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- Words: 1,843
- Pages: 34
15-03-2019
CEL213 - Engineering Geology
Dr. Srinivasan V Dept. Of Civil Engg.,
1
15-03-2019
Structural geology is a branch of geology where geological structures in rocks and sediments are studied to understand their Origin, Occurrence, time frame and Causes
ATTITUDES IN THE ROCKS
2
15-03-2019
ATTITUDES IN THE ROCKS • Rock masses (layered or stratified) when subjected to tectonic forces – they will either get tilted or folded • This tilting or inclination results into two important structural elements which defines the attitude of beds or strata w.r.t horizontality i.e. Strike and Dip
OUTCROPS Any geological formations exposed on the surface is called outcrop
ATTITUDE I. Strike II. Dip
3
15-03-2019
STRIKE • Is an imaginary line on the surface that marks the direction of intersection of the bedding plane with an horizontal plane – the compass direction is usually expressed as a bearing e.g. N300E; N300W or N1200
DIP •Is an imaginary line constructed down slope on a bedding plane that marks the direction of inclination. •Dip direction is always taken perpendicular to the strike direction - True Dip. And is usually expressed in bearing and an angle of tilt (dip) •Dip angle is the angle between the inclined bed and the horizontal plane. It is expressed as 250SE or 250SW etc.
4
15-03-2019
True Dip and Apparent Dip
• True dip is the dip angle of a bed which is measured right angle to the strike of that bed • The dip angle measured in any other direction w.r.t the strike (other than the true dip direction) is Apparent dip. • Apparent dip is always less than True dip
aa’-strike direction bb’- true dip direction cc’; dd’- apparent dip direction
b
c’ Clinometer a’ Is the dip angle
d’ a
d c
b’
• True dip(A) • Apparent dip (B), which is less than the true dip.
5
15-03-2019
6
15-03-2019
7
15-03-2019
Different types of structures
1. FOLDS 2. FAULTS 3. JOINTS
FOLDS
A buckling up in the rock strata of the Earth’s crust
• Folds occur where the strata are crumpled or squeezed under compressional tectonic environment, perhaps due to plate movements i.e., collision; subduction etc. • Folds may occur in various dimensions e.g. few centimeter long or runs for hundreds of km. • Bend of the fold may be gentle or steep, this depends upon – – – –
the strength of the forces involved the ability of the rock to resist being deformed the arrangement of the rock layers and also the nature of movement that cause the folding
8
15-03-2019
Parts of Folds Limbs: The sides of the fold are called the flanks or limbs
Hinge line
Hinge line: Is a line of the maximum curvature in the folded bed. Fold Axis: Is a line parallel to the hinges. Axial Plane: Is the surface connecting all the hinges. It may be straight or curved; vertical or inclined. Attitudes of AP is defined by its strike and dip.
anticline
syncline
9
15-03-2019
NOMENCLATURE OF FOLDS • Folds are classified on the basis of
• the inclination of the Axial Plane • Orientation of limbs • Stratigraphic position of the strata
10
15-03-2019
Anticline
• Is a fold which is convex upward • Axial plane is vertical • Two limbs dips away from each other at same angle or in same direction at different angle w.r.t. axial plane • Older rocks are in the core (center) of the fold
Syncline
• Convex downward
• Younger rocks are in the core (center) of the fold. If the both the limb angles are similar with respect to the vertical AP then they are known as Symmetrical anticline or Symmetrical syncline
11
15-03-2019
• If the axial plane is inclined and the limbs dip in the same direction at different angle– Overturned fold
12
15-03-2019
FOLD ANGLE
•
• • • •
BASED ON INTERLIMB ANGLE THE FOLDS ARE CLASSIFIED AS:
1800 900-1700 100-900 00-100
: : : :
Homocline or Flat lying fold Open fold Tight fold or Close fold Isoclinal fold
13
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14
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Isoclinal folds: Both the limbs dips at the same angle and in same direction AP is inclined: Inclined Isoclinal fold AP is horizontal: Recumbent Isoclinal fold
15
15-03-2019
Monoclinal fold
• One limb may dip up to 900 where as the other will be nearly horizontal. Normally seen in Plateau regions. • (Plateau: An elevated flat land which drops abruptly on one or more sides)
Plunging fold
• The attitude of the Fold Axis or the Hinge Line is defined by two measurements: the bearing (strike) and its projection (inclined or horizontal) • So if the Fold Axis is inclined– Plunging Fold
16
15-03-2019
Drag folding competent incompetent competent
• This type of folds are formed when a competent bed slides over and incompetent bed. • They are minor folds, confined to the limbs of the major fold
17
15-03-2019
Chevron folding • Hinges are sharp and angular in shape
Chevron folding
18
15-03-2019
19
15-03-2019
Part II
20
15-03-2019
Is this our fault???
