RELATION BETWEEN VOLCANISM AND RIFT SEGMENT IN THE EAST AFRICAN RIFT: INSIGHTS FROM A COMPARISON WITH ULTRASLOW OCEANIC RIDGES

Oceanic spreading center and continental rifts are two manifestations of plate divergence. One major difference is evidently that oceanic spreading center have achieved “drift”, that is, the preexisting plate has been sufficiently stretched that it is completely replaced by newly formed lithosphere. From a mechanical standpoint, though, “old” lithosphere is similar to “new” lithosphere. Unlike continental rifts, the mechanical lithosphere has essentially zero thickness at the axis of oceanic spreading centers.

In that respect, ultraslow spreading centers like the Southwest Indian Ridge (SWIR) blur the difference between oceanic and continental divergent zones. At spreading rates less than 10 mm/yr, heat conduction is no longer negligible compared to mantle upwelling, so that the mechanical lithosphere immediately underneath the ridge axis adopts a steady-state thickness that can reach 30 km. These ultraslow ridges differ markedly from the textbook view of mid-ocean ridges: the ridge axis may not be orthogonal to plate motion, its orientation can changes abruptly along strike, without intervening transform faults, and volcanism is no longer continuous. Instead, localized volcanic centers are associated with changes in ridge azimuth. Volcanic activity can be explained by considering the effects of varying lithospheric thickness on melt migration.

The East African Rift (EAR), especially around the Tanzania Craton, bears some similarities with the characteristics of the SWIR. Different segments have different degrees of obliquity with respect to the plate divergence direction, transform faults are absent, and volcanism is punctuated geographically. I calculate the effective spreading rate (the parameter controlling lithosphere thickness at oceanic spreading centers) of each segment of the EAR and discuss its relation with volcanic activity. If the analogy with the SWIR holds, it implies that the mantle in the Northern half of the Tanzania Craton is significantly hotter than in its southern half and underneath mid-ocean ridges. This conclusion is consistent with the involvement of a mantle plume in facilitating rifting. It implies that the thermal effects of the plume are still important today and that they are responsible for the volcanism EAR.