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Structural evolution of southern coastal Tanzania since the Jurassic

Nicholas, Christopher J., Pearson, Paul Nicholas, McMillan, Ian Kenneth, Ditchfield, Peter W. and Singano, Joyce M. 2007. Structural evolution of southern coastal Tanzania since the Jurassic. Journal of African Earth Sciences 48 (4) , pp. 273-297. 10.1016/j.jafrearsci.2007.04.003

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Following the rifting of Madagascar away from East Africa in the Triassic and Jurassic, a passive margin developed along what is now the Tanzanian coast. This period of tectonic stability lasted from the Late Cretaceous to Early Oligocene and was marked in southern Tanzania by the accumulation of a thick, outer shelf, clay-dominated succession now recognised as the Kilwa Group. Offshore rifting during the Miocene opened a series of basins along the margin parallel to the modern coastline. However, our recent field surveys along the southern coast between the towns of Kilwa and Lindi, which have focussed primarily on the Kilwa Group, have yielded diverse evidence for structural reactivation and inversion which we present here to give a more detailed interpretation of the structural evolution of this region since the Jurassic. Following extensive surface sampling and shallow coring in 22 boreholes, we present new planktonic foraminiferal biozone maps for Kilwa and Lindi which help to resolve the deformation in these two key areas. Growth faults active during the Miocene can be identified in the field at both Kilwa and Lindi, but there is also evidence of extensional reactivation of older, pre-Kilwa Group faults. Post-Miocene ‘flower’ structures, high-angle reverse faults and thrusts indicate that the area has undergone compression and strike-slip deformation in the past �6 Ma. Present day topography is also controlled by the regional fault pattern, or in some cases by relatively recent movement on faults. For instance, Singino, Mpara and Kimamba Hills in the Kilwa area are ‘pop-up’ structures. We use this observation to identify potential faults on satellite imagery in the eastern Mandawa basin (between Kilwa and Pande) and north-east Ruvuma basin (Lindi Bay) and combine this with new seismic interpretations across key lines to confirm faults and project them subsurface. We suggest that the Triassic–Jurassic regional north–north-west fault trend was reactivated at least twice. Miocene extension may have led to new faults, but certainly reactivated the older Mesozoic faults. Subsequently, both Miocene and older faults were reversed during Pliocene positive inversion, possibly related to doming from a mantle plume beneath the Tanzanian craton(s). We suggest here that reversal was either accompanied by, or followed by, strike-slip movement on these faults to develop the ‘flower’ structures and ‘pop-ups’ seen in the field and on seismic sections. This motion could be sinistral and linked to periodic stretching in the eastern arm of the East African Rift System over the past �1.2 Ma, combined with an east-south-east movement with minor clockwise rotation of the ‘Rovuma Plate’ of central and southern Tanzania.

Item Type: Article
Status: Published
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Uncontrolled Keywords: Kilwa Group; Neogene; Mandawa basin; Ruvuma basin; Growth faults; ‘Flower structures’; Fault reactivation; Inversion; East African Rift System
Publisher: Elsevier
ISSN: 0899-5362
Last Modified: 04 Jun 2017 03:00

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