The Lithospheric Architecture of the Conjugate South Atlantic Passive Margins – Implications for Asymmetric Rift Initiation
Peter Hass, Institute of Geosciences, Christian-Albrechts-Universität Kiel
le jeudi 14 septembre, 14h30, amphi A, Institut Universitaire Européen de la Mer
The lithospheric architecture of passive margins is crucial for understanding the tectonic processes that caused the breakup of Gondwana.
We highlight the evolution of the South Atlantic passive margins by a simple thermal lithosphere-asthenosphere boundary (LAB) model based on onset and cessation of rifting, crustal thickness, and stretching factors.
Our model captures the entire South Atlantic passive margins, ranging from the Agulhas – Falkland FZ in the South up to the conjugate Demerara/Guinea Plateaus in the North. We simulate lithospheric thinning and select the LAB as the T = 1,330°C isotherm, which is calculated by 1D advection and diffusion. Stretching factors and margin geometry are adjusted to state-of-the-art data sets, giving a thermal LAB model that is especially designed for the continental margins of the South Atlantic.
Our LAB model shows distinct variations along the passive margins that are not imaged by global LAB models, indicating different rifting mechanisms. For example, we model up to 200 km deep lithosphere in the South American Santos Basin and shallow lithosphere less than 60 km in the Namibe Basin offshore Africa. These two conjugate basins reflect a strong asymmetry in LAB depth that resembles variations in margin width.
In a Gondwana reconstruction, we discuss these patterns together with seismic velocity perturbations for the Central and Austral Segments of the margins. The shallow lithosphere in the Namibe Basin correlates with signatures of the Angola Dome, attributed to epeirogenic uplift in the Neogene, suggesting an additional component of post-breakup lithospheric thinning.
The lithospheric model can be used to trace the depth structure of local tectonic features along the passive margins related to the break-up of Gondwana, for example Transform Marginal Plateaus. Here, we investigate if (conjugate) Transform Marginal Plateaus have an anomalous lithospheric thickness pattern and discuss the implications for Gondwana breakup.