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

The preservation and exhumation of high-pressure rocks is an important obse rvation in understanding the geodynamics of orogenic processes. A numerical tool is developed to estimate quantitatively the effect of the complex int erplay between the mechanical and thermodynamical behaviour, and to assess under which conditions the preservation of metastable denser phases is poss ible. A finite difference numerical method is used to solve the continuity, Navier-Stokes and thermal equations for a Newtonian compressible fluid med ium. In the model we take into account a typical forcing induced by a subdu ction process in a collisional environment according to a corner flow model . We follow the evolution of different phases in the crust including a pres sure-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 therma lly activated diffusion and of the nucleation processes are taken into acco unt in the modelling of the phase transition. Our simplified model does not explicitly take into account the presence of water. It assumes that the ra te of phase transformation can be computed from a knowledge of pressure, te mperature and phase content. The parameters of the kinetic equations are em pirically chosen to reproduce qualitatively the typical pressure-temperatur e-time paths recorded in the Alpine belt. To obtain significant concentrati ons of high-pressure phases at the surface, different activation energies f or the prograde and retrograde reactions are needed. This difference may be related to changes in the water content of the crust between its burial an d its exhumation.