Controls of basement fabric on rift coupling and development of normal fault geometries: Insights from the Rukwa – North Malawi Rift

Abstract

The Rukwa Rift and North Malawi Rift Segments (RNMRS) both define a major rift-oblique segment of the East African Rift System (EARS), and although the two young rifts show colinear approaching geometries, they are often regarded as discrete rifts due to the presence of the intervening Mbozi Block uplift located in-between. This problem has been complicated by the dominance of the Rungwe volcanic features along the northeastern boundary of the Mbozi Block and lack of distinct normal faults along the southwestern boundary of the block. Here, we investigate the coupling of discrete rift segments during the onset of continental rifting, modulated by the control of pre-existing basement fabrics on the development of the border fault geometries and linkage across the intra-rift transfer zone. We utilized the Shuttle Radar Topography Mission Digital Elevation Models (SRTM-DEM) to investigate the morphological architecture of the rift domains; and aeromagnetic data and SRTM-DEM to assess the relationships between the rift structures and the pre-existing basement fabric (in plan-view). Our results show that the present-day morphology of the RNMRS is characterized by along-rift alternation of rift shoulder polarity, characteristic of coupled rift segments. Careful interpretation of filtered aeromagnetic maps along the northeastern and southwestern boundaries of the RNMRS reveal striking alignment of the rift-bounding faults with colinear NW-SE-trending pre-existing basement fabrics. We find that rift coupling along the northeastern boundary of the Mbozi Block transfer zone is accommodated by magmatism utilizing pre-existing fault systems, whereas, coupling along the southwestern boundary is accommodated by a new-found dextral strike-slip fault. Additionally, we show how the configuration of the pre-existing basement fabrics may influence the development of rectilinear or curvilinear normal fault geometries (plan-view) along the rifts, and the formation of basin-scale rift bifurcation around basement inter-rift transfer zones. In summary, we suggest that the structural continuation of the boundary faults along the RNMRS, and their alignment with colinear basement fabrics demonstrate the influence of structural inheritance on the coupling and amalgamation of approaching rift segments.

The origin of high helium concentrations in the gas fields of southwestern Tanzania

“…The proximity to the Cenozoic volcanism in the RVP likely provided the necessary heating for the release of accumulated radiogenic He (King and Metcalfe, 2013). Several deep faults are present in the basin and we suggest that Ivuna and Itumbula (RRB), which have the highest 4 He and 40 Ar*, may be connected to axial Itumbula rift faults, which focus radiogenic volatiles ( 4 He and 40 Ar) in the respective hydrothermal systems (Heilman et al, 2018; Figure 1; Supplementary Figure 1a). The LHS gas seeps have higher mantle He contributions (f mantle ) of ~12% versus those from the RRB consistent with the lower 4 He concentrations and higher CO2 concentrations.…”

“…To the southwest, the RRB is bounded by the Ufipa fault, and to the northeast by the Lupa Border fault, which is associated with the distribution of LHS seeps. To the south, the basin bifurcates into the Songwe half-graben and Msangano trough, which is separated from Songwe trough by the Mbozi plateau (Heilman et al, 2018). The volcanic deposits of RVP bind the RRB in the southeast end of the basin.…”