Overthrust

A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger. Thrust faults are the result of compressional forces.

Thrust faults are a special class of "reverse faulting" that typically have low dip angles. It is often hard to recognize thrusts because their deformation and dislocation can be difficult to detect when they occur within the same rocks without appreciable offset of lithological contacts.

If the angle of the fault plane is low (generally less than 20 degrees from the horizontal) and the displacement of the overlying block is large (often in the kilometer range) the fault is called an overthrust. Erosion can remove part of the overlying block, creating a fenster (or window) when the underlying block is only exposed in a relatively small area. When erosion removes most of the overlying block, leaving only island-like remnants resting on the lower block, the remnants are called klippen (singular klippe).

If the fault plane terminates before it reaches the Earth's surface, it is referred to as a blind thrust fault. Because of the lack of surface evidence, blind thrust faults are difficult to detect until they rupture. The destructive 1994 quake in Northridge, California was caused by a previously-undiscovered blind thrust fault.

Because of their low dip, thrusts are also difficult to appreciate in mapping, where lithological offsets are generally subtle and stratigraphic repetition difficult to detect especially in peneplanated areas.

Thrust faults, particularly those involved in thin-skinned style of deformation, have a so-called ramp-flat geometry. Thrusts mostly propagate along zones of weakness within a sedimentary sequence, such as mudstones or salt layers, these parts of the thrust are called decollements. If the effectiveness of the decollement becomes reduced, the thrust will tend to cut up the section to a higher stratigraphic level until it reaches another effective decollement where it can continue as bedding parallel flat. The part of the thrust linking the two flats is known as a ramp and typically forms at an angle of about 15°-30° to the bedding. Continued displacement on a thrust over a ramp produces a characteristic fold geometry known as a ramp anticline or, more generally, as a fault-bend fold.

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