Steady-state rift migration explains origin and asymmetry of highly thinned continental crust

Many magma-poor continental margins feature highly thinned continental crust (thickness less than 10 km) that is partitioned between conjugate margin pairs with pronounced asymmetry. A holistic scheme of margin evolution that explains the generation of these highly extended continental margins and their asymmetric distribution is still missing. This study utilizes knowledge gathered from two key examples of asymmetric margin pairs, Iberia-New Foundland and Southern Africa-Brazil. We investigate the rift evolution by means of thermo-mechanical forward simulation. We constrain our experiments with detailed plate kinematic history of the pre-break up and early seafloor spreading phase, laboratory-based rheology, and melt fraction evaluation of mantle upwelling. Our results are consistent with observed fault patterns, crustal thickness, and basin stratigraphy. We find that two processes are fundamental for the formation of highly thinned crust and the associated margin asymmetry: (i) Strain hardening takes place in the rift center due to cooling of upwelling mantle material to a depth of ~10 km. (ii) The formation of a low viscosity crustal pocket adjacent to the rift center is caused by heat transfer from the mantle and viscous strain softening. Both mechanisms generate a lateral strength contrast that promotes rift migration in a steady-state manner. A major conclusion is that the rate of extension has paramount control on margin width. We argue that high extension velocities lead to hotter rift flanks and a larger low-viscosity pocket which facilitates rift migration and results in significantly wider margins. The South Atlantic conjugate margins are an excellent natural benchmark for this model prediction as reconstructed extension velocities for the South Atlantic rift increase from North to South. As predicted by our numerical model, the maximum present-day margin width increases almost linearly from the conjugate Equatorial margin segments to the Florianopolis/Walvis ridge. Although the polarity of the magma-poor South Atlantic margins alternates, the asymmetry and the width of the wider margin are in very good agreement with our numerical modelling results. The process of steady-state rift migration has three important implications which challenge current understanding of passive margin evolution: Firstly, the rift migrates laterally into the previously undisturbed narrow margin, so that a constant transfer of crustal material takes places across the evolving plate boundary. This means that large parts making up the crust of the wide margin originate from its conjugate. Secondly, the migration of deformation across the highly extended crust results in faulting in the distal parts of the margin postdating that of the proximal parts by as much as 10 Million years while continuous tectonic erosion and deformation affects the narrow margin until breakup. Thirdly, this pronounced diachroneity of deformation results in drastically different peak heat flow histories which mirror the migration of the deformation focus.