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Shear zones and shear sense indicators Please read (D&R, pp. 493-551)
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Shear zone: zone of highly strained rocks A fault zone is a shear zone formed in the brittle regime Can also have a purely ductile shear zone Or even a zone with a mixture of brittle and ductile deformation- due to composition (feldspar or qtz) or strain rate (silly putty analogy)
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Shear zones form in the deeper crust of all structural systems: thrust, strike-slip and normal But how are they brought to the surface (exhumed)?
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Thrusting must be accompanied by erosion to cause exhumation
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"Tectonic Exhumation" Removal of overburden by normal faulting. Erosion plays a role, but is not required!
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The architecture of many extensional shear zones brittle detachment fault
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A ledge of fault breccia below the detachment
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thick zone of mylonitic rocks below the detachment
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Ahh, sweet mylonites! What is sense of shear?
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Even deeper- a less deformed "injection complex"
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Undeformed granite in core Mylonite
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Shear sense indicators in shear zones
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brittle fault offsets- be careful!!!! 1) faults postdate ductile deformation 2) sense of shear not always clear
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ductile offsets within shear zone- better!
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another example of a small ductile shear zone
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Folds, transposition, and ambiguous sense of shear
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sheath folds (tongues that point in direction of shear)
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Cross section of a sheath fold
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Mylonites are EXCELLENT! What is sense of shear? Porphyroclasts
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Sigma structure- wings step up in direction of shear
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Delta structure: porphyroclast rotates faster than wings
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A delta + sigma structure- what is sense of shear?
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Mica-fish fabrics. Typical for sheared rocks with muscovite and/or biotite. A special form of S-C fabric. Again wings/tails step up in direction of shear
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Summary of shear fabrics
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So far, we have talked about shear fabrics related to noncoaxial deformation (simple shear). Some "shear" zones are not due to simple shear, but rather coaxial deformation (pure shear) What do some of these structures look like?
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symmetric boudins due to flattening/stretching
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Symmetric pressure shadows due to flattening (pure shear) strain
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Next Lecture: Metamorphism, deformation and time
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