Flat Slab Formation and Evolution below Cratonic Lithosphere: Insights from 3D Time-Dependent Modeling

Abstract:

Several mechanisms have been suggested for the formation of flat slabs including buoyant features on the subducting plate, trenchward motion of the overriding plate and the thermal or cratonic structure of the overriding plate. Analysis of episodes of flat subduction indicate that not all flat slabs can be attributed to only one of these mechanisms and that it is likely that multiple mechanisms work together to create the necessary conditions for flat slab subduction. In this study we examine the role of localized regions of cratonic lithosphere in the overriding plate in the formation and evolution of flat slabs. We use time-dependent simulations with three-dimensional variation in overriding plate structure. We find that there are two modes of flat subduction: 1) permanent underplating occurs when the slab suction in the mantle wedge is sufficient to fully couple the subducting plate to the overriding lithosphere, and 2) transient flattening occurs when there is sufficient negative-slab buoyancy available along strike to overcome the increased suction on the slab beneath the craton. Our models show how regions of the slab adjacent to the sub-cratonic flat portion continue to pull the slab into the mantle leading to highly contorted slab shapes with apparent slab gaps beneath the craton. These results show how the interpretation of seismic images of subduction zones can be complicated by the occurrence of either permanent or transient flattening of the slab, and how the signature of a recent flat slab episode may persist as the slab resumes normal subduction. Our models suggest that permanent underplating of slabs may preferentially occur below thick and cold lithosphere providing a built-in mechanism for regeneration of cratons.