Ahtanum Ridge is a long anticline mountain ridge in Yakima County in the U.S. state of Washington . It is located just south of the city of Yakima , and much of its length is at the northern edge of the Yakama Indian Reservation . Its name comes from the Sahaptin toponym [átanɨm].
31-750: Ahtanum Ridge is part of the Yakima Fold Belt of east-tending long ridges formed by the folding of Miocene Columbia River basalt flows . Ahtanum Ridge separates Yakima Valley on the south from Ahtanum Valley on the north. At the eastern end of Ahtanum Ridge the Yakima River flows through Union Gap. The ridge continues east of Union Gap as the Rattlesnake Hills . 46°30′25″N 120°48′56″W / 46.50694°N 120.81556°W / 46.50694; -120.81556 This Yakima County, Washington state location article
62-479: A fold and, as a result, they are directly related to the kinematic components in a fold. In this sense, the term vergence , has implications of movement and, thus, has become controversial in geology, because vergence, in Stille's original definition, only describes geometrical relationships, and many geologists believed it should not be used as a primary tool for describing movement. As a result of this controversy,
93-453: A fold, can assist geologists in classifying a fold in terms of its style (antiform or synform), as well as classifying it as an anticline or syncline, which holds stratigraphic significance. One of the most important uses of vergence is that it gives geologists a sense of the geometric property of symmetry in folds. In the case of asymmetric folds, vergence can be observed and recorded in structures known as fold pairs. Fold pairs help illustrate
124-705: A large area of the Columbia Basin . The extent of these lava flows was limited to the west and north by the rising Cascade Mountains and the Wenatchee Mountains . The lava flows extend east well beyond this image, but the Yakima Folds do not. The northernmost 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
155-422: A more definitive definition of vergence. The now, more widely accepted definition of vergence, which is used to help describe the geometric component of asymmetry in a fold, is the horizontal direction in which the upper component of rotation in a fold is directed. In this definition, the concept of vergence is distinguished from and independent of the facing component of a fold. Based on this independence, much of
186-578: Is a stub . You can help Misplaced Pages by expanding it . Yakima Fold Belt 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 14,000 km (5,400 sq mi) structural-tectonic sub province of the western Columbia Plateau Province resulting from complex and poorly understood regional tectonics. The folds are associated with geological faults whose seismic risk
217-510: Is a result of the common definition of fold-facing in geology, which is described as the direction (normal to the axis of a fold and corresponding to the axial plane) that points towards younger beds . As a result of this definition, two folds, which possess identical asymmetry, can be seen as facing opposite directions in relation to the stratigraphic elements of the structure, resulting in differently interpreted directions of vergence. To clear up this confusion, geologists have attempted to create
248-607: Is of particular concern to the nuclear facilities at the Hanford Nuclear Reservation (immediately northwest of the Tri-Cities) and major dams on the Columbia and Snake Rivers . 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
279-416: Is sometimes classified as sinistral or dextral. This is because of the relationship between vergence and rotation, as in its definition, vergence is used to determine the direction of rotation of the upper component of a fold. Vergence can be classified as dextral, when there is apparent clockwise rotation, or sinistral, when there is apparent anticlockwise rotation. Although there is a shared relationship in
310-456: Is used to provide an overall characterization, in the symmetry (or asymmetry ) of folds, and can be used to observe changes in small-scale structures in relation to the axis of a large fold. The vergence of a fold lies parallel to the surrounding surfaces of a fold, so if these surrounding surfaces are not horizontal, the vergence of the fold will be inclined. For a fold, the direction and the extent to which vergence occurs can be calculated from
341-478: The Hanford Site . Vergence (geology) In structural geology , vergence refers to the direction of the overturned component of an asymmetric fold . In simpler terms, vergence can be described as the horizontal direction in which the upper component of rotation is directed. Vergence can be observed and recorded in folds to help a geologist determine characteristics of larger fold areas. Vergence
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#1732793665924372-405: The strike and dip of the axial surfaces, along with that of the enveloping surfaces. These calculations can be handy for geologists in determining the overall elements of larger areas. Vergence is a term that has been commonly confused and, as a result, somewhat misunderstood throughout its history of use. The earliest form of use, and the recognized introduction to the term vergence , came from
403-699: 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, and the orientation and spacing of some of
434-903: The Folds influenced by, 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. It is the central portion of the Olympic–Wallowa Lineament , referred to as the Cle Elum-Wallula deformed zone (CLEW), constising 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. In most parts of
465-473: The German geologist, Hans Stille , in 1924. Stille originally used the term to describe the direction of the overturning of minor folds, as well as to describe the "up-dip" direction, which is directly related to the strike and dip of a fold. These descriptions of certain physical components of a fold, determined by vergence, were used by Stille in the description of the overall direction of the over- thrusting in
496-733: The Tri-Cities the rampart of the Horse Heaven Hills extends for a short distance past the Columbia River. The ends of these ridges mark the edge of a block of continental crust (part of the North American craton , indicated by the dashed orange line) that has resisted the tectonic compression that formed the ridges. The southernmost ridge of the Yakima Fold Belt is the Columbia Hills on the north side of
