Forward modeling of non-steady-state deformations and the 'minimum strain path'

Structural analyses of shear zones often rely on an assumption of steady-state behavior, i.e. that the ratio of pure shear strain rate(s) to simple shear strain rate(s) remains fixed throughout deformation. However, geological deformations are not necessarily steady state. Non-steady-state deformation paths can be theoretically modeled if certain deformation parameters, such as strain or offset, are specified. We have analyzed a two-dimensional case of specified offset and geometry, termed the minimum strain path. The minimum finite strain needed to produce a fixed offset across a shear zone is neither simple shear nor pure shear, but a combination of the two (the minimum strain path). If this deformation accumulates with a steady-state deformation, the kinematic vorticity number (Wk) Of the minimum strain path varies with the amount of finite offset, although W-k approaches 0.7 at high offset values. Because of this relation between W-k and finite offset, the minimum strain path is better modeled as a non-steady-state deformation, in which case deformation history starts close to simple shear but rapidly changes to a more pure shear dominated deformation. It is expected that the minimum strain path is applicable to geological deformation zones; with relaxed boundary conditions, such as basal parts of spreading nappes or extensional detachment systems. (C) 1997 Elsevier Science Ltd.