Transpression vs. simple shear superimposed on inherited pure shear – methodology for a more refined kinematic analysis

English Abstract:

Determination of the extent of shortening that accompanies simple shearing in transpressional shear zones is of great importance to regional deformation studies. A kinematic method is developed here to discriminate between combined pure and simple shear deformation (i.e. general shear - transpression/transtension), and cases where the simple and pure shear occurred at distinct times in a specific lithology within the shear zone. The following assumptions should be valid for the selected lithology: (i) the non-coaxial shearing is the last deformation event; (ii) the strain field prior to this last shearing event was homogeneous; (iii) no strain recovery has occurred during or after the last deformation event; and (iv) the last shearing event operated at constant strain rate. The method involves determination of the states of finite strain at different locations within the selected sheared lithology as input parameters. The finite incremental strain is then calculated from the finite strain using any standard methodology, e.g. Horsman & Tikoff (2006). Thereafter, assuming constant strain rate, more precise estimates of the infinitesimal incremental strain can be obtained by taking roots of the finite incremental strain. Restriction of finite strain determination to a specific lithology eliminates errors arising from the presence of pre-shearing finite strain inhomogeneities in lithologies of varying competencies. Thus, this method can be used to distinguish whether the last shearing event within a lithology of a particular shear zone was one of general shear, or a case of simple shearing superimposed on earlier, inherited pure shear deformation. This method has been applied to a quartzite unit in a major, crustal-scale shear zone in the eastern Indian shield, to demonstrate that shortening features in the unit are temporally unrelated to the later, dominantly simple shear strike-slip event.