Convective Patterns under the Indo-Atlantic box

Using recent fluid mechanics results as a framework, we reinterpret the images of the Indo-Atlantic mantle obtained from global and regional tomography studies together with geochemical, geological and paleomagnetic observations to unravel the pattern of convection in the Indo-Atlantic box and its temporal evolution over the last 260 Myr. Seismic tomography sections at different depths show that the Earth's mantle seems to be divided in two boxes by the subducted plates, the Pacific and the Indo-Atlantic boxes. The latter presently contains a) a broad slow seismic anomaly at the CMB which has a similar shape to Pangea 250 Myr ago, and which divides into several branches higher in the lower mantle, b) one superswell centered on the western edge of South Africa, c) at least 6 primary hotspots with long tracks related to traps, and d) numerous smaller hotspots. Moreover, in the last 260 Myr, this mantle box has undergone 10 traps events, 7 of them related to continental break up. Several of these past events are spatially correlated with present-day seismic anomalies and/or upwellings, suggesting episodicity. Laboratory experiments show that superswells, long-lived hotspot tracks and traps may represent three evolutionnary stages of the same phenomenon, i.e. the episodic destabilization of a hot, chemically heterogeneous thermal boundary layer, close to the bottom of the mantle. When scaled to the Earth's mantle, the recurrence time of this phenomenon is on the order of 100-200 Myr. Also, at any given time, the Indo-Atlantic box should contain 3 to 9 of these instabilities at different stages of their development. This is in agreement with observations. The return flow of the downwelling slabs, although confined to two main boxes by subduction zone geometry, may therefore not be passive, but rather take the form of active thermochemical instabilities.