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34-466: The coal and other overlying rock has been removed by mining down to a resistant sandstone bed, revealing the three-dimensional structures of folding and faulting caused by the Alleghany Orogeny . Students of geology have visited the location for decades due to the quality of exposures. The central anticline in the valley is often called the "Whaleback". The sequence of structural deformation

68-531: A fold is a stack of originally planar surfaces, such as sedimentary strata , that are bent or curved ( "folded" ) during permanent deformation . Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur as single isolated folds or in periodic sets (known as fold trains ). Synsedimentary folds are those formed during sedimentary deposition. Folds form under varied conditions of stress , pore pressure , and temperature gradient , as evidenced by their presence in soft sediments ,

102-609: A hydrocarbons trap , oil accumulating in the crest of the fold. Most anticlinal traps are produced as a result of sideways pressure, folding the layers of rock, but can also occur from sediments being compacted. Chevron (geology) Chevron folds are a structural feature characterized by repeated well behaved folded beds with straight limbs and sharp hinges. Well developed, these folds develop repeated set of v-shaped beds. They develop in response to regional or local compressive stress . Inter-limb angles are generally 60 degrees or less. Chevron folding preferentially occurs when

136-421: A fold axis is called a cylindrical fold . This term has been broadened to include near-cylindrical folds. Often, the fold axis is the same as the hinge line. Minor folds are quite frequently seen in outcrop; major folds seldom are except in the more arid countries. Minor folds can, however, often provide the key to the major folds they are related to. They reflect the same shape and style, the direction in which

170-444: A hinge need to accommodate large deformations in the hinge zone. This results in voids between the layers. These voids, and especially the fact that the water pressure is lower in the voids than outside of them, act as triggers for the deposition of minerals. Over millions of years, this process is capable of gathering large quantities of trace minerals from large expanses of rock and depositing them at very concentrated sites. This may be

204-427: A mechanism that is responsible for the veins. To summarize, when searching for veins of valuable minerals, it might be wise to look for highly folded rock, and this is the reason why the mining industry is very interested in the theory of geological folding. Anticlinal traps are formed by folding of rock. For example, if a porous sandstone unit covered with low permeability shale is folded into an anticline, it may form

238-622: A planar detachment without further fault propagation, detachment folds may form, typically of box-fold style. These generally occur above a good detachment such as in the Jura Mountains , where the detachment occurs on middle Triassic evaporites . Shear zones that approximate to simple shear typically contain minor asymmetric folds, with the direction of overturning consistent with the overall shear sense. Some of these folds have highly curved hinge-lines and are referred to as sheath folds . Folds in shear zones can be inherited, formed due to

272-524: A sinusoidal geometry. In a stratigraphic sequence, beds are geometrically and physically constrained by their neighbours. Similarity must be maintained. To accommodate such constraints while maintaining sinusoidal geometry, less competent layers would need to be subjected to extensive flow. Kinked, yielding and highly localized hinges with straight limbs greatly reduce the geometrical need for deformation. Chevron folds are energetically preferred to conventional sinusoidal folds as they minimize ductile flow to

306-541: A strong axial planar cleavage . Folds in the rock are formed about the stress field in which the rocks are located and the rheology , or method of response to stress, of the rock at the time at which the stress is applied. The rheology of the layers being folded determines characteristic features of the folds that are measured in the field. Rocks that deform more easily form many short-wavelength, high-amplitude folds. Rocks that do not deform as easily form long-wavelength, low-amplitude folds. Layers of rock that fold into

340-435: A thrust fault cuts up section from one detachment level to another. Displacement over this higher-angle ramp generates the folding. Fault propagation folds or tip-line folds are caused when displacement occurs on an existing fault without further propagation. In both reverse and normal faults this leads to folding of the overlying sequence, often in the form of a monocline . When a thrust fault continues to displace above

374-504: Is accommodated by slip between the pages of the book. The fold formed by the compression of competent rock beds is called "flexure fold". Typically, folding is thought to occur by simple buckling of a planar surface and its confining volume. The volume change is accommodated by layer parallel shortening the volume, which grows in thickness . Folding under this mechanism is typical of a similar fold style, as thinned limbs are shortened horizontally and thickened hinges do so vertically. If

