Faulting
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A fault, strictly speaking, is a planar fracture through a rock, wherein the motion along the fracture is in the plane of the fracture. If there is no relative sliding motion along a fracture, that is, it only opens, it is a joint.
Faulting occurs when shear stress on a rock overcomes the cohesion and frictional force on the rock. The fracture itself is called a fault plane, although it is not necessarily planar. When it is exposed at the earth's surface, it may form a cliff or steep slope called a fault scarp.
The angle between the fault plane and an imaginary horizontal plane is called the dip angle of the fault. Faults may dip shallowly or steeply.
Faults are generally categorized in three types, depending on the orientation of the fault plane, relative to the direction of motion on the fault.
A normal fault is one where the fault dips toward the downthrown block. Motion on normal faults produce overall extension and thinning of the crust. Normal faults characterize rifted terranes, such as Mid-Ocean Ridges, the African Rift, and the Basin and Range Province of western North America. It is shown on geologic maps as a black line with either a block pattern on the downthrown side, or the letters U/D showing the upthrown and downthrown sides.
A reverse fault (if steeply dipping) or thrust fault (if shallowly dipping)[1] is a fault where the fault plane dips toward the upthrown block. It is shown on the geologic map with triangular teeth pointing toward the upthrown side of the fault. Reverse and thrust faults effectively shorten (horizontally) and thicken the crust. They are characteristic of collisional origins.
A transform (oceanic) or strike-slip (continental fault is one where the relative motion is horizontal. The fault plane is usually vertical. Famous examples of these include the San Andreas Fault of California, the Alpine Fault of New Zealand's south island, and the Anatolian Fault in Turkey.
