FAULTS. Mukul Dey M.Sc Tech in Applied Geology, 2023-26 IIT (ISM) DHANBAD, JHARKHAND. 

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Mukul Dey   M.Sc Tech in Applied Geology, 2023-26   IIT (ISM) DHANBAD,  JHARKHAND

Fault:. A fault is a fracture discontinuity along which the rocks on either side have moved past each other.. 

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A fault is a fracture discontinuity along which the rocks on either side have moved past each other.

The trace of the fault on the Earth's surface

If a fault is not vertical, the side above the fault plane is called the hanging wall, and the side below the fault plane is called the foot wall.

Net slip:. The relative displacement of two adjoining points on either side of the plane is known as net slip. The components of the net slip along the strike and the dip of the fault plane are known the strike-slip and dip-slip components, respectively. The dip slip component may in turn be described as sum of a vertical component and a harrizontal component, which are sometimes called the throw and the heave, respectively. Hade is the angle between the fault plane and a vertical plane that strikes parallel to the fault. Hade= 90° - angle of dip. 

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The relative displacement of two adjoining points on either side of the plane is known as net slip. The components of the net slip along the strike and the dip of the fault plane are known the strike-slip and dip-slip components, respectively. The dip slip component may in turn be described as sum of a vertical component and a harrizontal component, which are sometimes called the throw and the heave, respectively. Hade is the angle between the fault plane and a vertical plane that strikes parallel to the fault.  Hade= 90° - angle of dip

Classification of faults based on relative movement between walls:. 

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Classification of faults based on relative movement between walls:

(1) Dip-slip faults:  are those which have a dominant dip-slip component or when the net slip is parallel to the true dip line. Depending upon the sense of movement, two types of dip-slip faults are recognized —— Normal faults- In normal faults the hanging wall moves down relative to the foot wall. eg- horst and graben etc. Reverse faults- In reverse faults the hanging wall moves up relative to the foot wall. A reverse faults in which dip is less than 45° is called a thrust faults. eg- imbricate fan, duplex etc.

(2) Strike-slip faults: are those which have a dominant strike-slip component or when the net slip is parallel to the strike line. eg- transform faults, transcureent faults, flower structures, pull apart basins etc. Depending upon the sense of movement, two types of strike-slip faults are recognized —— If looking across the fault plane the opposite wall appears to have moved towards the right, the strike-slip fault is called right lateral or right handed or dextral. If looking across the fault plane the opposite wall appears to have moved towards the left, the strike-slip fault is called left lateral or left handed or sinistral.. 

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(2) Strike-slip faults:  are those which have a dominant strike-slip component or when the net slip is parallel to the strike line. eg- transform faults, transcureent faults, flower structures, pull apart basins etc. Depending upon the sense of movement, two types of strike-slip faults are recognized —— If looking across the fault plane the opposite wall appears to have moved towards the right, the strike-slip fault is called right lateral or right handed or dextral. If looking across the fault plane the opposite wall appears to have moved towards the left, the strike-slip fault is called left lateral or left handed or sinistral.

(3) Oblique-slip faults: when the net slip is neither parallel to the strike nor parallel to the true dip line. Depending upon whether the opposite wall moves towards right or left and whether the hanging wall moves relatively up or down, oblique-slip faults can be further subdivided into four types —— a. Right normal, b. Right reverse, c. Left normal, and d. Left reverse. 

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(3) Oblique-slip faults:  when the net slip is neither parallel to the strike nor parallel to the true dip line. Depending upon whether the opposite wall moves towards right or left and whether the hanging wall moves relatively up or down, oblique-slip faults can be further subdivided into four types —— a. Right normal, b. Right reverse, c. Left normal, and d. Left reverse

Classification of faults based on fault pattern:. 

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Classification of faults based on fault pattern:

(1) Parallel faults:  A series of faults running parallel to each other.

(2) Step faults: A series of faults, but successive foults parallel are downthrown more and more.

(2) En-echelon faults: Parallel but overlapping faults. .

(4) Radial faults: A system of faults that radiate out from a point. 

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(4) Radial faults:  A system of faults that radiate out from a point

(5) Peripheral faults: Circular faults or arcuate faults that bound a cincular area an part of a cincular area. .

