Wide Versus Narrow Back-Arc Rifting: Control of Subduction Velocity and Convective Back-Arc Thinning

Back-arc basins such as the ones behind the island-arcs of the Western Pacific Ocean or the ones in the Mediterranean Sea are ubiquitous structures of the Earth. They are extensional basins forming in the overriding plate behind subduction zones and similarly to continental rifts, they can exhibit different structural styles from narrow, localized rifting to wide-rift extension. While these different structural styles have been long recognized, the factors controlling the style of extension in these basins have not been explored properly. We use thermo-mechanical models to investigate how the relative rates of progressive build-up of slab-pull force and of convective thinning and thermal weakening of the overriding plate control the style of back-arc rifting. Following subduction-initiation, a high subducting plate velocity results in rapid build-up of the slab-pull force. The relatively low rate of convectively thinning and associated moderate weakening of the overriding plate require slab-pull to build up to close to its maximum value to overcome the high back-arc integrated strength resulting in a narrow back-arc rift. In turn a low subducting plate velocity in comparison with the timescale of convective thinning of the overriding plate allows for significant back-arc weakening before the slab-pull force becomes large enough to drive back-arc extension. In this case, the back-arc exhibits a wide rifting style as extension occurs at significantly reduced overriding plate integrated strength. Our model results provide an explanation why some subduction zones exhibit wide, distributed extension in the overriding plate such as for instance observed in the Pannonian basin.