Dynamic pressure variations in the lower crust caused by localized fluid-induced weakening

Abstract

Metamorphism of dry, lower crust within orogens is often localized in shear zones composed of mechanically weaker rocks. Several field-based studies suggest shear zone development is preceded by brittle faults which provide the fluid pathways required for metamorphism and weakening. However, a unifying model which couples long-term geological deformation to fluid migration and metamorphic reactions does not exist. Here, we present a visco-elasto-plastic model where the most pertinent features observed in transformed lower crust emerge from basic mechanical principles during the deformation of a coherent rock volume with associated fluid introduction. These include a strikingly dynamic and heterogeneous pressure distribution in the reacting and deforming rock volumes. Lower crustal pressure variations may reach 1 GPa at the same depth. This will have first order effects on the pattern of fluid migration in the lower crust, and may explain the apparent discrepancies between the relevant tectonic settings and petrologically-inferred burial depths.