Influence of overriding plate velocity changes on slab dip and deformation: insights from laboratory models
Résumé
Over geological times, plate reorganization associated with mantle convection led to changes in absolute plate
velocities, which may in turn have impacted the geometry of the subducting plate as well as the overriding plate
regime of deformation. Indeed, previous studies have shown a very good correlation between the absolute motion
of the overriding plate on one hand and slab dip and overriding plate deformation on the other hand: extension
and steep slab are associated with an overriding plate moving away from the trench while shortening and shallow
slab occur if the upper plate goes the other way. However, these correlations are established when subduction has
reached a steady-state regime and for a constant motion of the overriding plate over the subducting plate, which
may not always be the case on Earth. The response of the subduction system to changes in absolute overriding
plate velocity still remain an open question.
In this study, we conducted a set of 3-D mantle-scale laboratory models of subduction in which we incrementally
changed the velocity of the overriding plate to reproduce changes of velocities that may arise from variations of
far-field boundary conditions in Nature. We first show that strain rates in the overriding plate are correlated with
overriding plate absolute velocity but also that the regime of deformation adjusts rapidly to changes of velocity.
This may explain for instance why despite the subduction has been continuous beneath South America since at
least the middle Jurassic, shortening along its active margin is only recorded episodically, the main phases of
Andean orogeny roughly corresponding to periods of South American plate westward acceleration. We also show
that slab dip adjusts to changes of overriding plate velocity but it requires several Myr before it stabilizes. It may
explain why the correlation between absolute overriding plate motion and slab dip from the analysis of present-day
subduction zones is only moderate, part of the subduction zones possibly not being at steady-state at present-day.