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Modifcation of Rocks by Folding and Fracturing
DEFORMATION Modifcation of Rocks by Folding and Fracturing Chapter 7
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Folded
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Tilted
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Faulted
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Mangled!
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Strike and Dip N W Dip direction Strike direction N W Dip angle E S 45° E S 45° Dip angle A special compass is used by geologists for the purpose of recording the orientation and dip of planar rock structures like folds, beds, faults, and metamorphic laminations. These measurements are reported on 2-D geological maps to help geologists see how the structure is are oriented relative to the north pole, and the surface of the earth.
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slope parallel to dip. The strike is oriented 90 degrees to the dip.
W Dip direction Strike direction N W Dip angle E S 45° E S 45° Dip angle Water trickles down slope parallel to dip. The strike is oriented 90 degrees to the dip.
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Rocks strain when subjected to stress
Rocks strain when subjected to stress. The strain is released through folding (plastic deformation) or faulting (brittle deformation). An undeformed sample
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Under conditions representative of the shallow crust, the marble is brittle.
the deeper crust, marble is ductile. An undeformed sample
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Normal fault TENSION Fault plane Extension
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Index Fossils and Biostratigraphy
The shorter the time-stratigraphic range of the fossil, the more precise are the correlations that can be made Fossils representing free swimming or drifting organisms can be found world wide and deposited in a wide variety of rock type (different depositional environments). These are the most useful for global correlation of the stratigraphic records. They are called Index Fossils.
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Reverse fault COMPRESSION Compression
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Thrust fault COMPRESSION Compression
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Left-lateral strike-slip fault
SHEAR
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Right-lateral strike-slip fault
SHEAR
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Oblique-slip faulting is caused by a combination of forces.
SHEAR TENSION
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Tectonic Forces Determine the Style of Faulting
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Youngest rock Anticline Syncline Oldest rock
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Horizontal fold Fold axis Axial plane Limb Limb
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Plunging fold Horizontal Axial plane 45° Limb Limb
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Symmetrical folds Axial plane Anticline Syncline
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Asymmetrical folds
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Limb Limb Limb Axial plane
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Tensional tectonics Ductile lower crust
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Sinai Saudi Arabia Egypt RED SEA Rift valley Continental crust Downfaulted blocks Oceanic crust Asthenosphere
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The African Plate and the Arabian
Plate are drifting apart. Sinai Saudi Arabia Egypt RED SEA Rift valley Continental crust Downfaulted blocks Oceanic crust Asthenosphere
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The African Plate and the Arabian
Plate are drifting apart. Sinai Saudi Arabia Egypt RED SEA Rift valley Continental crust Downfaulted blocks Oceanic crust Asthenosphere Tensional forces have created a rift valley.
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Compressive tectonics
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Shearing tectonics A left bend in the fault
results in local compression. A right bend in the fault results in local extension.
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Lavas offset Offset of 315 km from volcano Volcano Stream Offset
San Francisco San Andreas fault Lavas offset from volcano Offset of 315 km Volcano Los Angeles Northwest Stream Southeast Offset of 130 m Stream Pacific Plate North American Plate
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Unravelling Geological History C D B A
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C D B A Compressive forces create a fault. 1
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C D B A Compressive forces create a fault. 1 D C D C B B D C A B 50 km
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overlie younger layers. 2
C D B A Compressive forces create a fault. 1 Old layers now overlie younger layers. 2 D C D C B B D C A B 50 km
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overlie younger layers. 2
C D B A Compressive forces create a fault. 1 Old layers now overlie younger layers. 2 D C D C B B D C A B 50 km D B C B C D B A
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overlie younger layers. 2
C D B A Compressive forces create a fault. 1 Old layers now overlie younger layers. 2 D C D C B B D C A B Erosion reveals the view we see today. 3 50 km D B C B C D B A
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Keystone thrust fault, southern Nevada
B D
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TIME 1 Sediments are deposited on the seafloor.
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TIME 2 Compressive forces cause folding and faulting. Compressive forces Faults
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TIME 3 Uplift is followed by erosion.
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TIME 4 Volcanic eruptions flood the new surface with lava sheets. Lava flows
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TIME 5 Tensional forces cause normal faults, creating downfaulted blocks. Tensional forces Normal faults
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