Seismotectonics and active faulting of Usangu basin, East African rift system, with implications for the rift propagation
Introduction
The southward propagation of the Western branch of the East African Rift System (EARS) through Uganda, Rwanda and Burundi into western and southern Tanzania in the Rungwe Volcanic Province (RVP), where the rift zone forms the triple junction, has been seismically well studied (Camelbeeck and Iranga, 1996, Brazier et al., 2005, Ferdinand and Arvidsson, 2002) (Fig. 1). The RVP lies between three major border faults systems (Livingstone, Rukwa and Usangu border faults) forming the triple junction, all of which have been active in Miocene–Recent times (Fontijn et al., 2010, Fontijn et al., 2012). The triple junction in the RVP links the NW trending South Rukwa and North Malawi rift basins with the NE trending Usangu basin. The Usangu basin is located at the northern outskirts of the RVP (Fig. 1), where the nature of the seismicity and stress regime and the locus of rifting are, by contrast, less well understood. The basin, like the Rukwa/Songwe and north Malawi/Karonga basins, has been described to develop in three stages that include initial border fault development, asymmetric basin subsidence/flank uplift, and development of monoclines opposite the border faults; and continued subsidence and tilting along intrabasinal faults with flexural upwarping of the rift flanks, enhancing basinal asymmetries (Ebinger et al., 1989). The Usangu basin lies within the proposed tectonic boundary between the Victoria and Ruvuma microplates, which are rotating in counterclockwise and clockwise directions, respectively, with respect to Nubian Plate (Calais et al., 2006; Stamps et al., 2008; Saria et al., 2013). The basin is the rift basin located close to the intersection of the North Malawi and South Rukwa rift basins. It is suggested to undergo strike slip deformation transfer fault by accommodating the relative movement between the southern-most sub-basin of the Rukwa rift and northern-most sub-basin of the Malawi rift (Delvaux et al., 1992; Mbede, 2002).
The seismicity and stress regime in Western branch and its southern propagation along the south Tanganyika–Rukwa–north Malawi rift segment (TRM) have been studied in great detail (e.g., Camelbeeck and Iranga, 1996; Brazier et al., 2005; Delvaux and Barth, 2010; Ferdinand and Arvidsson, 2002; Delvaux et al., 2012), but the seismicity and stress regime of the NE-SW extending Usangu basin of the Western branch north to northeast of the RVP has not been similarly investigated and is therefore less well understood. This is mainly because until recently, there have been few seismic networks within the area to record local seismicity. In this paper, data from temporary and permanent seismic stations in and surrounding southern Tanzania operated under the SEGMeNT project together with gravity/magnetic data are used to investigate the seismicity and regional stress regime and evaluate the role played by the Usangu basin in accommodating the relative movement between the two sub-basins of the Rukwa rift and the Malawi rift.
Section snippets
Geology and tectonic setting
The regional tectonic and topography, and local geology of the study area are shown in Fig. 1, Fig. 2 respectively, which include the area to the south of the Tanzania Craton along the southern side of the East African Plateau. Regionally the Tanzania Craton forms the core of the Precambrian tectonic framework of eastern Africa and is flanked by several Proterozoic mobile belts (Fig. 1, Fig. 2). To the west and southwest of the Craton lie the Ubendian Belt whereby in the eastern and
Seismic data
The data used in this study are broadband seismic data from temporary and permanent networks in Tanzania operated under the SEGMeNT project (Shillington et al., 2016; Fig. 1, Fig. 4). The installation of these stations was comprised of 24-bit Reftek data recorders with Streckeisen STS-2, Guralp 3 T, and 40 T broadband seismometers coupled with GPS clocks. The data were recorded continuously in three components at 50 samples per second. The data used are from two temporary SEGMeNT seismic
Seismicity and depth distribution
A total of 143 events around and within the Usangu basin were located and are shown in Fig. 4. The spatial distribution of the events in the basin shows that the events are distributed throughout the southwestern half of Usangu basin but concentrated along the Usangu border fault and within the southwestern part of the basin. Limited seismicity is observed in the southern part of the basin (Fig. 4). The temporal distribution of the events does not show any correlation within the region, and the
Discussion
In summary, the two main findings from the seismicity pattern are; (1) most of the seismicity along the Usangu border fault and in the southwest of the basin, and (2) there is a more limited amount of seismicity within the basin and in the southern end of the basin. The depth distribution of the seismicity shows distribution throughout the crust with events occurring as deep as ~39 km (Fig. 5). Focal mechanisms from selected events along the Usangu border and intrabasinal transfer faults as
Summary
From our analyses, it has been found that ~70% of the seismicity follows a northeast pattern into the southwestern end of the basin. In contrast, a lesser amount of seismicity (~30%) occurs in the south to southwest towards RVP. Structural results from aeromagnetic data show variations of faults and magnetic lineaments that trend in NE-SW, NNE-SSW, E-W, N-S and NW-SE directions. These findings suggest that the Usangu border fault and transfer faults within the basin are directly associated with
Credit author statement
Mulibo, G.D: All the work of this paper has been done by the author which include Conceptualization, Data curation, Formal analysis, Funding acquisition, Writing - original draft, Writing - review and editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work has been supported by the University of Dar es Salaam through the Directorate of Research and Publication (grant number CoNAS-GY18093). The study has used data from temporary and permanent seismic stations from SEGMeNT project and AfricaArray respectively. I thank all institutions involved in this project and many individuals from those institutions for their assistance with the data collection and archiving. Waveform data were obtained from the IRIS Data Management Center (IRIS–DMC).
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