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1、4 FaultsA fault is a planar discontinuity between blocks of rock that have been displaced past one another, in a direction parallel to the discontinuity. A fault zone is a tabular region containing many parallel or anastomosing faults (Fig. 7.9.b). A shear zone, in the sense preferred here, is a zon
2、e across which blocks of rock have been displaced in a faultlike manner, but without prominent development of visible faults. Shear zones are thus regions of localized ductile deformation, in contrast to fault zones that are regions of localized brittle deformation. Another distinction is the normal
3、 component of displacement, which is negligible for faults and fault zones, may be appreciable for shear zones. The displacement across a shear zone can be inclined at any angle, other than 90, to the boundaries of the zone. In another common usage Dennis (1967, p. 133) the term shear zone refers to
4、 a tabular region of pervasively faulted rock, that is, a fault zone containing a very large number of closely spaced and anastomosing fault surfaces.The rock immediately above and below any nonvertical fault is referred to, respectively, as the hanging wall and the footwall of the fault. The displa
5、cement vector connecting originally contiguous points in the hanging wall and footwall is called the net slip. The components of the net slip parallel to the strike and dip of the fault are the strike slip and the dip slip.The offset shown by a planar feature in a vertical cross section perpendicula
6、r to the fault is called the dip separation. The vertical and horizontal components of the dip separation are the throw and the heave. Notice, by comparing Figure 7.11 with Figure 7.10，that the dip separation is not equivalent to the dip slip, the former depending on the orientation of the offset su
7、rface as well as on the nature of the fault displacement. A fault with dominant strike slip displacement is called a strike slip fault. A fault with dominant dip slip displacement is a dip slip fault. Strike slip faults usually have very steep or vertical dips and are then referred to as transcurren
8、t faults or wrench faults. A large transcurrent fault that terminates in another large structure, such as an oceanic ridge or trench or triplejunction, is called a transform fault. Transform faults are discussed in detail in Section 10.2.3.The sense of the strike slip part of displacement on a fault
9、 is described by the terms sinistral and dextral, or alternatively, left lateral and right lateral. A fault is sinistral or left lateral if, to an observer standing on one block and facing the other, the opposite block appears to have been displaced to his left. The strike slip component of the disp
10、lacement in Figure 7.10 is sinistral.Faults dipping more or less than 45are called, respectively, high angle faults and low angle faults. A normal fault is a high angle, dip slip fault on which the hanging wall has moved down relative to the footwall. A fault of similar type but with a dip less than
11、 45is sometimes called a lag Rickard (1972). A reverse fault is a dip slip fault, either high or low angle Gill (1971), on which the hanging wall has moved up relative to the footwall. The terms normal fault and reverse fault, while strictly defined for faults with zero strike slip displacement, can
12、 also be used for faults with small strike slip displacements accompanying much larger dip slip displacements Rickard (1972). Where the strike slip and dip slip displacements are similar in magnitude, as in Figure 7.10, the fault can be called an oblique slip fault.A (relatively) downfaulted topogra
13、phic trough between high angle faults is a graben. An upfaulted block between high angle faults is a horst. The faults bordering horsts and grabens are usually normal faults.A thrust fault is a low-angle reverse fault, according to one common usage see Dennis (1967). The term is also used by many ge
14、ologists for low angle faults that are presumed to have involved reverse dip slip displacement but where this has not been demonstrated. A window (or fenster) is an exposure of the rock below a thrust fault that is completely surrounded by rock above the thrust. A klippe is an exposure of the rock a
15、bove a thrust completely surrounded by rock below it.Fault planes are commonly filled with fragmental material known as fault breccia, or microbreccia if the fragments are microscopic. Some microbreccias are soft and are designated by terms such as pug or gouge. Others, particularly in metamorphic r
16、ocks, are hard and characterized by a platy or streaky flow structure in thin section. Such microbreccias, which occur as fault fillings and in wider zones of intense deformation (Section 9. 14), are called mylonites Lapworth (1885); Waters and Campbell (1935); Christie, (1960). Some rocks that look like mylonites in the field are seen in thin section to be highly recrystallized, so that it is not clear whether or not they were ever microbreccias. The original meaning of th
