Mechanisms of strain-rate- and reaction-dependent permeability in the mid-crust of the Southern Alps: insight from three-dimensional mechanical models

We used three-dimensional mechanical modelling to explore the interplay of rheology, modes of permeability creation and fluid flow in the mid-crust of an oblique orogen. We used the central Southern Alps and the Otago Schist of New Zealand to constrain our models. We also compared our models with the magnetotelluric survey along the Rangitata–Whataroa rivers that imaged a U-shaped zone of high conductivity, interpreted as interconnected fluids, beneath the central Southern Alps. Modelling was carried out using the numerical code FLAC^3D. We used a number of simple assumptions: an initially homogeneous starting material, deformation boundary conditions based on the tectonics of the South Island, the capability of the modelled material to develop an anisotropic permeability structure, strain rate and reaction-induced permeability increases, initial saturation and lithostatic pore pressures as a basis for our models. The initial isotropic permeability was 10⁻¹⁸ m². We modelled two possible mechanisms of permeability increase: (i) strain-rate dependent and (ii) reaction dependent. For a strong mid-crust, the models showed enhanced permeability and hence fluid interconnectivity in a symmetric region beneath the model main divide, both ends were turned up towards the brittle–ductile transition and fluid flow was the greatest in the across strike direction. For a weak mid-crust, the region of enhanced permeability was asymmetric and turned up towards the brittle–ductile regime close to the Alpine Fault. The strong mid-crust model reproduces the features that are common to all interpretations of the MT soundings and is our preferred model for the central Southern Alps.