Geodynamic implications of the subduction of an active spreading ridge: the Chile Triple Junction case study

The temporal evolution of subduction zones involves processes of subduction initiation/cessation and self-sustaining subduction; geodynamic scenarios that have been studied for a broad range of initial conditions, geometries, and rheology. Nonetheless, the subduction of an active spreading ridge implies a particular geodynamic scenario given by the interaction of a buoyant and hot magma chamber located along the ridge axis and the opening of a slab window beneath the continent linked to complex asthenosphere dynamics. We used numerical simulations in order to unravel the long-term geodynamics conditions and thermal structure associated with the subduction of an active spreading ridge, using as a working case the Chile Triple Junction (CTJ) in southern Chile (Nazca-Antarctica-South America). Here, the oblique subduction ridge started about 10-12 Ma at the latitude of the Madre de Dios Island, to further migrate northwards until its present-day position 40km north of the Taitao Peninsula. The area is characterized by elevated heat flow values (>120 mW/m2) and the emplacement of the Taitao ophiolite. The numerical simulations are performed using the I2ELVIS code, with an initial setup considering: (1) the subduction of the Nazca plate beneath South America; (2) the subduction of an active spreading ridge, represented as a buoyant magmatic chamber due to their thermal and compositional nature; and (3) the post ridge subduction, with the subduction of young oceanic lithosphere that gets older with time. Preliminary results provide likely scenarios about the size and density contrast between the magmatic chamber and the asthenosphere. The geodynamic evolution of the ridge subduction is thus mainly controlled by the density contrast due to compositional effects and modified by thermal buoyancy. Our simulations are consistent with the expected detachment of the Nazca plate and the opening of the slab window where the buoyancy of the magmatic chamber plays a key role in the asthenosphere dynamics. FONDECYT PROJECT 1201354