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408-417: Is also a feature of many igneous intrusions and glacier ice. Folding of rocks must balance the deformation of layers with the conservation of volume in a rock mass. This occurs by several mechanisms. Flexural slip allows folding by creating layer-parallel slip between the layers of the folded strata, which, altogether, result in deformation. A good analogy is bending a phone book, where volume preservation

442-410: Is outlined as follows: 40°45.8′N 76°35.7′W  /  40.7633°N 76.5950°W  / 40.7633; -76.5950 This article about a specific mine is a stub . You can help Misplaced Pages by expanding it . This Northumberland County, Pennsylvania state location article is a stub . You can help Misplaced Pages by expanding it . Fold (geology) In structural geology ,

476-425: Is relatively weak. When rock behaves as a fluid, as in the case of very weak rock such as rock salt, or any rock that is buried deeply enough, it typically shows flow folding (also called passive folding , because little resistance is offered): the strata appear shifted undistorted, assuming any shape impressed upon them by surrounding more rigid rocks. The strata simply serve as markers of the folding. Such folding

510-454: Is the midpoint of the limb. The axial surface is defined as a plane connecting all the hinge lines of stacked folded surfaces. If the axial surface is planar, it is called an axial plane and can be described in terms of strike and dip . Folds can have a fold axis . A fold axis "is the closest approximation to a straight line that when moved parallel to itself, generates the form of the fold". (Ramsay 1967). A fold that can be generated by

544-441: The accommodation of strains between neighboring faults. Fault-bend folds are caused by displacement along a non-planar fault. In non-vertical faults, the hanging-wall deforms to accommodate the mismatch across the fault as displacement progresses. Fault bend folds occur in both extensional and thrust faulting. In extension, listric faults form rollover anticlines in their hanging walls. In thrusting, ramp anticlines form whenever

578-493: The axis of the fold. Those with limbs of relatively equal length are termed symmetrical , and those with highly unequal limbs are asymmetrical . Asymmetrical folds generally have an axis at an angle to the original unfolded surface they formed on. Vergence is calculated in a direction perpendicular to the fold axis. Folds that maintain uniform layer thickness are classed as concentric folds. Those that do not are called similar folds . Similar folds tend to display thinning of

612-403: The bed and the thickness of competent beds further determines the structural stability. A 1:10 ratio between the thickness of competent beds and the length appears to be the threshold required for the formation of chevron folds. Smaller ratios require too much flow in the more ductile layers. Given high length to thickness and low high-competency to low-competency thickness ratios, irregularities in

646-585: The bedding regularly alternates between contrasting competences . Turbidites , characterized by alternating high-competence sandstones and low-competence shales , provide the typical geological setting for chevron folds to occur. Perpetuation of the fold structure is not geometrically limited. Given a proper stratigraphy , chevrons can persist almost indefinitely. In response to compressional stress, geological beds fold in order to minimize dissipation of energy. Given an unconstrained bed , folding does so by correspondingly minimizing bending and thus develops

680-412: The closures of the major folds lie, and their cleavage indicates the attitude of the axial planes of the major folds and their direction of overturning A fold can be shaped like a chevron , with planar limbs meeting at an angular axis, as cuspate with curved limbs, as circular with a curved axis, or as elliptical with unequal wavelength . Fold tightness is defined by the size of the angle between

714-409: The effects of a high-level igneous intrusion e.g. above a laccolith . The fold hinge is the line joining points of maximum curvature on a folded surface. This line may be either straight or curved. The term hinge line has also been used for this feature. A fold surface seen perpendicular to its shortening direction can be divided into hinge and limb portions; the limbs are the flanks of

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748-408: The expense of localized bending. Four stages mark development of chevron folds: sinusoidal nucleation, concentric folding , straightening of limbs/sharpening of hinges, and tightening of the chevron fold. When inter-limb angles approach 60 degrees, frictional forces limit simple shear and flow deformation in less competent layers and favors pure shear of the whole stratigraphic complex. Therefore,

782-450: The fold's limbs (as measured tangential to the folded surface at the inflection line of each limb), called the interlimb angle. Gentle folds have an interlimb angle of between 180° and 120°, open folds range from 120° to 70°, close folds from 70° to 30°, and tight folds from 30° to 0°. Isoclines , or isoclinal folds , have an interlimb angle of between 10° and zero, with essentially parallel limbs. Not all folds are equal on both sides of

