Subduction with Variations in Slab Buoyancy: Models and Application to the Banda and Apennine Systems

Abstract

Temporal variations in the buoyancy of subducting lithosphere exert a fi rst-order control on subduction rate, slab dip and the position of the associated vol- canic arc. We use a semi-analytic, three-dimensional subduction model to simulate “unforced” subduction, in which trench motion is driven solely by slab buoyancy. Model rates of subduction and model slab dip respond almost immediately to changes in the buoyancy of the subducting lithosphere entering the trench; as more buoyant slab segments correlate with slower subduction rates and steeper slab dip. The results are largely consistent with observations from the Banda and southern Apennine sub- duction systems, where subduction slowed and ended shortly after the entry of conti- nental lithosphere into the trench. Over a 2 m.y. period, model subduction rates decrease from ~70 mm/year to ~30 mm/year for the Banda Arc, and from ~40 mm/ year to ~20 mm/year for the Apennine Arc. Increases in model slab dip and decreases in arc-trench distance are likewise consistent with hypocenter locations and volcanic arc position along the Banda and Sunda arcs. In contrast, a time period of ~10 m.y. is needed for model subduction rates to slow to near zero, much longer than the ~3 m.y. upper bound on the observed slowing and cessation of trench motion in the Apennine and Banda systems. One possible explanation is that slab break-off, or the formation of large slab windows, occurred during the last stages of subduction, eliminating toroidal fl ow around the slab and allowing the slab to steepen rapidly into its fi nal position.