Fault (geology)
In geology, a 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 movements.
[8] Due to friction and the rigidity of the constituent rocks, the two sides of a fault cannot always glide or flow past each other easily, and so occasionally all movement stops.
[2] Strain occurs accumulatively or instantaneously, depending on the liquid state of the rock; the ductile lower crust and mantle accumulate deformation gradually via shearing, whereas the brittle upper crust reacts by fracture – instantaneous stress release – resulting in motion along the fault.
Slip is defined as the relative movement of geological features present on either side of a fault plane.
[11] The vector of slip can be qualitatively assessed by studying any drag folding of strata, which may be visible on either side of the fault.
[14] This terminology comes from mining: when working a tabular ore body, the miner stood with the footwall under his feet and with the hanging wall above him.
This class is related to an offset in a spreading center, such as a mid-ocean ridge, or, less common, within continental lithosphere, such as the Dead Sea Transform in the Middle East or the Alpine Fault in New Zealand.
A sequence of grabens and horsts on the surface of the Earth produces a characteristic basin and range topography.
They can also form where the hanging wall is absent (such as on a cliff), where the footwall may slump in a manner that creates multiple listric faults.
[22][23] Thrust faults typically form ramps, flats and fault-bend (hanging wall and footwall) folds.
Continued dip-slip displacement tends to juxtapose fault rocks characteristic of different crustal levels, with varying degrees of overprinting.
The level of a fault's activity can be critical for (1) locating buildings, tanks, and pipelines and (2) assessing the seismic shaking and tsunami hazard to infrastructure and people in the vicinity.
In California, for example, new building construction has been prohibited directly on or near faults that have moved within the Holocene Epoch (the last 11,700 years) of the Earth's geological history.
[28] Also, faults that have shown movement during the Holocene plus Pleistocene Epochs (the last 2.6 million years) may receive consideration, especially for critical structures such as power plants, dams, hospitals, and schools.
Geologists assess a fault's age by studying soil features seen in shallow excavations and geomorphology seen in aerial photographs.
[31] Further south in Chile Los Bronces and El Teniente porphyry copper deposit lie each at the intersection of two fault systems.
It has been proposed that deep-seated "misoriented" faults may instead be zones where magmas forming porphyry copper stagnate achieving the right time for—and type of—igneous differentiation.
[32] At a given time differentiated magmas would burst violently out of the fault-traps and head to shallower places in the crust where porphyry copper deposits would be formed.
[32] As faults are zones of weakness, they facilitate the interaction of water with the surrounding rock and enhance chemical weathering.
