clockwise as a single coherent block. (Other possible rotation mechanisms are discussed by Globerman et al. (1982). See also Wells & Heller (1988)).

Has Siletzia turned around the north or the south end? This question has received considerable attention, and a rotation around the north end has long been assumed. (Second model from Simpson & Cox (1977), refined by Hammond (1979). Various objections to a north pivot point were made by Magill et al. (1981) who preferred an initial phase of rotation with a south pivot point. Some obvious palinspastic ones Contradictions related to the Clarno Formation in north-central Oregon appear to have been resolved by Grommé et al., 1986. A major problem for a southern pivot point is that it implies rotation during merging with the continent, while most Research suggests that most or all of the rotation happened after the assumed merger.) One key to demonstrating this is that the Crescent Formation was laid over sediments (the Blue Mountain Unit) that originated from the continent, including the about 65 million year old boulders made of quartz diorite. This was initially interpreted to mean that the Crescent Formation was formed near the continent. (See also Babcock et al. (1994) and McCrory & Wilson (2013b)) 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 were pushed underneath at a later date. Such under-pushing implies that the north end of Siletzia was originally farther from the continent and allows radial movement around a more southerly or easterly pivot point near the present-day Washington-Oregon border, as was recently suggested.

This model assumes that Siletzia was formed at the edge of the continental plate, along the zone now called Olympic-Wallowa Lineament (OWL; a zone of topographical features of unknown age and of tectonic importance) and with the southern end of Siletzia and the Klamath Mountains ( united with Siletzia) near the Idaho batholith in central Idaho. Further evidence for this comes from the sands of the Tyee Formation, which overlay the Roseburg Formation. Not only does this sand have the same isotopic composition as the rocks of the Idaho batholith (as does the sand displaced by the Snake and Columbia Rivers today), it also appears not to have been transported far from its source. This implies that the Tyee Formation was much closer to the Idaho batholith while it was being deposited until it was eventually turned away. Geodetic surveys indicate that the region continues to rotate, likely due to the extent of the Basin and Range Province and asthenospheric flow around the southern boundary of the subducted Juan de Fuca Plate.

North of the Columbia River, things are much more complicated. First, the rotation observed in southwest Washington is only half the size of that of similar-aged rocks in Oregon. This forms the basis for the assumption that the Crescent Terran broke off from the Siletzian Terran (perhaps because they were formed on different oceanic plates) and underwent a different rotational history. Second, there is greater variation in the degree of rotation and more faults in Washington, leading to speculation that the crescent terran has broken into eight or nine blocks of crust.

In present-day Bremerton on the east side of the Olympic Mountains, the measured rotation is low and within the statistical error limits to zero; otherwise further north near Port Townsend , where the rotation was slightly counterclockwise. On Vancouver Island, the paleorotations are counterclockwise, and further evidence shows that the tip of the island was bent, possibly due to a collision with Siletzia. The northwest tip of the Olympic Peninsula also shows a counterclockwise rotation of about 45 degrees. This begs the question of how much of the curved outline of the crescent formation is due to the loss of material from the center after the uplift of the Olympic Mountains, and how much of it reflects oroclinal bends.

origin

The origin of Siletzia has not yet been determined and is (as of 2017) discussed controversially. Bromley (2011) recently said "There is no definitive answer". Theories continue to be developed and even the details on which the theories are based "remain a mystery". Several of the most notable models are considered below.

