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

Abstract

Faulting and earthquakes occur extensively along the flanks of the East African Rift System, including an offshore branch in the western Indian Ocean, resulting in remobilization of sediment in the form of landslides. To date, constraints on the occurrence of submarine landslides at margin scale are lacking, leaving unanswered a link between rifting and slope instability. Here, we show the first overview of landslide deposits in the post-Eocene stratigraphy of the Tanzania margin and we present the discovery of one of the biggest landslides on Earth: the Mafia mega-slide. The emplacement of multiple landslides, including the Mafia mega-slide, during the early-mid Miocene is coeval with cratonic rifting in Tanzania, indicating that plateau uplift and rifting in East Africa triggered large and potentially tsunamigenic landslides likely through earthquake activity and enhanced sediment supply. This study is a first step to evaluate the risk associated with submarine landslides in the region.

Architecture, structural and tectonic significance of the Seagap fault (offshore Tanzania) in the framework of the East African Rift

“…The DFZ was reactivated by localized extensional (e.g. Kerimbas and Lacerda Basin, see Maselli et al, 2020, Vormann & Jokat, 2021a) and contractional structures of Hauterivian‐Aptian age (e.g. Intawong et al, 2019; Mahanjane, 2014, Roche & Ringenbach, 2022) (Figure 2).…”

“…Grimison and Chen (1988) analysed a swarm of 5 < mb <6.4 earthquakes with normal faulting mechanisms along N‐S striking nodal planes, and they interpreted the Davie Ridge as the eastern edge of the EARS. Although gravity and magnetic anomalies and their derivatives have been used to link the south Somalian Wadu Ridge structure to the DFZ (Gaina et al, 2013; Reeves & de Wit, 2000), seismic reflection data do not show clear evidence of the DFZ north of 9° S (Klimke & Franke, 2016; Maselli et al, 2020; Sauter et al, 2018). Vormann et al (2020) trace the DFZ south to 27°S latitude as a bathymetric ridge (Figures 1 and 2).…”

“…The area of investigation comprises a series of slope‐deep water channel complexes which reflect both the sediment supply from the Ruaha/Rufiji and Rovuma rivers and the influence of the bottom currents which are northward directed (Sansom, 2018). By integrating seismic and gamma ray data from 2 wells, Maselli et al (2020) identified 13 depositional cycles and defined three main units bounded by four horizons covering, from the base of the Eocene to the Quaternary, a 45‐million‐year time span including the reflectors M1, M2, M2a (Figure 3). From the Eocene, the offshore Tanzania passive margin has recorded a sediment influx initiated by plateau uplift (e.g.…”

“…Along the coastal zone of the passive margin, since the Miocene, a series of normal fault footwalls formed topographic highs such as Zanzibar, Pemba and Mafia islands, which further re‐routed the main deep‐water drainage (Dottore Stagna et al, 2022). Between 22.9 Ma and 19.8 Ma a submarine landslide of area more than 11,000 km 2 , named the Mafia mega‐slide, was emplaced above the reflector M2 (Maselli et al, 2020). The landslide was most likely a consequence of initial volcanism, earthquakes, and drainage changes in the Rukwa‐Malawi rift zone, and dynamic uplift of the East African margin in the Western Indian Ocean (e.g.…”

“…Well‐to‐seismic tie and biostratigraphy of Well‐1 and Well‐2. Well‐to‐seismic tie of Well‐1 and Well‐2 with dated stratigraphic horizons (M1‐M2–M2a), seismic amplitude at the well sites, gamma‐ray (GR), velocity model from check‐shots, depth below sea level in two‐way travel time (TWT, milliseconds) and true vertical depth (ssTVD, metres), and depositional units (DU) from Maselli et al (2020). Stratigraphic sequences (Pg1 to Ng2) are from Sansom (2018).…”