Shear (geology) - Ductile Shear Microstructures

Ductile Shear Microstructures

Very distinctive textures form as a consequence of ductile shear. An important group of microstructures observed in ductile shear zones are S-planes, C-planes and C' planes.

• S-planes or schistosité planes are generally defined by a planar fabric caused by the alignment of micas or platy minerals. Define the flattened long-axis of the strain ellipse.
• C-planes or cisaillement planes form parallel to the shear zone boundary. The angle between the C and S planes is always acute, and defines the shear sense. Generally, the lower the C-S angle the greater the strain.
• The C' planes, also known as shear bands and secondary shear fabrics, are commonly observed in strongly foliated mylonites especially phyllonites, and form at an angle of about 20 degrees to the S-plane.

The sense of shear shown by both S-C and S-C' structures matches that of the shear zone in which they are found.

Other microstructures which can give sense of shear include:

• sigmoidal veins
• mica fish
• rotated porphyroclasts
• asymmetric boudins (Figure 1)
• asymmetric folds