Siletzia
Siletzia is a massive formation of early to middle
Siletzia corresponds geographically to the Coast Range Volcanic Province (or Coast Range basalts),
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
Exposures and discovery
The rock of Siletzia has been exposed in various places by tectonic uplift (as around the periphery of the
The discovery of Siletzia began in 1906 with Arnold's description and naming of a small exposure on the north side of the
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.[15] The Siletz River Volcanics was described in 1948 by Snavely and Baldwin after exposures near the Siletz River, Oregon,[16] and the Roseburg and related formations in southern Oregon described in various reports from the 1960s on.[17]
Etymology
"Siletzia" was coined in 1979 by Irving, (based in turn on the Siletz River and the Siletz Reservation), to describe the full extent of these Eocene basalts and interbedded sedimentary formations.[18]
Extent
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.[19]
There are only two exposed contacts of Siletzia with the older (pre-
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
Further south, near Mount St. Helens, is a similar situation, where the St. Helens Fault Zone (SHZ) is believed to be the eastern edge of the Crescent Formation,[25] but the pre-Cenozoic continental basement is near Mount Rainier. Separating these is the marine sedimentary formation known as the Southern Washington Cascades Conductor (SWCC); it is possible that it was deposited over a fragment of Siletzia.[26] Or not: the oldest parts of the SWCC likely predate Siletzia,[27] 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
In southern Oregon, Siletzia has been thrust against the Mesozoic Klamath Mountains of southern Oregon along the Klamath—Blue Mountain Lineament (KBML).[29] Near Roseburg this contact is exposed at the Wild Safari Fault where the Late Jurassic Dothan Formation has been thrust over the Roseburg Formation.[30]
Off the coast of southern Oregon, the western edge of Siletzia is the
The way the Crescent Formation wraps around the Olympic Mountains ("Oly" on the map) may reflect oroclinal bending as a result of being crushed against Vancouver Island.[34] It has also been attributed to loss of the deposits originally overlying the Olympics prior to their uplift,[35] 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
Composition
The various formations of Siletzia are characterized as marine
On the Olympic Peninsula the Blue Mountain unit at the base of the Crescent Formation includes sediments (including large boulders of quartz diorite) of continental origin, suggesting that the continent was close by;[40] other sediments were eroded from the pre-Cenozoic rock of Vancouver Island and the northern Cascade Range.[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.[43]
Age
Eruption of the Siletzia basalts has been placed roughly in the late
Dating from 2010 based on 40Ar-39Ar, U-Pb (uranium-lead), and
Size
Siletzia is massive: over 400 miles (600 kilometers) long, almost half that much across (and likely further at depth). The original deposits were from 16 to 35 kilometers thick.[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.[52] Snavely et al., recognizing at least 10,000 feet of thickness, and as much as 20,000 feet under eruptive centers, estimated the volume to be in excess of 50,000 cubic miles (over 200,000 km3).[53] Duncan (1982) estimated around 250,000 km3 (about 60,000 cubic miles), which exceeds the volume of most continental rift zones, and some flood basalt provinces.[54] A recent estimate put the volume at 2 million cubic km.[55]
Paleorotation
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.[56] 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.[57]
Did Siletzia pivot about its northern end or southern end? This question has attracted considerable attention, with the evidence long suggesting a northern pivot.[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. This was previously interpreted as requiring the Crescent Formation to have formed close to the continent.[59] 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.[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.[61]
This model has Siletzia forming on the continental margin along what is now the
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),[65] and undergone a different rotational history.[66] 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.[67]
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.[68] 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.[69] 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.[70]
Origin
Siletzia's origin is not yet determined, and (as of 2017) remains controversial.[71] Theories are still being developed, and even the details the theories depend on "have remained enigmatic".[72] Following are several of the most notable models.
Models of how Siletzia formed are of two general types:
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).
