The African continental divide: Indian versus Atlantic Ocean spreading during Gondwana dispersal

It is well established that plate-tectonic processes operate on a global scale and that
spatially separate but temporally coincident events may be linked. However, identifying
such links in the geological record and understanding the mechanisms involved
remain speculative. This is particularly acute during major geodynamic events, such as
the dispersal of supercontinents, where multiple axes of breakup may be present as well
as coincidental collisional events. To explore this aspect of plate tectonics, we present a
detailed analysis of the temporal variation in the mean half rate of seafloor spreading
in the Indian and Atlantic Oceans, as well as plate-kinematic attributes extracted from
global plate-tectonic models during the dispersal of Gondwana since ca. 200 Ma. Our
analysis shows that during the ~20 m.y. prior to collision between India and Asia at
ca. 55 Ma, there was an increase in the mean rate of seafloor spreading in the Indian
Ocean. This manifests as India rapidly accelerating toward Asia. This event was then
followed by a prompt deceleration in the mean rate of Indian Ocean seafloor spreading
after India collided with Asia at ca. 55 Ma. Since inception, the mean rate of seafloor
spreading in the Indian Ocean has been generally greater than that in the Atlantic
Ocean, and the period of fastest mean half spreading rate in the Indian Ocean was coincident
with a slowdown in mean half seafloor spreading rate in the competing Atlantic
Ocean. We hypothesize that faster and hotter seafloor spreading in the Indian Ocean
resulted in larger ridge-push forces, which were transmitted through the African plate,
leading to a slowdown in Atlantic Ocean spreading. Following collision between India
and Asia, and a slowdown of Indian Ocean spreading, Atlantic spreading rates consequently
increased again. We conclude that the processes in the Indian and Atlantic
Oceans have likely remained coupled throughout their existence, that their individual
evolution has influenced each other, and that, more generally, spreading in one basin
inevitably influences proximal regions. While we do not believe that ridge push is the
main cause of plate motions, we consider it to have played a role in the coupling of the
kinematic evolution of these oceans. The implication of this observation is that interaction
and competition between nascent ocean basins and ridges during supercontinent
dispersal exert a significant control on resultant continental configuration.

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