Controls on back-arc basin formation

The relationship between subduction and back-arc spreading has been well known since the early days of plate tectonics. However, the reasons why back-arc basins are associated with some subduction systems but not all has remained elusive. We examine the kinematic controls on subduction and back-arc basins for both the present-day and Cenozoic to differentiate between the major competing hypotheses for back-arc basin formation and to explain their temporal and spatial distribution. Our new data set of subduction and back-arc basin parameters uses a new set of paleo-oceanic age grids (Muller et al., 2005) associated with a moving Atlantic-Indian Ocean hot spot reference frame (O'Neill et al., 2005). The plate model includes detailed reconstructed spreading histories of back-arc basins based on marine geophysical and satellite gravity data. Our combined rotation and oceanic paleo-age model provides the age distribution of subducting lithosphere through space and time, convergence rates, and the absolute motion of the downgoing and overriding plates. We find that back-arc basins develop when the age of subducting normal oceanic lithosphere is greater than 55 million years. Additionally, we establish an age-dip relationship showing that the intermediate dip angle of the subducting slab is always greater than 30 degrees with back-arc spreading. Our results suggest that back-arc basin formation is always preceded by an absolute motion of the overriding plate away from the subduction hinge, thereby creating accommodation space between the overriding and subducting plates. Once back-arc extension is established, it continues regardless of overriding plate motion, indicating back-arc spreading is not a simple consequence of overriding plate behaviour. The landward migration of the overriding plate as a precursor to back-arc extension may indicate that extension on the overriding plate is influenced by the oceanward lateral flow of the mantle. However, once back-arc extension is established, rollback of the subduction hinge appears to be the primary force responsible for the continued creation of accommodation space. Our analysis indicates the driving mechanism for back-arc extension is a combination of surface kinematics, properties of the downgoing slab, the effect of lateral mantle flow on the slab, and mantle wedge dynamics.