Simpson & Cox 1977: Two models
Seeking to explain the observed clockwise paleorotation, and noting that Siletzia appeared to have rotated as a rigid block,
Offshore model: A captured island chain?
An early and widely cited paper by
This study has been criticized on multiple grounds, particularly regarding the ages. Duncan himself noted that measurement of the northern ages may have been affected by loss of argon due to low-grade metamorphism, and that there might be bias in respect of stratigraphic position.[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. Recent dating also shows a more monotonic trend of south to north age progression ("younging").[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.[83] This objection is attenuated somewhat in that the newer ages show a smaller range of ages.[84]
Inshore models
Various models have Siletzia forming inshore, on or near the continental margin. While all current models have Siletzia rifting away from the continent after accretion or formation, a subclass of "rifted" models consider the rifting to have caused the Siletzia eruptions.
Wells, et al., alternately suggested that as a terrane at the margin of the continent was pushed over the Yellowstone hotspot, it was rifted away from the continent by the upwelling magma, which then formed the Siletzia basalts.[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. One such effect is the formation of a slab window (or slab gap) which would allow increased upwelling of magma.[88]
Slab windows
That
Madsen et al. (2006) showed that most of the Eocene and subsequent magmatism from Alaska to Oregon "is explainable in terms of ridge subduction and slab window tectonics".[91] That is, a slab window — and a single subducted ridge can give rise to multiple slab windows — can provide adequate magmatism without having to invoke a hotspot (mantle plume). (So much so that it has been suggested that the Yellowstone hotspot may have been initiated by a slab window.)[92] Mantle plumes and slab windows both feature voluminous magmatism; the main difference is that slab windows would form only where the spreading ridge is subducted. This implies formation at the continental margin, and then rifting, in the manner of the second class of models.
Gulf of Alaska
Any model of the origin of Siletzia must account for interactions with plate boundaries that were being subducted under North America through the Eocene. Early studies were plagued by indeterminate locations for these boundaries, particularly of the Kula-Farallon (K-F) spreading ridge: basalts at the head of the Gulf of Alaska (along the Alaska panhandle) have ages and compositions corresponding to the Siletz volcanics, suggesting that the K-F ridge was offshore of the Yukon at the same time it was offshore of Washington. This can be resolved by assuming that by about 56 Ma the eastern part of the Kula plate had broken away to form the Resurrection plate, with the new Kula-Resurrection (K-R) spreading ridge running up the Gulf of Alaska towards Kodiak Island, and the former K-F (now R-F) ridge reaching Washington.[93] Subduction of this plate under western Canada was rapid, and it disappeared entirely with subduction of the K-R ridge about 50 Ma.[94]
This scenario then permits rapid transport north of crustal blocks such as the
After accretion: 50-42 Ma
Whether formed far offshore as seamounts, or close inshore by a slab window, the Siletzian basalts were laid down on a subducting oceanic plate: the Siletz terrane on the
As Siletzia accreted it also jammed the existing subduction zone, halting subduction of the Farallon plate. This terminated the
Subduction, having ceased at the existing zone, eventually reinitiated further to the west as the current Cascadia subduction zone.[104] Volcanism from the new subduction zone (such as the Grays River Volcanics[105] and Northcraft Volcanics)[106] reached the surface about 42 Ma, thereby initiating the rise of the ancestral Cascade Range.[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. On a broader scale, there was a change in absolute direction of the Pacific plate[110] (marked by the end of the bend in the Hawaiian-Emperor seamount chain), and a change in the convergence of the Kula plate with the North American plate.[111]
As subduction waned so did the force that had clamped Siletzia against the continent, and the tectonic regime shifted from compressional to extensional.[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.[114] What initiated rifting is unknown. Wells et al. (1984, p. 290) suggested that as the continent overrode the Yellowstone hotspot, the upwelling plume tore away a previously accreted terrane. Babcock et al. (1992) suggested a change in the rate at which the plates were converging, or the "kinematic effects" (such as a slab window) from the passage of the Kula-Farallon ridge (or Resurrection-Farallon ridge).[115]
See also
Notes
- ^ Snavely, MacLeod & Wagner 1968, p. 454; Phillips, Walsh & Hagen 1989, p. 209; Brandon & Vance 1992, p. 571; Trehu et al. 1994, p. 237.
