Stress and strain on rocks is responsible for the deformationstructures of rocks
Stress is a force that causes deformation and strain is a measurement of deformation
Stress – measured in Pa, shear stress: parallel to surface and normal stress: perpendicular to surface
Strain – unitless, permanent and elastic strain
Deformation Types
Rocks deform 3 different ways:
Elastically – When stress is removed, rock returns to its original shape. However, if the strain on the rock is great enough, the rock will fail resulting in earthquakes (stress is released)
Plastically – Where the rocks bend instead of break due to applied stress. The bonds of the rocks break under large temperatures. Resulting structures: Folds
Brittlely – Rocks break instead of bend, most rocks in the uppermost 10km of the surface experience brittle deformation. Resulting structures: Faults and fractures
Measuring stress and strain
Stress cannot be measured directly, but rather can be inferred from strain
Strain analysis of rocks measures the orientation, magnitude, and distribution of strain features such as folds to infer the stress regime that caused them.
This can then be used to infer the strength of stress on a rock based off of the unit’s elasticity, strength, and ductility
Stress Regimes
There are extensional, compressional and shear stress regimes
Extensional Stress Regimes
Extensional stress in faults leads to the production of normal faults and rift valleys
Compressional Stress Regime
Compressional stress in faults leads to reverse faults and thrust faults
Shear Fault Regimes
Where shear stress in faults leads to dextral and sinistral faults
Well vs Poorly Oriented Faults
Within constant stress regimes there can be both well and poorly orientated faults
Faults are typically well orientated when they have a dip less than 60˚. When faults have this dip, they are fewer perpendicular forces acting on the fault, and more shear, which results in less friction allowing the fault to slip easier
However, when the dip of a fault increases to above 60˚, the normal stress becomes increased, and the shear stress decreases. Resulting in the fault locking up and having less movement
Factors affecting strain behaviour
Temperature
Pressure
Deformation rate
Fluids
Competency
Young's Modulus
Young’s modulus:
o Describes the relationship between stress and strain for an object
o Each mineral has a different young’s modulus, therefore he mineral makeup of the rock has a strong control over how that rock will deform
If the rock is composed of higher competency minerals, then the rock will be stronger and a higher amount of stress will be required for brittle strain
Fluids in faults
Fluids such as water, or oil can find their way into faults
Faults are impacted by the influence of fluids. When fluids become involved on a fault plane, they reduce the amount of friction and allow the shear stress to more easily affect the plane. Making slip easier and more probable
Temperature, Pressure and Deformation Rate
Temperature increases the further you travel below the surface. Every km there is a temperature increase of approx. 20˚-30˚
Pressure also increases the further you travel into the crust
The rate of deformation depends on the stress regime and tectonic setting of the rocks