There are two fundamentally different types of models of the formation of Siletzia: Brandon & Vance (1992) call these the deep-sea mountain interpretation and the marginal basin interpretation . Chan et al. (2012) only count three general models ; they limit the former to hotspot volcanism on a mid-ocean ridge and name clod windows as the third model form. Eddy et al. (2017) offer a renewed summary. (1) The formation in the open ocean (possibly as submarine mountains as in the case of the Hawaii-Emperor chain or a hotspot on a mid-ocean ridge as in the case of Iceland ) as well as the collision with the continent; (2) the formation near the coast on or near the continental margin (perhaps as a result of leaf displacement or a clod window). All current models see Siletzia afterwards as drifting away from the continental margin, namely around a northern pivot point. (Some early models depicted Siletzia as rotating into the continent around a southern pivot point, so that merging with the continent was the culmination. The southern pivot point seems largely obsolete under discussion, in part because of various studies (e.g. Heller & Ryberg (1983), Wells et al. (1984), Heller et al. (1985)) represent most of the rotation as post-merge These models have been classified as either "fused" or "cracked", but this is imprecise , because the formation close to the coast can also involve a merging and all offshore models use a northern pivot point, which implies the rifting of a rift.) Investigations into the origin of Siletzia have generally been based on two principal observations: the great paleorotation (as described above) and the voluminous output (more than 50,000 cubic miles [approx. 200,000 0 km³], which exceeds the volume of most of the continental rift valley zones and some basalt flood provinces). Taking into account the observed basalt volumes requires an extensive magnetic source, for which most models assume either the presence of the Yellowstone hotspot or that of a clod window. The latter would come from the subduction of the Farallon and Kula plates (or perhaps the reappearance of the Farallon plate). The relationship with the mid-ocean ridge that separated the Kula and Farallon plates is a significant element in all models, although its location was not well determined during this period. Babcock et al. (1992) show the uncertainty of the position of the Kula Farallon Ridge 65 million years ago by specifying it somewhere between Mexico and the Queen Charlotte Islands . Figure 1 in Haeussler et al. (2003) shows this ridge alternately near Washington and near Anchorage .

Simpson & Cox 1977: Two models

In search of an explanation for clockwise paleorotation, Simpson & Cox (1977) noted that Siletzia appeared to have rotated as a rigid block and suggested two models. The first concerned the rotation around a southern pivot point in contact with the Klamath Mountains. This creates several problems, particularly because sediments and even boulders from the continent have been found at the base of the Crescent formation at the north end, suggesting that Siletzia was close to the continent from the start. In the second model (finally improved by Hammond (1979)), Siletzia was originally adjacent to the Olympic-Wallowa lineament, then was pushed away from the continent and rotated around a northern pivot point near the Olympic Peninsula. Because the sediments also suggest loose contact of the Klamaths from the start, this requires that the Klamaths have moved with Siletzia. Originally there was a dispute over when the Klamaths and, as the rotation progressed, the Clarno Formation in central Oregon were moved. Most of these were identified in a study of the Clarno formation by Grommé et al. (1986) clarified and illustrated with a Palinspastic reconstruction of the state 38 million years ago.

Offshore model: a trapped island chain?

An early and widely cited study by Duncan (1982) (based on features of the fairly new theory of plate tectonics) is an example of the offshore or " deep sea mountain " type of models. It offers a series of radiometrically (K-Ar and 40 Ar- 39 Ar measurements) determined age data that determined younger rocks in the center (for the Grays River volcanoes) and older rocks at the edges. This two-sided symmetrical age progression is strongly reminiscent of the pattern on mid-ocean ridges, where older rock is displaced by younger rock on both sides. Duncan considered five models (but none involving ripping or subduction of the chains) and favored one with a hotspot - presumably the Yellowstone hotspot - that shared the Farallon-Kula ridge (as in Iceland) by one To create chain of islands. These islands then merged with the continent when the underlying oceanic crust was subducted.

This study has been criticized for a number of reasons, particularly the age information. Duncan himself notes that the measurement of the age in the northern part may have been influenced by the loss of argon during the low temperature metamorphism and that there may have been some noise with regard to the stratigraphic position. The latter was demonstrated in a recent study that showed, on a geochemical basis, that the Grays River volcanoes are younger (with an age of 42… 37 million years, even much younger) than Siletzia and therefore not representative of the initial Siletzian phase Magmatism can be. The current measurements show a rather monotonous growth in age from south to north.

