3. Classification of faults and Field Recognition of faults

It includes classification and field recognition of faults

1. Fault Classification Bases of fault classification: Faults can be classified on the basis of their geometry and genesis. Geometric classification is easy to apply than genetic classification. A. Geometrical classification: B. Genetic classification: 1. the rake of the net slip. 2. the attitude of the fault relative to the attitude of the adjacent rocks. 3. the pattern of the faults. 4. the angle at which the fault dips. 5. the apparent movement on the fault. 1. classification based on relative movements. 2. classification based on absolute movements.

2. A. Geometrical classification: 1. the rake of the net slip. 2. the attitude of the fault relative to the attitude of the adjacent rocks. 3. the pattern of the faults. 4. the angle at which the fault dips. 5. the apparent movement on the fault.

3. A. Geometrical classification: 1. Classification based on the rake of the net slip. a. STRIKE SLIP FAULT: is one in which the net slip is parallel to the strike of the fault. b. DIP SLIP FAULT: is one in which the net slip is up or down the dip of the fault; that is the dip slip equals the net slip and there is no strike slip component. The rake of the net slip is therefore 90 degrees. c. DIAGONAL SLIP FAULT: is one in which the net slip is diagonally up or down the fault plane. There is both a strike slip and dip slip component, the rake of the net slip is greater than zero but less than 90 degrees.

4. A. Geometrical classification: 2. Classification based on the attitude of fault relative to the strike of the adjacent rocks. a. Strike Fault b. Bedding Fault c. Dip Fault d. Oblique / Diagonal Fault e. Longitudinal Fault f. Transverse Fault

5. A. Geometrical classification: 2. Classification based on the attitude of fault relative to the strike of the adjacent rocks. a. STRIKE FAULT: is one that strikes essentially parallel to the strike of the adjacent rocks. The strike of the adjacent rocks is ordinarily measured on the bedding, but if the bedding is absent, the strike may be measured on the schistosity of metamorphic rocks or on the flow structure of igneous rocks.

6. A. Geometrical classification: 2. Classification based on the attitude of fault relative to the strike of the adjacent rocks. b. BEDDING FAULT: is a variety of strike fault in which the fault plane is essentially parallel to the bedding .

7. A. Geometrical classification: 2. Classification based on the attitude of fault relative to the strike of the adjacent rocks. c. DIP FAULT: is one that strikes essentially parallel to the direction of dip of the adjacent beds; that is, its strike is perpendicular to the strike of the adjacent beds.

8. A. Geometrical classification: 2. Classification based on the attitude of fault relative to the strike of the adjacent rocks. d. OBLIQUE or DIAGONAL FAULT: is one that strikes obliquely or diagonally to the strike of the adjacent rocks.

9. A. Geometrical classification: 2. Classification based on the attitude of fault relative to the strike of the adjacent rocks. e. LONGITUDINAL FAULT: is one that strikes parallel to the strike of the regional structure. f. TRANSVERSE FAULT: is one that strikes perpendicularly or diagonally to the strike of the regional structure.

10. A. Geometrical classification: 3. Classification based on the pattern of the faults. a. Parallel faults b. En -echelon faults c. Peripheral faults d. Radial faults. A third geometrical classification is based on the pattern shown by the faults; ordinarily the classification is based on the pattern on a map, but it may be based on the pattern in a cross section. The attitude of the adjacent rocks is unimportant.

11. A. Geometrical classification: 3. Classification based on the pattern of the faults. a. Parallel faults b. En -echelon faults c. Peripheral faults d. Radial faults. A third geometrical classification is based on the pattern shown by the faults; ordinarily the classification is based on the pattern on a map, but it may be based on the pattern in a cross section. The attitude of the adjacent rocks is unimportant. a. Parallel faults: Parallel faults have essentially the same dip and strike; they thus belong to a set of parallel faults. If the strikes are the same but the dips differ the faults are assigned to two or more sets of parallel faults.

12. A. Geometrical classification: 3. Classification based on the pattern of the faults. a. Parallel faults b. En -echelon faults c. Peripheral faults d. Radial faults. A third geometrical classification is based on the pattern shown by the faults; ordinarily the classification is based on the pattern on a map, but it may be based on the pattern in a cross section. The attitude of the adjacent rocks is unimportant. b. En -echelon faults: are relatively short faults that overlap each other.

