The Olympic–Wallowa lineament (OWL) is a series of geologic structures oriented from northwest to southeast for across Washington and northeast Oregon in the United States, passing through the Seattle area and including notable features east of the Cascade Range such as the Yakima Fold Belt and Wallowa Mountains. It was first reported by cartographer Erwin Raisz in 1945 on a relief map of the continental United States. The origin of this feature in its entirety is not well understood. The Olympic–Wallowa lineament likely predates the Columbia River Basalt Group.
Location
Raisz located the OWL from Cape Flattery (the northwest corner of the Olympic Peninsula) and along the north shore of Lake Crescent, thence to the Little River (south of Port Angeles), Liberty Bay (Poulsbo), Elliott Bay (setting the orientation of the streets in downtown Seattle), the north shore of Mercer Island, the Cedar River (Chester Morse Reservoir), Stampede Pass (at the crest of the Cascade Range), the south side of the Kittitas Valley (I-90), Manastash Ridge, the Wallula Gap (on the Columbia River in southeastern Washington, where it approaches the Oregon state line), and then the South Fork of the Walla Walla River as it flows north into Washington from the northeastern corner of Oregon. After crossing the Blue Mountains, Raisz associated the OWL with a dramatic scarp on the north side of the Wallowa Mountains. Raisz 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. Subsequent geological investigations have suggested various refinements and adjustments.
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 – 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 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 the Cascadia subduction zone. These mountains are distinctly different on either side of the OWL, the material of the South Cascades being Cenozoic (<66 Ma) volcanic and sedimentary rock, and the North Cascades being much older Paleozoic (hundreds of millions of years) metamorphic and plutonic rocks.; . 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 Ma) 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 .; . 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 around 44 to about 41 million years ago; ; . (i.e., during the middle-Eocene epoch). And if the OWL is a strike-slip fault or megashear, as many have speculated,; ; . then it should offset the SCF, and whether the OWL offsets the SCF, or not, becomes an important test of just what the OWL is.
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,; . not a trace has been found.
If the SCF fault does not continue directly southward; . – and the utter lack of evidence that it does makes a case for evidence of lack – then where else might it be? suggest some possibilities: it may curve to the east, it may curve to the west, or it may just end.
Tabor mapped the SCF turning and merging with the Taneum fault (coincident with the OWL) south of Kachess Lake.; . This conforms with the general pattern seen in Lakes Keechelus, Kachess, and Cle Elum, and associated geological units and faults (see image, right): each is aligned north—south at the north end, but turns to the southeast where it approaches the OWL., , and . 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 right lateral (dextral), and incompatible with the geology to the southeast. Particularly, studies of the region to the southeast (in connection with Department of Energy activities at the Hanford Reservation) show no indication of any fault or other structure comparable to the SCF.E.g., , generally, and .
On the other hand, maps the SCF as proceeding southerly (without addressing the situation south of the OWL). (He has subsequently speculated, . that the missing part of the SCF may have been dextrally offset to become a southerly trending fault in the Puget Lowland. But same problem: later deposits cover any traces.) The seeming southeasterly curvature is possibly explained as a geometrical effect of foreshortening: it occurs in a belt of intense folding (much resembling a rug which has slid against a wall) which, if unfolded, could restore some of the "curves" to a linear position along the southerly extension of the SCF.See the maps of (DGER OFR 99-4) and (USGS Map I-2538); see also (USGS Map I-2940).
There seem to be no indications that the SCF turns to the west. Although such indications would mostly be buried, the general sense of the topography suggests no such turn. Displacement, to either the west or the east, seems unlikely in that certain effects that would be expected are not found.
Could the SCF just end? This is difficult to comprehend. If there is displacement along this fault, where did it come from? To quote Wyld et al. (albeit in the context of a different fault): "it cannot just end". Although the SCF has had substantial strike-slip displacement, claim that final major movement on the SCF (about 40 Ma ago) was dominantly dip-slip (vertical displacement). So perhaps the displacement came from the depths, and, as it was extruded, was eroded and redistributed as sediments. But this has not been established.
Another possibility is that the missing southern segment of the SCF is on a crustal block that rotated away from the OWL. There is evidence that around 45 million years ago much of Oregon and southwestern Washington rotated some 60° or more about a pivot somewhere in the Olympic Peninsula (see Oregon rotation, below). This would have left a large gap south of the OWL, which could explain why Cenozoic rocks are not found immediately south of the OWL. This suggests that a continuation of the SCF, if any, and the missing Cenozoic, might be somewhere southwest of Mount St. Helens, but this has not been observed.
Darrington–Devils Mountain Fault Zone The interaction of the Straight Creek Fault with the OWL has yielded practically no intelligible information, and remains as enigmatic as the OWL itself. More informative is the closely related Darrington—Devils Mountain Fault Zone (DDMFZ). It runs east from a complex of faults on the southern end of Vancouver Island to the town of Darrington, where it turns south to converge with the SCF (see map, above).
North of the DDMFZ (and west of the SCF) is the Chuckanut Formation (part of the "Northwest Cascade System" of rocks shown in green on the map), an Eocene sedimentary formation which formed adjacent to the Swauk, Roslyn and other formations (also in green) south of Mount Stuart; their wide separation is attributed to right-lateral strike-slip movement along the SCF. That the northern part of the DDMFZ shows left-lateral strike-slip movement is not the inconsistency it may initially seem – think of the motion on either side of an arrowhead.
It appears that what is now the DDMFZ was originally aligned on the OWL. Then about 50 Ma ago North America crashed into what is now the Olympic Peninsula along an axis nearly perpendicular to the OWL, pushing the rock of the Mesozoic (pre-Cenozoic) Western and Eastern Melange Belts (WEMB, blue on the map) across the OWL, bowing the DDMFZ, and initiating the SCF and thereby splitting the Chuckanut Formation. On the north side of the DDMFZ, and wrapping around a bit to the east side, is a suite of distinctive rocks – the Helena–Haystack mélange ("HH Melange" on the map) – which was collapsed into vertical folds. Similarly distinctive rock is found in Manastash Ridge (shown on the map, but almost too small to see) still lying on the OWL, just east of the SCF.
This can explain an early puzzle as to why the Mesozoic rocks just south of the DDMFZ – the Western and Eastern Melange Belts – have no counterpart on the east side of the OWL and offset to the south: they were not faulted by the SCF, but were pushed against it from the southwest.