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Faults are the places in the crust where brittle deformation occurs as two blocks of rocks move relative to one another. There are three major fault types: normal, reverse, and strike-slip. Normal and reverse faults display vertical, also known as dip-slip, motion. Dip-slip motion consists of relative up and down movement along a dipping fault between two blocks, the hanging wall, and the footwall. In a dip-slip system, the footwallis below the fault plane, and the hanging-wall is above the fault plane. An excellent way to remember this is to imagine a mine tunnel through a fault; the hanging wall would be where a miner would hang a lantern, and the footwall would be at the miner’s feet. Faults are more prevalent near and related to plate boundaries, but can occur in plate interiors as well. Faults can show evidence of movement along the fault plane. Slickensides are polished, often grooved surfaces along the fault plane created by friction during the movement. A joint or fracture is a plane of breakage in a rock that does not show movement or offset. Joints can result from many processes, such as cooling, depressurizing, or folding. Joint systems may be regional affecting many square miles.
Normal faults move by a vertical motion where the hanging-wall moves downward relative to the footwall along the dip of the fault. Tensional forces create normal faults in the crust. Normal faults and tensional forces are commonly caused at divergent plate boundaries and where the crust is being stretched by tensional stresses. Utah examples of normal faults are the Wasatch Fault, the Hurricane Fault, and other faults bounding valleys in the Basin and Range.
Grabens, horsts, and half-grabens are all blocks of crust or rock that are bounded by normal faults. Grabens drop down relative to adjacent blocks and create valleys. Horsts go up relative to adjacent down-dropped blocks and become areas of high topography. Where together, horsts and grabens create a symmetrical pattern of valleys surrounded by normal faults on both sides and mountains. Half-grabens are a one-sided version of a horst and graben, where blocks are tilted by a normal fault on one side, creating an asymmetrical valley-mountain arrangement. The mountain-valleys of the Basin and Range Province of Western Utah and Nevada consist of a series of full and half-grabens from the Salt Lake Valley to the Sierra Nevada Mountains. When the dip of a normal fault decreases with depth (i.e., the fault becomes more horizontal as it goes deeper), the fault is a listric fault. Extreme versions of listric faulting occur when large amounts of extension occur along very low-angle normal faults, known as detachment faults. The normal faults of the Basin and Range appear to become detachment faults at depth.
Reverse faults, caused by compressional forces, are when the hanging wall moves up relative to the footwall. A thrust fault is a reverse fault where the fault plane has a low dip angle (generally less than 45 degrees). Thrust faults bring older rocks on top of younger rocks and can cause repetition of rock units in the stratigraphic record. Convergent plate boundaries with subduction zones create a particular type of “reverse” fault called a megathrust fault. Megathrust faults cause the most significant magnitude earthquakes and commonly cause tsunamis.
Strike-slip faults have a side to side motion. In the pure strike-slip motion, crustal blocks on either side of the fault do not move up or down relative to each other. There is left-lateral, called sinistral, and right-lateral, called dextral, strike-slip motion. In left-lateral or sinistral strike-slip motion, the opposite block moves left relative to the block that the observer is standing on. In right-lateral or dextral strike-slip motion, the opposite block moves right relative to the observer’s block. Strike-slip faults are most commonly associated with transform boundaries and are prevalent in fracture zones adjacent to mid-ocean ridges.
Bends in strike-slip faults can create areas where the sliding blocks create compression or tension. Tensional stresses will create transtensional features with normal faults an2d basins like California’s Salton Sea, and compressional stresses will create transpressional features with reverse faults and small-scale mountain building, like California’s San Gabriel Mountains. The faults that play off of transpression or transtension features are known as flower structures.
An example of a right-lateral strike-slip fault is the San Andreas Fault, which denotes a transform boundary between the North American and Pacific plates. An example of a left-lateral strike-slip fault is the Dead Sea fault in Jordan and Israel.
Introduction to Physical Geography by R. Adam Dastrup is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.
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