Double subduction controls on long-lived continental tectonics and subcontinental mantle temperatures

The complexities of convergent margins commonly include the interactions of subduction zones, with many geological records of “double” subduction. Here, we build two-dimensional numerical models to explore the evolution of complex subduction systems by systematically testing single and inward-dipping double subduction beneath a continental upper plate and the impact of continental collision on these systems. When compared to single subduction models, the inward-dipping double subduction shows hindered trench migrations and larger volumes of upwelling mantle enhanced by excess sinking slab mass. Double subduction draws larger volumes of hotter mantle beneath the continent in an area much broader than the marginal basins of single subductions, contributing to subcontinental heating by ∼200 °C. As collision jams one margin of a double subduction system, the other margin follows the evolution of migrating single subduction zones, although characterized by persisting higher mantle temperatures and strong upwellings, inherited from the double subduction stage, and large-scale upper plate extension. The modeling outcomes are compared to scaling arguments to test the viability of the mechanism proposed for tectonics of the Cenozoic South China Sea and Neoproterozoic Yangtze Block (southeastern China), where the inward-dipping double subduction provides a context for protracted large-scale continental extension, hotter subcontinental temperatures, and channeled mantle flow not easily reconciled with the dynamics of single subduction zones.