Continental rifts most often nucleate within orogenic belts. However, some studies in the East African rift system have shown that continental rifts can also develop within cratons. This work investigated the ~1.5‐Ma Eyasi basin, which propagates in a WSW direction into the Tanzanian craton. The basin is located where the eastern branch of the East African rift system transitions from a narrow rift (~70 km wide) to the wider (~300 km wide) North Tanzanian Divergence. Unlike the rest of the eastern branch segments, the Eyasi basin does not follow the Mozambique orogenic belt located on the eastern margin of the Tanzanian craton. This work generated lithospheric‐scale sections across the basin using (1) digital elevation model to map surface rift‐related brittle structures; (2) aeromagnetic data to determine the depth to the Precambrian basement; and (3) World Gravity Model 2012 to estimate crustal and lithospheric thickness by applying the two‐dimensional radially averaged power spectral analysis and two‐dimensional forward gravity modeling. These cross sections show that the Eyasi basin nucleates within a previously unidentified suture zone within the Tanzanian craton and that this suture zone is characterized by thinner lithosphere that can be as thin as ~95 km. This zone of thinner lithosphere is offset southeastward from the surface expression of the Eyasi basin and might have facilitated the formation of other basins further south. Furthermore, the lithospheric thickness map indicates that the Tanzanian craton is heterogeneous and possibly composed of multiple smaller cratonic fragments.
We used aeromagnetic and satellite gravity data to investigate lithospheric structure beneath the Cretaceous Chilwa Alkaline Province (CAP) in southern Malawi and adjacent Mozambique. The CAP consists of granites, syenites, nepheline syenites, basanites, phonolite dikes, and minor carbonatite bodies. The intrusions were emplaced in the Precambrian (Mesoproterozoic-Neoproterozoic) terranes of the Southern Irumide and Mozambique orogenic belts. Aeromagnetic data show the CAP as overlapping circular anomalies typical of nested igneous ring complexes formed through caldera collapse mechanism. We used gravity data to infer that: (1) the CAP was sourced from~30-km wide igneous bodies now preserved in the upper crust at~5-km depth.(2) the CAP is underlain by up to~45-km thick crust (due to mafic magmatic underplating) and a lithosphere as thin as~90 km. These data suggest that mafic magmatic underplating and lithospheric thinning occurred during a Cretaceous rifting event. We propose, based on these results and taking into account previous petrographic, geochemical, geochronological, and isotopic studies, that the silica undersaturated magmatic phase of the CAP was due to flux melting of the asthenosphere. This was followed by silica-rich magmatic phase due to decompression melting of the asthenosphere as a result of lithospheric thinning. Lithospheric thinning and ascendance of the asthenospheric melt might have been facilitated by the presence of late Neoproterozoic to early Cambrian suture zone. The heat provided by the mafic magmatic underplating resulted in partial melting of the lower crust to form the silica-saturated CAP intrusions from mixed magma sources.
The Turkana Depression (TD) is a NW trending topographic corridor within the East African Rift System between the Ethiopia‐Yemen plateau in the northeast and the East African plateau to the southwest. The Anza rift within the TD is a NW‐SE trending failed arm of a late Jurassic rift‐rift‐rift triple junction. This rift is correlated with the Sudan and South Sudan rifts. The Anza rift is intersected by the East African Rift System represented by the N‐S trending Turkana rifted zone. We image the lithospheric structure beneath the TD using satellite gravity data. We also use these data to model crustal density distribution beneath the Kino Sogo fault belt, part of the Turkana rifted zone. Our results show thinner crust (23–28 km) and lithosphere (140–150 km) beneath the TD. We interpret this as due to extension that resulted in the formation of the Kenya‐Sudan and South Sudan rifts. Our results also show that crustal depth between 0 and 4.8 km is dominated by N‐S density contrast anomalies and between 4.8 and 14.5 km by E‐W anomalies. We interpret the N‐S anomalies as due to the presence of Precambrian structure that might have facilitated strain localization during the initiation of the Kino Sogo fault belt. Differently, we interpret the E‐W anomalies as due to the presence E‐W trending faults that were formed in association with the development of the Anza rift and/or Turkana rifted zone and were later filled with Mesozoic and Cenozoic mafic dikes.
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