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→‎References: Added two references.
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| url =
| url =
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* {{citation
| ref = CITEREFReidelet.al.1993
| last1 = Reidel
| first1 = S. P.
| last2 = Campbell
| first2 = N. P.
| last3 = Fecht
| first3 = K. R.
| last4 = Lindsey
| first4 = K. A.
| year = 1993
| date = Sept. 1993
| title = Late Cenozoic structure and stratigraphy of south-central Washington
| publisher = Westinghouse Hanford Company
| issue = Report WHC-SA-1764-FP
| doi = 10.2172/10193734
| url =
}}

* {{citation
* {{citation
| year = 1982
| year = 1982
Zeile 1.227: Zeile 1.247:
| title = Preliminary Interpretation of the Tectonic Stability of the Reference Repository Location, Cold Creek Syncline, Hanford site
| title = Preliminary Interpretation of the Tectonic Stability of the Reference Repository Location, Cold Creek Syncline, Hanford site
| publisher =
| publisher =
}}
* {{citation
| last1 = Saltus
| first1 = R. W.
| year = 1993
| date = Sept. 1993
| title = Upper-crustal structure beneath the Columbia River Basalt Group, Washingon: Gravity interpretation controlled by borehole and seismic studies
| journal = GSA Bulletin
| volume = 105
| issue =
| pages = 1247–1259
| doi = 10.1130/0016-7606(1993)105<1247:UCSBTC>2.3.CO;2
}}
}}


Version vom 4. April 2009, 00:58 Uhr

Location of the Olympic-Wallowa Lineament.
Is the OWL an optical illusion?

-- Parts of this article are currently being reorganized --

The Olympic-Wallowa lineament (OWL) – first noted by cartographer Erwin Raisz in 1945 [1] on a relief map of the continental United States – is a physiographic feature of unknown origin in the state of Washington (northwestern U.S.) running approximately from the town of Port Angeles, on the Olympic Peninsula to the Wallowa Mountains of northeastern Oregon.


Location

Raisz located the OWL particularly from Cape Flattery and along the north shore of Lake Crescent, thence the Little River (south of Port Angeles), Liberty Bay (Poulsbo), Elliot Bay (setting the orientation of the streets in downtown Seattle), the north shore of Mercer Island, the Cedar River (Chester Morse Reservoir), Stampede Pass (Cascade crest), the south side of the Kittitas Valley (I-90), Manastash Ridge, the Wallula Gap (on the Columbia River where it approaches the Oregon state line), and then the South Fork of the Walla Walla River into Oregon. After crossing the Blue Mountains the lineament follows a dramatic scarp on the north side of the Wallowa Mountains. Riasz observed that the OWL tends to have basins on the north side (Seattle Basin, Kittitas Valley, Pasco Basin, Walla Walla Basin) and mountains on the southern side (the Olympics, Manastash and Umtanum ridges, Rattlesnake Mountain, the Horseheaven Hills, the Wallowa Mountains), and noted parallel alignments at various points, generally about four miles north or south of the main line. The alignment of these particular features is somewhat irregular; modern maps with much more detail show a broad zone of more regular alignments.

Introduction to a puzzle

What triangle?

Most geological features are initially identified or characterized from a local expression of that feature. The OWL was first identified as a perceptual effect, a pattern perceived by the human visual system in a broad field of many seemingly random elements. But is it real? Or just an optical illusion, such as the Kanizsa triangle (see image), where we "see" a triangle that does not really exist?

Raisz considered whether the OWL might be just a chance alignment of random elements, and geologists since have not been able to find any common unitary feature, nor identify any connection between the various local elements. Yet it has been found to be coincide with many faults and fault zones, and to delineate significant differences of geology.[2] These are much too correlated to be dismissed as random alignments. But for all of its prominence, there is as yet no understanding of what the OWL is or how it came to be; it looms just beyond the horizon of current human knowledge.

The OWL piques the interest of geologically minded persons in part because its characteristic NW-SE angle of orientation – approximately 50 to 60 degrees west of north (a little short of northwest)[3] – is shared by many other seeming local features across a broad swath of geography. Around Seattle these include strikingly parallel alignments at the south end of Lake Washington, the northside of Elliot Bay, the valley of the Ship Canal, the bluff along Interlaken Blvd. (aligned with the Ship Canal, but offset slightly to the north), the alignment of Ravenna Creek (draining Green Lake southeast into Union Bay) and Carkeek Creek (northwest into Puget Sound), various stream drainages around Lake Forest Park (north end of Lake Washington), and (on the Eastside) the Northrup Valley (Hwy. 520 from Yarrow Bay to the Overlake area), and various smaller details too numerous to mention. All of these are carved into "recent" (less than 18,000 years old) glacial deposits, and it is difficult to conceive of how these could be controlled by anything other than a recent glacial process.

Yet the same orientation shows up in the Brothers, Euguene-Denio, and McLoughlin fault zones in Oregon (see map, below), which are geological features tens of millions of years old, and the Walker Lane lineament in Nevada.

