Thermo-mechanical controls of flat subduction: insights from numerical modeling

Abstract

Numerical experiments are used to investigate the thermo- mechanical controls for inducing flat subduction and why flat subduction is rare relative to normal/steep subduction. Our modeling results demonstrate that flat subduction is an end-member of a steady state subduction geometry and is characterized by a curved slab with a nearly-horizontal slab section. Intermediate cases between normal/steep and flat subduction appear to be transient in origin and evolve toward one of the stable end-members. Physical parameters inducing flat subduction can be classified into four categories: buoyancy of the subducting oceanic lithosphere (e.g., slab age, oceanic crustal thickness), viscous coupling between the overriding and downgoing plates (e.g., initial subduction angle), external kinematic conditions, and rheological properties of the subduction zone. On the basis of parameter sensitivity tests and the main characteristics of present-day flat subduction zones, positive buoyancy from either the young slab or the thickened oceanic crust are considered the primary controlling parameter. Our results show that the possibility of flat subduction is directly proportional to oceanic crustal thickness and inversely proportional to the slab age. Furthermore, oceanic crust must be thicker than 8 km to induce flat subduction, when the slab is older than 30 Ma with an initial subduction angle of ≥ 20°, and without absolute trenchward motion of the overriding plate. The lower the initial subduction angle or the thicker the overriding continental lithosphere, the more likelihood for flat subduction. The initial subduction angle is more influential for the development of flat subduction than the overriding lithospheric thickness, and a thick overriding lithosphere induces flat subduction only under the condition of an initial subduction angle of ≤ 25°, with a slab age of ≥ 30 Ma and without absolute trenchward motion of the overriding plate. However, when the initial subduction angle is increased to > 25°, no flat subduction is predicted. All the parameters are evaluated within the constraints of a mechanical framework in which the slab geometry is regarded as a result of a balance between the gravitational and hydrodynamic torque. Any factor that can sufficiently reduce gravitational torque or increase hydrodynamic torque will exert a strong effect on flat subduction development. Our results are consistent with the observations of modern flat subduction zones on Earth.