An extensional fault is a fault caused by stretching of the

Earth's crust Earth's crust is Earth's thin outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The ...

. Stretching reduces the thickness and horizontally extends portions of the crust and/or

lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...

. In most cases such a fault is also a

normal fault 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 ...

, but may create a shallower dip usually associated with a

thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...

. Extensional faults are generally

planar Planar is an adjective meaning "relating to a plane (geometry)". Planar may also refer to: Science and technology * Planar (computer graphics), computer graphics pixel information from several bitplanes * Planar (transmission line technologies), ...

. If the

stress field A stress field is the distribution of internal forces in a body that balance a given set of external forces. Stress fields are widely used in fluid dynamics and materials science. Consider that one can picture the stress fields as the stress cre ...

is oriented with the maximum stress perpendicular to the Earth's surface, extensional faults will create an initial dip of the associated

beds A bed is an item of furniture that is used as a place to sleep, rest, and relax. Most modern beds consist of a soft, cushioned mattress on a bed frame. The mattress rests either on a solid base, often wood slats, or a sprung base. Many be ...

of about 60° from the horizontal. The faults will typically extend down to the base of the

seismogenic layer In geophysics and seismology, the seismogenic layer covers the range of depths within the crust or lithosphere in which most earthquakes originate. The thickness is heavily dependent on location. For oceanic crust, the seismogenic layer thicknes ...

. As crustal stretching continues, the faults will rotate, resulting in steeply-dipping fault blocks between them.

References

Structural geology {{Tectonics-stub

See also

References