Back-arc basins: A global view from geophysical synthesis and analysis

This global study of 31 off-shore back-arc basins and subbasins (BABs) identifies their principal characteristics based on a broad spectrum of geophysical and subduction-related parameters. My synthesis is used to identify trends in the evolution of back-arc basins for improving our understanding of subduction systems in general. The analysis, based on the present plate configuration, demonstrates that geophysical characteristics and fate of the back-arc basins are essentially controlled by the tectonic type of the overriding plate, which controls the lithosphere thermo-compositional structure and rheology. The type of the plate governs the length of the extensional zone in back-arc settings along the trench, the efficiency of lithosphere stretching, and the crustal structure, buoyancy and bathymetry of the BABs. Subduction dip angle apparently controls the location of the slab melting zone and the efficiency of slab roll-back with feedback links to other parameters. By the tectonic nature of the overriding plate (the downgoing plate is always oceanic) the back-arc basins are split into active BABs formed by ocean-ocean, arc-ocean, and continent-ocean convergence, and extinct back-arc basins. By geophysical characteristics, BABs formed on continental plates are subdivided into active BABs with and without seafloor spreading, and extinct BABs are subdivided into the Pacific BABs, possibly formed on oceanic plates, and the non-Pacific BABs with reworked continental or arc fragments.

Six types of BABs are distinctly different. Extension of the overriding oceanic plate above a steeply dipping old oceanic plate, preferentially subducting nearly westwards, forms large deep back-arc basins with a thin oceanic-type crust. In contrast, BABs on the overriding continental or arc plates form at small opening rates and often by shallow subduction of younger oceanic plates with a random subduction orientation; these BABs have small sizes, shallow bathymetry, and hyperextended or transitional ∼20 km thick arc- or continental-type crust typical of passive margins. The presence of a 2-5 km thick high-Vp lowermost crustal layer, characteristic of BABs in all settings, indicates the importance of magmatic underplating in the crustal growth.

Conditions required for the initiation of a back-arc basin and transition from stretching to seafloor opening depend on the nature of the overriding plate. BABs formed on oceanic plates always evolve to seafloor spreading. BABs formed on continental or arc plates require long spreading duration with large (>8 cm/y) opening rates and a large crustal thinning factor of 2.8-5.0 to progress from crustal extension to seafloor spreading. On the present Earth such transition does not happen in the back-arc basins formed behind a shallow subduction (<45^o) of a young (<40 My) oceanic plate.

The nature of the overriding plate also determines the fate of back-arc basins after termination of lithosphere extension: the extinct Pacific back-arc basins with oceanic-type crust evolve towards deep old "normal" oceans, while the shallow non-Pacific BABs with low heat flow and thick crust are likely to preserve their continental or arc affinity. BABs do not follow the oceanic cooling plate model predictions. Distinctly different geophysical signatures for mid-ocean ridge spreading and for back-arc seafloor spreading are caused by principally different dynamics.