INITIATION AND TERMINATION OF SUBDUCTION: PREDICTIONS FROM NUMERICAL MODELS

Many hypotheses have been proposed for subduction initiation but no undisputable examples of the ongoing initiation are yet known. The major obstacle in the possibility to directly investigate subduction initiation is the uncertainty about localities where subduction starts now. As suggested by numerical experiments spontaneous subduction initiation at a passive margin is a long-term process with a “hidden” phase of initial movements that are not expressed in diagnostic subduction features such as trench and magmatic arc. The main challenge is, therefore, to identify possible subduction initiation localities in the world by evaluating numerically probability of subduction initiation at existing passive margins and oceanic plate boundaries (based on a local lithospheric/mantle structure and acting tectonic forces). Numerical analyses of American Atlantic margins gives very promissing results: subduction is likely to initiate now along south-eastern margin of Brasil where observation-based studies should concentrate.

Collision processes often culminate subduction episodes causing the fate of subduction, orogenesis, arc/continent assembly, continental subduction and subducted slab breakoff. We conducted a set of numerical experiments to study the evolution of subduction-collision system subject to spontaneous subduction termination associated with slab breakoff. Four different rheological end-members of this process were observed depending on the age of the slab and rate of subduction. Models involving different types of breakoff are subject to different topography evolution but always display a sharp breakoff signal. Post-breakoff uplift rates in foreland an hinterland basins ranges between 0.2 km/My for deep (400 km) detachment and 0.8 km/My for shallow (40 km) detachment. Continental crust subduction was observed in breakoff experiments in volving an oceanic lithosphere older than 30 My. Different exhumation processes such as subduction channel extrusion, slab retreat and eduction occurs according to depth of breakoff. Numerical miodelling of characteristic P-T paths associated with subduction termination reconsile with metamorphic record in collisional orogens including ultrahigh-pressure rock complexes.