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
S-C Fabrics
Mylonitic
foliation (S)
form along X-Y plane of strain ellipsoid
foliation(S) curves in & out of shear surfaces (C)
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 sense indicators
Sigma clasts - asymmetry
Shear band - deflection sense of mylonitic 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
Shear bands
Slip surfaces which obliquely transect & potentially backrotate foliation
Stretching lineation - angle btwn it & mylo foliation
Angle btwn lineation & mylonitic foliation
Allows inference shear sense/rotation
Represents orientation of SZ
Scale: up to entire plutons
Sigmoid shape on hill / headland adjacent in Calla D'agulle
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
Lattice Preferred orientation
Pre-deformation features i.e. cross beds
Lattice realigns in order to minimise stress
Analogue - ice crystals reorient to facilitate bassal slip