Friday Rocks #36: Conjugate Deformation Bands

[caption id="attachment_564" align="aligncenter" width="610"]Conjugate Deformation Bands Conjugate Deformation Bands[/caption]

Deformation bands are mm wide shear offsets that typically occur in  porous rock such as sandstone at shallow depths (Aydin 1978). Offsets on deformation bands are typically small and may be distributed across the entire length of the structure (cm to kms). Brittle deformation via breaking and crushing of grains creates deformation bands which are filled with a gouge or cataclasite. Deformation bands have important implications for reservoir permeability and will typically host a cement.

Fractures that are at angles less than 90 degrees to each other are called Conjugate fractures. The orientation of fractures and offsets may be used to determine the stress state in the rocks during deformation (Anderson 1942, Jaeger and Cook 1969, Olsson et al. 2004). Assuming the above photo is a 2-D example, how do you think Sigma_1 and Sigma_3 are oriented?
Anderson, E.M., 1942. The Dynamics of Faulting and Dyke Formation with Application to Britain, Oliver and Boyd, Edinburgh, London.

Aydin, Atilla. "Small faults formed as deformation bands in sandstone." Pure and Applied Geophysics 116.4-5 (1978): 913-930.

Jaeger, J.C., Cook, N.G.W., 1969. Fundamentals of Rock Mechanics, 2nd
ed, Chapman and Hall, London

Olsson, William A., John C. Lorenz, and Scott P. Cooper. "A mechanical model for multiply-oriented conjugate deformation bands." Journal of Structural Geology 26.2 (2004): 325-338.

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