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X. Deformation and . Mountain Building
A. Plate Tectonics and Stress
B. Rock Deformation
C. Geologic Structures
D. Origin of Mountains
E. Continental Crust
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Tectonic Stresses Large Scale Strain of the Crust i.e., Geologic Structures
Inner core: Solid iron Outer core: Liquid iron,
convecting (magnetic field)
Mantle (Asthenosphere) : Solid iron-magnesium silicate, plastic, convecting
Crust (Lithosphere): Rigid, thin5-30km
Crust: Rigid, Thin
Mantle: Plastic, Convecting
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Tectonics and Structural Geology
Tectonic Stresses resulting fromInternal Energy (heat driving convection) Strains (deforms) the Mantle and Crust
Bends Rocks, i.e., ductile strain (Folds)Breaks Rock, i.e., brittle strain (Joints) and Moves large blocks along Faults andReleases energy Earthquakes
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Fig. 10-CO, p. 216
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Folds and Faults (Palmdale, Ca)
See Fig. 10-2a, p. 219
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Eastern PennsylvaniaEastern Pennsylvania
NorthwesternAfrica
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Stresses at Plate
Boundaries Divergent (Tensional)
| Convergent (Compressional) | Transform (Shear)
e.g., Pacific NW
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Geologic Structures Different stresses result in
various forms of strain (geologic structures) Folds (compressive
stresses may cause ductile strain)
Faults (Any type of stress may cause brittle strain. The type of fault depends on the type of stress)
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Stikes and Dips are used to identify geologic structures
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Strike and Dip
Define and map the orientation of planar features Bedding planes (sedimentary rocks) Foliation Joints Faults Dikes Sills Ore Veins
Fig. 10-4, p. 221
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Strike and Dip
Strike: The line of intersection between the plane and a horizontal surface
Dip: Angle that the plane makes with that horizontal plane
Fig. 10-4, p. 221
Strike and Dip Map Symbol
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Sipping Bedding Planes
Youngest (top) P: Permian P: Pennsylvanian M: Mississippian D: Devonian S: Silurian O: Ordovician C: Cambrian
Oldest (bottom)
D
S
O
Sedimentary Rocks Dip in the direction of younger rocks
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Deciphering the Geology of OhioUsing Dipping Bedding Planes
Beds Dip 2o, West Younger rocks, West Mirror image east of
Sandusky?
Beds Dip 2o, West Younger rocks, West Mirror image east of
Sandusky?
Sandstone Shale Limestone
M O D2o2o2o
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Anticline (fold)
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Anticline (fold)
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Syncline (fold)
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Plunging Anticline
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Fold Terminology
Axis Axial Plane Plunging Age of rocks
and outcrops
Axis Axis
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Plunging Anticline, Colorado
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Eastern PennsylvaniaEastern Pennsylvania Folds and faults resulting from
compressive stresses Anticlines (many plunging) Synclines (many plunging) Reverse faults Thrust faults
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Domes and Basins
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Bedrock Geology of the Michigan Basin During and after
the deposition of Michigan’s sedimentary rocks
The crust warped downward
Exposing younger rocks in the center and
Older rocks on the rim (e.g. Toledo)
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When shallow crust is strained rocks tend to exhibit brittle strain
Brittle Strain Joints
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Sheet Joints
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Defining Fault Orientation
Strike of fault plane parallels the fault trace and fault scarp
Direction of Dip of the fault plane indicates the Hanging wall block
Fig. 10-11a, p. 227
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Fault: Movement occurring along a discontinuity Brittle strain and subsequent movement as a
result of stress Fault
terminology
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Faults
Fault: When movement occurs along a discontinuity
Fault type depends on the type of stress
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Normal Faults
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Normal Faults, Horsts and Grabens
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Structures at Divergent Boundaries
Tensional Stresses cause brittle strain and formation of sets of normal faults
i.e., Horsts and Grabens
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Horsts and Grabens Older Rocks are exposed along the ridges
formed by the horsts
Younger rocks lie beneath the grabens Sediment fills in the linear valleys
Horst GrabenHorst
Graben
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Nevada
“Washboard topography” is the result of Horsts and Grabens
A.k.a, Basin and Range E.g., Humbolt Range E.g., Death Valley
(Graben)
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Horst and Graben, Nevada
Humboldt Range, Northern Nevada Fig. 10-15b, p. 233
Graben
Horst
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Horst and Graben, Nevada
Humboldt Range, Northern Nevada
Graben
Horst
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Reverse and Thrust Faults
Compressive stress causes the hanging wall to move upward relative to the foot wall Reverse Fault
At convergent plate boundaries ancient rocks can be thrust over younger rocks Thrust Fault
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Structures at a Passive Continental Margin
Resulting from continental breakup E.g., The Americas and Africa
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Salt Domes: e.g., Texas
Rising of less dense salt
Stretches overlying crust
Forming normal faults and
Oil traps
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Structural Oil Traps
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Thrust Fault: Glacier NP, Montana
Old
Younger
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Structures at a Convergent Boundary
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Structures within Mountain Belts
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Compressional and TensionalStructures
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E.g., The Apls
Intense folding and thrusting of sedimentary rocks
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Strike Slip Faults
Physiographic Features
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San Andreas Fault
What type of fault is this? What other features are
associated with the fault?
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