Numerical modelling of slab breakoff and its magmatic effects

Abstract

The process of slab breakoff following the collision of continental plates has been invoked to account for many magmatic observations in post -collisional areas. These interpretations are often based upon the results of kinematic modelling of slab breakoff, which find this process can induce a thermal perturbation in the overriding plate, possibly generating lithospheric melting. Such a process requires that breakoff occurs at depths shallower than the base of the overriding lithosphere. This thesis aims to investigate more thoroughly the magmatic effects that breakoff may have. 2D numerical experiments are conducted, which make use of a thermodynamic database to determine the generation of melts and (de)hydration processes that occur within a collisional environment. A systematic parametric study is designed, which investigates those factors which may have a significant control on the breakoff depth. The breakoff timing and resulting mantle flow patterns are also investigated, as these may affect the magmatic processes. The strength of the subducting continental lithosphere is found to have a key control on the breakoff depth, whilst the timing and flow are significantly dependent upon the mantle rheology. By investigating further those conditions which promote shallow breakoff, it is found that breakoff appears unable to induce any significant melting of the overriding mantle lithosphere. In most cases , breakoff localises deeper than the overriding plate, preventing melting. Even following very shallow breakoff, the rapid descent of the detached slab gives way to a subsequent cooling of the overriding plate. A more commonly observed feature is the melting of continental crust upon breakoff, which may be a more reliable indicator of the occurrence of breakoff than lithospheric melting. Despite this, post-collisional magmatic observations show a wide range of compositions, timings, and localities. It is unlikely that melting across a post-collisional area can be attributed to any one geodynamic process.