Likewise to the east, where both the OWL and the Brothers Fault Zone become less distinct in Idaho where they hit the old North American continental craton and the track of Yellowstone hotspot. But some 50 miles to the north is the parallel Trans-Idaho Discontinuity, and further north, the Osburn fault (Lewis and Clark line) running roughly from Missoula to Spokane. And aeromagnetic[4] and gravitational anomaly[5] surveys suggest extension into the interior of the continent.

All of these alignments seem too strong to be random, but as yet it is quite a puzzle of how features millions of years old are linked with features only thousands of years young, and across hundreds of miles of diverse geology.

Structural relationships with other features

A problem in evaluating any hypothesis regarding the OWL is a dearth of evidence. Raisz suggested that the OWL might be a "transcurrent fault" (long strike-slip faults at what are now known to be plate boundaries), but lacked both data and competence to assess it. One of the first speculations that the OWL might be a major geological structure Vorlage:Harv – written when the theory of plate tectonics was still new and not entirely accepted – was called by the author "an outrageous hypothesis". Modern investigation is still largely balked by the immense span of geography involved and lack of continuous structures, the lack of clearly cross-cutting features, and a confusing expression in both rock millions of years old and glacial sediments only 16,000 years old.

Geological investigation of a feature begins with determining its structure, composition, age, and relationship with other features. The OWL does not cooperate. It is expressed as an orientation in many elements of diverse structure and compositions, and even as a boundary between areas of differing structure and composition; there is yet no understanding of what kind of feature or process – the "ur-OWL" – could control this. Nor are there particular "OWL" rocks which can be examined and radiometrically dated. We are left with determining its age by looking at its relationship with other features, such as which features overlap or cross-cut other (presumably older) features. In the following sections we will look at several features which might be expected to have some kind of structural relationship with the OWL, and consider what they might tell us about the OWL.

Cascade Range

Major geological structures in Washington and Oregon:  SCF – Straight Creek fault; SB – Snoqualmie batholith; OWL – Olympic-Wallowa lineament; L&C – Lewis and Clark line (gravity anomaly); HF – Hite fault; KBM – Klamath-Blue Mountains lineament; NC – Newberry caldera; BFZ – Brothers Fault zone; EDFZ –Eugene-Denio fault zone; MFZ – Modoc fault zone; WSRP – western Snake River Plain; NR –  Nevada Rift zone; OIG – Oregon-Idaho graben; CE – Clearwater Embayment; (From Vorlage:Harvnb, Fig. 1, PNNL)

The most notable geological feature crossing the OWL is the Cascade Range, raised up in the Pliocene (two to five million years ago) as a result of Cascadia subuction zone. These mountains are distinctly different on either side of the OWL, the material of the South Cascades being Cenozoic (<66 m.y.) volcanic and sedimentary rock, and the North Cascades being much older Paleozoic (hundreds of millions of years) metamorphic and plutonic rocks.[6] It is unknown whether this difference is in any way linked with the OWL, or is simply a coincidental regional difference.

Raisz judged the Cascades on the north side of the OWL to be offset about six miles to the west, and similarly for the Blue Mountains, but this is questionable, and similar offsets are not apparent in the older – up to 17 Ma (millions of years) old – Columbia River basalt flows. In general, there are no clear indications of structures offset by the OWL, but neither are there any distinct features crossing the OWL (and older than 17 my) that positively demonstrate a lack of offsetting.

Straight Creek Fault

The Straight Creek Fault (SCF) - just east of Snoqualmie Pass and running nearly due north into Canada - is a major fault notable for considerable identified dextral strike-slip offset (opposite side moving laterally to the right) of at least 85 km. [7]). Its intersection with the OWL (near Kachess Lake) is the geological equivalent of an atom smasher, and the results should be informative. For example, that the OWL is not offset suggests that it must be younger than the last strike-slip motion on the SCF, anywhere from 25 million [8] to about 41 million [9] years ago (i.e., during the Oligocene or Eocene epochs).

So does the OWL offset the SCF, or not? It is hard to say, as no trace whatsoever has been found of the SCF anywhere south of the OWL. While some geologists have speculated that it does continue directly south, albeit hidden under younger deposits, [10] not a trace has been found. Morever, the petrology to the south of the OWL does not seem to correspond with the petrology offset by the SCF.

It has been speculated that the missing part of the SCF might have been offset as far west as Puget Sound, but no evidence has been presented, and no indications found south of Olympia, where it should emerge from the glacial deposits that cover the bedrock of Puget Sound. [11]

Closer examination shows splays of the SCF turning and merging with the Taneum fault (coincident with the OWL) south of Kachess Lake. [12] This conforms with the general pattern seen in Lakes Keechelus, Kachess, and Cle Elum, and associated geological units: each is aligned northerly at the north end, but turns to the southeast where it approaches the OWL. This is suggestive of the OWL being a left lateral (sinistral) strike-slip fault that has distorted and offset the SCF. But that is inconsistent with the SCF itself and most other strike-slip faults associated with the OWL being dextral, and incompatible with the geology to the southeast.

