Kinematic and rheological controls on ultra-wide asymmetric rifted margins evolution

Abstract

Only a limited number of geodynamic numerical models have been successful in simulating both asymmetric and ultra-wide ($>$500 km) rifted margins. In this paper, we present a comprehensive suite of 72 thermo-mechanical geodynamic numerical simulations of rifting. These simulations revealed the impact of initial crustal thickness, variations in the ratios between the upper and lower crusts, rift velocity, structural inheritance, and shear heating on the asymmetry and width of conjugate rifted margins. Our findings highlight that initial crustal thickness is a primary controlling factor of conjugate rifted margin geometries. Specifically, initial crustal thicknesses ranging from 35 to 40 km generally do not result in margins exceeding 300 km in width, where wider margins are 1.5 times longer than the narrow corresponding conjugate. An initial crustal thickness of 45 km facilitates the formation of ultra-wide margins, especially when the upper crust is thicker than the lower crust. This is attributed to the greater concentration of heat-producing elements in the thicker upper crust, which accumulates heat, weakens the crust, and promotes ductile thinning. Additionally, high rifting velocities enhance advective heating from the upwelling asthenosphere. This further influences the development of the distal margin domains and results in highly asymmetric conjugate margin pairs with one ultra-wide margin. Structural inheritance also emerges as a critical factor, as it distributes strain within the pre-rift lithosphere, thereby enhancing the development of ultra-wide ($>$500 km) and highly asymmetric margins. Our results are compatible with conjugate rifted margins between Brazil and Africa, such as Espirito Santo - North Kwanza, Campos - South Kwanza and Santos - Benguela.