Faults and Fractures

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Faults and fractures

Fractures and faults are planes of tensile or shear failure at microscopic to regional scales in brittle rocks. Faults may constitute a single plane or comprise zones of parallel or oblique shear planes, fault breccia or gouge (finely ground rock ) across which there has been relative displacement of rocks on either side. Faults and fractures dominate approximately the upper 9 mi (15 km) of the earth's crust . Earthquakes are the expression of rapid displacement along faults. Most upper crustal rocks deform in a brittle manner at rapid deformation (strain) rates due to low temperature and confining pressure. Faults and fractures also develop in competent rocks at deeper crustal levels and in some dry rocks in the lower crust.

Fractures develop when the applied stress exceeds a rock's elastic limit. Regional stresses in plate interiors responsible for fracturing and faulting may represent far-field effects of tectonic forces acting on plate margins. They can also result

from gravitational instabilities (e.g., when a less dense, ductile rock such as salt is overlain by brittle, denser rocks), unloading (e.g., in response to removal of an ice sheet) or mantle plumes . Rocks may also fracture at regional stresses below their elastic limit if pore fluid pressure increases sufficiently. This phenomenon (called hydrofracturing or hydraulic fracturing) is used commercially in the petroleum industry to increase permeability of petroleum reservoirs. It may also be induced inadvertently, such as by building a large dam over a previously inactive fault zone, triggering fracturing that may potentially cause the dam to fail.

Tensile (extensional) fractures develop normal to the maximum extension direction. Tensile fractures may dilate and infill with minerals such as quartz and calcite precipitated from fluids within the rock to form extensional veins. The direction of vein opening (deduced from the orientation of quartz or calcite fibers that track the incremental extension direction) is orthogonal to fracture margins. Shear fractures (fractures along which lateral displacement occurs) develop oblique to the principal stresses. Shear fractures commonly develop in two preferred orientations where fractures with the opposite sense of displacement form at an angle of approximately 60° to each other, constituting a conjugate set. Conjugate fractures are bisected acutely by the maximum shortening direction and obtusely by the maximum extension direction. Their intersection parallels the intermediate principal stress. The acute angle between conjugate shear fractures and faults may be less than 60° if the rock is very brittle or greater than 60° if the rock is more ductile. One or more of the following types of minor fractures commonly develop prior to the formation of through-going shear fractures or faults:

  • tensile fractures that strike parallel to each other in both conjugate zones,
  • shear fractures along each zone that trend parallel to, and have the same sense of displacement as the conjugate zone,
  • shear fractures (called Riedel shears) that make an angle of approximately 15° to each zone. All three may step en échelon along incipient shear zones . The sense of stepping may be used to determine the sense of displacement along a fault where there is no clear offset of marker layers.

There are three end-member types of faults, with each type forming under different orientations of principal stresses:

  • Normal or extensional faults are inclined structures along which rocks above the fault plane (i.e. in the hangingwall) are displaced down the fault with respect to rocks beneath the fault plane (i.e. in the footwall). Such faults were called normal as they were the most commonly observed faults by Welsh coal miners who termed the name. They form when the maximum extension direction is horizontal during vertical shortening (due to the gravitation loading of overlying rock). Normal faults develop in sedimentary basins during rifting and in areas of localized horizontal extension, such as above salt diapirs or during collapse or slumping.
  • Reverse or contractional faults are moderately inclined structures along which the hanging wall is displaced up the footwall. The name reverse fault also comes from Welsh miners as they showed the opposite sense of displacement to the normal faults. Shallowly dipping faults with reverse displacement are called thrusts. Reverse and thrust faults imply horizontal shortening and vertical extension, and are commonly formed in convergent plate margins. Reverse faults in sedimentary basins may also form in the toes of deltas because of local shortening due to sediment loading or slumping or on the margins of laterally expanding salt diapirs.
  • Faults along which there has been lateral, sub-horizontal displacement are called strike-slip, transcurrent or wrench faults. They are generally steeply dipping structures. They also form during regional horizontal shortening, but where the maximum extension direction is horizontal. When the left side (observed in map view) is displaced towards the observer, the fault is said to show a sinistral or left lateral sense of displacement. When the right side is displaced towards the observer, the displacement is dextral or right-lateral. Oblique-slip faults have components of both transcurrent and either normal or reverse displacement. The direction of displacement along a fault is indicated by fine scratch or gouge marks (called slickenlines) and/or mineral fibers that infill spaces created by displacement of irregular, stepped surfaces. Fault planes that contain striations and/or fibers are called slickensides. Where the sense of displacement cannot be seen by the offset of markers, it can be determined from the sense of stepping of irregularities along the fault surface, the location of dilatational sites in which mineral fibers have grown, gouge marks formed by the incutting of rigid bodies in the rock, and the en échelon stepping of minor fractures (as described above).

Faults are important in mineral and petroleum exploration as they may either seal and act as a barrier to fluid flow (e.g., due to smearing of mud or shale along them), or may be important conduits for the migration of petroleum or mineralizing fluids. Many mineral deposits are fault and fracture controlled. Recognition of faults is also important in hydrogeological studies as fracturing along faults may produce hard-rock aquifers.

See also Petroleum detection; Plate tectonics