(5) Horst and Graben: Two separate normal faults dipping toward each other. create a downthrown block known as graben. A one sided graben is called as half graben. Normal faults dipping away from each other create a up thrown block known as horst. The largest faults in a faulted area, called masterfaults, are associated with minor faults that may be anthithetic or synthetic. An anthithetic fault dips toward the master fault. while an synthetic fault dips in the same direction as the master fault.

Classification of faults based on attitude of the fault surface:. 

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Classification of faults based on attitude of the fault surface:

High angle fault: A high angle fault is one that dips at angle greater than 45°. Low angle fault: A low angle fault is one that dips at angle smaller than 45.

Listric faults: A fault surface may be curved on planar. Curved faults are known as Listric faults. A listric faults may be translateral.

Translational faults: parallel lines on either side remain parallel after faulting.

Fig: Tranverse and longitudinal faults. Rotational faults: Parallel lines on either side are not remain parallel after faulting and have variable netslip in different part. This fault some time described as hinge fault or scissor fault or pivot fault.. 

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Rotational faults: Parallel lines on either side are not remain parallel after faulting and have variable netslip in different part. This fault some time described as hinge fault or scissor fault or pivot fault.

Transverse fault: A transverse fault is one striking the strike of the regional structure. perpendicularly an diagonally to the strike of the regional structure. Longitudinal fault: A longitudinal fault is one striking parallel to the strike of the regional structure.

Transform fault: Transform fault is form in two different plate, where each of them moving away from spreading centre of divergent plate boundaries. — Associated with plate boundaries. — Slip and separation are in opposite direction. — Amount of offset remain constant. — Usually on the ocean floor but few on land. — Fault is at high angles to the extension. — The strain which with they are associated is not plane strain.. 

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Transform fault: Transform fault is form in two different plate, where each of them moving away from spreading centre of divergent plate boundaries. — Associated with plate boundaries. — Slip and separation are in opposite direction. — Amount of offset remain constant. — Usually on the ocean floor but few on land. — Fault is at high angles to the extension. — The strain which with they are associated is not plane strain.

Transcurrent fault: When net slip is parallel to strike means two plate slide passed each other. — Associated within plate boundaries. — Slip and separation are in same direction. — Amount of offset increases with time. — Usually on land. — Fault is parallel to the extension. — Can be plane strain.

Anderson's theory of faulting:. Anderson made the assumption that, since there is no shear stress at Earth's surface (shear stress cannot occur in fluids), one of the principal stresses has to be vertical, implying that the other two ane horizontal. Depending upon which of the three principal stresses is vertical one, Anderson derfine three fault regime —. 

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Anderson made the assumption that, since there is no shear stress at Earth's surface (shear stress cannot occur in fluids), one of the principal stresses has to be vertical, implying that the other two ane horizontal. Depending upon which of the three principal stresses is vertical one, Anderson derfine three fault regime —

Anderson classification is strictly valid for in coaxial deformation regimes. Furthermore, the deforming rock must be isotropic.

Mechanism of faulting:. Because the rocks are hard and have friction, they can't slide or run past each other. Stress builds up in rocks, and when it hits a level that is higher than the strain threshold, the stored potential energy is released. This energy is then focused into a plane called a fault, which allows relative movement to happen. Depending on the rock's rheology, strain can build up over time or happen all at once. The lower ductile crust and mantle build up deformation slowly through shearing, while the brittle upper crust responds by cracking, which is an instantaneous release of stress that causes movement along the fault. Instantaneous strain release releases energy, which can cause earthquakes, which are common near transform borders. Faulting has a lot to do with fluids. They act as a lubricant in the fault zone because of their buoyancy, which lowers the shear stress needed for the fault to slip.. 

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Because the rocks are hard and have friction, they can't slide or run past each other. Stress builds up in rocks, and when it hits a level that is higher than the strain threshold, the stored potential energy is released. This energy is then focused into a plane called a fault, which allows relative movement to happen. Depending on the rock's rheology, strain can build up over time or happen all at once. The lower ductile crust and mantle build up deformation slowly through shearing, while the brittle upper crust responds by cracking, which is an instantaneous release of stress that causes movement along the fault. Instantaneous strain release releases energy, which can cause earthquakes, which are common near transform borders. Faulting has a lot to do with fluids. They act as a lubricant in the fault zone because of their buoyancy, which lowers the shear stress needed for the fault to slip.