The range of the original age was also a problem, as the rate of spread of the Kula-Farallon Ridge should have created a much longer chain of deep-sea mountains than the observed chain and would have been too far from the continent to explain the sediments of continental origin. This contradiction is weakened somewhat by the fact that the newer age regulations show a smaller range of ages.

Inshore models

Several models assume a formation of siletzia near the coast, on or near the continental margin. While all current models consider Siletzia to have split off after merging or formation, a subclass of rifting models considers this process of rift formation as the cause of the eruptions on Siletzia.

Wells et al. (1984) suggested that the Siletzia basalts could have “broken through” transform faults (perpendicular to a mid-ocean ridge) at the changes in direction of the tectonic plates. The extent of these eruptions and their location in the region is reported to be similar to that of the Yellowstone hotspot. This “transform breakthrough” theory appears to be broadly rejected, probably because the underlying model of plate motions has proven to be flawed.

Wells et al. suggested that a terran on the continental margin was pushed over the Yellowstone hotspot, this was pushed away from the continent by the rising magma, and finally formed the Siletzia basalts. This idea was borne out by Babcock et al. (1992) who suggested that rifting could have been initiated by changing the direction of movement of the plate. Kinematic effects of the migration of the Kula-Farallon Ridge along the continental margin are also possible. One such effect is the formation of a window (or a gap) in the subducted plate ("slab"), which would allow increased upwelling of magma.

Flounder Window

The fact that mid-ocean ridges could be subducted was observed early in the development of plate tectonics, but at the time there was little thought about the subsequent effects. In the 1980s the idea arose that the magma rising through the ridge from the asthenosphere did not reach the sea water and therefore could not cool down and close the gap. The continued expansion of the ridge would create a widened gap or "window" in the subducted plate through which more magma would escape. The importance of this for Siletzia was first demonstrated by Thorkelson & Taylor (1989) and Babcock et al. (1992) (based on the pioneering work of Dickinson & Snyder (1979), cited in Michaud et al. (2002) and Thorkelson (1996)). Breitsprecher et al. (2003) finally identified the fan-shaped awakening of volcanoes with characteristic geochemistry, followed by the widening of the floe window in the Kula-Farallon Plate across northeast Washington as far as Idaho. Madsen et al. (2006) showed that during most of the Eocene the subsequent magmatism from Alaska to Oregon “can be explained with terms of subduction and the floe window”. (Your model separates the northern part of the re-emergent plate about 47 million years ago to form the Eshamy plate.) This means that a clod window - and a single subducted ridge can contain several of them - can provide adequate magmatism, without being connected to a hotspot (a mantle plume ). (The magmatism can reach such proportions that it has also been suggested that the Yellowstone hotspot may have been initiated by a clod window.) Both mantle plumes and clod windows create voluminous magmatism; the main difference is that clod windows only form where the mid-ocean ridge is subducted. This implies formation on the continental margin and subsequent rifting in the manner of the second model class.

Gulf of Alaska

Any model of the origin of Siletzia must take into account the interactions with the plate boundaries that were pushed under the North American plate during the Eocene. Early studies got sick at undetermined locations of these borders, especially the Kula Farallon Ridge: Basalts on the outer borders of the Gulf of Alaska (along the Alaska Panhandle ) are as old and similarly composed as the volcanoes of Siletzia and suggest that the KF -Back far ahead of the Yukon Territory and at the same time far ahead of Washington. This can be resolved if one assumes that about 56 million years ago the eastern part of the Kula plate broke off and the Resurrection plate (about "resurrected plate") formed, while the new Kula Resurrection ridge (KR) formed the gulf stretched along from Alaska to Kodiak Island , and the former KF (now RF) ridge reached Washington. The subduction of this plate under what is now western Canada was rapid and was completed with the complete disappearance by the subduction of the KR ridge 50 million years ago.

This scenario also allows rapid transport north of the crustal blocks such as the Yakutat Terran. Located southeast of Cordova on the Gulf of Alaska today, paleomagnetic features indicate that it was formed at a latitude corresponding to present-day Oregon or Northern California. Various mica schists from Baranof Island are held analogously for the Leech River schists ( Leech River Complex ) on Vancouver Island with an age of 50 million years, which were then transported northwards with other elements of the Chugach-Prince-William-Terrane.

