Evidence of recent seismicity on the West African Craton: the seismites of Bamako, Mali

Résumé

The West African Craton (WAC) has been considered tectonically inactive and stable since the Proterozoic. The later findings of seismites embedded in the Quaternary lacustrine and fluvial deposits of the Niger River valley at Bamako, Republic of Mali, seem to certify the contrary. The seismites discovered in the deposits include three clastic dikes, soft-sediment faults, microfaults and folds, convolutes, and ball structures. The clastic dikes were studied during various fieldwork campaigns: samples were collected for grain-size and heavy-mineral analyses and OSL dating. The results show that 2 of the clastic dikes (clastic dike 01 and clastic dike 03) are oriented 285 degrees and are about 4 kilometers apart, but parallel. They measure 80 to 100cm wide. The third is oriented 170 degrees but is smaller, with a width of 40 cm. The grain-size analysis shows that the clastic dikes are polygenic, with three modes, and are entirely different from the lacustrine deposits they crosscut. The heavy-mineral analysis also confirms the markedly different mineralogical composition between the clastic dikes and the adjacent horizontal layers. The vertical clastic dikes have higher zircon, tourmaline, and rutile contents, attesting that they were extruded and remobilized from the adjacent lacustrine layers. OSL dating gives approximately the same ages for the lacustrine and clastic dikes, at 280 and 250 Ka, respectively. However, the event that triggered the formation of the clastic dikes is much younger than the lacustrine limit, and the fluvial deposits that it cross-cuts date to 17 Ka. Three main conclusions can be drawn from the clastic dikes. Their direction suggests two different seismic events. The two widest parallel dikes are from a seismic event different from the small one. The direction of the seismic waves can also be assessed. They are perpendicular to the dikes and are inferred to be 15 degrees NNE, with some easily identifiable faults present in the area. The epicenter that triggered the earthquake can also be set to a 20 km radius around the dikes. Lastly, the magnitude of the earthquake that caused them can also be inferred to be around M6.5 or M7, depending on their width, which is usually proportional to the earthquake’s magnitude. The presence of these seismites at Bamako, on the West African Craton, is very significant in certifying that the Craton is not as stable as previously thought and to put into context the contemporary earthquakes that have been reported on the area: Ghana (1939), Odienné, Ivory Coast (2007 and 2020), Kolokani, Mali (1999), Diangounté Kamara, Mali (2018).