THE DRAG OF MARKERS NEAR SHEAR ZONES AND THEIR KINEMATIC INTERPRETATION

The drag and shear of marker horizons like layering or foliation across discontinuities can lead to the development of several types of flanking structures (Passchier 2001, Grasemann & Stüwe, 2001). The development of such structures forming around slip surfaces (cross cutting elements CE) in a Newtonian viscous medium were investigated using a mechanical finite element model. The resulting structures can be classified on the basis of three criteria (Grasemann et al. in press): (1) the extensional or contractional offset of markers, (2) the co- or counter-shearing sense along CE and (3) the normal or reverse drag of markers relative to the shear sense along CE. As a function of the kinematic vorticity of the flow and the initial orientation of CE, with respect to the shear zone boundaries, three main types of structures, can develop: (1) shear bands developed along co-shearing extensional CEs; (2) a-type flanking folds developed along counter-shearing CEs; and (3) s-type flanking folds developed along co-shearing contractional CEs. All these structures can occur with either normal or reverse drag effects. Numerous examples of flanking structures have been observed in natural low- and high-grade metamorphic shear zones, brittle-ductile deformed rocks, fault gouges, rock salt, glacier ice or rocks affected by soft sediment deformation (Passchier, 2001), but their kinematic interpretation is not always fully understood: Because of their contractional offset, the shear sense of s-type flanking structures is always straightforward. However, shear bands and a-type flanking folds recording opposite shear senses can be geometrically very similar and consequently lead to kinematic misinterpretations. Especially in pure shear dominated shear zones both a-type flanking folds and shear bands developed as a conjugate set may record mirror symmetries. However, if the overall shear sense is known from other criteria, correctly identified flanking structures can provide quantitative information about the kinematic vorticity of the flow.

References: Grasemann, B., and Stüwe, K., 2001, Journal of Structural Geology, v. 23, p. 715-724. Grasemann, B., Stüwe, K. & Vannay, J.-C., in press. Journal of Structural Geology Passchier, C.W., 2001, Journal of Structural Geology, v. 23, p. 951-962.