Analysis of transpressional deformation from geometrical evolution of mesoscopic structures from Phulad shear zone, Rajasthan, India

Abstract

Transpressional deformation in ductile shear zones may be analyzed by simultaneous wall-parallel simple shear (with strain rate γ^•=γ_xy^•) along with three-dimensional coaxial deformation (with strain rates ε_x•, ε_y^• and ε_z^•). Several types of transpression may be distinguished on the basis of relative values of strain-rate ratios ε_x^•/γ^• (=a), ε_y^•/γ^• (=b) and ε_z^•/^•• (=c). It is possible to identify the nature of transpressional deformation from a detailed analysis of shear zone structures. In the Phulad shear zone of Rajasthan, India, evidence of general flattening, a thrusting sense of shear, occurrence of transport-parallel stretching lineation, frequent occurrence of sheath folds with apical direction parallel to the stretching lineation and occurrence of U-shaped lineation patterns indicate that the deformation was transpressional, with b/a ratio ranging between -1 and -2. The bulk deformation was not dominated by simple shear, but involved both simple and pure shear, with extrusion along the transport direction much greater than along the vorticity vector. Rotation of long tectonic clasts caused reorientation of monoclinic rolling structures, so that strongly monoclinic structures, with opposite senses of asymmetry in different domains, appear on subhorizontal outcrops parallel to the vorticity vector, although the component of simple shear is zero on these surfaces.