Week 4 Lecture: Structure Flashcards
What are the three processes of deformation?
- Compression
- Extension (tension)
- Shear
What process of deformation causes the Crust to fold and thicken?
Compression
What does tension do to the Crust and what formation is associated with it?
- Thins and stretches it
- Is associated with rifting
Describe the 2 names for the direction of shear faults
- Sinistral: To your left
- Dextral: To your right
What is the difference between Hydrostatic and Lithostatic pressure?
Hydrostatic: weight of overlying column of water
Lithostatic: weight of overlying column of rock (density x gravity x depth)
Lithostatic and Hydrostatic stress is equal in all _______, and produces a change in ________.
Lithostatic/hydrostatic stress is equal in all directions and produces a change in size (volume).
What is Differential Stress?
The difference between the greatest and least compressive stress experienced by an object. (sigma 1, 2, and 3)
If Differential stresses are not equal, what occurs?
A change in shape (Strain / Deformation)
What is strain / deformation?
- Change in shape
- Change in position
What are the 3 styles of strain (rock behaviour)?
- Elastic
- Brittle
- Ductile
Define Elastic Strain
Proportional to stress and reversible
Define Ductile strain
continuous, irreversible, dispersed strain, no breakage
What are two types of ductility?
Plastic behaviour - can retain shape
Viscous behaviour - cannot retain shape
Define Brittle strain
Irreversible strains that include discontinuities (breakage)
What is the primary control on the style of deformation?
The material or rock type (more specifically, mineral composition and texture)
What is stronger, basalt or schist?
basalt
What is more ductile, quartzite or marble?
marble
What is confining pressure?
The stress or pressure forced on a layer of soil or rock by the heaviness of the overlying substance
What does the amount of confining pressure do to the type of deformation of a rock?
High stresses - brittle
Higher stresses - ductile
How does temperature affect ductile deformation?
Low Temperature - fracturing
High Temperature - folding ductile shearing
How does history of a material affect it?
Pre-existing cracks, crystal defects, etc can affect its deformation
Do crustal rocks deform in a brittle or ductile manner?
Brittle. At higher depths, rocks begin to become more brittle
How does deformation affect the rock Mylonite?
It is strongly deformed and has undergone grain size reduction due to plastic deformation
TEST QUESTION (ACCORDING TO ELSBETH)
What are the two types of Fractures? Describe them.
Joints and Faults.
Joint: no measurable displacement
Fault: measurable displacement
When do Joints form?
When the elastic limit of the rock is exceeded and there is failure
Name and describe the 2 subcategories of Joint Arrays:
Joint Groups: repeated regularly across rock (parallel)
Joint Sets: two or more groups of joints (with different orientation)
Name and describe the 2 subcategories of Joint Sets:
Orthogonal sets: 90 deg to each other
Conjugate sets: between 30 and 60 deg to each other
List some natural processes that cause Joints
- Uplift and Erosion
- Cooling
- Hydraulic Fracturing
- Folding
What kind of joints are caused by uplift and erosion?
Exfoliation joints
What is hydrofracturing?
Where the pore pressure exceeds the lithostatic pressure causing brittle rupture
Describe what Folding does
Folding produces a region of tensile stress in the outer arc of the fold and forms joints.
List the key terminology of Fault geometry
- Footwall
- Hanging wall
- Strike Line
- Fault Plane
- Angle of dip
What is the Strike Line?
The horizontal line (with respect to Earth’s surface) of a rock layer that gives the angle from North. (It’s the compass direction on a horizontal line)
The Strike always is perpendicular to the direction of dip.
What is the Angle of dip?
The angle between a horizontal plane (the Earth’s surface) and the tilted rock layer.
What is Plunge?
Plunge is used to describe the tilt of any line on the dipping plane. (Can include the strike line and angle of dip line)
Name and describe two components of Plunge
- the angle of plunge: vertical angle
- the plunge direction (trend): the compass direction
List the following faults in order of how common they are (common to rare):
- Thrust
- Strike-slip
- Normal
- Oblique slip
- Reverse
- Normal
- Thrust
- Reverse
- Strike-slip
- Oblique slip
Describe a Reverse Fault
A fault that is undergoing horizontal compression with a dip from 45 to 60 deg. Hanging wall travels upwards
Describe a Thrust Fault
Similar to a Reverse Fault except that the angle of dip is only between 15 to 45 deg.
What kind of fault is tension responsible for?
Normal Fault
Describe what a horst and a graben is
When tension creates a normal fault, the block of land that drops down is the graben and the uplifted block is the horst
Name and describe the two types of strike-slip faults
If you’re standing on one side of a strike-slip fault, and look across at the block/plate on the other side:
- A Left-lateral strike-slip fault is where the block on the other side moves to the left
- A Right-lateral strike-slip fault is where the block on the other side moves to the right
What are folds?
Wave-like structures that result from the deformation of planar surfaces in a rock. Directly related to compression
What is the Axis of a fold?
The line that traces the crest or trough of a fold
What is an Axial plane?
Imaginary plane that includes the axis and divides the fold as symmetrically as possible
What is the hinge of a fold?
Tightest part of a fold
What is the limb of a fold?
Arm of the fold, intersects at the hinge
Differentiate an anticline from a syncline
An anticline has the shape of an upward arching fold with older rocks in the center/core of the arch and younger rocks along the perimeter
A syncline has the shape of a downward arching fold with older rock away from the core of the arch and younger rocks towards the core
What is an overturned fold?
A fold with an axial plane inclined so much that one limb is essentially leaning over another
What is a recumbent fold?
A fold with an essentially horizontal axial plane
What is one thing to remember when studying Folds?
They are 3d
What are Penetrative Fabrics and how are they caused?
The regularly spaced alignment of minerals. They are caused due to the stresses associated with plate tectonics.
What is structure when discussing rock bodies
Any definable shape or fabric in 3d
What are Planar fabrics?
- Cleavage
- Schistosity
- Gneissosity
Describe Linear fabrics?
Developed by stretching of pre-existing minerals, growth of new minerals, or by intersection of two planar fabrics
