Control of Plate Age at Trench on Subduction Dynamics and Surface Topography
Résumé
At present-day, the age of the subducting plate at trench largely varies among subduction zones, from 0 Ma where active spreading ridges are subducted (e.g., Patagonia, North America) to ∼150 Ma for the oldest subducted oceanic lithospheres. The age of the slab at subduction zones can also vary over time, implying a change in the slab thermal thickness and associated slab pull. Previous models have shown that such temporal changes can produce significant modifications of the geometry of the slab as well as changes of plates velocity and upper plate deformation. However, the evolution of plate age at trench also depends on the prior history of oceanic spreading, which may vary from one subduction zone to another.
Here, we systematically test with 2-D mantle-scale numerical models (MDOODZ) the effect of different rates of plate age decrease or increase at trench, within the range of values extracted from the Cenozoic evolution of major subduction zones (0 to ∼6 Ma/My). Models show that in the absence of external forces, a plate age decrease induces an increase of slab dip and a decrease of subduction velocity. In addition, the larger the rate of plate age change at trench is, the larger the evolution of the slab geometry and subduction velocity departs from the values expected from steady-state models. It thus results in a large variability of the slab dip (by as much as 30-40°) and subduction velocities for a same age of the plate at trench. Our study shows that steady-state subduction may only apply to a limited number of past and present-day subduction zones, which could in turn explain to a certain extent the lack of correlation between slab dip and plate age at trench. We also explore how the surface topography respond to the constantly-evolving plate thickness at trench and the associated changes in slab dip.