Określanie zwrotu ścinania

Zbigniew Cymerman



The aim of this paper is to describe criteria for the determination of the sense of shear. The determination of movement in shear zones is fundamental to the understanding of orogenes. U sing the criteria outlined herein, the correct sense of shear can be elucidated.
The most useful criteria in determining shear sense are: deflections, intrafolial folds, "porphyroclasts", "domino" type structures, extensional structures, SC structures, fabric asymmetries, "en echelon" fractures and tectonic inclusions. The sense of shear can often be determined from a reorientation (deflection) of pre-existing planar markers (foliations, veins) (Fig. 2). In some cases, however, traces of the initially planar markers on a map can shows an opposite the over-all sense of shear (Fig. 3). The shear sense can also be inferred by the direction of intrafolial fold vergence (Figs. 4, 5). Tails around "porphyroclasts" are in important feature that allows the determination of the sense of shear (Figs. 6-10). Due to a geometrical similarity of some σ - (augen structures) and δ - (pressure shadows) type "porphyroclasts" it is therefore very important for the deduction of shear sense to distinguish σ-from δ -types. (Figs. 6, 7). "Domino" type structures, •defined by displaced fragments of grains, may be used to determine the over-all shear sense (Figs. 11, 12). Extensional structures (asymmetrical pull-apart structures) are potentially useful kinematic indicators in the determination of shear sense (Figs. 13, 14). The relationship between S- and C-planes is the one which is commonly observed in shear zones; the sense of shearing being as indicated in Figs. 15-17. Crystallographic fabrics developing along a rotational strain path are markedly asymmetric in respect to S foliation and Le extensional lineation (Figs.18- 21). These fabrics criteria, however, cannot be used for all minerals and occasionally gives anomalous results. In highly sheared rocks, deduction of the sense of shear are often obliterated or become ambigous. In general, however, it may be necessary to examine several samples before the sense of shear can be determined with confidence.

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