After merging 50… 42 million years ago

Whether formed as deep sea mountains far from the coast or near the coast by a floe window, the Siletzian basalts were deposited on a subducting oceanic plate: the Siletz terran on the Farallon plate and the Crescent terran very likely on the adjacent Resurrection plate (after this from the Kula plate had broken off, which in turn had previously detached from the Farallon plate). In both cases, Siletzia was moved in the direction of the subduction zone, which possibly ran diagonally through what is now Washington, estimated at the position of the Olympic-Wallowa lineament. This would be the Challis subduction zone, but this raises some questions. However, Siletzia was too big to be subducted and merged with the continent. Fusion is sometimes referred to as "docking", but is more like a collision: several peripheral structures are first folded or crushed, then the main structures are deformed as soon as they come into contact and different parts are pushed over others; all of this takes many millions of years. As far as possible, most studies give the age of the merger of Siletzia with North America as 50 million years. Some early studies (e.g. Duncan (1982)) dated the amalgamation no later than 42 million years ago. A recent study suggests that it happened no earlier than 55 million years ago. This dating has additional significance as it coincides with the beginning of the change in direction of the Pacific plate , as seen in the loop of the Hawaii-Emperor chain, as well as the change in the Pacific Northwest from compressional to extensional tectonics. It could also coincide with the subduction of the remainder of the Resurrection Plate under British Columbia . The initiation of the north-trending right-hand Straight Creek Fault about 48 million years ago may have been caused by stress accumulation during the merging of Siletzia with the continent.

When Siletzia merged with the continent, it also blocked the existing subduction zone, thus stopping the subduction of the Farallon Plate. This ended the Laramian orogeny that had created the Rocky Mountains and was the trigger for the "Middle-Tertiary Rain of Fire", a wave of large-volume siliceous magmatism that swept across much of western North America between 50 and 20 million years ago today. This undoubtedly had an impact on the enigmatic and controversial Challis Arc , which stretches from southeastern British Columbia to the Idaho batholith, somewhat parallel to the Olympic Wallowa lineament; however, the details are unknown.

Subduction that stalled at the existing zone eventually reinitiated the Cascadia subduction zone further west. How this happened does not seem to have been explored in detail anywhere, but Figure 5 in Simpson & Cox (1977) suggests that the new subduction zone simply opened up next to the old one, starting from the south. The volcanism emanating from the new subduction zone (such as the Grays River volcanoes and the Northcraft volcanoes) reached the surface about 42 million years ago, thereby initiating the uplift of the Cascade Range.

Several other significant events occurred approximately 42 million years ago, including the end of the transformation of the Leech River Shales (which emerged from the Metchosin / Crescent Formation thrust beneath Vancouver Island) and the halt of the plunge-slide movement at the Straight Creek Fault ; these events could reflect the recent movements of Siletzia relative to North America. On a larger scale, there was a change in the absolute direction of the Pacific plate (marked by the end of the loop in the Hawaii-Emperor chain), as well as a change in the convergence of the Kula plate with the North America plate.

Just as the subduction died down, so did the force that was pressing Siletzia against the continent; the tectonic regime changed from a compressional to an extensional one. The deposition of sands from the then-neighboring Idaho batholith into the Tyee Formation in southern Oregon may have continued until 46.5 million years ago, but was interrupted when Siletzia drifted off the continent and began to rotate. (As explained earlier, the rotation appears to have been around a north pivot point.) What caused the rift is unknown. Wells et al. (1984) assumed that the continent slipped over the Yellowstone hotspot and that the rising plume tore away a previously merged terran. Babcock et al. (1992) proposed a change in the rate at which the plates converged, or "kinematic effects" (like a clod window) from the passage of the Kula Farallon plate (or the Resurrection Farallon plate).

