Fault

Fault, in geology, a fracture in the Earth's crust along which a section of the crust has been displaced relative to another section, in response to forces of tension or compression as a result of tectonic movement. This movement may be in a vertical or horizontal direction, or a combination of the two. The fracture may range from centimetres to hundreds of kilometres long, such as the San Andreas Fault in California. The movements at fault lines may be sudden if generated in a severe earthquake. This may even involve substantial displacement of the surface forms. Commonly, however, movement is long term, cumulative, slow, and imperceptible, as in mountain masses that have risen by fault movement. Even so it may, in time, result in thousands of metres of displacement. Over millions of years, horizontal movement along the San Andreas Fault, which forms a boundary between different plates of the Earth's crust, has shifted a section of California's coastal ranges far to the north-west relative to the rest of the region—and generated powerful earthquakes, such as the one that struck San Francisco in 1906.

Faults consist of a single, sharply defined plane, as a shear zone (a series of folds) of interleaved cracks and crushed rocks several metres wide, or as a zone of ducts in which rock layers flow, slide, and bend past each other. A fault is actually a surface known as a fault “plane”, but which is often non-planar in form. The plane may be vertical but commonly dips into the ground as a sloping surface. In such cases there will be a rock mass resting on the plane which is known as a hanging wall. The mass beneath the plane is the footwall. The fault plane is defined by an angle of dip, a strike direction, and an amount of displacement. If the movement is up or down the dip the fault is called a dip-slip fault. If the movement is along the strike a strike-slip or wrench fault occurs. A combination is an oblique-slip fault.

If the hanging wall rock mass moves down relative to the footwall mass then a normal fault is produced. Where the block moves up relative to the footwall a reverse or thrust fault occurs. Movement along the strike produces a right-lateral or dextral fault if the fault block moves to the right when observed across the fault line. If it moves to the left it is a left-lateral or sinistral fault. Where the amount of dip changes along the strike a hinge or rotational fault is defined.

There are three broad kinds of evidence for faults. First, movement will form characteristic structures such as polished surfaces, striations (scratches often caused by erosion), or slickensides (polished rock surfaces caused by friction). Material textures produced by the movement include broken pieces of rock or powder called breccia and gouge; rock melted by frictional heat called pseudotachylites; and, at depth, fine-grained rocks called mylonites. Secondly, movement causes omission, repetition, or truncation of strata (rock layers) so that the original layers are not continuous in section. There may be small drag fold features or other strata deformation. Thirdly, at the ground surface distinctive morphological (land-form) features occur. Movement can rupture the surface to form a cliff-like fault scarp. Erosion of this by streams flowing across the scarp produces truncated spurs or flat-iron forms, which are small triangular spur remnants of the original scarp. Continued erosion degrades the scarp away from the fault itself to leave a fault-line scarp. Fault traces, benches, hollows, and uphill-facing scarplets determine the limits of the fault line, as do offset streams and shutter-ridges. In plan the line may show sudden bends, ramps, and terminations, or widen into two fold forms and splay into multiple traces.