- ^ 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.
- ^ Brandon & Vance 1992, p. 571.
- ^ Brandon & Vance 1992, p. 571.
- ^ Phillips, Walsh & Hagen 1989, pp. 200, 205; Cady 1975, p. 573.
- ^ Phillips, Walsh & Hagen 1989. Authors have differed as to which formations are Siletzian. For a recent categorization see McCrory & Wilson 2013b, Table 1.
- ^ Bromley 2011, p. 9; McCrory & Wilson 2013b, para. 2.
- ^ Sharp & Clague 2006, p. 1283.
- ^ Gao 2011, pp. 44, 48.
- ^ Wells et al. 1984, Fig. 1; Trehu et al. 1994, note 9; McCrory & Wilson 2013b, § 2.1, Fig. 1, and Table 1.
- ^ Chan, Tepper & Nelson 2012, p. 1324; Rarey 1985, p. 87.
- ^ Arnold 1906.
- ^ Babcock, Suczek & Engebretson 1994, p. 144.
- ^ Henriksen 1956, pp. 22, 31.
- ^ Weaver 1939.
- ^ Originally named the "Siletz River Volcanic Series" by Snavely & Baldwin 1948, renamed by Snavely, MacLeod & Wagner 1968, p. 454.
- ^ Snavely, MacLeod & Wagner 1968; Baldwin 1974; Wells et al. 1984; Baldwin & Perttu 1989; Wells et al. 2000.
- ^ Irving 1979, p. 672.
- ^ Wells, Weaver & Blakely 1998, p. 760.
- ^ Wells et al. 2000, p. 15.
- ^ Massey 1986, p. 602.
- ^ Brandon & Vance 1992, p. 571.
- ^ The Leech River Fault/CRBF has also been aligned with possible faults in Discovery Bay and Puget Sound — see Puget Sound faults — but the evidence is rather against these possibilities. E.g., see Babcock et al. 1992, p. 6809 and Babcock, Suczek & Engebretson 1994, p. 149.
- ^ Finn 1990, p. 19,537.
- ^ Stanley, Finn & Plesha 1987, p. 10,179.
- ^ Miller et al. 1997, p. 17,869.
- ^ Stanley, Finn & Plesha 1987, p. 10,186; Stanley et al. 1996, pp. 4, 16.
- ^ Blakely 1994, p. 2771.
- ^ Blakely 1994, p. 2759; Gao et al. 2011, pp. 206, 208, 210.
- ^ Wells et al. 2000, p. 12, and figure 2, p. 31.
- ^ Snavely & Wells 1996, pp. 162, 171 and Fig. 64; Goldfinger et al. 1997, Fig. 2.
- ^ Fleming & Tréhu 1999, pp. 20, 442, 20, 432; Snavely & Wells 1996, pp. 172–173; Davis & Plafker 1986.
- ^ Goldfinger et al. 1997, p. 8228. Parsons et al. (1999) used seismic data to build a three-dimensional image of Siletzia beneath Washington, including the inferred western boundary.
- ^ Beck & Engebretson 1982, pp. 3757–56.
- ^ Cady 1975; Warnock, Burmester & Engebretson 1993, pp. 11, 735–11, 736.
- ^ Trehu et al. 1994; McCrory & Wilson 2013a. McCrory & Wilson (2013b, para. 7) say 27±5 km.
- ^ Graindorge et al. 2003, § 9.2; McCrory & Wilson 2013a, slides 15 and 17.
- ^ Cady 1975, p. 573 and following sources.
