Geologic fault

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Geologic faults or simply faults are planar rock fractures which show evidence of relative movement. Large faults within the Earth's crust are the result of shear motion and active fault zones are the causal locations of most earthquakes. Earthquakes are caused by energy release during rapid slippage along faults. The largest examples are at tectonic plate boundaries, but many faults occur far from active plate boundaries. Since faults usually do not consist of a single, clean fracture, the term fault zone is used when referring to the zone of complex deformation associated with the fault plane.

The two sides of a fault are called the hanging wall and footwall. By definition, the hanging wall occurs above the fault and the footwall occurs below the fault.

1 Slip, heave, throw
2 Fault types
3 Dip-slip faults
4 Strike-slip faults
5 Oblique-slip faults
6 See also
7 References

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Slip, heave, throw

The sense of slip is defined by the relative movements of geological features present on either side of the fault plane and is a vector. The sense of slip defines the type of fault. This is distinct from the throw of the fault, which is the vertical offset. Heave is the measured horizontal offset of the fault.

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Fault types

Faults can be categorized into three groups based on the sense of slip. A fault where the main sense of movement (or slip) on the fault plane is vertical is known as a dip-slip fault. Where the main sense of slip is horizontal the fault is known as a transform (or strike-slip) fault. Oblique-slip faults have significant components of both strike and dip slip.

For all naming distinctions, it is the orientation of the net dip and sense of slip of the fault which must be considered, and not the present day orientation, which may have been altered by local or regional folding or tilting.

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Dip-slip faults

Image:Fault types.png

Dip-slip faults include both normal and reverse.

A normal fault occurs when the crust is in extension. The hanging wall moves downwards relative to the footwall. The depressed ground between two parallel normal faults is called a graben. An upthrown block between two parallel normal faults is called a horst. Low-angle normal faults with regional tectonic significance may be designated detachment faults.

A reverse fault is the opposite of a normal fault - the hanging wall moves up relative to the footwall. Reverse faults are indicative of shortening of the crust. The dip of a reverse fault is relatively steep, greater than 45°.

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 foot wall) folds. Thrust faults are responsible for forming nappes and klippen in the large thrust belts.

The fault plane is the plane that represents the fracture surface of a fault. Flat segments of thrust fault planes are known as flats, and inclined sections of the thrust are known as ramps. Typically thrust faults move within formations by forming flats, and climb up section with ramps.

Fault-bend folds are formed by faults from the pressure of the hangingwall and footwall moving against one another.

diagrams explaining thrust fault geometries

In the image below, you see a normal fault at left. A reverse fault is the same, except the hanging wall moves up instead of down. At right, you see a thrust fault.

Image:Nor rev.png

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Strike-slip faults

The fault surface is usually near vertical and the footwall moves either left or right or laterally with very small vertical motion. Strike-slip faults with left-lateral motion are also known as sinistral faults. Those with right-lateral motion are also known as dextral faults. A special class of strike-slip faults is the transform faults which are a plate tectonics feature related to spreading centers such as mid-ocean ridges.

Image:Strike slip fault.png

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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 will have some component of both dip-slip and strike-slip, so defining a fault as oblique requires both dip and strike components to be measurable and significant. Most oblique faults occur within transtensional and transpressional regimes.

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