13. A. Geometrical classification: 3. Classification based on the pattern of the faults. a. Parallel faults b. En-echelon faults c. Peripheral faults d. Radial faults. A third geometrical classification is based on the pattern shown by the faults; ordinarily the classification is based on the pattern on a map, but it may be based on the pattern in a cross section. The attitude of the adjacent rocks is unimportant. c. Peripheral faults: are circular or arcuate faults that bound a circular area or part of a circular area.

14. A. Geometrical classification: 3. Classification based on the pattern of the faults. a. Parallel faults b. En-echelon faults c. Peripheral faults d. Radial faults. A third geometrical classification is based on the pattern shown by the faults; ordinarily the classification is based on the pattern on a map, but it may be based on the pattern in a cross section. The attitude of the adjacent rocks is unimportant. d. Radial faults: Radial faults belong to a system of faults that radiate out from a point.

15. A. Geometrical classification: 4. Classification based on value of dip of fault. a. High-angle faults b. Low-angle faults

16. A. Geometrical classification: 4. Classification based on value of dip of fault. a. High-angle faults b. Low-angle faults a. High-angle faults: are those that fault plane dip greater than 45 degrees. b. Low-angle faults: are those that fault plane dip less than 45 degrees.

17. A. Geometrical classification: 5. Classification based on apparent movement. a. Apparent normal fault b. Apparent thrust fault A fifth geometrical classification is based upon the apparent movement in vertical sections at right angles to the fault.

18. A. Geometrical classification: 5. Classification based on apparent movement. a. Apparent normal fault An apparent normal fault is one in which the hanging wall, in a vertical section at right angles to the strike of the fault, appears to have gone down relative to the footwall. A fifth geometrical classification is based upon the apparent movement in vertical sections at right angles to the fault.

19. A. Geometrical classification: 5. Classification based on apparent movement. An apparent thrust fault is one in which the hanging wall, in a vertical section at right angles to the strike of the fault, appears to have gone up relative to the footwall. b. Apparent thrust fault A fifth geometrical classification is based upon the apparent movement in vertical sections at right angles to the fault.

20. B. Genetic classification:

21. B. Genetic classification: The ideal and most satisfactory classification in natural sciences are those based on genesis. An ideal genetic classification of faults would be based primarily on the nature of forces involved. Such a classification should considered not only whether the forces were compressional, tensional, shearing or torsional; but it should consider also the direction in which those forces were acting. 1. Classification based on relative movements. 2. Classification based on absolute movements.

22. B. Genetic classification: 1. Classification based on relative movements. The most satisfactory genetic classification that can be established at present is based on the nature of relative movement along the fault. a. Thrust fault b. Normal fault c. Strike slip fault

23. B. Genetic classification: a. Thrust fault THRUST FAULT or thrust is a fault along which the hanging wall has moved up relative to the footwall. Three categories are usually recognized: A reverse fault is a thrust that dips more than 45 degrees , the term thrust is one that dips less than 45 degrees , but overthrust is used for a fault that dips less than about 10 degrees and has a large net slip. Overthrust has also been used with different connotation. Thrust faults indicate shortening of the rocks involved. Reverse fault Thrust fault Overthrust fault

24. B. Genetic classification: b. Normal fault NORMAL FAULT is a fault along which the hanging wall has moved relatively downward. It is also called gravity fault, but not generally followed by geologists. A detachment fault is a special category of low angle normal fault due to the downhill sliding of rocks from an uplifted region. Normal fault or Gravity fault Detachment fault

25. B. Genetic classification: c. Strike slip fault STRIKE SLIP FAULTS also called wrench faults, are those along which displacement has been essentially parallel to the strike of the fault --- that is, the dip slip component is small compared to the strike slip component. If an observer approaches a strike slip fault from one side, the opposite wall will have moved relatively either to the right or left. A right-slip fault is one in which the opposite wall moved relatively to the right. The terms dextral, right-handed and right-lateral have been used. A left-slip fault is one in which the opposite wall moved relatively to the left. Similarly, the terms sinistral, left-handed and left-lateral have been used.

