Thomas Theunissen

Topography during Pangea Breakup – Case of the South Atlantic Central Segment – 2-D thermo-mechanical modelling and implications for salt basin formation

In this study we use observations from the central South Atlantic conjugate margins to constrain

the structural style of rifting and its relation with sedimentary basin evolution during

the syn and early post-rift. Three synthetics transects from North (Gabon-Brazil) to South

(Angola-Brazil) are used to constrain fault distribution, margin width, crustal thickness,

distribution of magmatism, syn-rift sedimentary section thickness and paleo-environment

from the start of rifting in the Berriasian (145 Ma) until the early post rift in the Aptian (114 Ma).

This integrated study aims to understand variations in along strike structural style,

magmatic output, and sedimentary basin evolution to assess the contribution of

mantle processes on topography using forward 2-D thermo-mechanical modelling.

We design a model setup that reproduces South Atlantic central segment main characteristics

before rifting. We then explore the effect of sedimentation rate on subsidence and the effect of

mantle temperature on topography. The model accounts for decompression melting with feedbacks

on temperature, viscosity and density of the mantle. The subsidence in the thermo-mechanical

models is calibrated with a reference mid-ocean ridge elevation, where a 6 km thick oceanic crust

is predicted, and explained by the different contributions on buoyancy of rifted passive

margin during rifting. We discuss conditions to get magma-poor margins and conditions

to reach a small topographic gradient and shallow water environment between the proximal

and distal domains over more than 200 km of the wide margin during most of the syn-rift.