THE EFFECT OF SPREADING RIDGES AND BUOYANT CRUST ON THE DYNAMICS OF SUBDUCTION (Invited Presentation)

Although we often imagine subduction as a long-term steady-state process variations in the structures approaching and entering the subduction zone cause episodic changes to the force balance with effects ranging from small changes in plate speed to termination of subduction. The extent of these changes depends on how the force balance is modified either through changes in buoyancy or plate strength or both. Several recent studies have evaluated the effect of a buoyant feature such as an oceanic plateau or aseismic ridge on continued subduction. These studies show that rather than a simple comparison of incoming-plate buoyancy to slab-pull forces, one must also consider the evolving density structural and 3D geometry. For example, the transformation of basalt to eclogite removes the excess buoyancy due to thickened crust allowing much thicker crustal sections to be subducted. Similarly, sections of the subducted plate adjacent to the thickened region add to the slab-pull force from the slab directly down-dip. These results suggest that small buoyant features are unlikely to signficantly shallow slab dip, unless other factors such as a thick overriding plate, are also present. 3D simulations also show that the plate boundary can be significantly modified in response to accretion of buoyant structures. In the case of the approach of spreading ridge only the case of a spreading ridge parallel to the subduction boundary has been considered in fully-dynamic models. In this case, the integrated strength of the young subducting plate ahead of the spreading ridge becomes too small relative to the slab-pull forces and the slab breaks before the ridge can enter the subduction zone. Rapid heating of the slab during breakoff may be a source for adakitic volcanism, as observed Baja California following ridge-trench interaction in the Miocene. While there are not yet numerical models for the dynamic process of a spreading ridge entering the subduction zone at a high angle, by analogy with the models of plateau subduction, it is likely that the fate of such a spreading ridge will depend strong on the amount of slab-pull available from adjacent sections of the subduction boundary.