See also

Individual evidence

  1. ^ A b N. J. Silberling, DL Jones, MC Blake Jr., DG Howell: Lithotectonic terrane map of the western conterminous-United States . In: US Geological Survey . Miscellaneous Field Studies Map MF-1874-C, 1987, pp. 20 + 1.
  2. ^ A b R. E. Wells, CS Weaver, RJ Blakely: Forearc migration in Cascadia and its neotectonic significance . In: Geology . 26, No. 8, August 1998, pp. 759-762. doi : 10.1130 / 0091-7613 (1998) 026 <0759: famica> 2.3.co; 2 .
  3. ↑ a b c d e f g h i P. McCrory, DS Wilson: A kinematic model for the formation of the Siletz-Crescent forearc terrane by capture of coherent fragments of the Farallon and Resurrection plates. . In: Tectonics . 32, No. 3, June 2013, pp. 718-736. doi : 10.1002 / tect.20045 .
  4. ↑ a b c d e f g h i R. A. Duncan: A captured island chain in the coast range of Oregon and Washington . In: Journal of Geophysical Research . 87, No. B13, December 10, 1982, pp. 10.827-10.837. doi : 10.1029 / jb087ib13p10827 .
  5. ^ A b c d e C. F. Chan, JH Tepper, BK Nelson: Petrology of the Grays River volcanics, southwest Washington: Plume-influenced slab window magmatism in the Cascadia forearc . In: GSA Bulletin . 124, No. 7-8, July / August 2012, pp. 1324-1338. doi : 10.1130 / B30576.1 .
  6. ^ A b c d e P. D. Snavely, NS MacLeod, HC Wagner: Tholeiitic and alkalic basalts of the Eocene Siletz River Volcanics, Oregon Coast Range . In: American Journal of Science . 266, June 1968, pp. 454-481. doi : 10.2475 / ajs.266.6.454 .
  7. ↑ a b c d W. M. Phillips, TJ Walsh, RA Hagen: Eocene transition from oceanic to arc volcanism, southwest Washington . In: Muffler, LJ, Weaver, CS, Blackwell, DD (eds.): Proceedings of workshop XLIV: Geological, geophysical, and tectonic setting of the Cascade Range  (= US Geological Survey), Volume Open-File Report 89-176 December 1989, pp. 199-256.
  8. ↑ a b c d e f M. T. Brandon, JA Vance: Tectonic evolution of the Cenozoic Olympic subduction complex, Washington State, as deduced from fission track ages for detrital zircons Archived from the original on April 22, 2012. Info: The archive link was inserted automatically and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. In: AJS . 293, October 1992, pp. 565-636. doi : 10.2475 / ajs.292.8.565 . Retrieved November 30, 2018. @1@ 2Template: Webachiv / IABot / earth.geology.yale.edu
  9. ↑ a b c d A. M. Trehu, I. Asudeh, TM Brocher, JH Luetgert, WD Mooney, JL Nabelek, Y. Nakamura: Crustal architecture of the Cascadia forearc . In: Science . 266, October 14, 1994, pp. 237-243. doi : 10.1126 / science.266.5183.237 .
  10. ^ JR Magill, RE Wells, RW Simpson, AV Cox: Post-12 my rotation of southwest Washington . In: Journal of Geophysical Research . 87, No. B5, May 10, 1982, pp. 3761-3776. doi : 10.1029 / jb087ib05p03761 .
  11. ^ A b c d e f W. M. Cady: Tectonic setting of the Tertiary volcanic rocks of the Olympic Peninsula, Washington . In: USGS Journal of Research . 3, No. 5, September / October 1975, pp. 573-582.
  12. ↑ a b c S. A. Bromley: Evolution and Inheritance of Cascadia Sub-arc Mantle Reservoirs [masters thesis] . Oregon State University. June 2011. Retrieved September 5, 2018.
  13. ^ WD Sharp, DA Clague: 50-Ma Initiation of Hawaiian-Emperor Bend Records Major Change in Pacific Plate Motion . In: Science . 313, September 2006, pp. 1281-1284. doi : 10.1126 / science.1128489 .
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  16. ↑ PJ Rarey: Geology of the Hamlet-North Fork of the Nehalem River Area, Southern Clatsop and Tillamook Counties Northernmost, Northwest Oregon [masters thesis] . Oregon State University. 1985. Retrieved September 5, 2018.
