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Under the present-day scenario of doing everything on a computer monitor it has become imperative to emphasize an obvious fact, viz., faults must be recognized on geological criteria. Several categories of geological features are used to recognize faults.

They are not mutually exclusive; often more than one category of features are found associated with a single fault.

• Geomorphic features: The most obvious geomorphic feature that may betray the presence of a fault is fault scarp (Fig. 8.5). Fault scarps are continuous linear features characterized by sudden break in topographic slopes. They may indicate either active or inactive faults. However, the original scarp associated with an old and inactive fault may not survive erosion for very long. Regular fault scarps are more commonly associated with normal faults (Fig. 8.5a).

Subsidiary smaller scarps are usually present parallel to the main scarp. The uplifted block is usually cut by V-shaped side valleys. The erosional debris derived from the uplifted block and brought along the side valleys form alluvial fans on the downthrown block. Successive movements along faults may leave perched alluvial terraces on the side valleys. The scarps formed due to thrust faulting tend to be irregular (Fig. 8.5b). An interesting feature associated with thrusting is that the fault often overrides the debris derived from the uplifted hangingwall and deposited in front of the scarp. In strike-slip faulting the scarp is usually small and of local importance (Fig. 8.5b). Deflected or offset geomorphic features, such as, river channels, hogbacks and ridges may indicate the presence of strike-slip faulting. The scarps associated with large faults may show up as lineament satellite images and air photographs. But it must be remembered that a majority of lineaments drawn on satellite images are not faults. Unfortunately, there is a rising but very unscientific tendency in some quarters to draw lineaments on satellite images, construct a rose diagram and deduce stress axes. Surface exposures of faults must be sought out during

fieldwork because much critical information can be gathered through field

Figure 8.5. Geomorphic features associated with three main types of faulting.

2 km

Figure 8.6. Recognition of faults from map patterns. (a) Offsets of rock units trace faults. Symmetric repetition of rock units in the N-S direction around rocks 1 and 4 are due to folding. The fault surfaces trace lines of discontinuities along which three rock units meet. (b) Omission and repetition of rock units trace faults F1 and F2.

• Geological map and stratigraphy: Large faults are relatively easy to recognize in regions of moderate to excellent exposures of rocks through systematic mapping. One should be careful, however, because some of the geologic features are common to both faults and unconformities. Faults are recognized on the basis of truncation and offset of one or more rock units (Fig. 8.6a).

Truncation and offset occur along a line on a map that defines a discontinuity along which three rocks meet at a point. Such a map pattern may also indicate angular unconformity if rocks on two sides of the discontinuity are of different ages. In case of faults, same rock units should be present on both sides of the discontinuity. This relation may not be valid for large overthrusts, which may bring older rocks to lie over younger rocks or may even bring metamorphic rocks on top of sedimentary rocks. The map patterns due to truncation and offset vary considerably depending on the orientations of faults relative to the orientations of rock layers affected by faulting, and amount and direction of slip. The map patterns may become even more complex if the terrain had an earlier history of folding and faulting. A common effect of truncation and offset is an apparent horizontal shifting of the rock units. There may not be any truncation and offset if the strike of the fault is same as the strike of the rock units. In such cases, faults can be recognized on the basis of repetition and omission of strata if the stratigraphy of the rock units is known (Fig. 8.6b). The line along which a packet of rock units are repeated (fault F1 in Fig. 8.6b) or some ofthe rock units are omitted (fault F2 in Fig. 8.6b) marks a fault plane.

Symmetric repetition of rock units about a particular rock unit due to folding (see Fig. 8.6a) should not be confused with simple repetition due to faulting. In drill wells, missing or repetition of beds can be used to predict faults.

• Fault rocks: Large-scale faulting with significant amount of displacement lead to development of characteristic textures and structures within the rocks present in the fault zone. Rocks with such characteristic textures and structures are collectively called fault rocks (Wise et al. 1984), which can be broadly divided into cataclasite and mylonite. Cataclasites or cataclastic rocks is a general term that refers to rocks fractured into clasts or ground into powder and signify brittle deformation. Individual clasts are sharp, angular and internally deformed. Cataclasites usually do not have any planar and linear fabrics. Fault gouge and fault breccia are results of cataclastic deformation within fault zones.

Incohesive (i.e., friable) cataclasitic rocks are characteristic of faulting above

depths of 1-4 km; below this depth cataclastic rocks are cohesive. Frictional heating during brittle faulting can be sufficient high to melt a small portion of the rock. The melt may intrude into surrounding fractures and quenched to give veins of pseudotachylite. Mylonitic rocks form if faulting occurs at depths exceeding 10 to 15 km. These are fine-grained rocks with grain size reduction via dynamic recrystallization and neomineralization. Mortar texture with highly strained clasts in a matrix of fine-grained recrystallized grains is a typical texture in these rocks. Planar and linear fabrics are common in mylonites. This texture is a characteristic feature of ductile shear zones. The development of textures and structures in fault zones depend on several parameters including strain, strain rate, temperature, pressure and pore fluid pressure. Therefore, it is not possible to make simple correlation between depth of faulting and type of textures and structures in fault rocks.