Compressive stress often generates two cross‐cutting joint sets.A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake - or may occur slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of kilometers. Most faults produce repeated displacements over geologic time. During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between.Įarth scientists use the angle of the fault with respect to the surface (known as the dip) and the direction of slip along the fault to classify faults. Faults which move along the direction of the dip plane are dip-slip faults and described as either normal or reverse (thrust), depending on their motion. Faults which move horizontally are known as strike-slip faults and are classified as either right-lateral or left-lateral. Faults which show both dip-slip and strike-slip motion are known as oblique-slip faults. Tensional stress usually results in a single joint orientation that is perpcndicular to the direction of stress. A joint set is composed of a series of roughly parallel joints that occur in one direction. 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. Fractures if it is hard and brittle and subjected to sudden strain that overcomes its internal crystalline bonds.
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