Cold Plumes Initiated by Rayleigh-Taylor Instabilities in Subduction Zones, and Their Characteristic Volcanic Distributions: The Role of Slab Dip

Dehydration melting in subduction zones often produces cold plumes, initiated by Rayleigh-Taylor instabilities in the buoyant partially molten zones lying above the dipping subducting slabs. We use scaled laboratory experiments to demonstrate how the slab dip (alpha) can control the evolution of such plumes. For alpha > 0 degrees, Rayleigh-Taylor instabilities evolve as two orthogonal waves, one trench perpendicular with wavelength lambda(L) and the other one trench parallel with wavelength lambda(T) (lambda(T) > lambda(L)). We show that two competing processes, (1) lambda(L)-controlled updip advection of partially molten materials and (2) lambda(T)/lambda(L) interference, determine the modes of plume growth. The lambda(T)/lambda(L) interference gives rise to an areal distribution of plumes (Mode 1), whereas advection leads to a linear distribution of plumes (Mode 2) at the upper fringe of the partially molten layer. The lambda(T) wave instabilities do not grow when alpha exceeds a threshold value (alpha* = 30 degrees). For alpha > alpha*, lambda(L)-driven advection takes the control to produce exclusively Mode 2 plumes. We performed a series of 2-D and 3-D computational fluid dynamics simulations to test the criticality of slab dip in switching the Mode 1 to Mode 2 transition at alpha(*). We discuss the effects of viscosity ratio (R) and the density contrast (Delta rho) between the source layers and ambient mantle, source layer thickness (T-s), and slab velocity (U-s) on the development of cold plumes. Finally, we discuss the areal versus linear distributions of volcanoes from natural subduction zones as possible examples of Mode 1 versus Mode 2 plume products.