Protracted continental subduction, indentation and collisional boundary migration coupled with adjacent oceanic slab-rollback and slab detachment in large-scale buoyancy-driven 3D whole-mantle scale numerical models of subduction-and-collision.

Continental collision zones form at convergent plate boundaries after negatively buoyant oceanic lithosphere subducts entirely into the Earth's mantle, whereafter collision ensues, and colliding continents are sutured together. In models of free subduction, the volume of the preceding and adjacent negatively buoyant oceanic lithosphere controls the system's driving force and dynamics. To investigate the dynamics of long-term continental subduction, indentation and collisional boundary migration and associated slab dynamics we designed large-scale numerical models of subduction-and-collision including two sets of modelled depths: whole mantle (2880 km) and upper mantle + partial lower mantle (960 km) and varying the trench parallel length ratio (1.5 - 2) of the indenting continental lithosphere (~2300 km) and adjacent oceanic lithosphere. In this contribution, we present the contrasting evolution of continental subduction and indentation coupled with adjacent oceanic slab rollback focusing on the different slab dynamics observed by varying the depth of the mantle in the models. Intriguingly, the whole mantle models show sustained continental indentation and concurrent deep continental subduction to mid-low upper mantle depths resulting in deep slab tearing at the subducted continental margin and shallow slab tearing at the trench parallel boundaries of the continental plate. In addition, the models also show continental underthrusting beneath the overriding plate and underplating of the continent, coeval with indentation and adjacent oceanic slab rollback. Together, these results provide insights into the India-Eurasia collision zone where the prolonged northward indentation of India during the last 50 Myrs and the rollback of the Sunda slab appear linked.