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If a fracture is a fault, there will also be offsets of beds or other older surfaces that are cut. Under those circumstances it is better to use compass directions to identify the walls: e.g. If a fault is vertical, it is not possible to characterize a footwall and a hanging wall. The geologist’s feet will rest naturally on the footwall, while the hanging wall will overhang the geologist’s head. If these are not clear, think of a geologist standing in a small cave exactly on a fault plane that dips moderately. The wall located to the up-dip side of the fault is called the footwall. The wall located on the down-dip side of the fault is called the hanging wall. For example, if a fault dips east, then the east wall must overhang the west wall. If a fault has a dip (other than 90°) then one wall overhangs the other. The blocks of rock on either side of a fault plane are the walls of the fault. If the fault is curved, then structure contours may show changes in orientation and spacing. If the fault is planar, the strike and dip will be constant, and the structure contours will be parallel, straight, and equally spaced. For large faults, the orientation may be more easily determined by drawing structure contours. Large faults tend to be poorly exposed, because rocks close to the fault plane are fractured and broken, and therefore are easily weathered. For small faults, it may be possible to walk up to an outcrop and measure the orientation with a clinometer. Like any planar structure, it has an orientation that may be characterized by strike and dip. The grey diagram shows a map of the fault plane, known as a fault plane section.Ī fault is a planar geologic structure. Below, the same fault is shown as it might appear on a map and in a cross-section parallel to the dip-direction (a dip section). Block diagram of a simple fault offsetting a single surface (dark grey).

Faults can be recognized at a variety of scales from centimetre-scale offsets in an individual outcrop to faults that can be traced on the ground for tens to hundreds of kilometres such as the San Andreas Fault. The orientation of natural faults often varies and they eventually disappear along strike however, they do tend to have surfaces that are more planar than other types of geological surfaces, at least over distances of a few kilometres. At outcrop (mesoscopic) or microscopic scale they look rather different. It is likely that most faults that deform the upper crust pass at depth into shear zones in the lower crust.Īt map scale, faults and shear zones look the same – a line across which older structures are offset. The movement in a shear zone is distributed across the zone, rather than being restricted to discrete brittle faults. However, a shear zone is a ductile structure, typically formed at depth where the deformation of the mineral grains is plastic, not brittle. A shear zone is the ductile equivalent of a fault zone – a belt of rock across which movement has caused a significant amount of offset between the two sides. These add up across the fault zone to a much larger offset.Ī related structure is a shear zone. Each fault in the zone offsets the rocks on either side by a small amount.

The total offset of the fault zone is distributed across the zone. This type of fault array is called a fault zone. However, many mapped faults turn out to have multiple fault strands, all roughly parallel, but branching and joining (“anastomosing”) along their strike. Strictly speaking, a fault is a single fracture surface. \)įaults, fault zones, and shear zones Figure 1.