The impact of along-strike variations in overriding plate thickness on subduction dynamics and mantle flow

Abstract

Subduction zones are inherently three-dimensional systems and exhibit pronounced trench-parallel variability in key observables, including the deformation style of the overriding plate, trench migration rates, slab geometry, and mantle flow patterns. Geodynamic models typically invoke external mantle flow and/or along-strike variations in the properties of the subducting slab to explain this variability, often neglecting the influence of the overriding plate, despite growing evidence of its strong control on subduction dynamics. In this study, we use self-consistent three-dimensional numerical models to explore how along-strike heterogeneities in the overriding plate structure can generate significant variations in subduction dynamics and mantle flow. Our results demonstrate that trench-parallel variations in overriding plate thickness produce large along-strike differences in trench retreat velocities, leading to strongly arcuate trench geometries. We further conduct a suite of models incorporating a mechanically weak zone in the subducting plate, representing the subduction of a transform fault oriented perpendicular to the trench. These experiments show that along-strike variations in overriding plate thickness can promote vertical slab tearing and segmentation of the subduction system into distinct slab segments. Slab tearing facilitates focused mantle upwelling through the tear, potentially triggering tear-related magmatism during slab rollback. Natural examples of subduction zones characterized by vertical slab tears include the Melanesian subduction system, the South Shetland margin and the Tyrrhenian-Apennines collision system.