Bookshelf sliding

Some porphyroclastic minerals, such as mica and feldspar, tend to shear on discrete fractures or crystallographic planes to accommodate ductile deformation in the surrounding matrix. The individual crystal fragments may be rotated in the direction of shear like a collapsing set of books, hence the term "bookshelf sliding". However, microfractures may be antithetic or synthetic to the general sense of shear, which makes then litigious sense-of-shear indicators. Antithetic and synthetic fractures largely depend on the initial orientation of the clast with respect to the bulk stress direction and are identified once the general shear is established from more reliable kinematic indicators.

Antithetic fractures: Slip on antithetic fractures is opposite to the bulk sense of shear. This is possible if the fractures or mineral cleavage initially make a high angle with the shear plane but are at an angle to the bulk principal stress that satisfies micro-faulting in a sense opposite to the general shear direction. Mineral fragments between these micro-fractures are thus rotating in consistency with the rotation in the surrounding matrix.

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Morphologic features of bookshelf structure after activation of antithetic microfractures. From Jean-Pierre Burg.

Synthetic fractures: If fractures or mineral cleavage initially make a relatively small angle with the shear plane, then the shear sense on the fractures is the same as it is in the matrix, implying some back-rotation of the fragments. Further shearing motion can lead to separation of the individual fragments and displaced crystals show displacements consistent with the bulk sense of shear.

Morphologic features of bookshelf structure after activation of synthetic microfractures. 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
Orthose with bookshelf structure in a granitic mylonite. Cap de Creus, Spain. XPL image, 2x (Field of view = 7mm)	Orthose with bookshelf structure in a granitic mylonite. Cap de Creus, Spain. XPL image, 2x (Field of view = 7mm)	Plagioclase with bookshelf structure in a Deformed Granitic Dyke. Elba Island, Italy. XPL image, 2x (Field of view = 7mm)
Orthose with bookshelf structure in a Ultramylonite From Sardina (Italy). PPL image, 2x (Field of view = 7mm)	Orthose with bookshelf structure in a Ultramylonite From Sardina (Italy). PPL image, 2x (Field of view = 7mm)	Orthose with bookshelf structure in a Ultramylonite From Sardina (Italy). XPL image, 2x (Field of view = 7mm)
Pyroxene with bookshelf structure in a serpentinite from Sebino, Bergamo, Italy. PPL image, 1x (Field of view = 9mm)	Pyroxene with bookshelf structure in a serpentinite from Sebino, Bergamo, Italy. XPL image, 1x (Field of view = 9mm)	Pyroxene with bookshelf structure in a serpentinite from Sebino, Bergamo, Italy. PPL image, 2x (Field of view = 7mm)
Pyroxene with bookshelf structure in a serpentinite from Sebino, Bergamo, Italy. XPL image, 2x (Field of view = 7mm)	Pyroxene with bookshelf structure in a serpentinite from Sebino, Bergamo, Italy. PPL image, 2x (Field of view = 7mm)	Pyroxene with bookshelf structure in a serpentinite from Sebino, Bergamo, Italy. XPL image, 2x (Field of view = 7mm)
Pyroxene with bookshelf structure in a serpentinite from Sebino, Bergamo, Italy. PPL image, 2x (Field of view = 7mm)	Pyroxene with bookshelf structure in a serpentinite from Sebino, Bergamo, Italy. XPL image, 2x (Field of view = 7mm)	Pyroxene with bookshelf structure in a serpentinite from Sebino, Bergamo, Italy. PPL image, 2x (Field of view = 7mm)
Piroxene with bookshelf sliding. Mafic mylonite. PPL image, 2x (Field of view = 7mm)	Piroxene with bookshelf sliding. Mafic mylonite. PPL image, 2x (Field of view = 7mm)	Piroxene with bookshelf sliding. Mafic mylonite. XPL image, 2x (Field of view = 7mm)
Piroxene with bookshelf sliding. Mafic mylonite. PPL image, 2x (Field of view = 7mm)	Piroxene with bookshelf sliding. Mafic mylonite. PPL image, 2x (Field of view = 7mm)	Piroxene with bookshelf sliding. Mafic mylonite. XPL image, 2x (Field of view = 7mm)
Piroxene with bookshelf sliding. Mafic mylonite. XPL image, 2x (Field of view = 7mm)	Piroxene with bookshelf sliding. Mafic mylonite. XPL image, 2x (Field of view = 7mm)	Orthose with bookshelf structure in a Deformed Granitic Dyke. Elba Island, Italy. XPL image, 10x (Field of view = 2mm)