527-604: The Yakima Fold Belt is linked to active Puget Sound faults . 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. Studies of the motion of the Yakima Fold Belt have been undertaken to evaluate seismic hazards at
558-416: The angular relationship of the bedding and cleavage of the fold. They can be 'S-shaped', in which they are termed sinistral , or they can be 'Z-shaped', in which they are known as dextral . The conventional use of terms sinistral and dextral are used to describe vergence when the vantage point of observation is from above, or the observer is looking down on the fold. Vergence, similarly to fold pairs,
589-490: 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 . Fold lengths range from 1 km to 100 km with wavelengths from several kilometers to 20 km. A graben underlies nearly the entire Yakima Fold Belt and has been subsiding since Eocene time, and continues to sink at a slow rate. A 2011 report found aeromagnetic , gravity , and paleoseismic evidence that
620-541: The confusion has been cleared up, and both concepts of vergence and facing are of important use to geologists, especially in the analysis of more complex structures. One of the main uses of vergence is to give an idea of the overall geometry of a fold by describing the symmetry or asymmetry of a fold. To better understand the importance of this use, it is important to understand the different classifications of folds, based on their geometry. The most commonly used terms to describe and classify folds, based on their geometry, are
651-441: The definition to coincide with its original use by Stille, which was the direction of the up-dip direction of a fold on an axial surface. The disagreement likely derived, because Stille was not explicitly clear about the direction of the overturning of folds following the up-dip or down-dip of the fold. In his original use of the term, however, he did, in fact, use the up-dip direction of the fold. The main reason this creates confusion
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#1732793665924682-522: The description of both vergence and fold pairs, they are independent of each other, as vergence is defined as a direction and not a shape. The use of vergence in describing the asymmetry of a fold is more useful than simply describing the asymmetry as either S-shaped (sinistral) or Z-shaped (dextral) because vergence is independent of fold plunge variations. Fold plunge variations are relatively common in folds, and based on these variations, two folds with similar asymmetry can be classified differently in terms of
713-442: The enveloping surface of the folds (plane drawn tangential to the folds), the folds are considered symmetrical. When this is not the case, the folds are considered asymmetrical, and the asymmetry of these folds will vary systematically across the axial surfaces of the folds. Vergence can be used to, not only give an overall idea of the symmetry (or asymmetry) of a fold, but in the common case of asymmetric folds, it can be used to record
744-442: The larger area surrounding the fold. The vergence of a fold can help a geologist determine several characteristics of folding on a larger scale, including the style, position, and geometry of the folding. By observing vergence in a fold, geologists can record data that can be used to calculate the approximate position and geometry of a larger area, and therefore assist geologists in mapping that area. More specifically, geologists use
775-533: The only evidence of the existence of large-scale folding in the area is through the mapping of vergence zones. In areas of simple deformation, vergence can even be used as a tool to locate hinge zones of major folds, as well as the sense of shear . Using vergence as a tool to map out larger zones, should be used with caution in more complex areas where there have been multiple deformations. For instance, in areas of highly metamorphosed rocks, as well as in regions where tectonic movements of different ages are present,
806-399: The previously mentioned systematic variation of asymmetry of folds. This is very important to the use of vergence in the field for geologists when determining the micro-to-macro components of an area. One of the main applications of vergence in geology is the tool it provides geologists to describe the geometries of folds on a small scale. Vergence, as well as the application of the facing of
837-653: The property of vergence in smaller folds, to determine some of the physical properties of the larger, surrounding area where larger-scale folding is taking place. This is a common practice for geologists and is used in mapping out many areas such as the mapping of the Morcles Nappe in Switzerland . This process is especially useful in some instances, such as in the Otago Schists in New Zealand . Here,
868-403: The shape being sinistral or dextral based on their fold plunge. This can result in inaccuracies in determining the geometries of folds, and therefore affect the mapping of larger areas. Overall, vergence can be very useful in the analysis of a folded pair, as the vergence can give a better understanding of the geometry of the folded pair, as well as be used to make determinations on the geometry of
899-417: The terms anticline and syncline are used in the description of the stratigraphic significance of the fold. These phrases can be used in conjunction with one another to describe the geometric aspects of the shape and direction of the fold. In addition, folds can be referred to as either symmetrical or asymmetrical. When a group of folds, whose axial planes are found to be perpendicular to the plane representing
930-404: The terms antiforms and synforms, as well as anticlines and synclines . Although these terms sound similar, they mean very different things about the geometry of a fold. When the limbs of a fold converge upward, the fold is referred to as an antiform. Conversely, when the limbs of a fold converge downward, the fold is known as a synform. Not to be confused with these terms (antiform and synform),
961-423: The use of the term never reached international agreement, and consequently has been used in distinctly different ways, throughout history. Most of the confusion, about the definition of vergence, stems from the confusion and conflation of the concepts of fold-facing and fold-vergence. Some geologists began to reference the definition of vergence to being the direction in which a fold is facing, while others believed