- ^ Snavely, MacLeod & Wagner 1968, p. 480. A more detailed description of the Siletz River Volcanics can be found in Snavely, Wagner & MacLeod 1965, and of the Crescent Formation in Lyttle & Clarke 1975.
- ^ Cady 1975, p. 579.
- ^ Snavely & Wells 1996, p. 164.
- ^ Heller & Ryberg 1983, p. 380; Heller, Tabor & Suczek 1987, p. 1662; Grommé et al. 1986, p. 14,090; Goldfinger 1990, p. 12.
- ^ Heller et al. 1985, p. 779.
- ^ Duncan 1982.
- ^ Babcock et al. 1992, p. 6815. Variations in geochemical alteration may also have skewed the results. Duncan 1982, p. 10,828; Magill, Cox & Duncan 1981, p. 2956.
- ^ Pyle et al. 2009 (abstract); Wells et al. 2010 (abstract).
- ^ Eddy, Clark & Polenz 2017, Table 1.
- ^ Wells et al. (2014, Figure 4) calculated a maximal depositional age of about 48.7 Ma, while Eddy, Clark & Polenz (2017, Table 1) report four ages ranging from 44.7 Ma to 47.8 Ma.
- ^ Eddy, Clark & Polenz 2017, p. 662.
- ^ Trehu et al. 1994, Fig. 2.
- ^ Weaver 1939.
- ^ Quoted in Henriksen 1956, p. 111.
- ^ Snavely, MacLeod & Wagner 1968, p. 456.
- ^ Babcock et al. 1992, p. 6813.
- ^ Wells et al. 2010 (abstract).
- ^ Beck & Plumley 1980, p. 573; Bates, Beck & Burmester 1981, p. 188.
- ^ Beck & Plumley 1980, p. 573; Magill, Cox & Duncan 1981, p. 2958. Other possible rotation mechanisms are discussed by Globerman, Beck & Duncan 1982, p. 1156. See also Wells & Heller 1988.
- Clarno Formation of north-central Oregon (Simpson & Cox 1977, p. 588) appear to have been resolved by Grommé et al. (1986). A major problem for a southern pivot is that it implies rotation during accretion, while most studies indicate that most or all of the rotation occurred after the presumed accretion (Heller et al. 1985, p. 779).
- ^ Cady 1975, p. 579. See also Babcock, Suczek & Engebretson 1994, pp. 141, 144 and McCrory & Wilson 2013b, § 2.1.2.
- ^ Eddy, Clark & Polenz 2017.
- ^ Wells et al. 2014, pp. 707–708.
- ^ Heller et al. 1985, pp. 770, 773, 779; Dumitru et al. 2013
- ^ Wells & Simpson 2001.
- ^ Zandt & Humphreys 2009; Wells & McCaffrey 2013.
- ^ McCrory & Wilson 2013b, paragraphs 5, 50, 54, 63–66.
- ^ Magill & Cox 1981; Globerman, Beck & Duncan 1982, p. 1155; Wells et al. 1984, p. 280.
- ^ Blakely et al. 2002.
- ^ Beck & Engebretson 1982.
- ^ Johnston & Acton 2003.
- ^ Prothero, Draus & Burns 2009.
- ^ Eddy, Clark & Polenz 2017, p. 652. Bromley (2011, p. 9) has previously said "it lacks a definite answer."
- ^ McCrory & Wilson 2013b, para. 2.
- ^ Brandon & Vance (1992, p. 571) call these the seamount interpretation and the marginal basin interpretation. Chan, Tepper & Nelson (2012, p. 1324) count only three general models, restricting the first to hotspot volcanism on a spreading ridge, and counting slab windows as a third model.Eddy, Clark & Polenz (2017, p. 652) provide an updated summary.
