GRAIN-SCALE PRESSURE VARIATIONS: FEASIBILITY, THEORY AND APPLICATIONS

Geothermobarometry and phase equilibria modeling provide important insight about the pressure deep in the Earth's interior. Once the thermodynamic pressure is known, estimates of the depth of metamorphism are possible if certain assumptions are made concerning the state of stress. In the limit of lithostatic state of stress, pressure in minerals depends only on the weight of the overlying rocks. However, geophysical and petrological data provide us with important constraints on the state of stress of the crust and the lithosphere. The support of mountain ranges, the magnitude of stress drop during earthquakes, and the general force balance at the lithospheric scale suggest that the state of stress in the crust is not lithostatic. In addition, the presence of weak or strong heterogeneities, i.e. regions of lower or higher differential stress, in the crust supports the fact that pressure is neither homogeneously distributed nor lithostatic. The presence of stress variations leads to variations in pressure.

The existence of non-lithostatic state of stress in the crust brings some new challenges concerning the interpretation of metamorphic pressure. Different approaches have been suggested over the recent years in an attempt to conclude which stress should be taken as thermodynamic pressure. The results from rock deformation experiments do not support most of the proposed theories. In this work, we present some important constraints that are provided by thermodynamic equilibrium under pressure variations. It is shown that pressure variations commonly interpreted to occur over Ma of geologic history may also be interpreted to occur in space. However, distinguishing between the different interpretations is not that straightforward and textural criteria are needed. In this contribution, we present different metamorphic textures of apparent disequilibrium that can be interpreted as textures developed during pressure variations.