METAMORPHIC PETROLOGY (SHEAR SENSE INDICATORS) Flashcards
Rock structures or textural elements that provide information concerning the relative sense of displacement of rock components within fault zones or shear zones
Shear Sense Indicators
SSIs that indicated the sense of displacement in shear zones and may also provide info on displacement distance
Offset markers
Formed when linear features were cut by planar faults or shear zones
Piercing Points
asymmetrical porphyroclasts or porphyroblasts with mineral tails that point in the direction of shear
Grain Tail Complexes
Tail complexes form commonly about these minerals
Qtz and Felds
Processes that may form Grain tail Complexes
1 Plastic Flattening of qtz and felds
2 Pressire solution from the center
3 Dynamic recrystallization at the rim of the grain
4 Neocrystallization in pressure shadows around the grain
wedge-shaped shaped tails
do not cross the refrence plane of shear.
pressure solution and Slow grain rotation,
Sigma Grain Tail Complexes (Shear)
rapid grain rotation relative to tail growth rate
bending of earlier formed part of the tails/Wrapping around
crosses the reference plane
Delta Grain Tail Complexes (Ductile)
Factures which can develop preferred orientaions that indicate sense of shear
Fracture patterns
?oriented to the directions of shear and are
?inclined at low angles <45 deg to foliations and
?shows displacement consistent with the overall shear sense
Sythentic Fractures
?oreinted in a direction opossite to the sense of shear and are ?incline at high angles >45 deg to the foliation and
?display an opposite sense of movement to over all sense of shear
Antithetic Fractures
Develops in mylonitic, schistose and genissic rocks subject to ductile shear especially in dextral shear zone
S-C foliations
S in SC represents
Schistocity or Foliation Diagonal zones
C in SC represents
Cisaillment, french for Shear direction
Horizontal parallel zones
