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KINK-BANDS AND SYMMETRY

The conjugate kink-bands may have a symmetry of low or high order. If two of the principal stresses (or the principal strains) lie within the pre-deformed layering and the third normal to it, the symmetry of conjugate kink-bands is orthorhombic or of high order. If only one of the principal stress directions is contained within the pre-deformed layers and the other two are at angles other than 90, then the symmetry of resulting structures is monoclinic. If none of the principal stresses are symmetrically disposed relative to the layers before deformation, then the symmetry of the resulting conjugate kink-bands or single kink bands is said to be triclinic. We may conclude:

Case 1: s1 and s2 within layering= orthorhombic

Case 2: s1 within the layering, s2 and s3 not contained within the layers: Monoclinic

Case III; All three principal stresses neither parallel nor normal to layering: Triclinic

In the above three cases, s3 denotes the lease principal compressive stress, s2 denotes the intermediate principal compressive stress and s1 denotes the maximum principal compressive stress but in the figure shown the opposite or engineering convention is used. The symmetry can be known by direct observation or by plotting on an equal area net the orientations of axial planes of conjugate folds and the orientation of un-deflected layering. Depending on the variation in orientation of the pre-kink schistosity, only one set may be developed in an area with complete exclusion of the other set. Complementary set may appear in another part of the same region where orientation of the layering is favourable to the general applied stress. Roberts (1971) showed this for the Flornes and Meraker area in a part of the Norwejian Caledonides. There is no evidence to suggest that chevron folds are produced from the original conjugate kink-bands but chevron folds may be found associated with conjugate kink-bands in the same sequence of strata because of anisotropy differences between parts of the same complex.