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ABSTRACT

The transpressional (or transtensional) model of Sanderson and Marchini considered a horizontal displacement to be oblique to the walls of a vertical tabular shear zone that has no extrusion along the strike direction of the shear zone. In a more general model that includes lateral extrusion, the nature of deformation becomes much more complex. This chapter analyzes such a tabular shear zone in terms of simultaneous simple shearing and coaxial straining. In such a deformation, one of the principal axes of the strain ellipsoid is always vertical and parallel to the vorticity vector and the other two lie on the horizontal plane. I present a new approach for analyzing and classifying transpressional (and transtensional) tectonism. The classification enables us to compare the degree of similarity of the deformation types and to visualize their relation with the kinematic vorticity number. It has been shown that a transpression does not always produce a flattening. Similarly, a transtension does not always produce a constriction. The history of progressive deformation may be complex, involving switching of the direction of maximum stretching, development of a cleavage and its folding in the same continuous deformation, and folding of early boudin lines in later stages of deformation.

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