1
Normal FaultsExtensional Tectonics
Geol341
2016Many diagrams are from Earth Structure, van der Pluijm and Marshak, 2004
OutlineCh. 17
• Extensional Environments• Shape of normal faults
– Planar– Listric– Roll-over anticlines
• Block rotation• Growth faults• Detachment surfaces• Movie• Analog models
Tectonic Settings for Extension
• Divergent plate motions
• Gravitational collapse– Over-thickened crust
– Continental margins
• Salt Domes
Extensional TectonicsContinental Rift
Oceanic RiftNormal Faults Continental
Extension
2
Rifts
Bounded by normal faults
Extension in Convergent MarginsBack Arcs
Van der Pluijm and Marshak (2004)
Extension of the lithosphere 1
Van der Pluijm and Marshak (2004)
1300oC
Extension of the lithosphere 2
Van der Pluijm and Marshak (2004)
1300oC
Consequences of Lithospheric Extension
•Thinning of the lithosphere
•Normal faulting at the surface
•High heat flow (rinsing of the isotherms)
•Thermal subsidence during cooling after end of extension- > sedimentary basins
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North Sea Basin
Depth to basement
contour map
Norway
Scotland
Extensional Basin Evolution
Present Day North Sea Basin
Fossen, 2010
Structure of the base Cretaceous, east Viking graben, North Sea. (Fossen, 2010)
Norway
3D view Gulfaks Field, North Sea
Map viewGulfaks North Se
Pure shear vs. simple shear systems
Van der Pluijm and Marshak (2004)
Fossen, 2010
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Gravitational Collapse of Mt. Belts
Van der Pluijm and Marshak (2004)
Weak
Basin and Range
Province
New Mountains-Active Fault –Basin and Range Province
Two modes of extension
Van der Pluijm and Marshak (2004)
Deep seismic reflection data from extensional system(Basin and Range)
5
Parsons, 2010
Geometry of Extensional Belts
Sandbox- Block rotation
Sand above plasticine
Block Rotation
Van der Pluijm and Marshak (2004)
Domino block rotation in the Basin and Range
•Older faults rotate and become inactive•New faults cross-cut them•Result= large magnitude extension
Successive generations of normal faults
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Listric Normal Faults Listric (curved) Faults- roll over anticlines
Small roll-overs
Sedimentation during faultingGrowth faulting
Animation by R. Allmendinger http://www.geo.cornell.edu/geology/faculty/RWA/movies/
Curved faults – Block rotationGrowth Strata
3 km
0
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NW SE
Hammerfest Basin, Norway
50 km
CI = 300 m
Gulf of Mexico
Passive Continental Margin
Gulf of Mexico
Salt
Extension Above Salt Diapir
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Transfer zone in a riftMetamorphic Core Complex- Large magnitude extension
Take home points
• Normal faults form in divergent plate boundaries, areas of gravitational collapse and extension above salt domes
• Also, back-arc rifts form convergent plate boundaries• Thinning and cooling of the lithosphere after continental
rifting creates basins• The basin and range is an example of gravitational collapse
of a contractional orogen.• Series of normal faults rotate like books on a shelf• Curved normal faults (listric) produce roll-over anticlines• Sedimentary deposits are thick and tilted like a fan on the
hanging wall of listric normal faults• Continental passive margins are full of down to the basin
normal faults and often salt layers
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