Geodynamic regimes of subduction under an active margin: effects of rheological weakening by fluids and melts.

The dynamics of subduction under an active margin are analysed by using a 2D-coupled petrological-thermomechanical numerical model of an oceanic-continental subduction system. The model includes spontaneous slab bending, dehydration of the subducted crust, aqueous fluid transport, partial melting of both crustal and mantle rocks as well as melt extraction processes resulting in magmatic arc crust growth. Based on our models, the following five geodynamic regimes of subduction, which may potentially form at active margins, are identified: (i) stable subduction with no backarc spreading centre and without plumes in the mantle wedge; (ii) retreating subduction with a focused backarc spreading centre and without plumes; (iii) retreating subduction with distributed intra-arc extension and trans-lithospheric sedimentary plumes; (iv) advancing subduction with underplating (laterally extending) sub-lithospheric plumes; and (v) stable to advancing subduction with stationary (laterally limited) sub-lithospheric plumes. Transitions between these different regimes are mainly caused by the concurrence of rheological weakening effects of (i) aqueous fluids percolating from the subducting slab into the mantle wedge and (ii) melts propagating from the partially molten areas formed in the mantle wedge towards the surface. The aqueous fluids mainly affect the forearc region. Strong fluid-related weakening promotes plate decoupling and reduces subduction drag and thus results in stacking of sediments in the accretion prism. In contrast, reduced weakening by fluids results in strong coupling of the plates and leads to advancing collision-like subduction with enhanced subduction erosion. Thickening of the overriding plate and large sedimentary plumes in the mantle wedge are the consequences. On the other hand, melts, extracted from the hot regions above the slab, rheologically weaken the lithosphere below the arc, which thus controls overriding plate extension and shortening. Strong rheological weakening by melts in combination with weak plate coupling trigger retreating subduction with a pronounced backarc spreading centre. Also, weakening of the continental lithosphere by melts extracted from trans-lithospheric sedimentary plumes generate a weak channel through which these structures may be emplaced into subarc crust. If there is insufficient melt-related weakening, plumes cannot ascend but extend horizontally and thus underplate the lithosphere. [ABSTRACT FROM AUTHOR]