Dr. V. R Ghodake, Sinhgad College of Engineering, Pune.

Mob- +919764484757, Email- vyanky.g@gmail.com

OutcropOutcrop•Any Geological formation

exposed on the surface is called an outcrop.

Deformation of RocksDeformation of Rocks

• “Structural Geology” is the study of the deformation of rocks and its effects.

Orientation of Deformed RocksOrientation of Deformed Rocks

We need some way to describe the geometry of geologic structures. So we use the terms strike and dip.

Strike: Orientation of beds with respect to horizon.

Dip: Maximum inclinatioin of beds with respect to horizon,

Dip is always perpendicular to strike.

Strike and Dip on a Rooftop

Strike and Dip in a Rock Structure

Displaying Strike and Dip Displaying Strike and Dip on a Mapon a Map

35O

N

S

Direction of strike

E

Direction of down dip

W

What determines if a rock What determines if a rock Folds (bends) Folds (bends)

or faults (breaks)?or faults (breaks)?

• Type of force applied

• Pressure

• Temperature

• Rock (mineral) composition

Depth at Which the Deformation Depth at Which the Deformation Occurs is a Direct FactorOccurs is a Direct Factor

• At shallow crust depths, rock has a greater probability of breaking

• At deeper crust depths, rock usually deforms

StrengthStrength• Ability of an object to resist

deformation• In lab, marble was tested

Fractured Deformed

StrainStrainAny change in original shape or size of an object in response to stress acting on the object

Three Major TypesThree Major Typesof Directed Stressof Directed Stress

• Compression• Extension• Shear

DefDef- Folds may be defined as a curved or zigzag structures shown by - Folds may be defined as a curved or zigzag structures shown by rock beds. In other words the wavy undulations in the rock beds are rock beds. In other words the wavy undulations in the rock beds are called Folds. called Folds.

These bends or folds may develops in any type of rock and These bends or folds may develops in any type of rock and may be of any shapes may be of any shapes

Folds shows arches and troughs in alternate manner they are best Folds shows arches and troughs in alternate manner they are best displayed by the sedimentary rocks. Its size varies from few displayed by the sedimentary rocks. Its size varies from few centimeters to in kmcentimeters to in km

Parts of Folds•Limbs: •Axial Planes•Axis of Fold•Hinge Line

Anticline and SynclineAnticline and Syncline

Anticline Anticline – is a up fold where the limb dip away from axis of fold, these – is a up fold where the limb dip away from axis of fold, these shows convex upwards shows convex upwards

SynclineSyncline – is a down fold where the limb dip towards the axis of fold, these – is a down fold where the limb dip towards the axis of fold, these shows concave, or reversed anticline shape. shows concave, or reversed anticline shape.

Asymmetrical folds

Overturned folds

Plunging Folds: Plunging Folds: Folds having inclined axis are called Plunging folds.Folds having inclined axis are called Plunging folds.The angle of inclination of a fold axis with horizontal is called the angle of The angle of inclination of a fold axis with horizontal is called the angle of plunge. plunge.

CompressionCompressionAction of oppositely directed forces acting towards each other at the same time

TensionTensionAction of coinciding and oppositely directed forces acting away from each other

ShearShearAction of coinciding and oppositely directed forces acting parallel to each other across a surface

Types of DeformationTypes of Deformation

• ElasticElastic• Ductile (plastic)Ductile (plastic)• Brittle (rupture)Brittle (rupture)

Elastic DeformationElastic Deformation

Temporary change in shape or size that is recovered when the deforming force is removed

(Like squeezing a piece of rubber)

Ductile (Plastic) DeformationDuctile (Plastic) Deformation

• Permanent change in shape or size

• Under goes smooth and continuous plastic deformation under stress

• Does NOT recover original shape

Recumbent Fold – In recumbent folds thefolding is so intense that both the limbsBecomes almost horizontal in this case The axial plane also becomes nearly horizontal and lower limb gets overturned.

Types of foldsTypes of folds

Overturned foldOverturned fold

It is an asymmetrical It is an asymmetrical

fold whose one limb is fold whose one limb is

turned past the vertical. turned past the vertical.

In this case theIn this case the

axial plane is inclined axial plane is inclined

and both the limb dips in and both the limb dips in

the same directions In the same directions In

the overturnedthe overturned

fold the lower limb is fold the lower limb is

turned up side down.turned up side down.

Isoclinal FoldIsoclinal Fold

Folds with parallel Folds with parallel limbs are called limbs are called Isoclinal folds. Isoclinal folds. In this case limbs dips In this case limbs dips in same angle and in in same angle and in same direction.same direction.

Isoclinal folds have Isoclinal folds have three types three types

1)1)Inclined Isoclinal folds Inclined Isoclinal folds

2)2)Vertical Isoclinal folds Vertical Isoclinal folds

3)3)Recumbent or Recumbent or

horizontal Isoclinal fold.horizontal Isoclinal fold.

Some Special Type of FoldsSome Special Type of Folds

Anticlinorium and SynclinoriumAnticlinorium and Synclinorium

An anticlinorium is a large anticline running often An anticlinorium is a large anticline running often

for several hundred kilometers in length and for several hundred kilometers in length and

several kilometers in width which is further several kilometers in width which is further

thrown into smaller folds. Similarly a syclinorium thrown into smaller folds. Similarly a syclinorium

is a large syncline further consisting of smaller is a large syncline further consisting of smaller

folds.folds.

