Résumé
"Spontaneous subduction" corresponds to the gravitational instability of the more negatively buoyant plate, leading to subduction without convergence. This process has been argued to occur at transform faults, and was suggested for the initiation of the Izu-Bonin-Mariana subduction zone, based on a specific magmatic sequence including "forearc basalts" and boninites. Some thermo-mechanical models of plate gravitational instability have been tuned to help the triggering of the older and colder plate sinking, restricting the study of conditions yielding spontaneous subduction. We perform a more general 2D parametric study, by combining pseudo-brittle and ductile rheologies. We investigate large ranges of age contrast across the transform fault but also the role of the mechanical structure of the fault, assumed to be made of a weak layer. Slip is free on all sides of the simulation box. We observe three different behaviors depending on experimental set-up: overall static conductive cooling, spontaneous old plate sinking (OPS), and convective instability, or "dripping", of the young lithosphere. We first show that OPS can be modeled only if the vertical transform fault, but also a relatively wide domain on both sides of the fault are weak enough. Furthermore, simulations suggests that OPS is restricted to a limited range of plate age pairs, the thinner plate having to be younger than 15 Myr. The strength of the lithospheric mantle is a first order parameter resistant to old plate instability, as it controls the flexural rigidity competing against plate bending. In any cases, this mode of subduction initiation yields an instantaneous slab rollback associated with an extremely fast trench retreat, resulting in upper plate extension and asthenosphere upwelling along the slab top up to the surface. To conclude, the set of conditions necessary to trigger spontaneous subduction is not observed nowadays in nature, so that this process appears very unlikely in a cold Earth. Moreover, as when triggered, this subduction initiation is close to catastrophic, the typical magmatic sequence including boninites should erupt within a short amount of time. Geological records of subduction infancy in Izu-Bonin, approximately 52-50 Myr ago, attest for boninitic eruptions from 52 to 32 Ma, which is not compatible with a catastrophic process.