S-C fabric

S-C fabric is a metamorphic fabric formed by the intersection of shear surfaces within rocks affected by dynamic metamorphism. The foliation that develops in a shear zone is usually thought to trace the XY-plane of the strain ellipsoid. The sense of rotation of the foliation from the margin into the shear zone is generally a safe kinematic indicator. As strain accumulates, a set of slip surfaces or shear bands commonly forms parallel to the walls of the shear zone. These shear bands are called C (French cisaillement for shear, which relates to the movement of scissors) and the foliation is named S (for schistosity or schistosité), Fig.1.

S-C Fabric. From Jean-Pierre Burg.

C-surfaces are not really surfaces, but small-scale shear zones that affect the foliation within the main shear zone. In detail, the foliation curves into and out of the C-surfaces, and the sense of deflection shown by the curving foliation reflects the sense of shear of the entire shear zone. These discrete shear zones are visibly less penetrative than foliation planes S. The composite C-S fabric usually weakens outward (larger spacing between C-planes towards the shear zone boundaries) and ultimately disappears outside the shear zone, which suggests its direct affiliation to shearing. The C planes maintain a constant orientation, but the S planes rotate with increasing shear strain, so that the angle between the two sets reduces to close parallelism in highly sheared mylonites. The angle between S and C can vary but is typically about 25-45°.

C' and C'' surface: As high shear strain is approached, the C and S surfaces become sub-parallel, so that their separate identity is lost. In strongly foliated rocks, one or more sets of secondary, spaced planar elements may appear systematically oblique to both the early foliation and the shear plane and shear zone boundary (Fig.2). These micro shear zones are usually less continuous than C planes and delineate an extensional crenulation cleavage termed C' planes. They are essentially small-scale shear zones oblique to a pre-existing foliation, such that the displacement on this new spaced foliation results in net extension parallel to the earlier planar anisotropy. They may occur in a single set or in multiple sets (C'') at low angles to each other. The sense of shear is such that C’ surfaces tend to "crenulate" the older planar fabrics.

sc_fabric2020(1).png

C' extensional crenulation cleavage. From Jean-Pierre Burg.

Bibliography

• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.

Photo
S-C fabric in a mylonite from Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	S-C fabric in a mylonite from Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	S-C fabric in a mylonite from Sardinia (Italy). PPL image, 10x (Field of view = 2mm)
S-C fabric in a mylonite from Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	S-C fabric in a mylonite from Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	S-C fabric in a mylonite from Sardinia (Italy). XPL image, 10x (Field of view = 2mm)
C'shear bands (the high angle surfaces) in a mylonite. PPL image, 10x (Field of view = 2mm)	C'shear bands (the high angle surfaces) in a mylonite. PPL image, 10x (Field of view = 2mm)	C'shear bands (the high angle surfaces) in a mylonite. PPL image, 10x (Field of view = 2mm)