FAULTS • A fracture in a rock along which blocks of rock slip past each other is know as fault • Faults mainly occur as a zone called Fault Zone • Surface along which the block of rock slip is called Fault Plane • Surface expression in form of elevated cliff exposed at the surface due to faulting is known as Fault Scarp
21
15-03-2019
Parts of Faults (Dip)
H= Heave: is the horizontal component of the dip separation. It gives the amount of land shifted sideways
Hade
T= Throw: is the vertical component of the dip separation. It gives the amount of vertical displacement.
Fault Plane
Hade: is the angle of a fault plane w.r.t. vertical Dip: is the angle of a fault plane w.r.t. horizontal
Fault A fracture (crack) in the earth, where the two sides move past each other and the relative motion is parallel to the fracture.
Source: wikipedia
90˚ dip = vertical fault plane 0˚ strike = north parallel fault plane
44
22
15-03-2019
Surface Trace of a fault
Source: USGS public domain
45
Naming faults • Faults that we see in the field or on surface are named and classified on the basis of
• The dip of the fault plane • The direction • The sense of offset or movement • We determine these components to establish
• The direction of displacement • Sense of displacement • The amount or magnitude of displacement
23
15-03-2019
• These all parameters are related with the Slip : Is the actual relative displacement • On the basis of sense of slip the faults are categorized as • Strike-slip faults • Dip slip faults
Faults slip in response to stress - extension, compression, lateral shear
Geological structures are formed in response to forces acting on rocks and these give rise to stresses. In almost all geological situations stresses are always compressive but vary in different directions. Stresses can be evaluated as three principal stresses – where they are mutually perpendicular to each other
24
15-03-2019
Style of Deformation under Different Stress Fields
from Christopherson, 2001
Strike-slip faults • That accommodates horizontal slip between the adjacent blocks along the strike of the fault line
25
15-03-2019
Strike-slip faults
•That accommodates horizontal slip between the adjacent blocks
Strike-slip faults • Right lateral strike-slip fault- Dextral movement • Left lateral strike-slip fault- Sinistral movement
26
15-03-2019
Dip slip faults
DSF are the faults that accommodates translation or slip, upward or downward along the dip of the fault plane
Normal slip faults (moderate to steep angle 400 and 700), those dip < 450 are referred as low angle
normal faults
Thrust slip faults (dip < 450, usually at around 300)
Reverse slip faults (dip > 450)
Normal Faults
•
Normal faults may dip at a variety of angles, but they most typically dip between about 400 and 700.
27
15-03-2019
Normal Faults
Footwall Hanging wall
Footwall: The wall or the block of rock mass which does not move during the movement along the fault or it is a stationary block Hangingwall: The wall or the block of rock mass which moves during the movement along the fault w.r.t the footwall. The HW is always below the exposed fault plane.
Reverse and Thrust faults
28
15-03-2019
Reverse faults
Reverse Faults Hanging wall Footwall
•
Reverse faults (dip > 450) and Thrust Faults (dip < 450) are the result of Horizontal compressive stresses
•
Hanging wall moves up relative to the footwall
•
Cause shortening of the crust
•
Most of these faults place older rocks over younger rocks.