A deeper look reveals "a history of dominantly vertical movement along the Straight Creek Fault and its southeasterly splays." [13] It is conceivable that this vertical movement reflects one side being forced up a crustal ramp, where upon it was quickly eroded and distributed as sediments. (The observed curvature of the SCF splays might then be an artefact of geometrical perspective: not a curvature of direction but a curvature of how the shape of the emerging fault intercepts the surface.)

The possibility of the southerly moving eastern flank of the SCF [14] rising vertically and effectively disappearing illustrates a typical complication. Normally a feature is younger than the last movement of any strike-slip fault it crosses or offsets. And so the OWL should be younger than the last strike-slip movement on the SCF (sometime in the Oligocene). But if the OWL has some way of averting an on-coming strike-slip fault (such as ramping it skyward), then the normal relationship does not hold, and the OWL could be considerably older. So while the hypothesis suggested above can dispose of the problems of the missing section and of sinistral curving, it opens another possibility that (as has been suggested) "activity along the Straight Creek fault continued later than any throughgoing faulting on the lineament."[15] With so little data to sort out the many possible complications even the one datum we thought we had becomes less certain.

CLEW and Columbia Plateau

Further east is the "CLEW", the segment of the OWL from approximately the town of Cle Elum (marking the eastern limit of the Columbia River basalts) to the Wallula Gap (a narrow gap on the Columbia River just north of the Oregon border). This segment, and the associated Yakima fold belts, do include many northeast-trending faults crossing the OWL. However, these are largely dip-slip (vertical) faults, associated with compressional folding of the overlying basalt. As there are typically 3 km of sedimentary deposits separating the basalts (also about 3 km thick) from the basement rock[16], these faults are somewhat isolated from the deeper structure. The geological consensus is that any strike-slip activity on the OWL predates the 17 Ma old Columbia River Basalt Group.[17]

There is some evidence that some of the northwest-trending ridges may have some continuity with the basement structure, but the nature and details of the deeper structure is not known.[18] A 260 km long seismic refraction profile[19] showed a rise in the crustal basement beneath the OWL, but was unable to determine if that rise was aligned with the OWL, or just coincidentally crossed the OWL at the same location as the profile; gravity data suggested the latter. The seismic data showed a uniformity of rock type and thickness across the OWL that discounts the possibility of it being a boundary between continental and oceanic crust. The results were interpreted as suggesting continental rifting during the Eocene, perhaps a failed rift basin[20], possibly connected with the rotation of the Klammath Mountain block away from the Idaho Batholith (see "Columbia Embayment", below).

Hite Fault System

Past the Wallula Gap the OWL is identified with the Wallula Fault Zone, which heads towards the Blue Mountains. The Wallula Fault Zone is active, but whether that can be attributed to the OWL is unknown: it may be that, like the Yakima Fold Belt, it is a result of regional stresses, and is expressed only in the surficial basalt, quite independently of what ever is happening in the basement rock.

At the western edge of the Blue Mountains the Wallula Fault zone intersects the northeast-striking Hite Fault System (HFS). This system is complex and has been variously interpreted.[21] Although seismically active it appears to be offset by, and thus should be older than, the Wallula fault.[22] Reidel, et. al.[23] suggested that the HFS reflects the eastern margin of a piece of old continental craton (centered around the "HF" - Hite Fault - on the map) that has slipped south; Kuehn[24] attributed 80 to 100 kilometers of left-lateral displacement along the HFS (and significant vertical displacements).

The interaction of the Wallula and Hite Fault systems is not yet understood. Past the Hite Fault System the OWL enters a region of geological complexity and confusion, where even the trace of the OWL is less clear, even to the point where it has been suggested that both the topographic feature and the Wallula fault are terminated by the Hite fault.[25] The original topographic lineament as described by Raisz is along the scarp on the northeast side of the Wallowa Mountains. However, there is a sense that the trend of the faulting in that area turns more to the south; it has been suggested the faulting associated with the OWL takes a large step south to the Vale Fault Zone[26], which connects with the Snake River Fault Zone in Idaho.[27] Both of these lines introduce a bend into the OWL. The Imnaha Fault (striking towards Riggins, Idaho) is more nearly in line with the rest of the OWL, and in line with the previously mentioned gravitational anomalies that run into the continent.[28] Which ever way is deemed correct, it is notable that the OWL seems to change character after it crosses the Hite Fault System. What this says about the nature of the OWL is unclear, although Kuehn concluded that, in northeastern Oregon or western Idaho, it is not a tectonically significant structure.

