Fault (geology)
In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as the megathrust faults of subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes. Faults may also displace slowly, by aseismic creep.
A fault plane is the plane that represents the fracture surface of a fault. A fault trace or fault line is a place where the fault can be seen or mapped on the surface. A fault trace is also the line commonly plotted on geological maps to represent a fault.
A fault zone is a cluster of parallel faults. However, the term is also used for the zone of crushed rock along a single fault. Prolonged motion along closely spaced faults can blur the distinction, as the rock between the faults is converted to fault-bound lenses of rock and then progressively crushed.
Mechanisms of faulting
Due to friction and the rigidity of the constituent rocks, the two sides of a fault cannot always glide or flow past each other easily, and so occasionally all movement stops. The regions of higher friction along a fault plane, where it becomes locked, are called asperities. Stress builds up when a fault is locked, and when it reaches a level that exceeds the strength threshold, the fault ruptures and the accumulated strain energy is released in part as seismic waves, forming an earthquake.Strain occurs accumulatively or instantaneously, depending on the liquid state of the rock; the ductile lower crust and mantle accumulate deformation gradually via shearing, whereas the brittle upper crust reacts by fracture – instantaneous stress release – resulting in motion along the fault. A fault in ductile rocks can also release instantaneously when the strain rate is too great.
Slip, heave, throw
Slip is defined as the relative movement of geological features present on either side of a fault plane. A fault's sense of slip is defined as the relative motion of the rock on each side of the fault concerning the other side. In measuring the horizontal or vertical separation, the throw of the fault is the vertical component of the separation and the heave of the fault is the horizontal component, as in "Throw up and heave out".The vector of slip can be qualitatively assessed by studying any drag folding of strata, which may be visible on either side of the fault. Drag folding is a zone of folding close to a fault that likely arises from frictional resistance to movement on the fault. The direction and magnitude of heave and throw can be measured only by finding common intersection points on either side of the fault. In practice, it is usually only possible to find the slip direction of faults, and an approximation of the heave and throw vector.
Hanging wall and footwall
The two sides of a non-vertical fault are known as the hanging wall and footwall. The hanging wall occurs above the fault plane and the footwall occurs below it. This terminology comes from mining: when working a tabular ore body, the miner stood with the footwall under his feet and with the hanging wall above him. These terms are important for distinguishing different dip-slip fault types: reverse faults and normal faults. In a reverse fault, the hanging wall displaces upward, while in a normal fault the hanging wall displaces downward. Distinguishing between these two fault types is important for determining the stress regime of the fault movement.The problem of the hanging wall can lead to severe stresses and rock bursts, for example at Frood Mine.
Fault types
Faults are mainly classified in terms of the angle that the fault plane makes with the Earth's surface, known as the dip, and the direction of slip along the fault plane.Based on the direction of slip, faults can be categorized as:
- strike-slip, where the offset is predominantly horizontal, parallel to the fault trace;
- dip-slip, offset is predominantly vertical and/or perpendicular to the fault trace; or
- oblique-slip, combining strike-slip and dip-slip.
Strike-slip faults
A special class of strike-slip fault is the transform fault when it forms a plate boundary. This class is related to an offset in a spreading center, such as a mid-ocean ridge, or, less common, within continental lithosphere, such as the Dead Sea Transform in the Middle East or the Alpine Fault in New Zealand. Transform faults are also referred to as "conservative" plate boundaries since the lithosphere is neither created nor destroyed.
Dip-slip faults
Dip-slip faults can be either normal or reverse. The terminology of "normal" and "reverse" comes from coal mining in England, where normal faults are the most common.With the passage of time, a regional reversal between tensional and compressional stresses might occur, and faults may be reactivated with their relative block movement inverted in opposite directions to the original movement. In such a way, a normal fault may therefore become a reverse fault and vice versa.
Normal faults
In a normal fault, the hanging wall moves downward, relative to the footwall. The dip of most normal faults is at least 60 degrees but some normal faults dip at less than 45 degrees.Basin and range topography
A downthrown block between two normal faults dipping towards each other is a graben. A block stranded between two grabens, and therefore two normal faults dipping away from each other, is a horst. A sequence of grabens and horsts on the surface of the Earth produces a characteristic basin and range topography.Listric faults
A listric fault is a type of normal fault that has a concave-upward shape with the upper section near Earth's surface being steeper, becoming more horizontal with increased depth. Normal faults can evolve into listric faults with the fault plane curving into the Earth. They can also form where the hanging wall is absent, where the footwall may slump in a manner that creates multiple listric faults.Detachment faults
The fault planes of listric faults can further flatten and evolve into a horizontal or near-horizontal plane, where slip progresses horizontally along a decollement. Extensional decollements can grow to great dimensions and form detachment faults, which are low-angle normal faults with regional tectonic significance.Due to the curvature of the fault plane, the horizontal extensional displacement on a listric fault implies a geometric "gap" between the hanging and footwalls of the fault forms when the slip motion occurs. To accommodate into the geometric gap, and depending on its rheology, the hanging wall might fold and slide downwards into the gap and produce rollover folding, or break into further faults and blocks which fill in the gap. If faults form, imbrication fans or domino faulting may form.
Reverse faults
A reverse fault is the opposite of a normal fault—the hanging wall moves up relative to the footwall.Reverse faults indicate compressive shortening of the crust.
Thrust faults
A thrust fault has the same sense of motion as a reverse fault, but with the dip of the fault plane at less than 45°. Thrust faults typically form ramps, flats and fault-bend folds.A section of a hanging wall or foot wall where a thrust fault formed along a relatively weak bedding plane is known as a flat and a section where the thrust fault cut upward through the stratigraphic sequence is known as a ramp. Typically, thrust faults move within formations by forming flats and climbing up sections with ramps. This results in the hanging wall flat lying atop the foot wall ramp as shown in the fault-bend fold diagram.
Thrust faults form nappes and klippen in the large thrust belts. Subduction zones are a special class of thrusts that form the largest faults on Earth and give rise to the largest earthquakes.
Oblique-slip faults
A fault which has a component of dip-slip and a component of strike-slip is termed an oblique-slip fault. Nearly all faults have some component of both dip-slip and strike-slip; hence, defining a fault as oblique requires both dip and strike components to be measurable and significant. Some oblique faults occur within transtensional and transpressional regimes, and others occur where the direction of extension or shortening changes during the deformation but the earlier formed faults remain active.The hade angle is defined as the complement of the dip angle; it is the angle between the fault plane and a vertical plane that strikes parallel to the fault.