Mantle Wavespeed and Discontinuity Structure Below East Africa: Implications for Cenozoic Hotspot Tectonism and the Development of the Turkana Depression

Abstract

Ethiopia's Cenozoic flood basalt magmatism, uplift, and rifting have been attributed to one or more mantle plumes. The Nubian plate, however, has drifted 500–1,000 km north since initial magmatism at ∼45 Ma, having developed above mantle that now underlies the northern Tanzania craton and the low‐lying Turkana Depression. Unfortunately, our knowledge of mantle wavespeed structure and mantle transition zone (MTZ) topography below these regions is poorest, due to a historical lack of seismograph stations. The same data gap means we lack constraints on lithospheric structure in and around the NW–SE trending Mesozoic Anza rift. We exploit data from new seismograph networks in the Turkana Depression and neighboring northern Uganda to develop AFRP22, a new African absolute P‐wavespeed tomographic model that resolves whole mantle structure along the entire East African rift system. We also map MTZ thickness using Ps receiver functions. East Africa's thinnest MTZ (∼25 km thinning) underlies the northwest Turkana Depression. AFRP22 reveals a co‐located, previously unrecognized, slow wavespeed plume tail, extending from the MTZ, deep into the lower mantle. This plume may thus have contributed, along with the African Superplume, to the development of the 45–30 Ma flood basalt province that preceded extension. Pervasive sub‐lithospheric slow wavespeeds imply that Turkana's present‐day low elevation is explained best by Mesozoic and Cenozoic‐age crustal thinning. At ∼100 km depth, AFRP22 illuminates a fast wavespeed SE Ethiopian plateau. In addition to governing the northernmost limit of Mesozoic Anza rifting, the refractory nature of this lithospheric block likely minimized Cenozoic flood basalt magmatism there.

Seamless Map of Depth to the Moho Interface in the Afro‐Arabian Region Using Gravity Data Derived From EGM2008

“…35 Ma (Hofmann et al, 1997). The role of one or more mantle plumes for the initiation of rifting based on seismic imaging and geodynamical modeling is discussed in Boyce et al (2023), Chang et al (2020), andLin et al (2005).…”

Active Deformation Constraints on the Nubia‐Somalia Plate Boundary Through Heterogenous Lithosphere of the Turkana Depression

“…Regional body and surface wave models image details within the super plume province and reveal two or more whole-mantle slow wave speed anomalies (Boyce et al, 2023;Chang et al, 2020;Emry et al, 2019). These separately imaged plumes are suggested to have influenced the spatial and temporal variations in mantle sources for magma in the EAR (e.g., Boyce et al, 2023;Chang et al, 2020). Using the TRAILS database, Kounoudis et al (2021) imaged velocity variations in the mantle at depth ranges of ∼75-600 km.…”

“…Pre‐existing lithospheric terranes of northeastern Africa include the >150 km‐thick lithosphere of the Archean Tanzania craton and the ∼120 km‐thick Pan‐African orogenic belt (e.g., Boyce et al., 2023; Emry et al., 2019). The Pan‐African orogeny (700–550 Ma) led to the accretion of island‐arcs and micro‐continents to the Tanzania craton (e.g., Fritz et al., 2013; Stern et al., 2012; Teklay et al., 1998).…”

“…Global tomographic models support the hypothesis that the African superplume rises from the core mantle boundary beneath southern Africa to the base of the lithosphere beneath East Africa (e.g., French & Romanowicz, 2015; Ritsema et al., 2011). Regional body and surface wave models image details within the super plume province and reveal two or more whole‐mantle slow wave speed anomalies (Boyce et al., 2023; Chang et al., 2020; Emry et al., 2019). These separately imaged plumes are suggested to have influenced the spatial and temporal variations in mantle sources for magma in the EAR (e.g., Boyce et al., 2023; Chang et al., 2020).…”

“…The NW‐trending Turkana Depression, a failed Mesozoic rift system, separates the uplifted Ethiopia‐Yemen and East Africa plateaus that formed above one or more mantle plumes (e.g., Boyce et al., 2023; Chang & Van der Lee, 2011; George et al., 1998; Pik et al., 2006) (Figure 1a). The Oligocene‐Recent East African rift zones transecting the plateaus overlap and link within the 300 km‐wide Depression, but little was known of the age of structures, the kinematics of rift linkage, and the role of pre‐existing structures in strain evolution (e.g., Knappe et al., 2020).…”