Columbia Embayment and North Pacific Rim orogenic Stream

Most of the mountain ranges in the western United States – such as the North Cascades, the Rocky Mountains, and the Sierra Nevada mountains – are comprised of pre-Cenozoic (or pre-Tertiary[29]) rock, older than 66 million years. The exception is southwestern Washington and Oregon – particularly, most everything south of the OWL and north of the Klammath-Blue Mountains Lineament (see below)[30] – where almost no pre-Cenozoic rock is found. This is the Columbia Embayment, a large indentation into the North American continent characterized by oceanic crust covered by thick sedimentary deposits, formerly a shallow sea.[31] "Embayment" is perhaps a misleading term, in that it suggests a bowing of a coast line, which only seems so in the context of the modern coast. In the geological past, the coast of North America was in Idaho and Nevada, at the edge of the North American craton (the ancient, stable core of the continental crust). The context of this embayment is in what has recently been termed the North Pacific Rim orogenic Stream (NPRS) – the stream of crustal blocks (or terranes) slipping north along the edge of the craton.[32] The terranes constituting the Canadian Cordillera – from the North Cascades into Alaska – happen to include a lot of older continental crust. The OWL is approximately the forward edge of several blocks of oceanic crust, which have been collecting Cenozoic sediments, and which are being pushed northward by the Sierra Nevada block in California and the Pacific plate. As these blocks are jammed onto the edge of the North American plate the transform fault and any subduction zone at the edge of the plate steps outward. Even though these blocks are now jammed against the North American plate, they are subject to a lot of oblique force (as shown by intense folding and metamorphism, presumably because of the Cascadia subduction zone), and, north of the OWL, continue to slide north along a series of major faults.

The crustal blocks underlying Oregon and southwestern Washington have been snubbed against the craton (roughly Idaho, eastern British Columbia, and a corner of Washington) somewhat obliquely, and the forces pushing on them have caused them to rotate clockwise. Paleomagnetic measurements show that there was a rapid rotation of about 40° about 45 Ma ago (during the Eocene), and perhaps 30° since; the pole of rotation has been variously reported as around Portland, or perhaps to the north.[33] The implications of this regarding the Straight Creek Fault do not appear to have been studied.

Blue Mountains and Klamath Blue Mountain Lineament

The Blue Mountains are almost as enigmatic as the OWL. As part of the Klamath-Blue Mountain Lineament (KBML)[34] trending southwest to the Klamath Mountains on the Oregon coast, they cut off the western end of the Brothers Fault Zone. Like the OWL, they also seem to bound the Columbia Embayment, but whether there is any connection or significance is unknown. It has been suggested that the KBML might be a piece of an older margin to the North American plate,[35] its current orientation (southwest-northeast) resulting from the crustal rotation mentioned earlier. If the full 70 degrees of rotation is undone, then 45 million years ago (mid-Eocene) the KBML was not only oriented nearly the same as the OWL, but also aligned with it.[36] In other words, the KBML seems to be a piece of the OWL that has been bent approximately 70 degrees to the southwest. The cause of this is unclear; perhaps something to do with the subduction of the Farallon Plate. If the KBML was formerly a continental margin, then might the OWL also have been a continental margin? This implies a continental-oceanic crustal boundary (as has been suggested[37]). However, geophysical studies in the Columbia Plateau do not see the kinds of differences expected at such a boundary[38], and in the Puget Sound Lowland the OWL is seen to not follow the crustal boundary.[39] The OWL may well be associated with a continental margin, or even rifting, but such a "solution" is incomplete at best.

The apparent close affinity of the KBML with both the OWL and the similarly oriented Brothers, Euguene-Denio, and McLoughlin fault zones in southern Oregon raises an intriguing question of whether the KBML connects them in some way. But that possibility does not seem to have been raised even speculatively.

Puget Sound

Another notable feature that crosses the OWL is Puget Sound, and it is curious to consider the possible implications of a Puget Sound Fault. (Such a fault was once proposed[40] on the basis of certain marine seismic data, but the proposal was stiffly rejected, and now seems to have been abandoned.) Combined terrestrial and bathymetric topography shows a distinct lineament along the west side of Puget Sound from approximately Point Jefferson (just across from north Seattle) northward to Holmes Harbor (Whidbey Island) and on to approximately the Whidbey Island Naval Air Station, where it meets an east-west fault. This lineament can also be seen extending south of Point Jefferson 24 miles to Maury Island, but there it is shifted several miles to the west. Curiously, the southern section lies in the approximate zone of the OWL. This suggests dextral offset along a strike-slip fault. But if that is the case then there should be a major fault in the vicinity of Point Jefferson and crossing north Seattle (perhaps at the Ship Canal) – but for this there is even less evidence then there was for the Puget Sound fault.

Alternately – and this would seem very pertinent in regard of the OWL – perhaps some mechanism other than strike-slip faulting creates these lineaments.

Seattle Fault

A locally notable feature that crosses the zone of the OWL is the west-east Seattle Fault. This is not a strike-slip fault, but a thrust fault, where a relatively shallow slab of rock from the south is being pushed against and over the northern part. One model has the slab of rock being forced up by some structure about 8 km deep. Another model has the base of the slab (again, about 8 km deep) catching on something, which causes the leading edge to roll.[41] The nature of the underlying structure is not known; geophysical data does not indicate a major fault nor any kind of crustal boundary along the front of the Seattle Fault, nor along the OWL, but this could be due to the limited reach of geophysical methods. Recent geological mapping at the eastern side of the Seattle Fault[42] suggests a decollement (horizontal plane) about 18 km deep.

