Normal FaultsExtensional Tectonics

Tectonic Settings for Extension

• Divergent plate motions

• Gravitational collapse– Over-thickened crust– Continental margins

• Salt Domes

Extensional Tectonics

Divergent BoundaryNormal Faults, rifts

Continental Extension

Divergent BoundariesContinental Plate Separation

East AfricanRift Valley

Somali Subplate

Somali SubplateAfrican Plate

African Plate

Parallel valleys; volcanoes and earthquakes.

Rifts

Bounded by normal faults

Back Arc extension

Extension of the lithosphere 1

Extension of the lithosphere 2

Lithospheric Extension

•Thinning of the lithosphere

•Normal faulting at the surface

•High heat flow (rinsing of the isotherms)

•Thermal subsidence - > sedimentary basins

North Sea Extensional Basin

Pure shear vs. simple shear systems

Gravitational Collapse of Mt. Belts

Basin and Range

Province

New Mountains-Active Fault –Basin and Range Province

Two modes of extension

Sandbox- Block rotation

Sand above plasticine

Block Rotation

Domino block rotation in the Basin and Range

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

Gulf of Mexico

Passive Margin

Gulf of Mexico

Salt

Extension Above Salt Diapir

Transfer zone in a rift

•Older faults rotate and become inactive•New faults cross-cut them•Result= large magnitude extension

Successive generations of normal faults