816-409: The fold, and the limbs converge at the hinge zone. Within the hinge zone lies the hinge point, which is the point of minimum radius of curvature (maximum curvature) of the fold. The crest of the fold represents the highest point of the fold surface whereas the trough is the lowest point. The inflection point of a fold is the point on a limb at which the concavity reverses; on regular folds, this

850-439: The folding deformation cannot be accommodated by a flexural slip or volume-change shortening (buckling), the rocks are generally removed from the path of the stress. This is achieved by pressure dissolution , a form of metamorphic process, in which rocks shorten by dissolving constituents in areas of high strain and redepositing them in areas of lower strain. Folds generated in this way include examples in migmatites and areas with

884-471: The full spectrum of metamorphic rocks , and even as primary flow structures in some igneous rocks . A set of folds distributed on a regional scale constitutes a fold belt , a common feature of orogenic zones . Folds are commonly formed by shortening of existing layers, but may also be formed as a result of displacement on a non-planar fault ( fault bend fold ), at the tip of a propagating fault ( fault propagation fold ), by differential compaction or due to

918-494: The hinge. These fractures are commonly infilled with crystalline veins . The behavior of chevron folds are effectively controlled by the characteristics of the stratigraphy under deformation. Ideally, beds should alternate between high competence and low competence. The stability of chevron folding stringently requires regular thickness in the high-competence layers; conversely, regularity in low competence layers has been found to have very little effect on stability. The length of

952-507: The inter-limb angle, rapidly decreasing as a function of time given larger angles begins to stabilize as the angle nears 60 degrees. There is, however, no physical limitation on the acuteness of the fold. Saddle reef structures, hinge collapse and/or simply dilation of incompetent layer commonly accommodates the geometrical void created in the hinge during folding. While the incompetent layer deforms and flows, thus having complex cleavage patterns, competent layers tend to fracture radially at

986-548: The limbs and thickening of the hinge zone. Concentric folds are caused by warping from active buckling of the layers, whereas similar folds usually form by some form of shear flow where the layers are not mechanically active. Ramsay has proposed a classification scheme for folds that often is used to describe folds in profile based upon the curvature of the inner and outer lines of a fold and the behavior of dip isogons . that is, lines connecting points of equal dip on adjacent folded surfaces: (A homocline involves strata dipping in

1020-609: The mechanical layering and the contrast in properties between the layers. If the layering does begin to fold, the fold style is also dependent on these properties. Isolated thick competent layers in a less competent matrix control the folding and typically generate classic rounded buckle folds accommodated by deformation in the matrix. In the case of regular alternations of layers of contrasting properties, such as sandstone-shale sequences, kink-bands, box-folds and chevron folds are normally produced. Many folds are directly related to faults, associated with their propagation, displacement and

1054-540: The orientation of pre-shearing layering or formed due to instability within the shear flow. Recently deposited sediments are normally mechanically weak and prone to remobilization before they become lithified, leading to folding. To distinguish them from folds of tectonic origin, such structures are called synsedimentary (formed during sedimentation). Slump folding: When slumps form in poorly consolidated sediments, they commonly undergo folding, particularly at their leading edges, during their emplacement. The asymmetry of

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1088-400: The same direction, though not necessarily any folding.) Folds appear on all scales, in all rock types , at all levels in the crust . They arise from a variety of causes. When a sequence of layered rocks is shortened parallel to its layering, this deformation may be accommodated in a number of ways, homogeneous shortening, reverse faulting or folding. The response depends on the thickness of

1122-420: The slump folds can be used to determine paleoslope directions in sequences of sedimentary rocks. Dewatering: Rapid dewatering of sandy sediments, possibly triggered by seismic activity, can cause convolute bedding. Compaction: Folds can be generated in a younger sequence by differential compaction over older structures such as fault blocks and reefs . The emplacement of igneous intrusions tends to deform

1156-478: The surrounding country rock . In the case of high-level intrusions, near the Earth's surface, this deformation is concentrated above the intrusion and often takes the form of folding, as with the upper surface of a laccolith . The compliance of rock layers is referred to as competence : a competent layer or bed of rock can withstand an applied load without collapsing and is relatively strong, while an incompetent layer

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