The coupled evolution of forearc and back-arc basins: inferences from 2D and 3D numerical modelling

The subsidence history of forearc and back-arc basins reflects the relationship between subduction kinematics, mantle dynamics, magmatism, crustal tectonics, and surface processes. The distinct contributions of these processes to the topographic variations of active margins during subduction initiation, oceanic subduction, and collision are less understood.

We conducted a series of 2D and 3D thermo-mechanical numerical models with the codes 2DELVIS and 3DELVIS, based on staggered finite differences and marker-in-cell techniques to solve the mass, momentum and energy conservation equations. Physical properties are transported by Lagrangian markers that move with the velocity field interpolated from the fix Eulerian grid. We discuss the influence of different subduction obliquity angles, the role of mantle flow variations and their connection with sediment transport and upper plate deformation. Furthermore, slab tearing and the gradual propagation of slab break-off is modelled during collision.

The models show the evolution of wedge-top and retro-forearc basins on the continental overriding plate, separated by a forearc high. They are affected by repeated compression and extension phases. Compression-induced subsidence is recorded in the syncline structure of the retro-forearc basin from the onset of subduction. The 2–4 km upper plate negative residual topography is produced by the gradually steepening slab, which drags down the upper plate. Trench retreat leads to slab unbending and decreasing slab dip angle that leads to upper plate trench-ward tilting. Back-arc basins are either formed along inherited weak zones at a large distance from the arc or are connected to the volcanic arc evolution leading to arc splitting. Backarc subsidence is primarily governed by crustal thinning that is controlled by slab roll-back and supported by the underlying mantle convection. High subduction and mantle convection velocities result in large wavelength negative dynamic topography. Collision and continental subduction are linked to the uplift of the forearc basins; however, the back-arc records ongoing extension during a soft collision. During the hard collision, both the forearc and back-arc basins are ultimately affected by the compression. Our modeling results are compared with the evolution of Mediterranean subduction zones.