Enigmatic C’ shear bands: their evolution and demise

Sep 3, 2020

C' shear bands and S-C fabric in naturally deformed rocks in thin section (left) and outcrop (right). Images are from the Zanskar Shear Zone, NW Himalaya (St = staurolite, Bt = biotite, Qtz = quartz).
Schematic diagram of S-C fabric and C' shear bands in ductile shear zones under dextral simple shear in the XZ-plane of the finite strain ellipsoid

C' shear bands are a common feature of ductile shear zones and are widely used as kinematic indicators. They are also enigmatic: research on naturally- and experimentally- deformed rocks has suggested several theories as to why they form, but there is little consensus. C' shear bands dip at 15-35° from the shear zone boundary in the direction of shear and usually occur in concert with C planes, which are parallel to the shear zone boundary, and S planes, which dip in the opposite direction to C' shear bands (see figures above). Previous research has suggested that C' shear bands form when rocks contain a weak phase, that is, a mineral that is weaker than the other minerals that produces anisotropy in the rock. However, it is unclear why this causes C' shear bands to develop. It has also been difficult to pinpoint how much of that weak mineral is required to cause C' shear bands to develop, and exactly how weak the weak mineral needs to be.

This is the sort of question that is perfect for numerical modelling, where the proportion of the weak mineral and the strength contrast between minerals is easy to manipulate. We used the numerical modelling software Elle and simulated a rock with three minerals that approximated the properties of quartz (strong), feldspar (intermediate strength) and mica (weak).

Our models begin with an undeformed rock and the microstructure of the minerals develops slowly as slowly under simple shear. We record data that allows us to make movies of the grains deforming as well as the strain rate and stress distribution (see movies above). These movies provide a wealth of information about shear zone development and allow observation of the incremental development of all the structures typically seen in natural shear zones – asymmetric isoclinal folds, anastomosing shear planes, S-C fabric and, of course, C' shear bands. We found that C' shear bands actually begin as C planes but when there are changes in strain rate or stress and it becomes hard to shear on C planes, they rotate forwards, forming C' shear bands (see figure below).

The formation of C' shear bands by the rotation of a C plane forwards due to high strain rate in the shear band and high stress at the tip of the shear band. (a) Discontinuous shear band with section parallel to the SZB at high strain rate (red arrow) and high stress in the region at the end of the shear band (orange arrow). (b) A low strain rate section in the shear band is bracketed on either side by high strain rate sections (red arrows) and begins to rotate forwards. (c) C' shear band forms in low strain rate section (red dashed line). (d) Strain rate reduces in the shear band and the C' shear band has rotated back into parallelism with the SZB and C planes. The first column shows the grain microstructure, the second column shows the normalised von Mises strain rate and the third column shows the von Mises stress. Model shown is 15WP_High and images in the same row correspond to the same model and step. In the grain microstructure column, black grains are the weak phase, white are the intermediate strength phase and grey are the strong phase.

We found that C' shear bands form in models with as little as 5% weak phase when there is a moderate or high strength contrast between minerals, and when there is at least 15% weak phase they can form at low strength contrasts. C' shear bands are relatively short-lived phenomena in our models: once they are inactive they either rotate into the shortening field and collapse or else they rotate back into parallelism with the shear zone boundary. Our paper was published in the Journal of Structural Geology.

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