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  18. ↑ a b c R. S. Babcock, CA Suczek, DC Engebretson: The Crescent "Terrane", Olympic Peninsula and Southern Vancouver Island . In: R. Lasmanis, ES Cheney (Ed.): Regional Geology of Washington State  (= DGER Bulletin), Volume 80 1994, pp. 141-157.
  19. ↑ a b D. A. Henriksen: Eocene stratigraphy of the lower Cowlitz River – eastern Willapa Hills area, southwestern Washington . In: Washington Division of Mines and Geology Bulletin . 43, 1956.
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  23. ^ EM Baldwin, RK Perttu: Eocene unconformities in the Camas Valley quadrangle, Oregon . In: Oregon Geology . 51, No. 1, January 1989, pp. 3-8.
  24. ↑ a b c R. E. Wells, AS Jayko, AR Niem, G. Black, T. Wiley, E. Baldwin, KM Molenaar, KL Wheeler, CB DuRoss, RW Givler: Geologic Map and Database of the Roseburg 30 x 60 'Quadrangle, Douglas and Coos Counties, Oregon Archived from the original on February 28, 2013. Info: The archive link was automatically inserted and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. In: US Geological Survey . Open-File Report 00-376, 2000. Retrieved April 17, 2013. @1@ 2Template: Webachiv / IABot / geopubs.wr.usgs.gov
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  27. ↑ a b c d e f g h i j k l m n o p R. S. Babcock, RF Burmester, DC Engebretson, A. Warnock, KP Clark: A Rifted Margin Origin for the Crescent Basalt and Related Rocks in the Northern Coast Range Volcanic Province, Washington and British Columbia . In: Journal of Geophysical Research . 97, No. B5, 1992, pp. 6,799-6,821. doi : 10.1029 / 91jb02926 .
  28. ↑ C. Finn: Geophysical constraints on Washington convergent margin structure . In: Journal of Geophysical Research . 95, No. B12, November 10, 1990, pp. 19,533-19,546. doi : 10.1029 / jb095ib12p19533 .  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Dead Link / ftp.agu.org  
  29. ^ A b W. D. Stanley, C. Finn, JL Plesha: Tectonics and Conductivity Structures in the Southern Washington Cascades . In: Journal of Geophysical Research . 92, No. B10, September 1987, pp. 10,179-10,193. doi : 10.1029 / jb092ib10p10179 .
  30. ↑ KC Miller, GR Keller, JM Gridley, JH Luetgert, WD Mooney, H. Thybo: Crustal structure along the west flank of the Cascades, western Washington . In: Journal of Geophysical Research . 102, No. B8, August 10, 1997, pp. 17,857-17,873. doi : 10.1029 / 97jb00882 .
  31. ↑ WD Stanley, SY Johnson, AI Qamar, CS Weaver, JM Williams: Tectonics and Seismicity of the Southern Washington Cascade Range Archived from the original on August 20, 2011. Info: The archive link was automatically inserted and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. In: Bulletin of the Seismological Society of America . 86, No. 1A, February 1996, pp. 1-18. Retrieved April 17, 2013. @1@ 2Template: Webachiv / IABot / mahabghodss.net
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  33. ↑ a b c H. Gao, ED Humphreys, H. Yao, RD van der Hilst: Crust and lithosphere structure of the northwestern US with ambient noise tomography: Terrane accretion and Cascade arc development . In: Earth and Planetary Science Letters . 304, 2011, pp. 202-211. doi : 10.1016 / j.epsl.2011.01.033 .
  34. ↑ a b c P. D. Snavely, RE Wells: Cenozoic evolution of the continental margin of Oregon and Washington . In: AM Rogers, TJ Walsh, WJ Kockelman & GR Priest (eds.): Assessing Earthquake Hazards and Reducing Risk in the Pacific Northwest  (= Professional Paper 1560), Volume 1. US Geological Survey, 1996, pp. 161-182.
