Modeling lithospheric thickness along the conjugate South Atlantic
passive margins implies asymmetric rift initiation
Abstract
The lithospheric architecture of passive margins is crucial for
understanding the tectonic processes that caused the break-up of
Gondwana. We highlight the evolution of the South Atlantic passive
margins by a simple thermal lithosphere-asthenosphere-boundary (LAB)
model based on rifting time, crustal thickness, and stretching factors.
We simulate the different rifting stages that caused the opening of the
South Atlantic Ocean and pick the LAB as the T=1330 °C isotherm, which
is calculated by 1D advection and diffusion. In a synthetic example, we
demonstrate that the initial crustal thickness has the largest effect on
the thermal LAB. For the South American passive margin, our modeled LAB
shows a deep and smooth structure between 110-150 km depth at equatorial
latitudes and a more variable LAB between 50-200 km along the southern
part. This division reflects different stages of the South Atlantic
opening: initial opening of the southern South Atlantic causing
substantial lithospheric thinning, followed by rather oblique opening of
the equatorial South Atlantic accompanied by severe thinning. The
modeled LAB reflects a high variability associated with tectonic
features on a small scale. Comparing the LAB of the conjugate South
American and African passive margins in a Gondwana framework reveals a
variable lithospheric architecture for the southern conjugate margins.
Along selected conjugate margin segments stark differences up to 80 km
of the LAB depths correlate with strong gradients in margin width. This
mutual asymmetry suggests highly asymmetric melting and lithospheric
thinning prior to rifting.