Tianyang Lyu, Zhiyuan Zhu, Benjun Wu *
Geophysical Journal International
Lithospheric plates on the Earth's surface interact with each other, producing distinctive structures comprising two descending slabs. Double-slab subduction with inward-dipping directions represents an important multi-plate system that is not yet well understood. This article presents two-dimensional numerical models that investigate the dynamic process of double-slab subduction with inward dipping, focussing on slab geometry and mantle transition zone upwelling flow. This unique double-slab configuration limits trench motion and causes steep downward slab movement, thus forming fold piles in the lower mantle and driving upwards mantle flow between the slabs. The model results show the effects of lithospheric plate properties and lower mantle viscosity on subducting plate kinematics, overriding plate stress and upwards mantle flow beneath the overriding plate. Appropriate lower mantle strength (such as an upper–lower mantle viscosity increase with a factor of 200) allows slabs to penetrate into the lower mantle with periodical buckling. While varying the length and thickness of a long overriding plate (≥ 2500) does not have a substantial effect on slab geometry, its viscosity has a marked impact on slab evolution and mantle flow pattern. When the overriding plate is strong, slabs exhibit an overturned geometry and hesitate to fold. Mantle transition zone upwelling velocity depends on the speed of descending slabs. The downward velocity of slabs with a large negative buoyancy (caused by thickness or density) is very fast, inducing a significant transition zone upwelling flow. A stiff slab slowly descends into the deep mantle, causing a small upwards flow in the transition zone. In addition, the temporal variation of mantle upwelling velocity shows strong correlation with evolution of slab folding geometry. In the double subduction system with inward-dipping directions, the mantle transition zone upwelling exhibits oscillatory rise with time. During the backwards-folding stage, upwelling velocity reaches its local maximum. Our results provide new insights into the deep mantle source of intraplate volcanism in a three-plate interaction system such as the Southeast Asia region.