The main source for the crustal deformation is the tectonic forces.Tectonic forces can be defines as
- ”Those forces generated from within the earth that result in uplift,movement or deformation of the crust of the earth
- Tectonic forces inside the earth causes to break and move the lithosphere plate and particularly along plate margins.
- When studying deformed rocks.structure geologists typically refer to stress.
Stress: A force acting on a body or rock unit that tends to change the size or change the shape of that body or rock unit. or force per unit area
Types of stresses
Stresses is of two types:
(1) Uniformal stress
(2) Differential stress
1Uniform stress: That type of stress in which the forces act equally from all direction\side i.e The pressure due to the overlying rocks which is also known as confining pressure.
2Differential stresses:
Forces do not act equally from all directions.
There are three different types of differential stress
(a) Compressional/Compressive stress
(a) Compressional/Compressive stress
(b) Tensional/Extensional stress
(c) Shear stress
(a)Compressive stress:
That type of stress in which the forces act towards one another and also these stresses act along same line (coaxial stresses).Compression causes shortening of the rocks horizontally and thickening vertically.
The structures that are produced due to compressive stress are Folds and Thrust faults.
Folds thrust fault
The structures that are produced due to compressive stress are Folds and Thrust faults.
Folds thrust fault
The best example of compressive stress is subduction zone.
Himalayas is the example of compressive stress.
Himalayas is the example of compressive stress.
(b)Tensional/Extensional stress: That type of stress in which the forces act away from one another and also along the same line.
Tensional stress enlarges the rock horizontally and shortens/thin vertically.
Structures that is associated with tensional/extensional stress is Normal Faults.
The best example of tensional/extensional stress is MOR (mid oceanic ridges).
EXAMPLE is Ring of fire in Pacific Ocean.
(c) Shear stress:
Those types of stress in which forces act towards one another but not along the same line.These stresses are parallel to one another and are in opposite direction.
Structures that are associated with shear stress are STRIKE SLIP FAULT
Example: SAN ANDREAS FAULT
In Pakistan Chamman transform fault/boundary.
Result of stress:
Stresses result in the deformation, also known as strain.
STRAIN:
It is the change in shape or size (volume) of a rock in response to the stress.
- DILATION: change in volume.
- DISTORSION: change in shape.
TYPES OF STRAIN:
(1)ELASTIC STRAIN
(2)DUCTILE STRAIN
(3)BRITTLE STRAIN
(1)ELASTIC STRAIN:
That type of strain in which the rock body retain its original position after removing the stress is called elastic strain.
NO Structure formed from the result of elastic strain.
(2)DUCTILE STRAIN:
That type of strain in which the permanent and the body or rock flows like a viscous liquid.
Ductile behavior occurs in those rocks that are hot,such as those buried at a depth.At such high temperatures stresses may cause the minerals to recrystallize,in doing so the rocks becomes rearranged and slowly deform in a liquid like flow pattern.Ductile behavior also may happen without mineral recrystallization.At lower temperatures manu rocks(particularly sedimentary)exhibits folding without any mineral recrystallization.
(3)BRITTLE STRAIN:
That type of strain in which we have the breaking of rocks rather than bending.
Structures form due to brittle deformation are FAULTS,JOINTS and FRACTURS.
The behavior of rocks change with the depth.So on the basis of different rock behaviors at different depth zones we divided the crust into different structural zones
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