Models of convection and segregation in heterogeneous partially molten crustal roots with a VOF method – I: flow regimes.

We investigate numerically some thermomechanical conditions for the development of crustal scale diapirism and convection in a heterogeneous continental crust independently from the action of regional tectonics. Here, we consider a hot crust, with unmolten and partially molten domains of specific temperature and strain-rate dependent power-law rheologies. We take advantage of the volume of fluid (VOF) method to capture the coalescence and separation of deformable inclusions in the partially molten domain. The inclusions, of several hundred metres in size, are more or less dense and viscous with respect to the ambient medium (they also behave with a power-law rheology). We restrict our study to a 20 Myr time scale, during which gravitational dynamics may dominate over lateral tectonics and lithospheric thermal re-equilibration. The motion of these inclusions during the development of gravitational instabilities displays distinct flow regimes that depend on two Rayleigh numbers denoted Ra UM and RaPM , for the unmolten and partially molten rock properties, respectively. A 'suspension' regime occurs at high Ra UM and high Ra PM, in which most of the light compositional heterogeneities remain entrained in the convective cells. In contrast at low Ra UM and high Ra PM, a 'layering' regime is characterized by merging of the light inclusions as floating clusters below the rigid upper crustal lid. This regime occurs in association with a sharp viscosity gradient at upper-to-middle crust transitional depths. In these two regimes, the dense inclusions accumulate at the bottom of the partially molten zone. Finally at moderate Ra PM, a 'diapiric' regime reflects the segregation of the heavy and the light inclusions, respectively downward and upward, without global convection. These numerical experiments lead to a first order evaluation of the physical parameters required for the segregation of deformable inclusions of variable densities and convection, in a partially molten crust, and provide insights on the conditions for the development of migmatite domes. Geological data indicate that these processes likely occur in a large number of settings from Archean to Phanerozoic times, and contribute to the differentiation of the continental crust. [ABSTRACT FROM AUTHOR]