The presence of a fault may be indicated when there are abrupt terminations of strata against distinct beds. Silicification and mineralization can occur through the utilization of fault lines, broad fractures, or branching major fractures, which serve as conduits for the movement of solution. The proposed approach has the potential to substitute the existing country rock with fine-grained quartz, resulting in a process known as "silicification."This phenomena, while not inherently indicative of faulting, can be extremely suggestive in certain geographical areas.The occurrence of "mineralization" along faults is a common characteristic observed in numerous mining districts.

A sudden alteration in sedimentary facies can occur when distinct sedimentary facies of rocks that are of the same age are juxtaposed due to significant horizontal movement. 4. Features on the fault surface(plane): In a fault surface many features can be seen, that are — The grooves, which are oriented parallel to the direction of movement. The proliferation of fibrous minerals in a direction parallel to the movement. Slickensides refer to fault surfaces that have undergone polishing. Slickenlines refer to slender, intricate, and shallow grooves that typically exhibit a linear pattern. These features are found directly on the fault surface and serve as indicators of the direction of fault movement. Chatter marks refer to the presence of small, uneven step-like features that are commonly aligned perpendicular to striations. Small steps: Additionally, it can be utilized to ascertain the direction of motion of the opposing wall. 5. Displacement of recognizable marker such as fossil, colour, composition etc. 6. Physiographic criteria: Many topographic features indicate the presence of fault. Theyare — Fault scarps, also known as cliffs, are geological features characterized by steep surface slopes that can arise as a result of faulting, which involves the displacement of blocks. Triangular facets are formed along some scarps that are connected with gravity faulting. The truncation of buildings, beds, or rock units.. 

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A sudden alteration in sedimentary facies can occur when distinct sedimentary facies of rocks that are of the same age are juxtaposed due to significant horizontal movement. 4. Features on the fault surface(plane): In a fault surface many features can be seen, that are — The grooves, which are oriented parallel to the direction of movement. The proliferation of fibrous minerals in a direction parallel to the movement. Slickensides refer to fault surfaces that have undergone polishing. Slickenlines refer to slender, intricate, and shallow grooves that typically exhibit a linear pattern. These features are found directly on the fault surface and serve as indicators of the direction of fault movement. Chatter marks refer to the presence of small, uneven step-like features that are commonly aligned perpendicular to striations. Small steps: Additionally, it can be utilized to ascertain the direction of motion of the opposing wall. 5. Displacement of recognizable marker such as fossil, colour, composition etc. 6. Physiographic criteria: Many topographic features indicate the presence of fault. Theyare — Fault scarps, also known as cliffs, are geological features characterized by steep surface slopes that can arise as a result of faulting, which involves the displacement of blocks. Triangular facets are formed along some scarps that are connected with gravity faulting. The truncation of buildings, beds, or rock units.

The drainage pattern is altered due to topographic features, namely those influenced by fault lines and fault scarps. Springs located at the base of a mountain range can provide valuable indications of faulting, particularly when the water emanating from these springs is of elevated temperature. Scarplets, commonly referred to as "Piedmont scarps," are geological features that serve as indicators of ongoing faulting activity. This settlement is located in close proximity to the base of the mountain. Gourge is a type of fault rock that belongs to the breccia series. The fault rock typically observed along fault surfaces and within fault zones is characterized by its light color and clay composition. The grain size is smaller than 0.1mm. 7. Abrupt changes in vegetation pattern. 8. Inconsistent stratigraphic relationship.. 

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The drainage pattern is altered due to topographic features, namely those influenced by fault lines and fault scarps. Springs located at the base of a mountain range can provide valuable indications of faulting, particularly when the water emanating from these springs is of elevated temperature. Scarplets, commonly referred to as "Piedmont scarps," are geological features that serve as indicators of ongoing faulting activity. This settlement is located in close proximity to the base of the mountain. Gourge is a type of fault rock that belongs to the breccia series. The fault rock typically observed along fault surfaces and within fault zones is characterized by its light color and clay composition. The grain size is smaller than 0.1mm. 7. Abrupt changes in vegetation pattern. 8.  Inconsistent stratigraphic relationship.

1. Repetition of beds (Reverse Fault) 2. Omission of beds (Normal Faults) 3. Strike Separation 4. Thickness change across Fault trace

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Please read the following books for further knowledge:. 

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Please read the following books for further knowledge:

Structural Geology Fundamentals and Modern Developments By S.K. Ghosh Structural Geology By Haakon Fossen Structural Geology By Marland Pratt Billings Structural Geology By Robert J. Twiss and Eldridge M. Moores