Large Scale Convective Flows with a Brittle-Ductile Transition

Abstract

The aim of this paper is to investigate the effects of introducing a brittle-ductile transition zone (BDTZ) into models of the large scale hydrology of systems like the Taupo Volcanic Zone (TVZ) in New Zealand. The coupling between fluid/heat flow and rock mechanics is based on a simplified Mohr-Coulomb failure criterion. This is a partial coupling in that the local strain rate and lithostatic pressure are assumed to be known a priori. A simple empirical relationship between the permeabilities of rock in its brittle and ductile states is also assumed, which mimics the effect of slow thermal creep in closing fluid pathways by reducing the effective permeability. The TVZ provides a setting for this model. Localised sources of water and heat, representing sill-like magmatic bodies at a depth of 10 km, induce a BDTZ which shallows from 14 km to less than 10 km, consistent with geophysical estimates in the TVZ region. A common feature of the TVZ models is that strong downflows of surface water occur in permeable regions adjacent to the heat sources, which depress the BDTZ by several kilometres between the geothermal areas. Water with near-surface temperatures can thus exist at great depths in these regions.