These models were developed in study of the western segment of the Seattle Fault. In the center segment, where it crosses surface exposures of Eocene rock associated with the OWL, the various strands of the fault – elsewhere fairly orderly – meander. The significance of this and the nature of the interaction with the Eocene rock are also not known.[43]

Southern Whidbey Island Fault and Rattlesnake Mountain Fault Zone

The Southern Whidbey Island Fault (SWIF), running nearly parallel to the OWL from Victoria, B.C., southeast to the Cascade foothills to a point northeast of Seattle, is notable as the contact between the Coast Range block of oceanic crust to the west and the Cascades block of pre-Tertiary continental crust to the east.[44] It appears to connect with the more southerly oriented right-lateral Rattlesnake Mountain Fault Zone (RMFZ) straddling Rattlesnake Mountain (near North Bend), which shows a similar deep-seated contact between different kinds of basement rock.[45] At the southern end of Rattlesnake Mountain – exactly where the first lineament of the OWL is encountered – at least one strand of the RMFZ (the others are hidden) turns to run by Cedar Falls and up the Cedar River. Other faults to the south also show a similar turn,[46] suggesting a general turning or bending across the OWL, yet such a bend is not apparent in the pattern of physiographic features that express the OWL. With awareness that the Seattle Fault and the RMFZ are the edges of a large sheet of material which is moving north, there is a distinct impression that these faults, and even some of the topographical features, are flowing around the corner of the Snoqualmie Valley. If it seems odd that a mountain should "float" around a valley: bear in mind that while the surface relief is about three-quarters of a kilometer (half a mile) in height, the material flowing could be as much as eighteen kilometers deep.[47] (The analogy of ice-bergs moving around a submerged sandbar is quite apt.) It is worth noting that Cedar Butte – a minor prominence just east of Cedar Falls – is the southwestern-most exposure in the region of some very old Cretaceaous (pre-Tertiary) metamorphic rock.[48] It seems quite plausible that there is some well-founded and obdurate obstruction at depth, around which the shallower and younger sedimentary formations are flowing. In such a context the observed arcuate fault bends would be very natural.

Examination of the various strands of the Seattle Fault, particularly in the central section, is similarly suggestive of ripples in a flow that is obliquely crossing some deeper sill. This is an intriguing idea that could explain how local and seemingly independent features could be organized from depth, and even across a large scale, but it does not seem to have been considered. This likely due, in part, to a paucity of information on the nature and structure of the lower crust where such a sill would exist.

Broader context

[ Under construction! This section is being reorganized and rewritten. ]

It is generally assumed that the pattern of the OWL is a manifestation of some deeper physical structure or process (the "ur-OWL"), which might be elucidated by studying the effects it has on other structures. As has been shown, study of features that should interact with OWL has yielded very little: a tentative age range (between 45 and 17 million years), suggestions that the ur-OWL arises from deep in the crust, and evidence that the OWL is not (contrary to expecations) itself a boundary between oceanic and continental crust.

The lack of results so far suggests that the broader context of the OWL should be considered. Following are some elements of that broader context, which may – or may not – relate in some way to the OWL.

Plate tectonics

The broadest and fullest context of the OWL is the global system of plate tectonics, driven by convective flows in the Earth's mantle. The primary story on the western margin of North America is the accretion, subduction, obduction, and translation of plates, micro-plates, terranes, and crustal blocks between the converging Pacific and North American plates. (For an excellent geological history of Washington, including plate tectonics, see the Burke Museum web site.)

The principal tectonic plate in this region (Washington, Oregon, Idaho) is the North American plate, consisting of a craton of ancient, relatively stable continental crust and various additional parts that have been accreted; this is essentially the whole of the North American continent. The interaction of the North American plate with various other plates, terranes, etc., along its western margin is the primary engine of geology in this region.

Since the breakup of the Pangaea supercontinent in the Jurassic (about 250 million years ago) the main tectonic story here has been the North American Plate's subduction of the Farallon Plate (see below) and its remaining fragments (such as the Juan de Fuca, Gorda, and Explorer plates). As the North American plate overrides the last of each remnant it comes into contact with the Pacific Plate, generally forming a transform fault, such as the Queen Charlotte Fault running north of Vancouver Island, and the San Andreas Fault on the coast of California. Between these is the Cascadia subduction zone, the last portion of a subduction zone that once stretched from Central America to Alaska.

This has not been a steady process. Around 42 to 50 Ma (million years) ago[49] there was a change in the direction of motion of the Pacific plate (as recorded in the bend in the Hawaiian-Emperor seamount chain). This had repercussions on all the adjoining plates, and may have had something to do with initiation of the Straight Creek Fault ~48 Ma ago[50], and the end of the Laramide orogeny (uplifting of the Rocky Mountains). This event may have set the stage for the OWL, as much of the crust in which it is expressed was formed around that epoch (the early Eocene); this may be when the story of the OWL starts. Other evidence suggests a similar plate reorganization around 80 Ma[51], possibly connected with the start of the Laramide orogeny. Vorlage:Harvtxt claimed at least five "major chaotic tectonic events since the Triassic" at approximately 225, 152, 92, 44, and 15 Ma. Each of these events is a possible candidate for creating some condition or structure that affected the OWL or ur-OWL, but knowledge of what these events were or their effects is itself still chaotic.

