Deformation, lectures 9 & 10 Flashcards
Deformation takes place on a range of spatial scales…
2 examples
- atomic level in single crystals
2. regional scale within mountain belts
Deformation takes place on a range of temporal scales
2 examples
- seconds-minutes during seismic activity
2. Ma for mountain building (logarithmic scale used)
Equation for stress
force/area
same as pressure
Compression stress
acts to shorten an object
Tension stress
acts to lengthen an object
Normal stress
acts perpendicular to a surface (compression or tension)
Shear stress
acts parallel to a surface
The Earth is…
dynamic
The Earth is dynamic which means that
deformation occurred in the past and the present
Two types of compression stress
hydrostatic/uniform
directed/differential
Hydrostatic/uniform stress
type of compression stress
Uniform in all directions
Directed/differential stress
type of compression stress
isn’t uniform in all directions
Strain
A measure of the extent of deformation, eg: a measure of the change in shape or volume due to stress.
Strain
change in dimension…
change in dimension relative to the original dimension
Two types of deformation
plastic
brittle
Plastic deformation
results in folding
eg: folded dyke
Brittle deformation
results in fracturing
eg: fault in sedimentary rock
Faults and folds
Geologic features resulting from deformation, provide a record of the forces which operated in the past
Benefits of interpretation of geologic structures
3
- helps to understand the history of the Earth
- aids exploration of natural resources
- helps to select appropriate sites for infrastructure
Sources of information on deformation processes
4
- field observations
- lab experiments
- computer simulations
- theoretical calculations
Three factors which affect how minerals and rocks respond to stress
- temp. and pressure
- minerals/rock type
- strain rate (rate of deformation)
Temperature and pressure
factors which affect how minerals and rocks respond to stress
more likely to deform under high temps.
Mineral/rock type
factors which affect how minerals and rocks respond to stress
- hard igneous/metamorphic rocks tend to behave as brittle material (faults)
- softer sedimentary rocks tends to deform in a ductile manner (folds)
Strain rate (rate of deformation) (factors which affect how minerals and rocks respond to stress)
- rapid deformation may result in brittle failure/faulting
- slow deformation in plastic deformation/folding
Dip and strike of strata
used to describe the orientation of deformed strata
Strike
The direction of a line formed by the intersection of a horizontal plane with an inclined plane, such as a rock layer.
What is strike direction relative to?
North,
compass used
Dip
A measure of the maximum angular deviation of an inclined plane from the horizontal
What is dip measured as relative to?
perpendicular to the strike direction
Classification of folds
based on geomtery and symmetry, and orientation of fold axis
Monocline
A step-like fold in strata with a zone of steeper dip in approximately horizontal sequence
Mechanism of formation of monocline
2 steps
- Differential compaction over underlying structure at edge of basin and greater compatibility of the basin fill
- Reactivation if underlying fault coupled with uplift/ subsidence
Non-plunging fold
fold axis is horizontal
Plunging fold
fold axis is inclined
Features of a plunging fold
- plunging anticline or plunging syncline
- all rocks dip away from the axis
- oldest rocks are exposed in the centre
Dome and basin
circular or oval equivalents of antiform and synform respectively
Dome
beds dip away from a central point
Basin
beds dip in towards a central point
Classification of joints
based on geometry or process of formation
Classification of faults
based on orientation of fault plane …
Joints
Fractures along which little or no movement has occurred or movement is perpendicular to the surface.
Faults
Fractures along which blocks on opposite sides of the fracture plane have moved.
GPS & LPS
faults
greatest principle stress axis
least principle stress axis
Two types of joints
tensional
cooling
Cooling joints
commonly occur in basaltic lavas
Antiform/anticline
an ‘n-shaped/up-arched’ fold
Synform/syncline
A ‘u-shaped/down-arched’ fold
Symmetrical/upright fold
Axial plane is vertical and each limb dips at the same angle
Asymmetrical fold
Axial plane is inclined and limbs dips at different angles
Overturned fold
one limb has been rotated so much that it is now upside down, common in mountain ranges
Recumbent fold
Axial plane is horizontal, common in…
