Strain and kinematic analysis in general shear zones

We present a unifying theory for the development and distribution of strain markers and kinematic indicators in zones of general shear, and thus provide a framework in which data previously considered contradictory may be understood. Rigid and deformable porphyroclast systems, including σ, δ and complex σ-δ grains are potential indicators of both strain and flow. The shapes and distribution of such porphyroclast systems may be used to distinguish among different tectonic regimes. General shear is divided into two fields: sub-simple shear, in which the rotational component of the strain is less than that for simple shear, and super-simple shear, in which the rotational component is greater than for simple shear. Sub-simple shear may involve narrowing or broadening of shear zones. Super-simple shear regimes are possible in local regions such as the vicinities of deformable porphyroclasts, but must be enclosed by regimes of sub-simple shear. The polar Mohr constructions for finite deformation and flow are useful to analyze general shear in theory. The hyperbolic net is employed for practical plotting of real data and derivation of the kinematic vorticity number, Wn. This number represents the relative contributions of pure and simple shear in steady flow. In nature, deformation is thought to build up and decay by processes that may invalidate the assumption of constant flow regime. We therefore introduce the concept of accelerating deformation and analyze the implications of non-steady flow for the shearing histories of deformed objects.