structural geology geology 101, fall 2012. structural geology the study of the deformation and...
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Structural geology
Geology 101, Fall 2012
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Structural geology
• The study of the deformation and fabric of rocks in order to understand the tectonic forces
• Rheology is the study of the effect of stress on materials
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Stress-strain diagram
• Stress is measured as a force applied to a material
• Strain is the resulting change in volume of the material
• Elastic means that the material returns to its normal volume once the stress is removed; plastic (or ductile) means that it does not
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Earth’s interior structure
• In fact, using seismic studies, we can measure the depth at which the Earth’s materials reach the elastic/ductile point
• This is where the material becomes “gooey”
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The stress-strain diagram is misleading
• It shows only one type of tectonic stress -- compression
• Two other types -- extension and shearing
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“Classic” plate tectonic settings and mountain building
• Divergent boundary – fault-block mountains (horsts and grabens)
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Extensional stress
• Normal faults arise from extensional stress
• Called “normal” because of age relationship of rocks across the fault
• Detachment faults are low-angle normal faults
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Orogeny - horst and graben
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“Classic” plate tectonic settings and mountain building
• Divergent boundary – fault-block mountains (horsts and grabens)
• Collision (convergent) boundary – “fold-and-thrust” belts
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“Classic” plate tectonic settings and mountain building
• Divergent boundary – fault-block mountains (horsts and grabens)
• Collision (convergent) boundary – “fold-and-thrust” belts
• Subduction (convergent) boundary – volcanic arc mountains
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Compression leads to certain structures
• Specifically, ductile structures called folds
• Sedimentary rocks can be deformed this way, but some metamorphism may also occur
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Rock fabric
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Terms associated with folds
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How folds look on the surface
• The combination of folding and differential erosion creates interesting rock outcrop patterns, which help explain human history.
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Plunging folds• Because the whole
fold may be tilted perpendicular to the axial plane, folds may plunge
• The plunge is measured as an orientation and an angle off of horizontal
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But even the toughest rocks break, and the break is called a fault
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Faults• Faults are a break in
a rock along which offset has clearly occurred
• Breaks where there is no evidence of motion are called joints
• All types of tectonic stress may lead to faults
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Compressive stress
• Reverse faults result from compression
• Called “reverse” because of age relationship of rocks across the fault
• Thrust faults are reverse faults with a fault dip angle < 45°
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Orogeny - fold and thrust belt
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“Classic” plate tectonic settings and mountain building
• Divergent boundary – fault-block mountains (horsts and grabens)
• Collision (convergent) boundary – “fold-and-thrust” belts
• Subduction (convergent) boundary – volcanic arc mountains
• Transform boundaries are not associated with mountain-building
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Shearing stress
• Strike-slip faults result from shearing stress• Called “strike-slip” because motion is along
strike (horizontal orientation)• Types: left-lateral, right-lateral
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Orogeny - transpressional
• Mountain building occurs in strike-slip fault areas with some compression
• Called “oblique-slip”
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Origin of the Transverse Ranges
“Transpressional”
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Origin of the Sierra Nevada
Relict subduction zone