Under-Displaced Normal Faults: Strain Accommodation Along an Early-Stage Rift-Bounding Fault in the Southern Malawi Rift

Abstract

One of the fundamental problems in continental rift segmentation and propagation is how strain is accommodated along large rift-bounding faults (border faults) since the segmentation of propagating border faults control the expression of rift zones, syn-rift depo-centers, and long-term basin evolution. In the Southern Malawi Rift, where previous studies on the early-stage rifting only assessed border fault structure from surficial and topographic expression, we integrate surface and subsurface data to investigate border fault segmentation, linkage, and growth as proxies for strain accommodation along the Bilila-Mtakataka Fault (BMF) System. We used 30 m-resolution topographic relief maps, electrical resistivity tomography (ERT), and high-resolution aeromagnetic data to characterize the detailed fault geometry and provide a more robust estimate of along-fault displacement distribution. Our results reveal a discrepancy between sub-aerial segmentation of the BMF geometry (six segments), scarp height (five segments) reflecting the most recent episodes of fault offset, and cumulative throw (three composite segments) reflecting the long-term fault offset. We also observe that although the BMF exhibits continuity of sub-aerial scarps along its length, the throw distribution shows a higher estimate at the Northern-to-Central segment relay zone (423 m absolute, 364 m moving median) compared to the Central-to-Southern segment relay zone (371 m absolute, 297 m moving median). The ERT profiles across the relay zones suggest a shallower basement and a possible canyon-mouth alluvial fan stratigraphy at the Central-to-Southern segment relay zone, contrasting the deeper basement and “simpler” electrical stratigraphy at the Northern-to-Central relay. The results suggest a more complex long-term evolution of the BMF than was assumed in previous studies. A comparison of BMF’s maximum displacement-vs-length with those of other Malawi Rift border faults and global normal fault populations suggest that although the BMF has possibly reached its maximum length, it remains largely under-displaced as its 580–837 m maximum displacement is significantly lower than that of faults of equivalent length. We suggest that the BMF may continue to accrue significant strain as tectonic extension progresses in the Southern Malawi Rift, thus posing a major seismic hazard in the region.

Knickpoint morphotectonics of the Middle Shire River basin: Implications for the evolution of rift interaction zones

“…Hypothetical paleographical reconstruction of the Middle Shire rift interaction zone from (a) the Pliocene‐Early Pleistocene to(b) the Middle Pleistoceneand present‐day, connecting the Zomba Graben (in Southern Malawi Rift) and the Lower Shire Graben (in Shire Rift Zone), after Dulanya (2017), Kolawole et al (2021, 2022), and Ojo, Ohenhen, et al, 2022, Ojo, Thomson, & Laó‐Dávila, 2022). …”

Low Dissipation of Earthquake Energy Where a Fault Follows Pre‐Existing Weaknesses: Field and Microstructural Observations of Malawi’s Bilila‐Mtakataka Fault

“…Variations in fault displacement, pressure‐temperature ( P‐T ) conditions, and fabric composition can influence fault structure‐foliation interactions (see Section 1). However, the BMF has accommodated similar amounts of displacement at Kasinje and Mua (Hodge et al., 2018; Ojo et al., 2022), both sites reflect deformation at near‐surface P‐T conditions, and their metamorphic fabrics and composition are broadly similar. We therefore interpret that the differences in fault zone width between Kasinje and Mua are associated with the BMF’s local orientation relative to the surrounding foliation.…”

“…This reflects limited constraints on hanging-wall sediment thickness and challenges with distinguishing between fault-related footwall exhumation and pre-existing topography imposed by lithologic variations (Figure 1). Nevertheless, an estimate of maximum displacement, D max , can be made by considering the escarpment's maximum height (∼300 m, Figure 1), and that boreholes (Figure 1a; Dawson & Kirkpatrick, 1968;Walshaw, 1965), aeromagnetic data (Ojo et al, 2022), and the syn-rift sediment thickness across strike in Lake Malawi (Scholz et al, 2020) constrain The BMF scarp's six sections (Hodge et al, 2018) underlain by a 12 m resolution TanDEM-X digital elevation model, and (b) its surrounding geologic units and foliation orientations (Dawson & Kirkpatrick, 1968;Hodge et al, 2018;Walshaw, 1965). Inset in (a), the BMF location in the context of the East African Rift.…”

A review of structural inheritance in rift basin formation

“…Alternatively, pervasive lateral heterogeneities in the crust (e.g., a wide anastomosing shear zone) can promote distributed deformation involving migration of extensional strain from the basin boundary to intrabasin faults (Kolawole et al, 2018(Kolawole et al, , 2021bWedmore et al, 2020a). Normal fault lengthening by exploitation of pre-existing fabrics (Section 2.1.4; Walsh et al, 2002) may account for why faults in the EARS achieved their full length at an early stage of their displacement accumulation (Vétel et al, 2005;Accardo et al, 2018;Corti et al, 2019;Ojo et al, 2022) and exhibit narrow fault damage zones and large single-earthquake displacement-tolength ratios (Figure 11) (Hodge et al, 2020;Williams et al, 2022). An important observation at the < 100 km scale is therefore that although the EARS is a classic example of plate-scale structural inheritance and co-location with relatively weak lithosphere (e.g., Versfelt and Rosendahl, 1989), this inheritance is not synonymous with reactivation.…”