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  36. ↑ SW Fleming, AM Tréhu: Crustal structure beneath the central Oregon convergent margin from potential-field modeling: Evidence for a buried basement ridge in local contact with a seaward dipping backstop Archived from the original on October 29, 2013. Info: The archive link became automatic used and not yet tested. Please check the original and archive link according to the instructions and then remove this notice. In: Journal of Geophysical Research . 104, No. B9, September 10, 1999, pp. 20.431-20.447. doi : 10.1029 / 1999jb900159 . Retrieved October 24, 2013. @1@ 2Template: Webachiv / IABot / scholarsarchive.library.oregonstate.edu
  37. ↑ a b A. S. Davis, G. Plafker: Eocene basalts from the Yakutat Terrane; evidence for the origin of an accreting terrane in southern Alaska . In: Geology . November 14, 1986, pp. 963-966. doi : 10.1130 / 0091-7613 (1986) 14 <963: EBFTYT> 2.0.CO; 2 .
  38. ^ T. Parsons, RE Wells, MA Fisher, E. Flueh, US ten Brink: Three-dimensional velocity structure of Siletzia and other accreted terranes in the Cascadia forearc of Washington Archived from the original on October 29, 2013. Info: The archive link was inserted automatically and not yet tested. Please check the original and archive link according to the instructions and then remove this notice. In: Journal of Geophysical Research . 104, No. B8, August 10, 1999, pp. 18,015-18,039. doi : 10.1029 / 1999jb900106 . Retrieved October 24, 2013. @1@ 2Template: Webachiv / IABot / profile.usgs.gov
  39. ↑ a b M. E. Beck, DC Engebretson: Paleomagnetism of small basalt exposures in the west Puget Sound area, Washington, and speculations on the accretionary origin of the Olympic Mountains . In: Journal of Geophysical Research . 87, No. B5, May 10, 1982, pp. 3755-3760. doi : 10.1029 / JB087iB05p03755 .
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  42. ↑ D. Graindorge, G. Spence, P. Charvis, JY Collot, R. Hyndman, AM Trehu: Crustal structure beneath the Strait of Juan de Fuca and southern Vancouver Island from seismic and gravity analyzes Archived from the original on November 4, 2014. Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. In: Journal of Geophysical Research: Solid Earth . 108, No. B10, October 2003. doi : 10.1029 / 2002JB001823 . Retrieved December 15, 2013. @1@ 2Template: Webachiv / IABot / geoazur.oca.eu
  43. ↑ PD Snavely, HC Wagner, NS MacLeod: Preliminary data on compositional variations of Tertiary volcanic rocks in the central part of the Oregon Coast Range . In: The Ore Bin . 27, No. 6, June 1965, pp. 101-117.
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  45. ↑ a b c P. L. Heller, PT Ryberg: Sedimentary record of subduction to forearec transition in the rotated Eocene basin of western Oregon . In: Geology . July 11, 1983, pp. 380-383. doi : 10.1130 / 0091-7613 (1983) 11 <380: SROSTF> 2.0.CO; 2 .
  46. ↑ a b c P. L. Heller, RW Tabor, CA Suczek: Paleogeographic evolution of the US Pacific Northwest during Paleogene time . In: Canadian Journal of Earth Sciences . 24, No. 8, August 1987, pp. 1652-1667. doi : 10.1139 / e87-159 .
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  48. ↑ a b c d P. L. Heller, ZE Peterman, JR O'Neil, M. Shafiquallah: Isotopic provenance of sandstones from the Eocene Tyee Formation, Oregon Coast Range . In: GSA Bulletin . 96, No. 6, June 1985, pp. 770-780. doi : 10.1130 / 0016-7606 (1985) 96 <770: IPOSFT> 2.0.CO; 2 .
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