Dynamics of partially molten layers on subducting slabs in subduction zones: Insights from theory and experiments

Abstract:

Subduction zones generally involve dehydration melting in the mantle wedge to form partially molten layers above the subducting slabs. This study investigates the flow dynamics in such layers considering Rayleigh-Taylor instability (RTI) and slab-parallel updip advection as two competing material transport mechanisms. The RTIs are triggered in a partially molten layer due to density inversion, leading to the ascent of buoyant materials through the high-density overburden in the form of a row of cold plumes. In contrast, the advection mechanism results in the accumulation of partially molten materials in the updip region and localize isolated plumes. Our scaled analogue experiments show that the slab dip (α) is a crucial parameter to modulate the RTI versus advection mechanism. Low α condition favours the RTI process, leading to the formation of distributed plumes in the mantle wedge. This process is replaced by the updip advection mechanism as α exceeds a critical value (α*). Our experimental results are supported by natural scale 2D computational fluid dynamics (CFD) simulations. Using lubrication approximation, we solve Navier-Stokes equation to perform linear stability analysis for RTI characterization and develop a theory for determining α* following thin-film dynamics. The theoretical α* is validated with the experimental value. We discuss the implication of our study in interpreting the arc volcano patterns in subduction zones. For α < α*, the dominant role of RTI mechanisms gives rise to distributed volcanoes both along and across the trench. The advection mechanism transforms the distributed to localized volcanoes, forming a trench parallel linear pattern.