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Shear Sense Indicators (Sigma clast (Tail = asymmetric wrt mylonitic…
Shear Sense Indicators
Sigma
clast
Tail = asymmetric wrt mylonitic foliation
Stair-
stepping
geometry
Tails located either side of reference line - do not cross it
Asymmetry - result of rotational deformation
Indicates
TOP TO THE RIGHT
shear sense
Scale: up to entire plutons
Sigmoid shape on hill / headland adjacent in Calla D'agulle
S-C Fabrics
Mylonitic
foliation (S)
form along X-Y plane of strain ellipsoid
foliation(S) curves in & out of shear surfaces (C)
Represents orientation of SZ
Shear surfaces (C)
form parallel to shear zone walls as strain accumulates
early deformation - cut across foliation
continual deformation - shear bands & foliation near parallel
(indistinguishable?) --> becomes C'
Shear
surface C'
New, oblique shear bands (C') form
Backrotate CS foliation which is now termed 'S'
If strain is high enough
Shear bands
Slip surfaces which obliquely transect & potentially backrotate foliation
Shear sense indicators
Sigma clasts - asymmetry
Shear band - deflection sense of mylonitic foliation
Stretching lineation - angle btwn it & mylo foliation
Stretching lineations
Form in mineral aggregates that dynamically recrystallise during deformation
Alignment of quartz crystal c-axes
Long axes of deformed grains
oriented oblique to main foliation
Angle btwn lineation & mylonitic foliation
Allows inference shear sense/rotation
Marker & Foliation Deflection
Pre-existing markers/foliation become rotated in shear zones
∴ Rotation of marker from low to high strain area
= shear sense indicator
e.g.
Quarter Structures
Contraction/extension of structures around an object
Quarter sectors distinguished around object either:
2. Accretion
Layers thicken & fold as particle decelerates
after passing object
1.Thinning
Layers thin as particle accelerates
to bypass corner of object
Quarter folding
rotation of layering when passing top of porphyroclast
∴ folds in extensional direction
Mineral
Fish
Mineral porphyroclasts have tails curving systematically away from general orientation
Often found in shear bands
Asymmetry indicates sense of shear
Fold Asymmetry
Folding of foliation/existing feats
Foliation rotated through shear plane
Pre-deformation features i.e. cross beds
Lattice Preferred orientation
Lattice realigns in order to minimise stress
Analogue - ice crystals reorient to facilitate bassal slip