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9.5: Faults

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  • Block diagram of a normal fault.
    Figure: Common terms used for normal faults. Normal faults form when the hanging wall moves down relative to the footwall.

    Faults are the places in the crust where brittle deformation occurs as two blocks of rocks move relative to one another. 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 footwall. In a dip-slip system, the footwall is below the fault plane and the hanging wall is above the fault plane. A good way to remember this is to imagine a mine tunnel running along a fault; the hanging wall would be where a miner would hang a lantern and the footwall would be at the miner’s feet.

    Faulting as a term refers to the rupture of rocks. Such ruptures occur at plate boundaries but can also occur in plate interiors as well. Faults slip along the fault plane. The fault scarp is the offset of the surface produced where the fault breaks through the surface. Slickensides are polished, often grooved surfaces along the fault plane created by friction during the movement.

    A joint or fracture is a plane of brittle deformation in the rock created by the movement that is not offset or sheared. Joints can result from many processes, such as cooling, depressurizing, or folding. Joint systems may be regional affecting many square miles.

    9.5.1: Normal Faults

    Normal faults move by a vertical motion where the hanging-wall moves downward relative to the footwall along the dip of the fault. Normal faults are created by tensional forces in the crust. Normal faults and tensional forces commonly occur at divergent plate boundaries, where the crust is being stretched by tensional stresses (see Chapter 2). Examples of normal faults in Utah are the Wasatch Fault, the Hurricane Fault, and other faults bounding the valleys in the Basin and Range province.

    Roadcut outcrop of multicolor rock beds offset by a normal fault.
    Figure: Example of a normal fault in an outcrop of the Pennsylvanian Honaker Trail Formation near Moab, Utah.
    While the area extends, individual grabens drop down relative to the horsts.
    Figure: Faulting that occurs in the crust under tensional stress.

    Grabens, horsts, and half-grabens are blocks of crust or rock bounded by normal faults (see Chapter 2). Grabens drop down relative to adjacent blocks and create valleys. Horsts rise up relative to adjacent down-dropped blocks and become areas of higher topography. Where occurring 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.

    Normal faults do not continue to clear into the mantle. In the Basin and Range Province, the dip of a normal fault tends to decrease with depth, i.e., the fault angle becomes shallower and more horizontal as it goes deeper. Such decreasing dips happen when large amounts of extension occur along very low-angle normal faults, known as detachment faults. The normal faults of the Basin and Range, produced by tension in the crust, appear to become detachment faults at greater depths.

    9.5.2: Reverse Faults

    Block diagram of a normal fault.
    Figure: Simplified block diagram of a reverse fault.

    In reverse faults, compressional forces cause the hanging wall to move up relative to the footwall. A thrust fault is a reverse fault where the fault plane has a low dip angle of less than 45°. Thrust faults carry older rocks on top of younger rocks and can even cause the repetition of rock units in the stratigraphic record.

    Convergent plate boundaries with subduction zones create a special type of “reverse” fault called a megathrust fault where denser oceanic crust drives down beneath less dense overlying crust. Megathrust faults cause the largest magnitude earthquakes yet measured and commonly cause massive destruction and tsunamis.

    Block diagram of a thrust fault, where the hangingwall overlies the footwall.
    Figure: Terminology of thrust faults (low-angle reverse faults). A klippe is the remnant of the hangingwall (aka nappe), where the surrounding material has been eroded away. A window is where part of the hangingwall has been eroded away to expose the footwall (autochthon). Note the symbol shows flags on the overlying thrust plate.
    Beds of rock offset along a fault plane to where one section of the rock has been pushed up over itself.
    Figure: Ketobe Knob in the San Rafael Swell of Utah displays an example of a thrust fault.

    9.5.3: Strike-slip Faults

    Strike-slip faults have side-to-side motion. Strike-slip faults are most commonly associated with transform plate boundaries and are prevalent in transform fracture zones along mid-ocean ridges. In pure strike-slip motion, fault blocks on either side of the fault do not move up or down relative to each other, rather move laterally, side to side. The direction of the strike-slip movement is determined by an observer standing on a block on one side of the fault. If the block on the opposing side of the fault moves left relative to the observer’s block, this is called sinistral motion. If the opposing block moves right, it is dextral motion.

    Video showing motion in a strike-slip fault.

    Bends along strike-slip faults create areas of compression or tension between the sliding blocks (see Chapter 2). Tensional stresses create transtensional features with normal faults and basins, such as the Salton Sea in California. Compressional stresses create transpressional features with reverse faults and cause small-scale mountain building, such as the San Gabriel Mountains in California. The faults that splay off transpression or transtension features are known as flower structures.

    Flower structures created by strike-slip faults. Depending on the relative movement in relation to the bend in the fault, flower structures can create basins or mountains.
    Figure: Flower structures created by strike-slip faults. Depending on the relative movement in relation to the bend in the fault, flower structures can create basins or mountains.

    An example of a dextral, 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 sinistral, left-lateral strike-slip fault is the Dead Sea fault in Jordan and Israel.

    Video showing how faults are classified: