Deformation partitioning, shear zone development and the role of undeformable objects

Abstract

The formation of a mylonite zone by homogeneous progressive simple shear is highly unlikely because heterogeneous strain of grains with different compositions causes deformation partitioning and results in progressive inhomogeneous simple shear or non-coaxial progressive bulk inhomogeneous shortening. However the presence of rigid bodies in a zone of rock undergoing bulk simple shear spreads and homogenizes the stress field such that the instantaneous displacement field is essentially homogeneous at a scale up to at least four times the diameter of the rigid object. That is, the portion of rock surrounding the rigid body deforms by homogeneous progressive simple shear and the rigid object is forced to rotate. This has considerable implications for the rotation or lack thereof of unstrained porphyroclasts and porphyroblasts, and hence the information such grains preserve about the sense of shear on their margins or through inclusion trails. Geometric relationships between porphyroclasts or porphyroblasts and the surrounding matrix that are superficially similar can indicate exactly the opposite sense of shear depending on whether the porphyroclasts or porphyroblasts have or have not rotated leading to considerable problems in interpretation of the bulk movement in a shear zone. Resolution of these relationships enables ready distinction of whether the deformation history involved progressive bulk inhomogeneous shortening or progressive simple shear. In particular these results have considerable implications for the origin of blueschist minerals preserved in garnet porphyroblasts via the roles of strain softening and hardening during mylonitization due to the destruction of feldspar porphyroclasts and growth of garnet porphyroblasts respectively.