- ^ Some early models had Siletzia rotating into the continent about a southern pivot, accretion therefore being the culmination of rotation. The southern pivot seems to be largely abandoned, in part because various studies (e.g.: Heller & Ryberg 1983, p. 383; Wells et al. 1984, p. 280; Heller et al. 1985, p. 779) show most of the rotation was post-accretion. These classes of models have been classified as either "accreted" or "rifted," but this is inaccurate as inshore formation can still involve accretion, and all offshore accretion models using a northern pivot imply rifting.
- ^ Babcock et al. 1992, p. 6813.
- ^ Bromley 2011, p. 9.
- Queen Charlotte Islands. See also figure 1 in Haeussler et al. 2003, showing the K-F ridge alternately near Washington, or near Anchorage.
- ^ Cady 1975, p. 579.
- ^ Duncan 1982, p. 10,828.
- ^ Duncan 1982, pp. 10, 828, 10, 830.
- ^ Chan, Tepper & Nelson 2012, p. 1324. And substantially younger, at 42 to 37 Ma.
- ^ Pyle et al. 2009.
- ^ Wells et al. 1984, p. 280.
- ^ Wilson & McCrory 2010. See also McCrory & Wilson 2013b.
- ^ Wells et al. 1984, p. 289.
- ^ Lonsdale 1988, p. 752.
- ^ Wells et al. 1984, pp. 289–290.
- ^ Babcock et al. 1992, p. 6813.
- ^ Thorkelson 1996.
- ^ Michaud et al. 2002; Thorkelson 1996, p. 48.
- ^ Madsen et al. 2006, p. 31. Their model has the northern part of the Resurrection plate separating at about 47 Ma to form the Eshamy plate.
- ^ Babcock et al. 1992, p. 6819. See also Christiansen, Foulger & Evans 2002.
- ^ See figure 1 in Haeussler et al. 2003, p. 868.
- ^ Haeussler et al. 2003, p. 872.
- ^ Davis & Plafker 1986. See also Cowan 2003, p. 472.
- ^ Cowan 2003, pp. 465–471 and figure 4.
- ^ Simpson & Cox 1977, p. 588, figure 5; see also Hamilton 1969, fig. 4. This would be the Challis subduction zone, but there is some question about it. See Babcock et al. 1992, p. 6817; Brandon & Vance 1992, p. 570; Schmandt & Humphreys 2010, p. 7.
- ^ Heller & Ryberg 1983, p. 383; Phillips, Walsh & Hagen 1989, p. 199. Some early studies (e.g., Duncan 1982) dated accretion as late as 42 Ma. A recent study (Schmandt & Humphreys 2011) suggests it may have been as early as 55 Ma.
- ^ Wells et al. 1984, pp. 275, 290; Heller, Tabor & Suczek 1987, p. 1652; Babcock et al. 1992, p. 5814; Gao et al. 2011, pp. 1, 43, 58.
- ^ Haeussler et al. 2003, p. 872.
- ^ Vance & Miller 1994.
- ^ Gao 2011, p. 9; Schmandt & Humphreys 2011, p. 177.
- ^ Moye et al. 1988; Babcock et al. 1992, p. 6817; du Bray & John 2011, p. 1122.
- ^ Dickinson 1976, p. 1283; Oxford 2006, p. 12; Gao et al. 2011, p. 203. How this happened does not seem to be detailed anywhere, but figure 5 of Simpson & Cox (1977, p. 587) suggests that the new subduction zone may have simply unzipped from the old zone, starting from the south.
- ^ Chan, Tepper & Nelson 2012, p. 1324.
- ^ Babcock et al. 1992, p. 6817.
- ^ Babcock et al. 1992, p. 6813.
- ^ Clowes et al. 1987, p. 33.
- ^ Vance & Miller 1994.
- ^ Wells et al. 1984, p. 277.
- ^ Lonsdale 1988, p. 33.
- ^ Heller, Tabor & Suczek 1987, p. 1652.
- ^ Dumitru et al. 2013, p. 188.
- ^ As explained earlier, it appears the rotation was about a northern pole.
- ^ Babcock et al. 1992, pp. 6799, 6819, 6813.
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