Joints are generally the result of a rock mass adjusting to compressive or tensional stress or cooling. The hanging wall block and footwall block of a thrust fault are typically called the upper plate and lower plate, respectively (Figure 4). Thrust faults are reverse dip‐slip faults in which the hanging wall block has overridden the footwall block at a very shallow angle for tens of kilometers. A horst results when a block that is bounded by normal faults experiences a tensional force that forces the block upward, forming mountainous terrain (Figure 3). If the fault blocks show both horizontal and vertical displacement, the fault is termed an oblique‐slip.Ī graben is formed when a block that is bounded by normal faults slips downward, usually because of a tensional force, creating a valley-like depression. A right‐lateral strike‐slip fault is one in which the displacement appears to the right when looking across the fault (Figure 2). If a person is standing at the fault and looks across to see that a feature has been displaced to the left, it is called a left‐lateral strike‐slip fault. The blocks on either side of a strike‐slip fault move horizontally in relation to each other, parallel to the strike of the fault. A reverse dip‐slip fault is just the opposite: the hanging wall block has moved upward relative to the footwall block (Figure 1). A normal dip‐slip fault, or normal fault, is one in which the hanging wall block has slipped down the fault plane relative to the footwall block. The block that underlies an inclined dip‐slip fault is called the footwall the block that rests on top of the inclined fault plane is called the hanging wall. Movement in a dip‐slip fault is parallel to the dip of the fault plane in an “up” or “down” direction between the two blocks. Three kinds of fault movements are recognized: dip‐slip, strike‐slip, and oblique‐slip. A fault is generally considered active if movement has occurred along it during the past 10,000 years.įault movements. The rocks within a fault zone may also be hydrothermally altered or veined from hot solutions that have migrated up the fault zone. The broken material within a fault is called fault gouge. The fault can be merely a crack between the two sides of rock, or it can be a fault zone hundreds of meters wide that consists of rock that has been very fractured, brecciated, and pulverized from repeated grinding movements along the fault plane. Horizontal or vertical displacement along the fault plane can range from a few centimeters to hundreds of kilometers. If the rock has been displaced along a fracture, such as having one side that is moved up or down, the fracture is called aįault, and if there is no displacement along the crack, the fracture is called aįaults. Examples include the San Andreas Fault, California Anatolian Fault, Turkey.Fractures if it is hard and brittle and subjected to sudden strain that overcomes its internal crystalline bonds. The fault motion of a strike-slip fault is caused by shearing forces. If it moves to the right, the fault is called right-lateral. If the block on the far side of the fault moves to the left, as shown in this animation, the fault is called left-lateral (Figure 2). Strike-slip fault-movement of blocks along a fault is horizontal and the fault plane is nearly vertical. Examples include the Rocky Mountains and the Himalayan Mountains. A reverse fault is called a thrust fault if the dip of the fault plane is small. This fault motion is caused by compressional forces and results in shortening. Reverse fault -the block above the inclined fault moves up relative to the block below the fault. This fault motion is caused by extensional forces and results in extension. Normal fault -the block above the inclined fault moves down relative to the block below the fault. This clip includes selected excerpts from the more-in-depth animation, " Earthquake Faults, Plate Boundaries, & Stress" SEE TABS ABOVE for stand-alone versions of each fault type. Faults are categorized into three general groups based on the sense of slip or movement. A fault is a rock fracture where the two sides have been displaced relative to each other. Your browser does not support the video tag.
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