Spreading ridge subduction and geological evolution of the Southern Patagonia: insights from analogue modelling
Résumé
The Chile spreading ridge is subducting beneath Southern Patagonia since 18 Ma. Increasingly younger and thinner Nazca oceanic
plate subducted beneath that part of the Andes during the lower Miocene before ridge subduction, and an asthenospheric window
opened there following the ridge subduction. These modifications in the process of subduction may explain several of the geological
events that occurred in Southern Patagonia during the Neogene. We present 3D mantle-scale analogue models simulating the
arrival at trench of a spreading ridge to understand its possible effects on plate kinematics, slab geometry, and on the deformation
of the overriding plate. Oceanic and continental lithospheres are modeled using Newtonian viscous plates made of silicones with
different densities. They initially float above glucose syrup modeling the upper mantle. We reproduce the progressive thicknessdecrease
of the oceanic lithosphere arriving at the trench. Experiments reveal that the subduction of a thinning plate beneath a
freely moving overriding continent favors a decrease of the subduction velocity and an increase of the oceanic slab dip. When
the upper plate motion is imposed by lateral boundary conditions, the evolution of the subducting plate’s geometry largely differs
depending on the velocity of the overriding plate: the larger its trenchward velocity, the smaller the superficial dip of the oceanic
slab. A slab flattening episode may occur resulting from the combined effects of the subduction of an increasingly thinner plate
and a fast overriding plate trenchward motion. Slab flattening would be marked by an increase of the distance between the trench
and the volcanic arc in Nature. This phenomenon may explain the reported Neogene eastward motion of the volcanic arc in the
Southern Patagonia that occurred prior to the subduction of the Chile Ridge. Moreover, models also confirm that the early Miocene
episode of shortening observed in the Southern Patagonian Andes may partly result from the ridge approaching the trench, and
that the end of shortening in the Southern Patagonia coincides with ridge subduction.