Olympic–Wallowa lineament

The Olympic–Wallowa lineament (OWL) is a series of geologic structures oriented from northwest to southeast for 650 km (400 mi) 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.

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)[b] – 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 north side of Elliott Bay, the valley of the Ship Canal, the bluff along Interlaken Blvd.

Yet the same orientation shows up in the Brothers, Eugene-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.

[citation needed] One of the first speculations that the OWL might be a major geological structure (Wise 1963) – written when the theory of plate tectonics was still new and not entirely accepted[c] – was called by the author[who?]

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.

[16] 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.

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.

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.

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).

But similar rock also occurs in the Rimrock Lake Inlier, about 75 km south of the OWL and just west of the projected trace of the SCF, and also in the Klamath Mountains of southwestern Oregon.

[30] 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 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 superficial basalt, quite independently of what ever is happening in the basement rock.

Southwest of this section of the OWL is a region of grabens (where large blocks of crust have dropped) extending about 60 miles (97 km) south to the nearly parallel Vale Fault Zone (see diagram, below).

This is the Columbia Embayment, a large indentation into the North American continent characterized by oceanic crust covered by thick sedimentary deposits.

The variations are mainly in the region of the CLEW, where sediments are buried under the basalts of the Columbia Basin, and in Puget Sound, where the Cenozoic geology extends as far north as Vancouver Island.

One interpretation of this is that western Oregon and southwestern Washington have swung as a rigid block about a pivot point at the northern end, near the Olympic Peninsula.

This interpretation implies a "back arc" of magmatism, probably fed by a subduction zone, and possibly implicated with the intrusion of various plutons in the North Cascades around 50 Ma.

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.

[64] 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.

50 Ma (million years) ago there was a change in the direction of motion of the Pacific plate (as recorded in the bend in the Hawaiian-Emperor seamount chain).

[69][82] 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.

Unlike anything on the OWL, these lava flows can be dated, and they show a westward age progression from an origin at the McDermitt Caldera on the Oregon-Nevada border to the Newberry Volcano.

Near the town of Orofino (just east of Lewiston, Idaho) something curious happens: the craton margin makes a sharp right-angle bend to the west.

What actually happens is the truncation of the WISZ by the WNW-trending Orofino Shear Zone (OSZ), which can be traced west roughly parallel with the OWL until it disappears below the Columbia River Basalts, and southeast across Idaho and possibly beyond.

[93] East of the WISZ this turns to the southeast (much as the OWL may be doing past the Wallula Gap) to follow the Clearwater fault zone down the continental divide near the Idaho—Montana border to the northwestern corner of Wyoming.

Following the Great Divide Megashear into the mid-continent reveals something interesting: a widespread pattern of similarly trending (roughly NW-SE) fault zones, rifts, and aeromagnetic and gravitational anomalies.

[t][98] Although some of the faults are recent, the NW trending zones themselves have been attributed to continental-scale transcurrent shearing at about 1.5 Ga – that's billions of years ago – during the assembly of Laurentia (the North American continent).

It is now believed that these two predominant patterns reflect ancient weaknesses in the underlying Precambrian basement rock,[101] which can be reactivated to control the orientation of features formed much later.