Metastable phase transition models and their bearing on the development of atlantic-type geosynclines

Abstract

The transformation of metastable basalt to stable garnet granulite or eclogite is assumed to be a possible mechanism involved in the development of sedimentary basins adjacent to passive continental margins. It might play a significant role for both the subsidence of those regions, as recorded by the sedimentary fill, as well as for the crustal structure with respect to the observed gradational crust-mantle boundary indicated by intermediate seismic velocities in those regions. In order to investigate the phase transition beneath a sedimentary layer, we combine a one-dimensional thermal model of the crust with different diffusion models for the reacting grains. The first model describes the movement of the boundary between the metastable and the stable phase as a function of the activation energy for diffusion, the thermal properties of the crust and the time function of sedimentation. The second model gives the degree of transformation of the grains in metastable state to the stable state, and shows the existence of an extended reaction zone. The results of the numerical calculations encourage one to draw some conclusions which support the above assumption. Thus, the reaction rate and the sedimentation rate appear to be nearly equal, but it seems to be likely that subsidence and sedimentation occur step by step. The beginning of the reaction requires a minimum thickness of sedimentary deposits: between 3 and 6 km for the given parameters. Finally, the derived reaction zone has an extension of several kilometers depending on the activation energy. This zone might give an explanation for the intermediate seismic velocities found at Atlantic-type continental margins.