10(l) Crustal Deformation Processes: Folding and Faulting
Folds are bends in rocks that are due to compressional forces. Folds are most visible in rocks that layered (also known as sedimentary rocks). Folds are formed . related strike-slip detachments, highly curvilinear folds, oblique thrust faults and a clockwise-transecting cleavage. A series . although unconformable relationships with overlying from both the John Ray Trust and the Annie Greenly Fund. Folding and faulting are both caused by stress in the Earth's crust exceeding the strength of the material.
Topographic relief of the Earth's terrestrial surface and ocean basins. Ocean trenches and the ocean floor have the lowest elevations on the image and are colored dark blue. Elevation is indicated by color. The legend below shows the relationship between color and elevation. Changes in shape and volume occur when stress and strain causes rock to buckle and fracture or crumple into folds. A fold can be defined as a bend in rock that is the response to compressional forces.
Folds are most visible in rocks that contain layering. For plastic deformation of rock to occur a number of conditions must be met, including: The rock material must have the ability to deform under pressure and heat. The higher the temperature of the rock the more plastic it becomes.
Pressure must not exceed the internal strength of the rock. If it does, fracturing occurs.
Deformation must be applied slowly. A number of different folds have been recognized and classified by geologists. The simplest type of fold is called a monocline Figure 10i This fold involves a slight bend in otherwise parallel layers of rock. An anticline is a convex up fold in rock that resembles an arch like structure with the rock beds or limbs dipping way from the center of the structure Figure 10l Note how the rock layers dip away from the center of the fold are roughly symmetrical.
These conditions exist in the orogenic belts that result from either two continental tectonic collisions or from subduction zone accretion. The resultant compressional forces produce mountain ranges. The Himalayasthe Alpsand the Appalachians are prominent examples of compressional orogenies with numerous overthrust faults. Thrust faults occur in the foreland basin which occur marginal to orogenic belts. Here, compression does not result in appreciable mountain building, which is mostly accommodated by folding and stacking of thrusts.
Instead thrust faults generally cause a thickening of the stratigraphic section. Foreland basin thrusts also usually observe the ramp-flat geometry, with thrusts propagating within units at a very low angle "flats" at 1—5 degrees and then moving up-section in steeper ramps at 5—20 degrees where they offset stratigraphic units.
Identifying ramps where they occur within units is usually problematic. Thrusts and duplexes are also found in accretionary wedges in the ocean trench margin of subduction zones, where oceanic sediments are scraped off the subducted plate and accumulate.
Here, ramp flat geometries are not usually observed because the compressional force is at a steep angle to the sedimentary layering. McConnell in the Canadian Rockies. Geikie in coined the term thrust-plane to describe this special set of faults.
By a system of reversed faults, a group of strata is made to cover a great breadth of ground and actually to overlie higher members of the same series. The most extraordinary dislocations, however, are those to which for distinction we have given the name of Thrust-planes.
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More complex fold types can develop in situations where lateral pressures become greater. The greater pressure results in anticlines and synclines that are inclined and asymmetrical. A Fault is the fracturing of rocks in the Earth causing breakage and slip parallel to the fracture. Caused by compressional, extensional and shear forces.
Thrust fault - Wikipedia
Occurs close to surface of the Earth. Faults form in rocks when the stresses overcome the internal strength of the rock resulting in a fracture. A fault can be defined as the displacement of once connected blocks of rock along a fault plane. This can occur in any direction with the blocks moving away from each other.
Faults occur from both tensional and compressional forces. What is responsible for the difference between folding and faulting? Confining stress, strain rate and temperature.
Faulting occurs where strain rates are large and there are relatively low temperatures and confining pressures at shallower d…epths within the earth's lithosphere. These conditions allow fractures to propagate through the rock causing faulting. Folding tends to occur at small strain rates the speed at which the material changes shape or is squashed is very smallat higher confining pressures and with higher temperatures at greater depths in the Earth typically the asthenosphere and mesosphere.
All of these factors act to stop the formation of fractures within the material leading to ductile deformation which is why the rocks can fold rather than break. See the related questions for more information.