The evolution of hyperextended rifted margins : linking variations on the width, asymmetry, and strain distribution to lithospheric strength and geodynamic processes

The goal of the work presented here is to improve the understanding of the processes controlling the styles of rifting. A large focus is on the structural and thermo-mechanical evolution of magma-poor margins. Applying parameters values that encompass those inferred from Atlantic margins to geodynamic numerical experiments of lithospheric extension successfully reproduce the variety of crustal thicknesses, widths and asymmetries observed at those margins. The results are grouped into four end members of margins for varying initial lithospheric strength and extension rates. The first two end members are narrow and asymmetric, and narrow and near-symmetric conjugate margins. The other two are wide extensional systems that evolve into asymmetric conjugate margins with one side <100 km wide, and the other >100-300 km wide; and highly asymmetric conjugate margins wherein the wide conjugate is 200 km to > 350 km across. All margins described above form by depth-dependent stretching, and polyphase sequential faulting, including detachment faults. In addition to different distributions of thinning, total crustal thinning across conjugate margins is not always balanced by an equal magnitude of distributed plastic deformation within the lithospheric mantle. The unbalanced thinning is associated with small-scale convection developed in the later stages of extension in the models. Mantle rheology and the continuous weakening of the lithosphere dominate the evolution of narrow systems. The formation of the wide asymmetric systems occurs due to deformation migration – diachronous rifting - wherein neither the upper nor the lower crustal deformation remains fix. Such extension is controlled by both the crustal and mantle rheology, and the initial lithospheric strength is preserved throughout most of the margin evolution. Two effects of bending stress at adjacent areas – one strengthening the fault, and the other weakening the rift flank – may contribute to the deformation migration. The change in curvature may help localize new faults near the rift-flank inflexion point. Despite the simplified lithosphere initial configuration assumed, the evolving extension results in complex rifted margins. Making further predictions of subsidence and thermal histories of margins will require integrating geodynamic modeling results with kinematic subsidence and heat flow studies in order to develop tectono-sedimentary models in closer agreement with observations.