Complicating the geology is a stream of terranes – crustal blocks – that have been streaming north along the continental margin[52] for over 120 Ma[53] (and probably much, much earliar), what has recently been called the North Pacific Rim orgenic Stream (NPRS) [54]. However, these terranes may be incidental to the OWL, as there are suggestions that local tectonic structures may be substantially affected by deeper and much older (e.g., Precambrian) basement rock, and even lithospheric mantle structures.[55]

Subduction of the Farallon Plate

120 million years ago (the Cretaceous Period) the entire Pacific coast of North America, from Alaska to Central America, was a subduction zone, as the North American Plate overran and subducted the Farallon Plate. The Farallon plate is notable for having been very large, and for subducting nearly horizontally under much of the United States and Mexico; it is likely connected with the Laramide Orogeny.[56]

About 85 Ma ago things got more complicated when the Kula plate separated from the Farallon plate; the location the Kula/Farallon boundary is unknown.[57] There may have been other plates present, such as the Resurrection Plate,[58] and smaller plates of which there is no record. All of these plates added to the complications of regional volcanism and orogeny.

Around 30 Ma ago part of the spreading center between the Farallon Plate and Pacific Plate was subducted under California, putting the Pacific plate into direct contact with the North American plate and creating the San Andreas Fault. The remainder of the Farallon Plate split, with the part to the north becoming the Juan de Fuca Plate; parts of this subsequently broke off to form the Gorda Plate and Explorer Plate. By this time the last of the Kula Plate had been subducted, initiating the Queen Charlotte transform fault on the coast of British Columbia; coastal subduction has been reduced to just the Cascadia Subduction Zone under Oregon and Washington.[59]

The fragmentation and subduction of these plates, along with the accretion of terranes to the continent, caused the subduction zones, with their consequent oceanic trenches and inland volcanism, to jump around, greatly complicating the geological development of the western United States. This is the backdrop into which any explanation of the OWL must be fitted.

Orofino Shear Zone

(Being revised.) In much of the foregoing the OWL can be seen to be contextually related to broader and older features and developments. Stepping back a little back and taking a slightly broader (and trans-border) view shows that the west end of the OWL makes a smoothly curving alignment with the west side of Vancouver Island and then the more northerly trending coast of British Columbia. (Or it could be aligned with the Kodiak Seamounts.[60]) Much of this is a transform fault where the oceanic plate slides past the accreted terranes at the edge of the North American continent. The continuity with the OWL suggests that it, too, might have been associated with such a fault.

Such a possibility is somewhat complicated in Idaho, near the eastern end of the OWL, where the pre-Tertiary boundary between accreted ocean rock and continental rock - believed to be the edge of the continent for much of the Cretaceous period - makes a sharp right turn, and puts a kink into any idea of this being a transform fault.

This bend is where the north-trending Western Idaho Shear Zone (WISZ; also known as the Salmon River suture zone) meets the east-trending Orofino Shear Zone (OSZ; connected with the Trans-Idaho Discontinuity, roughly parallel to the OWL and just north of it). The co-similarity of the OSZ with the similar trending OWL, that the WISZ seems to terminate the OWL, and that this junction is just beyond the apex of the Columbia Embayment, all suggest that there is some kind of relationship between these features, and shows that understanding the OWL requires a much better understanding of its broader context.

Recent (2007) work[61] shows that the OSZ and WISZ are separate structures of different ages. U-Pb radiometric dating shows that magmatism and deformation on the WISZ (when it was involved with northward migrating terranes) dates to 120 to 90 Ma ago. Its truncation by the OSZ is dated at 90 to 70 Ma. (These dates are mid- and late-Cretaceaous.) If the OWL is directly connected with active deformation on the OSZ (instead of, say, both being independently derived from some persistent master structure or process) it would then be twice as old as most of the rock underlying western Washington, and older than the Straight Creek Fault. Perhaps this is possible, perhaps the OWL reflects some deep and persistent structure in the underlying lithosphere. But this is currently rather more impossible than most geologists will accept.

Other recent (2007) work[62] incorporates the Trans-Idaho Discontinuity and other zones of faulting and deformation into the Great Divide megashear, runnng over 500 kilometers (300 miles) from eastern Washington through Idaho and along the continental divide to the northeastern end of the Snake River Plain. This is interpreted as an ancient (Mesoproterozoic – about a billion years ago) left–lateral shear of the continental craton. [63] That it also seems to show 'recent' (a mere 90 Ma ago) shearing (see above) illustrates how complex geology can become.


