chapter 10 – crustal deformation · 2016-11-18 · grand tetons, wy - fault block mountains....
TRANSCRIPT
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Crustal Deformation AKA – Structural geology
(adapted from Brunkel, 2012)
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Study the architecture and processes responsible for deformation of Earth’s crust.
Folding and Faulting
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How Rocks Deform: 4 Controls• Rock Type – i.e., sandstone is more brittle than shale. • Temperature – higher T = more ductile• Confining Pressure – high lithostatic stress = more ductile • Time – more time = more ductile (i.e., karate chop)
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Stress and Strain Relationships
The result of rock deformation can be seen at the surface as folds and faults
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A few things we need to know
• Law of original horizontality• Superposition• Cross-cutting relationships• Strike and dip
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Law of original Horizontality
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Superposition• Youngest on the top
• Oldest on the bottom
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i.e., These sandstone beds were deposited as horizontal layers before they were faulted.
Principle of Cross-cutting Relationships
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Strike and Dip- when rocks are no longer horizontal
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Strike and Dip- how do we describe their orientation
W E
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Strike and dip rules
• Strike is the direction on the surface of the rock formation – described by two directions ie. N-S, E-W, NE-SW
• Dip is always perpendicular to strike and is described by only one direction – N, S, E, W or NW, SE etc.
• Often it is easier to find the dip of a rock unit first and then describe the strike
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Strike and Dip
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Strike and Dip
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Folds
• How do rocks fold?• Ductile deformation• What environments lead to ductile
deformation?
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Folds• Folds wave-like undulations in rock that form
mainly from compressional stress that shortens and thickens the crust
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Fold Parts
• Limbs –the two planar sides of a fold
• Axis – imaginary line marking the crest or trough of each layer
• Axial plane – an imaginary plane of symmetry through the center of the fold
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Types of folds
• Anticlines – “A” shape
Fold Axis
limb limb
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Folds
FoldAxis
Syncline- think of a sink
limb limb
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Paired and tilted anticline and syncline
Fold Axis
Fold Axis
Fold Axis
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folds
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folds
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Overturned folds
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folds
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Folding on a large scale to produce large landforms
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• Sheep Mountain, WY: Plunging Anticline & Syncline– Note Outcrop “V”s, Plunge Arrows, Anticline Symbol, Syncline Symbol– Note Oldest & Youngest Layers
MississippianRocks
TriassicRocks
TriassicRocks
JurassicRocks
JurassicRocks
CretaceousRocks
CretaceousRocks
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Plunging folds
• Fold axis dips below the surface
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• Anticline – upfold– Oldest rock in
center– Point of mapped
outcrop “V” in the direction of plunge.
• Syncline - downfold – Youngest rock in
center– Open end of
mapped outcrop “V” is in the direction of plunge.
arrow on end of fold axissymbol indicates plunge direction
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DOME
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Sinclair Dome, WY
oldest rocks
younger rocks
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Folds in map viewAnticlines - eroded tops of anticlines reveal a characteristic map pattern of rock ages
• Oldest rocks exposed in the middle with bands getting younger as you go out
• e direction of dip of the bed will provide clues to what type of structure it is
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Folds in map view
Synclines -eroded synclines reveal a characteristic map pattern of rock ages
• Youngest rocks exposed in the middle with bands getting older as you go out
• The direction of dip of the bed will provide clues to what type of structure it is
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Geologic Mapssyncline axis symbolanticline axis symbol
Strike & dipsymbol
contactbetween
rock unitsof different
ages
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Faults
• Faults - fractures in rocks along which appreciable displacement has taken place – brittle deformation of the rock or layers of rock– 2 basic Types:
• Dip Slip – Movement is mainly parallel to the dip of the fault surface
• Strike Slip - Movement is mainly parallel to the strike of the fault surface
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Dip-Slip Faults
• Two main types –• Normal – Hanging wall moves down in
relation to foot wall• Reverse or Thrust – Hanging wall moves
up relative to footwall
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• Tensional forces cause normal faultingFaults
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Normal Faults
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Scarps
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• Form fault-block mountains
• Horst = high upthrown block
• Graben = low downthrown block
Normal Faults
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Grand Tetons, WY- fault block mountains
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Reverse Faults
Thrust Fault
Reverse Fault
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• Thrust Faults are a low angle reverse fault
Faults
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Overlapping thrust sheets build up mountain ranges
Thrusts are low angle reverse faults
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The Canadian Rockies were built up as a series of thrust sheets
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• Shear stresses cause strike-slip faultingFaults
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Strike-Slip Faults
• Right-lateral – as you face the fault, the block on the opposite side of the fault moves to the right
• Left-lateral – as you face the fault, the block on the opposite side of the fault moves to the left
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Engineering and Faults
• Fractures to Faults• Shear Zones
Roof
Floor