29
15-03-2019
STRUCTURES RELATED TO NORMAL FAULTING (EXTENSIONAL ENVIRONMENT)
• Horst: •
A block of rock thrown up between two normal faults
• Graben: •
A block which is thrown down between two normal faults
•
Most of the geologists believe that the horst structures are formed by compression, as two reverse faults are thrusted up, where as some believe that Horst and graben pairs can occur in areas of rifting, where the crust of the Earth is being pulled apart and normal faults are formed.
30
15-03-2019
Blind Thrust
31
15-03-2019
Oblique Slip
• ACTIVE FAULTS: Are faults which are likely to have another earthquake in the future. The Active Faults are commonly considered to be active if they have moved one or more times in the last 10,000 years. • ASEISMIC: Is the term used to describes a fault on which no earthquakes have been observed. • COSEISMIC: Is the term use to describe the instantaneous deformation during earthquake
32
15-03-2019
•
BLIND FAULTS. A terminology used to describe the rupture along the (thrust or reverse faults) that does not rupture up to the surface. It is "buried" under the uppermost layers of rock in the crust or no surface evidence.
•
FAULT CREEP: Slow and more or less continuous movement on faults due to ongoing tectonic deformation. Such creeping movement does lead to large magnitude earthquakes.
•
PRECAUTIONS TO IMPROVE THE FAULT SITE
•
First it is always desirable to avoid the site where faulting has occurred, but it is always not possible to fight with nature
•
1. Tectonic history of the area should be studied carefully
•
2. Depending upon the magnitude of the past earthquakes the structure should be made or planned in such a way that it can withstand the shaking.
•
If the faulting has not recurred in the recent past, then the damage caused to the region should be carefully assessed and proper treatment be given
•
1. Sealing the faults: Grouting is done by injecting a thin slurry of cement under pressure. This means that the strength of the faulted ground has been improved considerably and site is made suitable for construction.
•
2. If there is lot of shattering in the area due to past faulting, the whole loose mass is removed and refilled with concrete.
33
15-03-2019
Courtesy Prof. Javed Malik – IIT Kanpur
34
CEL213 - Engineering Geology
Dr. Srinivasan V Dept. Of Civil Engg.,
1
15-03-2019
Structural geology is a branch of geology where geological structures in rocks and sediments are studied to understand their Origin, Occurrence, time frame and Causes
ATTITUDES IN THE ROCKS
2
15-03-2019
ATTITUDES IN THE ROCKS • Rock masses (layered or stratified) when subjected to tectonic forces – they will either get tilted or folded • This tilting or inclination results into two important structural elements which defines the attitude of beds or strata w.r.t horizontality i.e. Strike and Dip
OUTCROPS Any geological formations exposed on the surface is called outcrop
ATTITUDE I. Strike II. Dip
3
15-03-2019
STRIKE • Is an imaginary line on the surface that marks the direction of intersection of the bedding plane with an horizontal plane – the compass direction is usually expressed as a bearing e.g. N300E; N300W or N1200
DIP •Is an imaginary line constructed down slope on a bedding plane that marks the direction of inclination. •Dip direction is always taken perpendicular to the strike direction - True Dip. And is usually expressed in bearing and an angle of tilt (dip) •Dip angle is the angle between the inclined bed and the horizontal plane. It is expressed as 250SE or 250SW etc.
4
15-03-2019
True Dip and Apparent Dip
• True dip is the dip angle of a bed which is measured right angle to the strike of that bed • The dip angle measured in any other direction w.r.t the strike (other than the true dip direction) is Apparent dip. • Apparent dip is always less than True dip
aa’-strike direction bb’- true dip direction cc’; dd’- apparent dip direction
b
c’ Clinometer a’ Is the dip angle
d’ a
d c
b’
• True dip(A) • Apparent dip (B), which is less than the true dip.
