Can subducting slab dynamics induce intraplate shortening at the trailing passive margins?

The pull force exerted by the down-going subducting oceanic slabs is the primary force driving the motion of the tectonic plates. This force has been shown to generate tensional stresses within the trailing part of subducting plates, which can induce extensional reactivation of inherited discontinuities and weaknesses, such as passive margins. Surprisingly, compressive intraplate stress conditions have also developed during subduction at the trailing passive margins, such as in northern Africa, resulting in spectacular geological fold-and-thrust belts. Whether these compressional features were formed due to the processes acting at the subduction plate boundary (e.g., the arrival of continental fragments into the subduction zone) or, instead, are related to the dynamics of the leading oceanic slab (e.g., the arrival of the down-going slab to the 660-km-deep mantle discontinuity) is unclear.

            Here we present a series of 2D numerical subduction models, utilizing the ASPECT geodynamic code. The models are kinematically driven, mimicking the far-field boundary forces acting on the subducting plate. We track the evolution of stresses and strains within the trailing passive margins, incorporated as a weak and thin crust between the oceanic and continental domains. Our preliminary results suggest that the stress field in the trailing passive margin responds to the behavior of the slab at depth. During the slab’s free sinking phase or during slab rollback, slab sinking rates across the upper mantle exceed the prescribed plate velocity, resulting in extensional stresses that are transmitted to, and concentrated at, the passive margin. In contrast, during the anchoring of the slab to the lower mantle (i.e., at 670 km depth) and during slab folding, the rates at which the leading slab is sinking in the upper mantle are lower than the prescribed plate velocity, inducing intraplate shortening at the trailing passive margin. The timescales and temporal behavior of passive margin deformation match those of slab dynamics, with fast slab buckling behavior leading to likewise fast oscillating stress changes in the margins. Our results may help explain the observed switches between tensional and compressional phases at the northern African passive margins and the overall heterogeneity of passive margin deformation styles within subducting plates, ranging from normal faulting and magmatism to shortening and folding.