Effect of the Reunion plume head on Indo-Atlantic plate motions: evidence from the bend along the Southwest Indian ridge fracture zones

It was recently shown that the arrival at the Earth's surface of the Reunion plume head had a major effect on the motions of the Indo-Atlantic plates between 68 and 45 Ma (Cande and Stegman, 2011). The primary evidence for this is the observation that the motions of the Indian and African plates appear to have been coupled during this period: when the Indian plate speeds up between 68 and 66 Ma (from 80 to 180 mm/yr relative to Africa), the convergence of Africa with Eurasia slows down and perhaps stops, and when the Indian plate slows down between 52 and 45 Ma, Africa-Eurasia convergence speeds up. The fast motion of India relative to Africa is well documented by seafloor spreading anomalies and can be divided into three stages: 1) between 66 and 63 Ma, corresponding to the formation of the Deccan traps, when India moves superfast (roughly 180 mm/yr), 2) between 63 and 52 Ma when the speed of India is roughly 100 mm/yr, which is fast but not superfast, and 3) between 52 and 45 Ma when India's motion gradually slows from 100 to 40 mm/yr. The changes in Africa's motion are best recorded in the systematic "swings" of the Euler stage poles constraining the motion of Africa relative to North America, South America and Antarctica. The stage poles (relative to Africa) on all three plate pairs move along arcs first away from and then back towards the Africa-Eurasia Euler pole, constraining a major slowdown of the African plate relative to Eurasia which reaches its nadir between 57 and 53 Ma before returning to its pre-slowdown motion around 45 Ma. The systematic slowing and speedup of Africa is recorded in the great bends of the fracture zones in the Central Atlantic, South Atlantic and Southwest Indian ridges. Since the Indian and African plates were on opposite sides of the Reunion plume relative to Eurasia, the synchroneity of the plate motion changes strongly suggests that the plume head was a major driving force and, in particular, that the slow down of India between 52 and 45 Ma was due to the waning of this plume head effect. The slowdown and possible cessation of Africa's convergence with Eurasia is synchronous with the Paleocene period of "tectonic quiescence" in the Alps and extension in the Sirte Basin in North Africa. Seafloor spreading rates between Australia and Antarctica and between South America and Antarctica also slow down during this period. These observations provide valuable constraints for the geodynamic modeling of mantle convection. However, it was not clear if the maximum swing in the stage poles on the Southwest Indian ridge occurred at the same time as on the Central Atlantic and South Atlantic ridges. We are currently trying to better document certain aspects of these observations by re-examining motion on the Southwest Indian ridge. Cande, S.C. and D. R. Stegman. Indian and African plate motions driven by the push force of the Reunion plume head, Nature, v. 475, 47-52, 2011.