5
15-03-2019
6
15-03-2019
7
15-03-2019
Different types of structures
1. FOLDS 2. FAULTS 3. JOINTS
FOLDS
A buckling up in the rock strata of the Earth’s crust
• Folds occur where the strata are crumpled or squeezed under compressional tectonic environment, perhaps due to plate movements i.e., collision; subduction etc. • Folds may occur in various dimensions e.g. few centimeter long or runs for hundreds of km. • Bend of the fold may be gentle or steep, this depends upon – – – –
the strength of the forces involved the ability of the rock to resist being deformed the arrangement of the rock layers and also the nature of movement that cause the folding
8
15-03-2019
Parts of Folds Limbs: The sides of the fold are called the flanks or limbs
Hinge line
Hinge line: Is a line of the maximum curvature in the folded bed. Fold Axis: Is a line parallel to the hinges. Axial Plane: Is the surface connecting all the hinges. It may be straight or curved; vertical or inclined. Attitudes of AP is defined by its strike and dip.
anticline
syncline
9
15-03-2019
NOMENCLATURE OF FOLDS • Folds are classified on the basis of
• the inclination of the Axial Plane • Orientation of limbs • Stratigraphic position of the strata
10
15-03-2019
Anticline
• Is a fold which is convex upward • Axial plane is vertical • Two limbs dips away from each other at same angle or in same direction at different angle w.r.t. axial plane • Older rocks are in the core (center) of the fold
Syncline
• Convex downward
• Younger rocks are in the core (center) of the fold. If the both the limb angles are similar with respect to the vertical AP then they are known as Symmetrical anticline or Symmetrical syncline
11
15-03-2019
• If the axial plane is inclined and the limbs dip in the same direction at different angle– Overturned fold
12
15-03-2019
FOLD ANGLE
•
• • • •
BASED ON INTERLIMB ANGLE THE FOLDS ARE CLASSIFIED AS:
1800 900-1700 100-900 00-100
: : : :
Homocline or Flat lying fold Open fold Tight fold or Close fold Isoclinal fold
13
15-03-2019
14
15-03-2019
Isoclinal folds: Both the limbs dips at the same angle and in same direction AP is inclined: Inclined Isoclinal fold AP is horizontal: Recumbent Isoclinal fold
15
15-03-2019
Monoclinal fold
• One limb may dip up to 900 where as the other will be nearly horizontal. Normally seen in Plateau regions. • (Plateau: An elevated flat land which drops abruptly on one or more sides)
Plunging fold
• The attitude of the Fold Axis or the Hinge Line is defined by two measurements: the bearing (strike) and its projection (inclined or horizontal) • So if the Fold Axis is inclined– Plunging Fold
16
15-03-2019
Drag folding competent incompetent competent
• This type of folds are formed when a competent bed slides over and incompetent bed. • They are minor folds, confined to the limbs of the major fold
17
15-03-2019
Chevron folding • Hinges are sharp and angular in shape
Chevron folding
18
15-03-2019
19
15-03-2019
Part II
20
15-03-2019
Is this our fault???
FAULTS • A fracture in a rock along which blocks of rock slip past each other is know as fault • Faults mainly occur as a zone called Fault Zone • Surface along which the block of rock slip is called Fault Plane • Surface expression in form of elevated cliff exposed at the surface due to faulting is known as Fault Scarp
21
15-03-2019
Parts of Faults (Dip)
H= Heave: is the horizontal component of the dip separation. It gives the amount of land shifted sideways
Hade
T= Throw: is the vertical component of the dip separation. It gives the amount of vertical displacement.
Fault Plane
Hade: is the angle of a fault plane w.r.t. vertical Dip: is the angle of a fault plane w.r.t. horizontal
Fault A fracture (crack) in the earth, where the two sides move past each other and the relative motion is parallel to the fracture.
