Impact of the East African Rift System on the routing of the deep‐water drainage network offshore Tanzania, western Indian Ocean

Abstract

The East African Rift System (EARS) exerted a major influence on river drainage basins and regional climate of east Africa during the Cenozoic. Recent studies have highlighted an offshore branch of the EARS in the western Indian Ocean, where the Kerimbas Graben and the Davie Ridge represent its sea floor expression. To date, a clear picture of the impact and timing of this EARS offshore branch on the continental margin of the western Indian Ocean, and associated sediment dispersal pathways, is still missing. This study presents new evidence for four giant canyons along the northern portion of the Davie Ridge offshore Tanzania. Seismic and multibeam bathymetric data highlight that the southernmost three canyons are now inactive, supra-elevated relative to the adjacent sea floor of the Kerimbas Graben and disconnected from the modern slope systems offshore the Rovuma and Rufiji River deltas. Regional correlation of dated seismic horizons, integrated with well data and sediment samples, proves that the tectonic activity driving the uplift of the Davie Ridge in this area has started during the middle-upper Miocene and is still ongoing, as suggested by the presence of fault escarpments at the sea floor and by the location and magnitude of recent earthquakes. Our findings contribute to placing the Kerimbas Graben and the Davie Ridge offshore Tanzania in the regional geodynamic context of the western Indian Ocean and show how the tectonics of the offshore branch of the EARS modified the physiography of the margin, rerouting the deep-water drainage network since the middle Miocene. Future studies are needed to understand the influence of changing sea floor topography on the western Indian Ocean circulation and to evaluate the potential of the EARS offshore tectonics in generating tsunamigenic events.

Large-scale mass wasting in the western Indian Ocean constrains onset of East African rifting

“…Growing datasets indicate the diachroneity of magmatism, plateau uplift, and basin and flank formation spanning at least 40 million years to present day. Offshore data provide key new insights into the timing of rift initiation [13][14][15] .…”

“…The chronology of horizons M1-M4 ( Fig. 2) has been derived through biostratigraphic data from the exploration wells (Supplementary Tables 1-4) and correlation with dated horizons presented in previous studies 13,14,39,[43][44][45][46] . Horizons M1 and M2 encounter both wells where biostratigraphic data are available; M1 dates back to the Priabonian (upper Eocene), as suggested by nannofossil evidence indicative of Zone NP20-NP19 47 , while M2 dates to the Rupelian-Chattian (Oligocene) Zone NP24 47 (Fig.…”

“…Horizon M4 is not biostratigraphically constrained in the wells, but it correlates with horizon H3 of ref. 14 that was dated to the early Pliocene by using foraminifera assemblages. M4 marks the base of sequence Ng2 defined in ref.…”

“…1 ). Moving upward in the stratigraphy, the development of channel complexes became affected by the topography generated by Neogene to Quaternary tectonic activity in the offshore branch of EARS 43 , which led to the formation of the Kerimbas Graben and uplift of the Davie Ridge 13 , 14 .…”

“…Microfaunal and pollen assemblages sampled in exploration wells from the Rovuma delta offshore Mozambique indicate a change towards a cooler and wetter climate through the middle Eocene to Oligocene 50 , during the crustal uplift related to the EARS 41 . Recent studies from the western Indian Ocean also highlight the EARS’ influence on offshore sedimentation along the margins of Tanzania and Mozambique 13 , 14 . The evolution of broad flexural uplifts on the rift shoulders promoted the formation of endorheic basins and reorganized pre-existing basins, such as the reactivated Rukwa and Malawi basins, redirecting the flow of the Ruaha–Rufiji and Rovuma Rivers to the east 23 , 33 , 51 , 52 .…”