EESA06H3 Chapter Notes -Convergent Boundary, Strike And Dip, Compressive Stress

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24 Apr 2012
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Chapter 11 - Page 1 of 7
Chapter Eleven Geologic Structures
Structural geology: branch of geology concerned with the shapes, arrangement, and
interrelationships of bedrock units and the forces that cause them
What causes rocks to bend and break?
Stress and strain in the Earth’s lithosphere
Tectonic forces move and deform parts of the lithosphere, especially along plate margins
Deformation may cause change in orientation, location, and shape of a rock body
Stress expressed as the force per unit area at a particular point
o Hard to measure stress in rocks that are currently buried
We can observe effects of past stress when rock bodies are exposed after uplift and erosion
Strain is the change In size (volume) or shape, or both, in response to stress
Pushed together or squeezed from opposite directions compressive stress
o Common along convergent plate boundaries and typically results in rocks being
deformed by a shortening strain
o In figure 11.2A, an elongate piece of dough may shorten by bending, or folding, whereas
a ball of dough will flatten by shortening in the direction parallel to the compressive
stress and elongating or stretching in the direction perpendicular to it. Rocks that have
been shortened or flattened are typically found along convergent plate boundaries
where rocks have been pushed or shoved together
Tensional stress caused by forces pulling away from one another in opposite directions
o Results in a stretching or extension of material
o Ball of dough will elongate or stretch parallel to the applied stress
o If applied rapidly, the dough will first stretch and then break apart
o At divergent plate boundaries, the lithosphere is undergoing extension as the plates
move away from one another
o Because rocks are very weak when pulled apart, fractures and faults are common
structures
When stresses are parallel to a plane, shear stress is produced
o Holding a deck of cards and moving your hands in opposite directions
o Results in a shear strain parallel to the direction of the stresses
o Occur along actively moving faults
How do rocks behave when stressed?
Rocks behave as elastic, ductile, or brittle materials depending on the amount and rate of stress
applied, the type of rock, and the temperature and pressure under which the rock is strained
If a deformed body recovers its original shape after the stress is reduced/removed, the
behaviour is elastic
o Tensional stress on a rubber band
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Chapter 11 - Page 2 of 7
o Most rocks behave like this at very low stress (a few kilobars)
o Once the stress exceeds the elastic limit, the rock will deform in a permanent way
A rock that behaves in a ductile or plastic manner will bend while under stress and doesn’t
return to its original shape after relaxation of the stress
o Pizza dough behaves like this unless the strain is rapid
o Rocks exposed to elevated pressure and temperature during regional metamorphism
also behave in a ductile manner and develop a planar texture, or foliation, due to
alignment of minerals
o Don’t require much of an increase in stress to continue to strain (flat-ish curve)
o Results in permanently deformed rocks (folding or bending)
A rock with brittle behaviour will fracture at stresses higher than its elastic limit, or once the
stresses are greater than the strength of the rock
o Rocks behave like this near the earth’s surface where temps/pressures are low
o Rocks here favour breaking rather than bending
o Faults and joints form due to the brittle behaviour of the crust
Sedimentary rock at the earth’s surface is brittle and will fracture if hit. If it’s bent then it means
that either stress increased very slowly or the rock was deformed under considerable confining
pressure (buried under rock) and higher temperatures
How do we measure and record geologic structures?
Geologic Maps and Field Methods
A geologic map, which uses standardized symbols and patterns to represent rock types and
geologic structures, is typically produced from the field map for a given area
o Plots type and distribution of rock units, the occurrence of structural features, ore
deposits, etc.
o Sometimes surficial features, such as deposits by former glaciers, are included, but
these may be shown separately on a different type of geologic map
Strike and Dip
When originally horizontal rocks are found tilted, it indicates that tilting must have occurred
after deposition and lithification
Strike is the compass direction of a line formed by the intersection of an inclined plane with a
horizontal plane
Angle of dip measured downward from the horizontal plane to the bedding plane (an inclined
plane); perpendicular to the bedding and horizontal planes
Direction of dip compass direction in which angle of dip is measured; direction in which a ball
would roll down the surface
Dip angle is always measured perpendicular to the strike line
Beds can dip away from the strike line in one of two directions important to specify the
direction
Right hand rule used to record azimuthal strikes and dips
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