The dynamical control of subduction parameters on surface topography

Abstract

The long-wavelength surface deflection of Earth’s outermost rocky shell is mainly controlled by large-scale dynamic processes like isostasy or mantle flow. The largest topographic amplitudes are therefore observed at plate boundaries due to the presence of large thermal heterogeneities and strong tectonic forces. Distinct vertical surface deflections are particularly apparent at convergent plate boundaries mostly due to the convergence and asymmetric sinking of the plates. Having a mantle convection model with a free surface that is able to reproduce both realistic single-sided subduction and long-wavelength surface topography self-consistently, we are now able to better investigate this interaction. We separate the topographic signal into distinct features and quantify the individual topographic contribution of several controlling subduction parameters. Results are diagnosed by splitting the topographic signal into isostatic and residual components, and by considering various physical aspects like viscous dissipation during plate bending. Performing several systematic suites of experiments, we are then able to quantify the topographic impact of the buoyancy, rheology, and geometry of the subduction-zone system to each and every topographic feature at a subduction zone and to provide corresponding scaling laws. We identify slab dip and, slightly less importantly, slab buoyancy as the major agents controlling surface topography at subduction zones on Earth. Only the island-arc high and the back-arc depression extent are mainly controlled by plate strength. Overall, his modeling study sets the basis to better constrain deep-seated mantle structures and their physical properties via the observed surface topography on present-day Earth and back through time. Plain Language Summary Earth’s partially molten mantle flows over millions of years due to the high temperatures, the enormous pressures and the tiny defects in crystals that occur in the Earth’s rocky interior. This global flow of mantle material, in particular in regions where cold, heavy plates sink back into the mantle (i.e., subduction), causes visible variations of surface elevation. These topographic variations potentially allow us to better understand the hidden structures and dynamics in Earth’s interior. We have developed an efficient model of Earth’s interior and can therefore run numerical experiments to better understand this interaction of mantle flow and surface topography. We test the numerous, to some extend still unknown, dynamic, rheologic and geometric properties of subduction zones and quantify their potential to deflect the plate at the surface. We find that the angle of the sinking plates and their relative weight compared to their surrounding have a major control on lifting and depressing the plate surface. Our results will finally allow us to better understand the interior of a planet on present day, but also back through time by simply looking at either the present surface topography or at the available data back through geologic time.