HOT SLAB MELTS FROM OLD SUBDUCTING LITHOSPHERE: SPONTANEOUS ARC RIFTING CAUSED BY TIME-DEPENDENT EVOLUTION OF THE MANTLE WEDGE CORNER

Backarc spreading centers commonly initiate as forearc or volcanic arc rifting events that occur when extension localize within lithosphere weakened by hydrous fluids (forearc) or melting (volcanic arc). Two models have been proposed for triggering forearc/arc rifting through extension: roll-back of the subducting plate causing trench retreat, or change in motion of the overriding plate away from the subduction zone. Here, using 2D fully-dynamic models of subduction, I show that there is a third mechanism for forearc/arc rifting caused by an in situ instability in the mantle wedge corner. During long-term subduction, a thin, cold high-viscosity boundary exists between weakened forearc crust and the hot, buoyant mantle wedge. However, as subduction evolves with both advancing and retreating motion of the slab, the thin high-viscosity boundary can be advected away. When this occurs, the hot, buoyant material in the mantle wedge corner can rapidly rise up into the weak forearc/arc lithosphere causing rifting and driving the overriding plate away from the subducting plate. During this process, the crust of the subducting plate undergoes rapid heating from less than 500^oC up to 800^oC at depths of 25-50 km. The hot slab crustal temperatures continue for almost 5 my during the rifting process. Such rapid heating of the crust at shallow depths could lead to the formation of hot slab melts (i.e., adakites) erupting in the fore-arc region. At the same time, regions of the overriding plate lithosphere are heated to over 1000^oC at depths of 25-50 km predicting melting of metasomatised peridodite and the formation of boninites. Therefore, this new model for arc rifting also suggests a mechanism for the formation of these unusual types of volcanism in supra-subduction settings accompanying the formation of back arc spreading centers. For such spontaneous forearc/arc rifting to occur 1) a broad region of weakened material must form along the plate interface, and 2) the overriding or subducting plate must be free to move or deform in response to the buoyancy forces driving arc rifting.