Week 4 Lecture: Structure Flashcards by Joe E

What are the three processes of deformation?

Compression
Extension (tension)
Shear

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What process of deformation causes the Crust to fold and thicken?

Compression

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What does tension do to the Crust and what formation is associated with it?

Thins and stretches it
Is associated with rifting

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Describe the 2 names for the direction of shear faults

Sinistral: To your left
Dextral: To your right

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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)

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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).

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What is Differential Stress?

The difference between the greatest and least compressive stress experienced by an object. (sigma 1, 2, and 3)

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If Differential stresses are not equal, what occurs?

A change in shape (Strain / Deformation)

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What is strain / deformation?

Change in shape
Change in position

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What are the 3 styles of strain (rock behaviour)?

Elastic
Brittle
Ductile

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Define Elastic Strain

Proportional to stress and reversible

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Define Ductile strain

continuous, irreversible, dispersed strain, no breakage

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What are two types of ductility?

Plastic behaviour - can retain shape
Viscous behaviour - cannot retain shape

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Define Brittle strain

Irreversible strains that include discontinuities (breakage)

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What is the primary control on the style of deformation?

The material or rock type (more specifically, mineral composition and texture)

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What is stronger, basalt or schist?

basalt

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What is more ductile, quartzite or marble?

marble

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What is confining pressure?

The stress or pressure forced on a layer of soil or rock by the heaviness of the overlying substance

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What does the amount of confining pressure do to the type of deformation of a rock?

High stresses - brittle

Higher stresses - ductile

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How does temperature affect ductile deformation?

Low Temperature - fracturing

High Temperature - folding ductile shearing

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How does history of a material affect it?

Pre-existing cracks, crystal defects, etc can affect its deformation

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Do crustal rocks deform in a brittle or ductile manner?

Brittle. At higher depths, rocks begin to become more brittle

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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