Geodynamic Modeling of Slab–Slab Interactions in the Manila–Taiwan–Ryukyu Subduction System

The Manila–Taiwan–Ryukyu subduction system provides a natural laboratory for investigating the transition from active oceanic subduction to mature arc–continent collision at a complex plate junction. While dense geophysical observations across Taiwan are commonly interpreted in terms of arc–continent collision and crustal-scale orogenic processes, the contributions of underlying mantle circulation and interacting slabs remains poorly quantified. This deficiency is critical because subduction-driven mantle flow can influence regional stress and deformation over distances of up to ~600 km, the ~500-km-long Taiwan orogen—particularly where neighboring slabs interact through slab edges, slab gaps, and potential tearing or detachment. Here, we investigate these processes using three-dimensional finite-element geodynamic models ASPECT. Starting from an simplified double-subduction configuration, we isolate the first-order signatures of slab–slab interactions from the complexity of regional tectonics. Systematic sensitivity tests varying inter-trench distance and convergence geometry are conducted to quantify their effects on mantle flow and regional stress–strain patterns. To connect model dynamics to seismological observables, we further predict seismic anisotropy by tracking the development of crystal preferred orientation within the modeled mantle flow. Model prediction of stress, strain, and seismic anisotropy are compared with earthquake focal mechanisms, island-wide GNSS-derived strain rates and SKS splitting observations. These comparisons constrain the extent to which double-subduction–driven mantle flow contributes to geophysical observables, and they identify which observables are most sensitive to specific subduction parameters and slab–slab interaction geometries.