Newberry Hotspot Track – Brothers Fault Zone

The Newberry Hotspot Track – a series of volcanic domes and lava flows closely coincident with the Brothers Fault Zone (BFZ) - is of interest because it is parallel to the OWL, has significant magmatic activity at its western end (the Newberry Volcano), and has been associated with the Yellowstone hotspot. But like the OWL, the orientation of the BFZ is inconsistent with motion of the continental plate over the mantle. Also, and very significantly, the age progression of lavas along the BFZ is westward from the Oregon-Nevada border (where the Yellowstone Hotspot is believed to have started); the general consensus is that Newberry volcanics are associated with the Yellowstone hotspot. [64]

Why the Newberry volcanics are organized linearly is still an open question. Suggestions include interactions with the edge of the subducted Farallon Plate (a.k.a. "Vancouver slab"), faulting in the underlying lithosphere, or exploitation of existing faulting, possibly associated with extension of the Basin and Range province. [65] The last possibility, that "recent" (circa 17 Ma old) volcanic activity might be following a much older line of weakness, possibly a fault of some kind, at great depth (and therefore below the various terranes) is intriguing: it suggests how features of disparate age might be linked.

Lithospheric rifting

Oceanic crust rises at, and then flows out from, spreading centers (such as the Mid-Atlantic Ridge or East Pacific Rise). These spreading centers frequently offset, forming long transform faults aligned with the flow of the crust away from the spreading center. Intriguingly, the OWL seems to be aligned (as well as the Brothers and other fault zones) with the transform faults from the Juan de Fuca spreading ridge.

Puzzle pieces

How any of the broader context relates to the OWL is unknown, as nothing is known of the nature of the ur-OWL. When the story is finally known it may be that the significance of some of these elements is in their non-significance.[66]

In attempting to solve puzzles it is often the case that pieces seem impossible to fit – until we discover we have been holding them the wrong way. It is often the same way with scientific puzzles. In an article titled "The value of outrageous geological hypotheses", Vorlage:Harvtxt mentioned "the Wegener outrage of wandering continents". It is instructive to note that the similarity of the western and eastern shores of the Atlantic Ocean - obvious even to a schoolchild - was long a puzzle. It seemed to require choosing between obvious impossibilities: either some kind of control of shore-line building processes over thousands of miles apart, or "wandering continents". Either of these seemed outrageous. Wegener's theory of continental drift, first published in 1912, was rejected on the basis of impossibility until the theory of plate tectonics explained how continents could 'wander'. Such reinterpretation of concepts and evidence seems characteristic of the resolution of puzzles; it is intriguing to consider what modifications and extensions will result when the puzzle of the OWL is finally solved.

Retrospectively the solution may seem "obvious", with future generations wondering why we could not sort it out. Perhaps it will be "obvious" – once we get all the pieces turned around the right way up.

Summary: What we know about the OWL

  • First reported by Erwin Raisz in 1945.
  • Seems to have more depressions and basins on the north side.
  • Associated with many right-lateral strike-slip fault zones.
  • Seems to be expressed in Quaternary (recent) glacial deposits.
  • Does not offset Columbia River Basalts, so older than 17 million years.
  • Not offset by the Straight Creek Fault, so probably younger than 45 million years. (Maybe.)
  • Approximately separates oceanic-continental provinces.
  • Not an oceanic-continental crustal boundary. (Maybe.)
  • Not a hotspot track. (Maybe.)

