Inherent gravitational instability of hot continental crust: Implications for doming and diapirism in granulite facies terrains

Abstract

Modeling of in situ rock properties based on a Gibbs free energy minimization approach shows that regional metamorphism of granulite facies may critically enhance the decrease of crustal density with depth. This leads to a gravitational instability of hot continental crust, resulting in regional doming and diapirism. Two types of crustal models have been studied: (1) lithologically homogeneous crust and (2) heterogeneous, multilayered crust. Gravitational instability of relatively homogeneous continental crust sections is related to a vertical density contrast developed during prograde changes in mineral assemblages and the thermal expansion of minerals with increasing temperature. Gravitational instability of lithologically heterogeneous crust is related to an initial density contrast of dissimilar intercalated layers enhanced by high-temperature phase transformations. In addition, the thermal regime of heterogeneous crust strongly depends on the pattern of vertical interlayering: A strong positive correlation between temperature and the estimated degree of lithological gravitational instability is indicated. An interrelated combination of two-dimensional, numerical thermomechanical experiments and modeling of in situ physical properties of rocks is used to study the processes of gravitational redistribution within a doubly stacked, heterogeneously layered continental crust. It is shown that exponential lowering of viscosity with increasing temperature, in conjunction with prograde changes in metamorphic mineral assemblages during thermal relaxation after collisional thickening of the crust, provide positive feedback mechanisms leading to regional doming and diapirism that contribute to the exhumation of high-grade metamorphic rocks.