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Shear Zones: Descriptors
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The mylonite, or mylonitic zone, corresponds to the central part of the shear zone where deformation is the most intense. In a mylonite, the grain size is significantly reduced so that mineral clasts appear to float in a fine-grained matrix of recrystallized grains.
The protolith corresponds to the undeformed rock.
Two reference frames are used to describe shear zones. The first reference frame is related to the strain. That frame consists of three orthogonal axes: X, Y, Z (or λ1, λ2, λ3). Together X and Y define the flattening plane (foliation plane). X is parallel to the direction of maximum elongation (stretching), therefore it is parallel to mineral and stretching lineation. Z is the pole of the foliation plane.
The second framework is related to the displacement field, it is called the kinematic frame. It consists of three orthogonal axes: a, b, c. Together a and b define the shear plane in which a corresponds to the shear direction. This plane is parallel to the mylonitic zone. The axes a and c define the movement plane. This plane is perpendicular to the shear plane and parallel to the shear direction.
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| The kinematic reference frame remains constant in orientation thorughout the shear zone. In contrast, the strain reference frame progressively rotates around the axis Y to parallel the kinematic frame in the mylonitic zone. The sense of rotation is a kinematic criteria. |
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