Ablative subduction: A two‐sided alternative to the conventional subduction model

Abstract

Subduction is conventionally modelled as a convergence and overlapping of two semirigid plates; such a convergence is one‐sided, in that one lithospheric plate descends while the other remains on the surface. We suggest an alternative model, one based on the dynamics of fluids. In this model, the viscous lower lithosphere flows downward, and the brittle upper lithosphere deforms in passive response. This process is potentially double‐sided, for we find that even a buoyant plate can be dragged downward by a dense, descending neighbor. Thus an apparent overriding plate may be worn away by a process of viscous ablation, with the rate of ablation a function of plate buoyancy. We call this process “ablative subduction”. Ablative subduction allows us to simply interpret observations concerning slab profiles, interplate seismicity, back arc tectonics, and complex processes such as double subduction and subduction polarity reversal. In performing experiments modelling the evolution of simple, fluid “slabs”, we find that slab profile is strongly influenced by ablation in the overriding plate. When ablation is weak, as when a buoyant continent borders the trench, deformable slabs adopt shallow, Andean‐style profiles. These profiles develop over time from an initially steep shape. More vigorous ablation rates, as might occur in an ocean‐ocean convergence, yield steeper, Marianas‐style profiles. Thus differences between Marianas‐style and Andean‐style slabs may result from differences in ablation rate and subduction duration. The occurrence of ablation might not be easily detected at depth, because material from subducting and ablating plates adhere closely as a single slab. These slabs show downdip deformations that are consistent with seismic focal mechanisms. Plate deformations associated with ablative subduction are also consistent with observed patterns of seismicity. In particular, the relative aseismicity of Marianas‐style plate boundaries is consistent with the ablative model; we may thus explain how plates can converge aseismically, without requiring that these plates be decoupled. Ablation should be weakest in Andean‐style subduction, although pulses of rapid ablation might lead to episodes of crustal shortening; in oceanic regions, the manifestation of ablation may depend upon tectonic regime. In an extensional area like the Marianas, ablation may be temporally self‐limiting, and ablative cycles may explain conflicting observations of tectonic erosion and accretion. In a compressional regime, vigorous ablation might lead to observations of double subduction and subduction reversal. In each case, the behavior of the lower lithosphere is relatively simple; what varies is the rate of ablation and the response of the upper lithosphere.