1. Field Recognition of Faults
Criteria for Faulting
Field Recognition of Faults
Numerous criteria demonstrate faulting.
Few criteria may be present in a particular case.
For some faults many criteria may be available, whereas for others very little evidence may be obtained.
Criteria depend in part on the size of the area involved.
Small faults are easy to identify in natural and artificial exposure.
Large scale faults in most cases are not visible, and are deduced on structural, stratigraphic and physiographic evidences.
Faults may be difficult to distinguish from unconformities.
Systematic geologic mapping has proved to be an effective method in recognizing faults.

2. Criteria for Faulting
The criteria for recognition of faults may be considered under the following headings:-
1. Discontinuity of structures 2. Repetition or omission of strata 3. Features characteristics of fault planes 4. Silicification and mineralization 5. Sudden changes in sedimentary facies 6. Physiographic criteria

3. Criteria for Faulting 1. Discontinuity of structures
If a strata suddenly end against different beds, a fault may be present. Dykes, veins or older faults also may end suddenly along some line, and the displaced parts may appear elsewhere. Moreover, discontinuity of structures is, in itself, not proof of faulting; the truncation of structures is also typical of unconformities, intrusive contacts, and, on a small scale, cross-bedding.
Discontinuity of structures is characteristic of faults, but it is a proof of faulting only if other possible interpretations have been eliminated.

4. Criteria for Faulting
1. Discontinuity of structures

5. Criteria for Faulting 2. Repetition or omission of strata
Repetition or omission of strata may indicate faulting. The following figure represents a geological map of a region of folded and faulted sedimentary rocks. A syncline lies near the centre of the map, as is shown by the dips, and the formations are progressively younger from a to e. In certain places, however, one or more formations are missing, as, for example, along the line FF, where formation b is absent, and along the line F’F’, where c and d are missing. The lines FF and F’F’ must be the traces of faults.
The omission of strata, however, may be due to an unconformity.

6. Criteria for Faulting 3. Features characteristics of fault planes
Following are the features developed during faulting:-
a. Slikensides
b. Grooves or furrows
c. Drag along fault
d. Gouge
e. Breccia
f. Mylonite
These features are conclusive proof of faulting, but some of them may be confused with phenomena of a different origin.

7. Criteria for Faulting 3. Features characteristics of fault planes
a. Slikensides: Slickensides are polished and striated surfaces that result from friction along the fault plane. The scratches or striations are parallel to the direction of movement, but caution is necessary in employing such information because some faults show many slickensided layers, in each of which the striations have different trends. Moreover, a slickenside layer may record only the last movements along the fault, and earlier displacements may have been in some other direction.
Many slikensided surfaces are accompanied by sharp, low steps that trend at right angles to the striations. These steps and striated marks can be used to determine the relative movement along the fault plane.
A person with sensitive fingers may be able to tell the direction of movement. The surface feels smooth if the fingers slide in the direction that the missing block was displaced, whereas in reverse direction the fault feels rough.

8. Criteria for Faulting 3. Features characteristics of fault planes
a. Slikensides:

9. Criteria for Faulting 3. Features characteristics of fault planes
b. Grooves or furrows : Some faults show large grooves or furrows several feet from crest to crest and several inches deep; they are parallel to the direction of displacement.

10. Criteria for Faulting 3. Features characteristics of fault planes
c. Drag : Drag is in some cases an aid in divulging the relative motion along the fault. Because of friction the beds in the up throw block are dragged up, whereas the beds in the down throw are drag down.

11. Criteria for Faulting 3. Features characteristics of fault planes
d. Gouge : Some of the rock along a fault may be pulverized to a fine-grained gouge, which looks and feels like clay.

12. Criteria for Faulting 3. Features characteristics of fault planes
d. Gouge :

13. Criteria for Faulting 3. Features characteristics of fault planes
e. Breccia : these consists of angular to subangular fragments of various sizes, characteristically associated with a more finely crushed matrix. The fragments typically range from an inch to several feet in diameter, but much larger blocks may occur. Fault breccias may be many tens of feet thick.

14. Criteria for Faulting 3. Features characteristics of fault planes
e. Breccia :

15. Criteria for Faulting 3. Features characteristics of fault planes
f. Mylonite : A mylonite is a microbreccia that maintained its coherence during the deformation. It is characteristically dark and fine-grained and may be difficult to distinguish from sedimentary or volcanic rocks.

16. Criteria for Faulting 3. Features characteristics of fault planes
f. Mylonite : A mylonite is a microbreccia that maintained its coherence during the deformation. It is characteristically dark and fine-grained and may be difficult to distinguish from sedimentary or volcanic rocks.
In general, gouge and breccia form near the surface of the earth, where the confining pressures are comparatively small, and mylonite forms at greater depth, where the confining pressure forces the rocks to retain their coherence.