DEFINITION OF FAULT

Discrete surfaces or planes across which two bodies

of rock have moved or slid past each other. They are disconti-

nuities, and they contrast with deformation by homogeneous

strain, folding, or buckling. Faults form at relatively high

crustal levels and low temperatures and are characterized by

brittle deformation. They are sites of rock crushing, grinding,

and fluid circulation. Faulting usually occurs in discrete

events or earthquakes, or by slower creep. Because the rocks

are easily crushed, faults tend to erode, forming gullies,

ravines, or valleys with no exposed rock, across which there

is a discontinuity in the geology.

Faults may be recognized by the presence of one or several

structural elements. They may cause structural or lithological

discontinuity, fault zone deformation, deformation of the land

surface (scarps, offset features); they may be associated with

fault-related sediments and basins, or stratigraphic repetition,

omission, or discontinuity. Recent fault traces in tectonically

active areas produce ground breaks or other recognizable features

associated with the fault trace. The most prominent of

these features are the fault scarps, where part of the land’s surface

is displaced relative to a part that used to be at the same

level. Lines of vegetation may be associated with increased

water permeability along the fault, and they may form ridges

and depressions as different rock slivers are uplifted or downsagged

between bifurcating fault segments. Notches and saddles

in ridges in mountainous areas may mark the trace of a

fault, since fault zones typically contain rocks that are crushed

and easily eroded. Offset stream channels are common along

strike-slip faults such as the San Andreas.

Naming faults is based on the orientation of the fault

planes and the direction of slip or displacement on the fault.

Faults may slip parallel to dip (dip-slip faults), parallel to

strike (strike-slip faults), or obliquely. The area above a fault

is the hanging wall, whereas the block below the fault is the

footwall. Steeply dipping faults are known as high-angle

faults, whereas shallow dipping faults are known as lowangle

faults. Contractional faults shorten or contract a bed

when they move, whereas extensional faults extend or lengthen

beds when they move. The most common classification is

into normal, thrust (or reverse), and strike-slip faults. Normal

faults are dip-slip faults in which the hanging wall drops relative

to the footwall. Thrust faults are dip-slip faults in which

the hanging wall moves up relative to the footwall. Strike-slip

faults are steeply dipping faults that have horizontal slips parallel

to the fault trace.

Faulting often creates topographic differentials across the

fault, creating opportunities for sedimentation. Faults may

form scarps with breccias and olistostromes deposited along

their edges. Some sedimentary basins have faulting occurring

throughout deposition, forming growth faults, characterized

by curving surfaces with thicker layers of sedimentary strata

deposited on the downthrown side of the fault.

Fault zones are dominated by brittle structures and solution

transfer structures (stylolites, veins), with minor

intracrystalline ductile deformation. They normally form

under brittle conditions in the upper 10 kilometers in the

crust, but transitions do exist with ductile shear zones and

mylonites. Frictional sliding along faults typically produces

polished and striated surfaces known as slickensides. The striations

on the surface are known as slickenlines and are

formed parallel to the direction of movement on the fault.

The steps on the slickenside surface face in the direction of

movement of the opposite block. Slickenlines may be either

grooves, scratched into the surface during movement, or

fibers grown within small spaces that opened within the fault

plane during movement. Fault breccias are angular fragments

of rocks formed within fault zones by the crushing of rocks

from either side of the fault. Fault gouge is a fine, sandy to

clay-like ground-up rock powder formed by intense grinding

and milling of the adjacent rocks. Pseudotachylite is an

extremely fine-grained glassy rock typically found as dikes

and intrusive veins along faults, formed by frictional melting

of rocks along the fault. Various types of vein systems are

common along faults because of the increased fluid flow

along the faults. Mylonite is a ductile shear zone rock,

formed at deeper crustal levels than the faults, although there

is a transition zone where they both may occur.

See also BRECCIA; MYLONITE; STRUCTURAL GEOLOGY.

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