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 In a
strike-slip fault (also known as a
wrench fault,
tear fault or
transcurrent fault), the fault surface (plane) is usually near vertical, and the footwall moves laterally either left or right with very little vertical motion. Strike-slip faults with left-lateral motion are also known as
sinistral faults and those with right-lateral motion as
dextral faults. Each is defined by the direction of movement of the ground as would be seen by an observer on the opposite side of the fault. 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 view, along a plane perpendicular to the
fault plane, illustrating normal and reverse dip-slip faults
Dip-slip faults can be either
normal ("
extensional") 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 (or vice-versa) might occur, and faults may be reactivated with their relative block movement inverted in opposite directions to the original movement (fault inversion). 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 (such as on a cliff), where the footwall may slump in a manner that creates multiple listric faults. File:Listric fault diagram.jpg|Cross-section diagram of a listric fault File:Listric walls in a cliff wall.jpg|Cross-section diagram of multiple listric faults in a cliff wall
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. File:Rollover.png|Cross-section diagram of a listric fault (red line), with a resulting rollover fold File:Listric faults and imbrication fan.jpg|Cross-section diagram showing how an imbrication fan forms The fault planes of multiple listric faults can flatten and connect at depth into a common
décollement, .
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 (hanging wall and footwall) 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 (or a portion thereof) 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.
Ring fault Ring faults, also known as
caldera faults, are faults that occur within collapsed volcanic
calderas and the sites of
bolide strikes, such as the
Chesapeake Bay impact crater. Ring faults are the result of a series of overlapping normal faults, forming a circular outline. Fractures created by ring faults may be filled by
ring dikes.
Synthetic and antithetic faults Synthetic and
antithetic are terms used to describe minor faults associated with a major fault. Synthetic faults dip in the same direction as the major fault while the antithetic faults dip in the opposite direction. These faults may be accompanied by
rollover anticlines (e.g. the
Niger Delta structural style). ==Fault rock==