Modelling of silicic intrusions in Alpine type orogens

The emplacement of huge intrusive bodies such as batholiths is a fundamental issue in geology. To improve our understanding of the dynamics of batholith emplacements under an active continental margin, a set of numerical experiments have been performed using a 2D geochemical - petrological - thermomechanical numerical model. Based on finite differences and marker in cell techniques, the model simulates subduction of an oceanic plate beneath an active continental margin. The model includes spontaneous slab retreat and bending, dehydration of subducted crust, aqueous fluid transport, partial melting, melt extraction and melt emplacement. A systematic parameter study has been carried out by varying the rheological weakening effect imposed by fluids and melts. Aqueous fluids percolating from the subducting slab into the mantle wedge mainly affect the forearc region, whereas extracted melts propagating from the mantle wedge toward the surface have a major impact on the lithosphere below the arc. The results indicate that several tectonic regimes exist, some of which show the development of silicic plumes. It was found that the rheological weakening effect of melts and fluids is of major importance for the formation and emplacement of silicic plumes. As a function of fluid and melt weakening, silicic plumes may (1) ascend and intrude into the crust, (2) extend horizontally and remain beneath the continental lithosphere (underplating) or (3) be emplaced at a later stage. Strong fluid related weakening promotes stacking of sediments and therefore results in a large accretion wedge. In this case, insufficient sediments are being subducted and plumes do not form. Small amounts of fluid weakening, on the other hand, results in strong coupling of the subducting and overriding plates in a collision-like subduction setting. In this case, large quantities of sediments are being subducted and sedimentary plumes are formed. However, if the continental lithosphere is insufficiently weakened by extracted melts, these sedimentary plumes cannot ascend. Our results thus suggest that batholiths may be the result of the emplacement of silicic plumes into the crust and their emplacement dynamics is strongly affected by the rheological weakening effect of fluids and melts.