Notes

Vorlage:Reflist

References

Note: some catalogs mis-identify this item as edited by K. A. Bergstrom. Also, another item with the same editor, title, and year (report SD-BWI-TI-111, 175p.) is actually the rough-draft of this item.
  1. ↑ Vorlage:Harvnb.
  2. ↑ For instance, in the Columbia Plateau the OWL marks a difference in structural expression, with strike-slip faulting and rotation predominate to the southwest but subordinate to the northeast Vorlage:Harv.
  3. ↑ Estimating the northing and westing from a map and applying the usual trignometric methods gives an angle of 59 degrees west of north (azimuth 301 deg.) from Wallula Gap to Cape Flattery. There is a bit of a bend east of Port Angeles – the shore line between Pillar Point to Slip Point has a more westerly angle of 65 degrees - but that section is so short that the angle from Wallula Gap to Port Angeles is still 57 degrees. A line run from the strong relief at Gold Creek to the mouth of Liberty Bay and beyond &nash; a line that runs along several seeming OWL features – has an angle of 52 deg. In Seattle the angle of the Ship Canal (which is a reasonably close proxy for the natural feature it lies in) has an angle of 55 degrees. It is possible that what ever causes the OWL is straight, but at depth, and its expression towards the surface is deflected by other structures. E.g., the Olympic Mountain batholith might be pushing Gold Creek out of alignment. And perhaps the Blue Mountains cause a similar bend. But this is entirely speculative.
  4. ↑ Vorlage:Harvnb.
  5. ↑ Vorlage:Harvnb.
  6. ↑ Vorlage:Harvnb.
  7. ↑ Vorlage:Harvnb.
  8. ↑ Vorlage:Harvnb.
  9. ↑ Vorlage:Harvnb.
  10. ↑ Vorlage:Harvnb, p. 282.
  11. ↑ Still searching for a citable source.
  12. ↑ Vorlage:Harvnb. But Vorlage:Harvtxt holds that the SCF strikes southerly.
  13. ↑ Vorlage:Harvnb, p. 26.
  14. ↑ The North American plate - a.k.a. "stable continental craton" - to the east is usually taken as a reference point, which makes the relative motion of the western side of the SCF "to the north". But the North American plate is also in motion, relative to the framework of hotspots. If the Pacific place is taken as the reference point then the east side motion is southerly.
  15. ↑ Vorlage:Harvnb.
  16. ↑ Vorlage:Harvnb.
  17. ↑ Vorlage:Harvnb.
  18. ↑ Vorlage:Harvnb.
  19. ↑ Vorlage:Harvnb.
  20. ↑ But questioned by others. See Vorlage:Harvnb, and also Vorlage:Harvnb.
  21. ↑ Vorlage:Harvnb, p. 9.
  22. ↑ Vorlage:Harvnb; Vorlage:Harvnb, p. 97. But see also Vorlage:Harvnb, p. 90.
  23. ↑ Vorlage:Harvnb, p. 9.
  24. ↑ Vorlage:Harvnb, p. 95.
  25. ↑ Vorlage:Harvnb, p. 2-17.
  26. ↑ Vorlage:Harvnb.
  27. ↑ Vorlage:Harvnb.
  28. ↑ Vorlage:Harvnb.
  29. ↑ The Cenozoic era is everything since the dinosaurs died out; the Tertiary period is all of the Cenozoic except the last 1.6 million years. For present purposes these are interchangeable.
  30. ↑ Locating the southern side of the Columbia Embayment is somewhat problematical, as it seems to have evolved over time. During the Tertiary Period the Blue Mountains seem to have been the effective edge of the Columbia Embayment. On a broader scale the Blue, Klamath, and Sierra Nevada Mountains can be viewed as seamounts passing through a region of oceanic crust.
  31. ↑ Vorlage:Harvnb; Vorlage:Harvnb.
  32. ↑ Vorlage:Harvnb; Vorlage:Harvnb.
  33. ↑ Vorlage:Harvnb; Vorlage:Harvnb.
  34. ↑ Vorlage:Harvnb.
  35. ↑ Vorlage:Harvnb.
  36. ↑ Vorlage:Harvnb.
  37. ↑ Vorlage:Harvnb, Vorlage:Harvnb, and others.
  38. ↑ Vorlage:Harvnb; Vorlage:Harvnb. Vorlage:Harvtxt is sometimes cited in this regard, but the single sentence he had to say on this was summarizing Caggiano and Duncan.
  39. ↑ Vorlage:Harvnb.
  40. ↑ Vorlage:Harvnb.
  41. ↑ Vorlage:Harvnb.
  42. ↑ DGER Geological Map Vorlage:Harvnb.
  43. ↑ Vorlage:Harvnb.
  44. ↑ Vorlage:Harvnb.
  45. ↑ DGER Geological Map Vorlage:Harvnb.
  46. ↑ DGER Geological Map Vorlage:Harvnb. Recent mapping (DGER Geological Map Vorlage:Harvnb) shows a multiplicity of fault strands; it is possible that these seemingly arcuate faults may artefacts of slightly confused mapping.
  47. ↑ DGER Geological Map Vorlage:Harvnb.
  48. ↑ DGER Geological Map Vorlage:Harvnb.
  49. ↑ Vorlage:Harvnb.
  50. ↑ Vorlage:Harvnb.
  51. ↑ Vorlage:Harvnb
  52. ↑ Vorlage:Harvnb;Vorlage:Harvnb; Vorlage:Harvnb.
  53. ↑ Vorlage:Harvnb.
  54. ↑ Vorlage:Harvnb.
  55. ↑ Vorlage:Harvnb;Vorlage:Harvnb.
  56. ↑ Vorlage:Harvnb; Burke Museum.
  57. ↑ Vorlage:Harvnb; Vorlage:Harvnb; Vorlage:Harvnb; Vorlage:Harvnb; Vorlage:Harvnb.
  58. ↑ Vorlage:Harvnb.
  59. ↑ Vorlage:Harvnb; Vorlage:Harvnb; Burke Museum.
  60. ↑ Vorlage:Harvnb.
  61. ↑ Vorlage:Harvnb. See also Vorlage:Harvtxt, who also derive a chronology of several events including accretion of terranes.
  62. ↑ Vorlage:Harvnb.
  63. ↑ Very similar to the proposal of Vorlage:Harvtxt, except for the reversed sense of slip.
  64. ↑ Vorlage:Harvnb; Vorlage:Harvnb; Vorlage:Harvnb.
  65. ↑ Vorlage:Harvnb.
  66. ↑ Like in Arthur Conan Doyle's story "White Blaze", where Sherlock Holmes refers to "the curious incident of the dog in the night-time." "The dog did nothing in the night-time", says Inspector Gregory. "That was the curious incident."
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