Source: wikipedia
90˚ dip = vertical fault plane 0˚ strike = north parallel fault plane
44
22
15-03-2019
Surface Trace of a fault
Source: USGS public domain
45
Naming faults • Faults that we see in the field or on surface are named and classified on the basis of
• The dip of the fault plane • The direction • The sense of offset or movement • We determine these components to establish
• The direction of displacement • Sense of displacement • The amount or magnitude of displacement
23
15-03-2019
• These all parameters are related with the Slip : Is the actual relative displacement • On the basis of sense of slip the faults are categorized as • Strike-slip faults • Dip slip faults
Faults slip in response to stress - extension, compression, lateral shear
Geological structures are formed in response to forces acting on rocks and these give rise to stresses. In almost all geological situations stresses are always compressive but vary in different directions. Stresses can be evaluated as three principal stresses – where they are mutually perpendicular to each other
24
15-03-2019
Style of Deformation under Different Stress Fields
from Christopherson, 2001
Strike-slip faults • That accommodates horizontal slip between the adjacent blocks along the strike of the fault line
25
15-03-2019
Strike-slip faults
•That accommodates horizontal slip between the adjacent blocks
Strike-slip faults • Right lateral strike-slip fault- Dextral movement • Left lateral strike-slip fault- Sinistral movement
26
15-03-2019
Dip slip faults
DSF are the faults that accommodates translation or slip, upward or downward along the dip of the fault plane
Normal slip faults (moderate to steep angle 400 and 700), those dip < 450 are referred as low angle
normal faults
Thrust slip faults (dip < 450, usually at around 300)
Reverse slip faults (dip > 450)
Normal Faults
•
Normal faults may dip at a variety of angles, but they most typically dip between about 400 and 700.
27
15-03-2019
Normal Faults
Footwall Hanging wall
Footwall: The wall or the block of rock mass which does not move during the movement along the fault or it is a stationary block Hangingwall: The wall or the block of rock mass which moves during the movement along the fault w.r.t the footwall. The HW is always below the exposed fault plane.
Reverse and Thrust faults
28
15-03-2019
Reverse faults
Reverse Faults Hanging wall Footwall
•
Reverse faults (dip > 450) and Thrust Faults (dip < 450) are the result of Horizontal compressive stresses
•
Hanging wall moves up relative to the footwall
•
Cause shortening of the crust
•
Most of these faults place older rocks over younger rocks.
29
15-03-2019
STRUCTURES RELATED TO NORMAL FAULTING (EXTENSIONAL ENVIRONMENT)
• Horst: •
A block of rock thrown up between two normal faults
• Graben: •
A block which is thrown down between two normal faults
•
Most of the geologists believe that the horst structures are formed by compression, as two reverse faults are thrusted up, where as some believe that Horst and graben pairs can occur in areas of rifting, where the crust of the Earth is being pulled apart and normal faults are formed.
30
15-03-2019
Blind Thrust
31
15-03-2019
Oblique Slip
• ACTIVE FAULTS: Are faults which are likely to have another earthquake in the future. The Active Faults are commonly considered to be active if they have moved one or more times in the last 10,000 years. • ASEISMIC: Is the term used to describes a fault on which no earthquakes have been observed. • COSEISMIC: Is the term use to describe the instantaneous deformation during earthquake
32
15-03-2019
•
BLIND FAULTS. A terminology used to describe the rupture along the (thrust or reverse faults) that does not rupture up to the surface. It is "buried" under the uppermost layers of rock in the crust or no surface evidence.
•
FAULT CREEP: Slow and more or less continuous movement on faults due to ongoing tectonic deformation. Such creeping movement does lead to large magnitude earthquakes.
•
PRECAUTIONS TO IMPROVE THE FAULT SITE
•
First it is always desirable to avoid the site where faulting has occurred, but it is always not possible to fight with nature
•
1. Tectonic history of the area should be studied carefully
•
2. Depending upon the magnitude of the past earthquakes the structure should be made or planned in such a way that it can withstand the shaking.
•
If the faulting has not recurred in the recent past, then the damage caused to the region should be carefully assessed and proper treatment be given
•
1. Sealing the faults: Grouting is done by injecting a thin slurry of cement under pressure. This means that the strength of the faulted ground has been improved considerably and site is made suitable for construction.
•
2. If there is lot of shattering in the area due to past faulting, the whole loose mass is removed and refilled with concrete.
33
15-03-2019
Courtesy Prof. Javed Malik – IIT Kanpur
34
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