Influence of Lower Crustal Rheology on Rifted Margin Evolution and Oceanization: A Case Study for the Campos/Angola and Camamu/South Gabon Margins

Abstract

Here we use dynamical models of rifting to show how lower crustal rheology affects conjugate margin architecture and the nature of the continent-ocean transition (COT). We explore the behavior of South Atlantic conjugate margins, specifically the Campos/Angola, which developed along the Late Proterozoic Ribeira/Kaoko fold belt, to the Camamu/South Gabon basins, formed on the São Francisco/Congo Craton. Along these conjugate margins the degree of asymmetry increases southwards. Additionally, in Campos the crust tapers smoothly towards break-up, faults show small offsets and the area of hyper-extended crust (< 10 km thickness) is very wide (~ 200 km). To the North, in the Camamu, faults have much larger offsets, crustal thinning is abrupt and the margin much narrower. Our models show that a strong lower crustal rheology, which would be expected for a craton-like setting of the Camamu, effectively couples deformation in upper crust and mantle, leading to rapid crustal break-up and subsidence through crust-cutting faults. These crust-cutting faults allow serpentinisation to start before break-up, and produce narrow margins with only slight degrees of asymmetry. Coupling of lithospheric layers leads to quick upwelling of the asthenosphere and melting. For slow extension velocities, such as those prevalent in this area, < 5 mm/yr, melting starts after the onset of serpentinisation. The resulting COT consists of exhumed and serpentinised mantle, underlain by a thin layer of frozen magma. For the same extension velocities, when the lower crust is weak as anticipated for a fold belt setting such as in Campos/Angola, rifting starts with a prolonged phase characterized by minor faulting distributed over a wide area and moderate crustal thinning. Continuing extension leads to strain localization, coupling of lithospheric layers, more pronounced crustal thinning and the emergence of an array of sequential, oceanward younging faults, and produces wide, hyper-extended and asymmetric margins (Brune et al., 2014). In this case, serpentinisation is insignificant because active faults do not reach the mantle. Asthenospheric upwelling is less pronounced, and the onset and amount of melting is delayed with respect to the previous case. When crustal break-up occurs, magma rises to form oceanic crust and a narrow continent-ocean transition. Thus during rifting, melting and oceanization is not only controlled by extension velocity but also by the rheology of the lower crust.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015