Fracturing

A rock fractures if it is hard and brittle and subjected to sudden strain that overcomes its internal crystalline bonds. If the rock has been displaced along a fracture, such as having one side that is moved up or down, the fracture is called a fault, and if there is no displacement along the crack, the fracture is called a joint.

Faults. Horizontal or vertical displacement along the fault plane can range from a few centimeters to hundreds of kilometers. The fault can be merely a crack between the two sides of rock, or it can be a fault zone hundreds of meters wide that consists of rock that has been very fractured, brecciated, and pulverized from repeated grinding movements along the fault plane. The broken material within a fault is called fault gouge. The rocks within a fault zone may also be hydrothermally altered or veined from hot solutions that have migrated up the fault zone. A fault is generally considered active if movement has occurred along it during the past 10,000 years.

Fault movements. Three kinds of fault movements are recognized: dip-slip, strike-slip, and oblique-slip. Movement in a dip-slip fault is parallel to the dip of the fault plane in an “up” or “down” direction between the two blocks. The block that underlies an inclined dip-slip fault is called the footwall; the block that rests on top of the inclined fault plane is called the hanging wall. A normal dip-slip fault, or normal fault, is one in which the hanging wall block has slipped down the fault plane relative to the footwall block. A reverse dip-slip fault is just the opposite: the hanging wall block has moved upward relative to the footwall block (Figure 1 ).

Figure 1 Dip-Slip Faults

The blocks on either side of a strike-slip fault move horizontally in relation to each other, parallel to the strike of the fault. If a person is standing at the fault and looks across to see that a feature has been displaced to the left, it is called a left-lateral strike-slip fault. A right-lateral strike-slip fault is one in which the displacement appears to the right when looking across the fault (Figure 2 ).

Figure 2 Strike-Slip Faults

A graben is formed when a block that is bounded by normal faults slips downward, usually because of a tensional force, creating a valley-like depression. A horst results when a block that is bounded by normal faults experiences a compressive force that forces the block upward, forming mountainous terrain (Figure 3 ).

Figure 3 A Graben and Horst

Thrust faults are reverse dip-slip faults in which the hanging wall block has overridden the footwall block at a very shallow angle for tens of kilometers. The hanging wall block and footwall block of a thrust fault are typically called the upper plate and lower plate, respectively (Figure 4 ).

Figure 4 A Thrust Fault

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