Geanticline and GeosynclineGeanticline and Geosyncline

An anticline of excessively large dimensions is An anticline of excessively large dimensions is

called a ‘geanticline’ and similar a syncline of called a ‘geanticline’ and similar a syncline of

huge dimensions is known as a ‘geosyncline’huge dimensions is known as a ‘geosyncline’

And Even More Fold TerminologyAnd Even More Fold Terminology

DomeDome: a sequence of : a sequence of folded rocks in which folded rocks in which all the beds dip away all the beds dip away from a central pointfrom a central point

BasinBasin: a sequence of : a sequence of folded rocks in which folded rocks in which all the beds dip all the beds dip towards a central towards a central pointpoint

• Fault planeFault plane: Surface that : Surface that the movement has taken the movement has taken place within the fault. On place within the fault. On this surface the dip and this surface the dip and strike of the fault is strike of the fault is measured.measured.

• Hanging wall: Hanging wall: The rock The rock mass resting on the fault mass resting on the fault plane.plane.

• Footwall: Footwall: The rock mass The rock mass beneath the fault plane.beneath the fault plane.

Parts of The FaultsParts of The Faults

Normal FaultNormal Fault

footwall

hanging wall

Cross Section

hanging wall

footwall

Before After

Normal FaultNormal Fault

hanging wall

footwall

cross section

Hanging wallFoot wall

Dip-slip FaultsDip-slip Faults

• Motion of the fault blocks is parallel to the dip direction

• Two types:Normal – movement is down dipReverse – movement is up dip

Types of Faults Types of Faults

Normal FaultsNormal Faults

Thrust FaultThrust Faults: In the thrust faults the

hanging wall has moved up

relative to the footwall (dip angle

30º or less)

Reverse Faults: Are similar to the

thrust faults regarding the sense

of motion but the dip angle of the

fault plane is 45º or more

Thrust faults usually formed in areas of

comperssional regime.

Thrust Thrust FaultFault

Thrust Faults

Strike-slip FaultsStrike-slip Faults

Motion of the fault blocks is parallel to the strike direction

There are 2 types

Right-lateral

Left-lateral

Strike-Slip FaultStrike-slip Faults: Are faults that have

movement along strikes.

There are two types of strike slip faults:

A] Right lateral strike-slip fault (dextral): Where the side opposite the observer moves to the right.

B] Left lateral strike-slip fault (sinistral): Where the side opposite the observer moves to the left.

Note that the same sense of movement will

also be observed from the other side of the

fault.

Strike-Slip Faults

Left-lateral

Transform FaultsTransform Faults: Are a type of strike-

slip fault (defined by Wilson 1965).

They form due to the differences in

motion between lithospheric

plates. They are basically occur

where type of plate boundary is

transformed into another.

Main types of transform faults are:

• Ridge-Ridge

• Ridge-Arc

• Arc-Arc

Types of Faults•Dip-slip faults have movement

parallel to the dip of the fault plane

–In normal faults, the hanging-wall block has moved down relative to the footwall block

–In reverse faults, the hanging-wall block has moved up relative to the footwall block

Types of Faults•Dip-slip faults have

movement parallel to the dip of the fault plane

–Fault blocks, bounded by normal faults, that drop down or are uplifted are known as grabens and horsts, respectively

•Grabens associated with divergent plate boundaries are called rifts

–Thrust faults are reverse faults with dip angles less than 30° from horizontal

Types of Faults•Strike-slip faults have movement

that is predominantly horizontal and parallel to the strike of the fault plane

–A viewer looking across to the other side of a right-lateral strike-slip fault would observe it to be offset to their right

–A viewer looking across to the other side of a left-lateral strike-slip fault would observe it to be offset to their left

•Oblique-slip faults have movement with both vertical and horizontal components

Right-lateral San Andreas Fault

Displacement both vertically and horizontally

Different Type of Faults

Rift Valley

Rift Valley

San Andreas Fault (right-lateral)

Joints

Definition:Definition:

When rock masses are subjected to tensional or When rock masses are subjected to tensional or

compressional forces regular or irregular fractures compressional forces regular or irregular fractures

develops in them. Such fractures along which their has develops in them. Such fractures along which their has

been no relative displacement are called Joints.been no relative displacement are called Joints.

joints occurs in almost all types of rocks they may joints occurs in almost all types of rocks they may

be vertical inclined or horizontal be vertical inclined or horizontal

Commonly rock contains a large number of joints which lie parallel Commonly rock contains a large number of joints which lie parallel

to one another. These parallel joints together forms a joint set. Two to one another. These parallel joints together forms a joint set. Two

or more joints sets are called joint system.or more joints sets are called joint system.

Classification of Joints Classification of Joints •Tension Joints: Tension joints are those which are formed as a result of tensional forces. These joints are relatively open and have rough and irregular

surface.

•Shear Joints: Shear joints are those which are formed due to compressional forces involved in the folding and faulting of rocks. This type of joints are clean and tightly closed

Classification of joints :

1)Strike joints- The joints are parallel to strike of the country rocks 2)Dip joints- The joints are parallel to direction of dip of country rocks 3)Oblique joints - Runs oblique to strike and Dip directions of country rocks.

Common Type of jointsCommon Type of joints

Sheet JointSheet JointHorizontal set dividing rock massLayered appearanceCaused due to weathering,Removal of overlying rockIt causes expansion of underlying igneous rocks

Dr. N. J. Sathe, SCOE, Pune. 78Photograph showing MURAL jointsPhotograph showing MURAL joints

• Also called as PRISMATIC jointsAlso called as PRISMATIC joints• Divides the rock into polygonal blocksDivides the rock into polygonal blocks• Each block is bounded by 3 to 8 Each block is bounded by 3 to 8

sidessides• 5 & 6 sided blocks are common5 & 6 sided blocks are common• Vertical or perpendicular jointsVertical or perpendicular joints• Varying depth Varying depth

Columnar JointsColumnar Joints

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