Yakima Fold Belt

Vorlage:Infobox landform The Yakima Fold Belt of south-central Washington, also called the Yakima fold-and-thrust belt, is an area of topographical folds (or wrinkles) raised by tectonic compression. It is a Vorlage:Convert structural-tectonic sub province of the western Columbia Plateau Province.Vorlage:SfnVorlage:SfnVorlage:Sfn

Location

Shaded-relief map showing ridges of the Yakima Fold Belt of south-central Washington, mostly between Interstate 90 (red line) and the Columbia River (bottom). Red square in center is the city of Yakima, red rectangle at lower right is the Tri-Cities, triangles are the Mount Rainier, Mount Saint Helens, and Mount Adams volcanoes of the Southern Washington Cascades. Yellow line marks the approximate location of the Olympic-Wallowa Lineament (OWL), crossing the Columbia River at the Wallula Gap (lower-right). Orange line is approximate location of edge of the North American craton. The smoothness of the central and eastern areas is due to infilling by volcanic flows of the Columbia River Basalts.

The topographical distinctness of the Yakima Folds (see the shaded-relief image) is due to their formation in a layer of lava flows and sedimentary deposits that have filled-in and generally smoothed the topographic surface of a large area of the Columbia Basin. These lava flows, part of the Columbia River Basalts (CRB), ponded against the rising Cascade Mountains in the west and the Wenatchee Mountains in the north, starting about 17 million years ago. Though the Columbia River Basalts extend east well beyond the this image, the Yakima Folds do not. The northern-most fold seen here (Frenchman Hills) ends at the Potholes Reservoir, another (Saddle Mountains) terminates just south of there, near the town of Othello (red circle). South of the Tri-Cities the rampart of the Horse Heaven Hills extends for a short distance past the Columbia River. These ridges terminate against the edge of a block of continental crust (part of the North American craton), indicated by the dashed orange line. West of that line the basement rock is oceanic basalts of the Siletzia terrane, which accreted against the continent approximately 48 million years ago. Tectonic compression of that terrane has caused the overlying layer of basalt and sedimentary deposits to form wrinkles — the Yakima Folds — whilst the stronger continental crust has resisted compression, so the deposits overlying it have not formed folds.

The southernmost ridge of the Yakima Fold Belt is the Columbia Hills on the north side of the Columbia River. The pattern of folding continues with the Dalles-Umatilla Syncline just south of the Columbia River, and further into Oregon with the Blue Mountains anticline, which approximately parallels the Klamath-Blue Mountain Lineament that marks the southeastern edge of Siletzia (see geological map, below).

The Yakima Fold Belt is also located on the Olympic-Wallowa Lineament (OWL), a broad zone of linear topographical features (dashed yellow line) extending from the Olympic Peninsula in northwestern Washington to the Wallowa Mountains in northeastern Oregon. The OWL is not fully understood. It may (or may not) mark the edge of the continent prior to the accretion of Siletiza, or perhaps of the subduction zone in front of the continental margin. Why the Yakima Folds along and immediately north of the OWL should parallel it is unknown.

Located within the Yakima Fold Belt are the Hanford Nuclear Reservation (immediately northwest of the Tri-Cities) and six major dams on the Columbia and Snake Rivers.

Geology

Yakima Fold Belt in context of major geological structures in Washington and Oregon. Belt of anticlines runs along Olympic-Wallowa Lineament (OWL [purple line]) in south-central Washington.

It is the central portion of the Olympic-Wallowa Lineament, referred to as the Cle Elum-Wallula deformed zone (CLEW),Vorlage:Sfn consising of a series of generally east-trending narrow asymmetrical anticlinal ridges and broad synclinal valleys formed by folding of Miocene Columbia River basalt flows and sediments.Vorlage:SfnVorlage:Sfn In most parts of the belt the folds have a north vergence (Columbia Hills' south vergence is an exception) with the steep limb typically faulted by imbricate thrust faults.Vorlage:SfnVorlage:Sfn Fold lengths range from 1 km to 100 km with wavelengths from several kilometers to 20 km.Vorlage:Sfn

A graben underlies nearly the entire Yakima Fold Belt and has been subsiding since Eocene time, and continues to sink at a slow rate.Vorlage:Sfn

A 2011 report found aeromagnetic, gravity, and paleoseismic evidence that the Yakima Fold Belt is linked to active Puget Sound faults.Vorlage:Sfn

Geodesy

Geodetic studies of the Oregon Rotation show that Oregon is rotating about a point somewhat south of Lewiston, Idaho compressing the Yakima fold an average of 3 millimeters per year, and the Washington Pacific coast about 7 millimeters per year.Vorlage:Sfn

Studies of the motion of the Yakima Fold Belt have been undertaken to evaluate seismic hazards at the Hanford Site.Vorlage:Sfn

See also

• Columbia Plateau (ecoregion)#Yakima Folds bioregion

Notes and sources

Sources

• Vorlage:Citation.
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• Vorlage:Citation, p. 91.
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Further reading

• Vorlage:Citation
• Vorlage:Cite thesis
• Vorlage:Citation
• Vorlage:Citation — includes flythrough animations