High-pressure/low-temperature metamorphism and the dynamics of an accretionary wedge

Abstract

The preservation and exhumation of high-pressure rocks is an important observation in understanding the geodynamics of orogenic processes. A numerical tool is developed to estimate quantitatively the effect of the complex interplay between the mechanical and thermodynamical behaviour, and to assess under which conditions the preservation of metastable denser phases is possible. A finite difference numerical method is used to solve the continuity, Navier-Stokes and thermal equations for a Newtonian compressible fluid medium. In the model we take into account a typical forcing induced by a subduction process in a collisional environment according to a corner flow model. We follow the evolution of different phases in the crust including a pressure-temperature-dependent phase transition in the numerical code. Although eclogite is formed at depth when the phase diagram is only prescribed from thermodynamics, it cannot reach the surface. The kinetic effects of thermally activated diffusion and of the nucleation processes are taken into account in the modelling of the phase transition. Our simplified model does not explicity take into account the presence of water. It assumes that the rate of phase transformation can be computed from a knowledge of pressure, temperature and phase content. The parameters of the kinetic equations are empirically chosen to reproduce qualitatively the typical pressure-temperature-time paths recorded in the Alpine belt. To obtain significant concentrations of high-pressure phases at the surface, different activation energies for the prograde and retrograde reactions are needed. This difference may be related to changes in the water content of the crust between its burial and its exhumation.