Types of Geological Faults

A Geological 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 movement.

Allochthon The thrust sheet above a detachment is the allochthon (meaning that it is composed of allochthonous rock; i.e., rock that has moved substantially from its place of origin).

Autochthon The footwall below a detachment is the autochthon; it is composed of autochthonous rock, or rock that is still in its place of origin.

Contractional fault A contractional fault is one whose displacement results in shortening of the layers that the fault cuts, regardless of the orientation of the fault with respect to horizontal.

Detachment fault This term is used for faults that initiate as a horizontal or subhorizontal surface along which the hanging-wall sheet of rock moved relative to the footwall. An older term “overthrust” is a regional detachment fault on which there has been a thrust sense of movement.

 Décollement The French word for detachment.

Types of Geological Faults

Overthrust faults are listric, and on some detachments, regional normal-sense displacement occurs.

Dip-slip fault The slip direction on a dip-slip fault is approximately parallel to the dip of the fault (i.e., has a rake between ?80° and 90°).

Extensional fault An extensional fault is one whose displacement results in extension of the layers that the fault cuts, regardless of the orientation of the fault with respect to horizontal.

Normal fault A normal fault is a dip-slip fault on which the hanging wall has slipped down relative to the footwall.

Oblique-slip fault The slip direction on an oblique-slip fault has a rake that is not parallel to the strike or dip of the fault. In the field, faults with a slip direction between ?10° and ?80° are generally called oblique-slip.

Overthrust fault This is an older term that you may find in older papers on faults, but is no longer used much today. The term is used for thrust faults of regional extent. In this context, “regional extent” means that the thrust sheet has an area measured in tens to hundreds of square km, and the amount of slip on the fault is measured in km or tens of km. Today, such faults are generally called regional detachments. Par-autochthonous If a fault block has only moved a small distance from its original position, the sheet is parautochthonous (literally, relatively in place).

 Reverse fault A reverse fault is a dip-slip fault on which the hanging wall has slipped up relative to the footwall. Scissors fault On a scissors fault, the amount of slip changes along strike so that the hanging-wall block rotates around an axis that is perpendicular to the fault surface.

 Strike-slip fault The slip direction on a strike-slip fault is approximately parallel to the fault strike (i.e., the line representing slip direction has a rake [pitch] in the fault plane of less than ?10°). Strike-slip faults are generally steeply dipping to vertical.

 Transfer fault A transfer fault accommodates the relative motion between blocks of rock that move because of the displacement on other faults.

 Transform fault In the preferred sense, transform faults are plate boundaries at which lithosphere is neither created nor destroyed. In a general sense, a transform fault links two other faults and accommodates the relative motion between the blocks of rock that move because of the displacement on the other two faults. However, we reserve the term transfer fault for this general type of displacement, independent of scale.