Out of Sequence Thrust Faults
Out of sequence faults are a type of thrust fault found commonly in fold and thrust belts. The Kvitvola Thrust sheet in southern Norway and the Numidian Thrust sheet in north Africa are good examples. In-sequence faults are the more common types of faults and are characterized by ramp and flat geometry. These faults tend to exhibit foreland propagation which is thought to be the normal thrust belt deformation sequence (Morley 1988).
Out of sequence faults show hinterland propagation. They are caused by reactivation of older in-sequence thrusts or by the development of new thrust faults through a deformed thrust sheet (Morley 1988). In-sequence thrusts place older rocks on top of younger rocks as they cut upsection in the transport direction (Fox 1969). Out of sequence thrusts result in younger rocks being on top of older rocks as they cut upsection and downsection. (Morley 1988).
Out of sequence faults are found in the wedge of the thrust sheet and are responsible for internal deformation. They serve to maintain the critical taper of the wedge (Chapple 1978). Foreland propagation seen in thrusting does not show continuous development. Many things causes the thrust sheet to stick. In order to overcome the sticking of the sheet an out of sequence fault is formed (Morley 1988). Out of sequence faults are also found associated with deformation seen in the sedimentary rocks that come from the underlying plate as the thrust sheet spreads toward the foreland (England 1982).