Shear zones and shear sense indicators Please read D

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Shear zones and shear sense indicatorsPlease
read (DR, 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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(No Transcript)
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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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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