Search

A new series of two- and three-dimensional deformation regime maps facilitating the simultaneous display of petrological, microstructural and rock-mechanical information illustrates the pressure, temperature and grainsize dependencies of brittle, crystal-plastic and granular creep regimes in the lithosphere. The predictions made using the extrapolated laboratory data are in qualitative agreement with inferences drawn from microstructural studies in naturally deformed monomineralic rocks. The deformation regime maps are used to show that the rheological evolution of a fault zone is related both to its location within a compositionally heterogeneous lithospheric section and to the pressure-temperature-time history of that section. Strain localization into ductile shear zones is accelerated if grainsize reduction leads to a change from grainsize-insensitive to grainsize-sensitive creep. In rocks whose rheology is approximated by the flow of one weak mineral, this process is favored at high stress-low temperature and/or retrograde thermal conditions. For polymineralic rocks in either prograde or retrograde thermal settings, reaction-induced or -enhanced switches to grainsize-sensitive creep can occur at depths corresponding to the intersection of the ambient geothermal gradient or pressure-temperature-time path with the limits of mineral stabilities for a specified rock composition. The consideration of the effects of pressure, temperature, grainsize and syntectonic reactions on fault rheology results in an evolutionary rheological model of an extending lithosphere which is more complex than existing models.