26. B. Genetic classification: 2. Classification based on absolute movements. A more elaborate classification would be based on absolute movements relative to some datum plane, such as sea level. a. Normal faults b. Thrust faults c. Upthrust d. Underthrust d. Overthrust

27. B. Genetic classification: a. Normal faults Based on absolute movements five kinds of normal faults might be recognized: i ) those in which the foot wall did not move, but in which the hanging wall moved down; ii) those in which the foot wall moved up, while the hanging wall remained stationary; iii) those in which the hanging wall moved down and the footwall moved up; iv) those in which both blocks moved down, but in which the hanging wall moved a greater amount; and v) those in which both blocks moved up, but in which the hanging wall moved less than footwall.

28. B. Genetic classification: b. Reverse faults Similarly, five kinds of thrust / reverse faults might be established. These are illustrated in the following figures:

29. B. Genetic classification: c. Upthrust UPTHRUSTS are high-angle faults along which the relatively uplifted block has been the active element. If the hanging wall of a high-angle thrust fault has moved up while the footwall stayed in place, or the footwall of a high-angle normal fault has moved up while the hanging wall stayed in place, the fault is an upthrust. Stationary block active block Stationary block

30. B. Genetic classification: d. Underthrust UNDERTHRUST is used for those thrust faults in which the footwall has been the active element. Stationary Block Active Block

31. B. Genetic classification: e. Overthrust OVERTHRUST is used for those thrust faults in which the hanging wall has been the active element. Stationary Block Active Block

32. Field Recognition of Faults 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

33. 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.

34. 2. Repetition or omission of strata The omission of strata, however, may be due to an unconformity.

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

36. 3. Features characteristics of fault planes 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. 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, Slikensides

37. 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.

38. 3. Features characteristics of fault planes c. Drag : Drag is in some cases an aid in recognizing 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.

39. 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. e. Breccia : these consists of angular to subangular fragments of various sizes, characteristically associated with a more finely crushed matrix. 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.

40. These are identified on the basis of size and percentage of matrix

41. 5. Sudden changes in sedimentary facies Different sedimentary facies of rocks of the same age may be brought into juxtaposition by large horizontal displacements. 4. Silicification and mineralization Faults, because they are extensive fractures or branches of large fractures, are often the avenues for moving solutions. The solutions may replace the country rock with fine-grained quartz, causing silicification. Sandstone Shale Limestone A B C D E

42. 6. Physiographic criteria Physiographic features are the most effective criteria for recognizing faults. The following physiographic features can be used as evidence of faulting:- a. Offset ridge b. Fault scarp c. Offset stream a. Offset ridge: Resistant sedimentary formations are generally expressed topographically by ridges. A dip-slip fault or diagonal fault will displace the strata, and consequently, the ridge held up by some resistant bed will be discontinuous, and an offset ridge will result.

43. 6. Physiographic criteria b. Fault scarp: A scarp is a relatively steep, straight slope that may range in height from a few feet to thousands of feet. A scarp is not, of course, proof of the presence of fault, because scarps may originate in other ways, erosion by wave action.

44. 6. Physiographic criteria C. Offset stream: Offset streams are found along active strike-slip faults. If a easterly flowing stream is offset by displacement along a north-south trending steep strike-slip fault, along which the east wall moved relatively south, the stream on the west wall will be offset relatively toward the north.

45. Mechanical model of faulting/ Anderson theory of Faulting Tectonic Stress = Mainly due to movement of plate Can be based on stress ellipsoid σ 1> σ 2 > σ 3 axis σ 1 = Max Tensile Stress (Found in MOR) σ 3 = Max Compressive Stress (Convergent Plate margin ) This theory is based on certain assumptions: Since ground surface is a free surface, one of the principal stress axes will always be perpendicular to the ground surface. Fault plane or fracture will initiate parallel to σ 2 stress axis. Fault plane will make an angle of ~ 30 angle w.r.t. σ 3 stress axis

46. Case I σ1 is vertical Due to horizontal compression; Reverse fault hence they are common in region of orogenic belts (Fold-thrust belts). Dip is gentle. Likely to be shear fracture.

47. Case 2 σ 2 is vertical Fault plane is vertical and hence dextral strike slip fault is formed. Left and Right lateral depends on the position of σ 1 and σ 3

48. Case 3 σ 3 is vertical Prevalent in zone where we have horizontal extension. Characteristic of divergent plate margin. This will result in development of normal fault there will be change in topography. Dip is Steep.