Intrarift fault fabric, segmentation, and basin evolution of the Lake Malawi (Nyasa) Rift, East Africa

Abstract

<jats:p>The Lake Malawi (Nyasa) Rift, in the East African Rift System (EARS), is an ideal modern analogue for the study of extensional tectonic systems in low strain rate settings. The seismically active rift contains the 700-m-deep Lake Malawi, one of the world’s oldest and largest freshwater lakes with one of the most diverse endemic faunal assemblages on Earth. Modern and reprocessed legacy multi­channel seismic-reflection data are constrained by velocity information from a wide-angle seismic experiment to evaluate variability in extension, segmentation, and timing of fault development along the 550-km-long rift zone. Fault geometries and patterns of synrift sediment fills show that the Lake Malawi Rift is composed of three asymmetric rift segments, with intervening accommodation zone morphologies controlled by the degree of overlap between segment border faults. Most extension occurs on the basin border faults, and broadly distributed extension is only observed at one accommodation zone, where no border fault overlap is observed. Structural restorations indicate a weakly extended rift system (~7 km), with dimin­ishing values of extension and thinner rift fill from north to south, suggesting a progressively younger rift to the south. There is no evidence of diking, sill injection, or extrusives within the synrift fill of the Lake Malawi Rift, although the volcanic load of the Rungwe magmatic system north of the lake and related subsidence may explain the presence of anomalously thick synrift fill in the northernmost part of the lake. The thickest synrift depocenters (~5.5 km) are confined to narrow 10- to 20-km-wide zones adjacent to each rift segment border fault, indicating concentration of strain on border faults rather than intrarift faults. Intrarift structures control axial sediment delivery in the North and Central rift segments, focusing sediment into confined areas resulting in localized overpressure and shale diapirs. The asymmetric, basement-controlled relief was established early in rift development. When over­printed with frequent high-amplitude hydroclimate fluctuations, which are well documented for this basin, the resulting highly variable landscape and lake morphometry through time likely impacted the diverse endemic faunas that evolved within the basin. New seismic-reflection data, augmented by wide-angle seismic data and age constraints from drill core, offer the most highly resolved 3D view to date of latest Cenozoic extensional deformation in East Africa and provide a foundation for hazards analysis, resource assessments, and constraining deformation in a low strain rate, magma-poor active rift.</jats:p>

Thermochemical Modification of the Upper Mantle Beneath the Northern Malawi Rift Constrained From Shear Velocity Imaging

“…The upper crust is highly heterogeneous, with surface exposures of metamorphic terranes, young lavas, and broad deep, fault-bounded sedimentary basins, with abrupt transitions between them. Such structures are not well described by a 0.2 o x0.2 o model and we leave detailed discussion to other analyses (Accardo et al, 2018;Shillington et al, 2020;Scholz et al, 2020). At mid-to-lower crustal depths (16, 22 km), the shear-velocity model captures a localized, low-velocity anomaly beneath RVP with velocities in the range of 3.5-3.6 km/s, surrounded by crust that shows small-amplitude, large-scale variations in the range of 3.7-3.9 km/s; Figs.…”

“…At shallow depths, the Malawi Rift is made up of three border-fault-bounded basins: North, Central, and South (Scholz et al, 2020;Fig. 1b).…”

“…SEGMeNT enables high-resolution constraints on faulting and rift-basin structure and evolution (Accardo et al, 2018;Ebinger et al, 2019;Scholz et al, 2020), crustal and lithospheric architecture (Borrego et al, 2018;Hopper et al, 2018), and crust and mantle lateral heterogeneity and anisotropy (Accardo et al, 2017;Grijalva et al, 2018;Tepp et al, 2018).…”

The Entire Crust can be Seismogenic: Evidence from Southern Malawi

“…For southern Malawi, the extension rates are about 2 mm/yr, roughly ENE-WSW in orientation (Saria et al, 2014;Stamps et al, 2018). The Malawi Rift propagated southward (C. Scholz et al, 2020), with the start of rifting in the North Basin of Lake Malawi around 9-20 Ma (Ebinger et al, 1993;Mortimer et al, 2016), and the Central Basin roughly 4.6 Ma (McCartney & Scholz, 2016). Earthquake mechanisms from the gCMT catalog (Ekström et al, 2012) are all normal faulting, striking similar to the border faults, NNW-SSE and N-S.…”

Rift Transfer Zones and the Stages of Rift Linkage in Active Segmented Continental Rift Systems

“…The southern Malawi Rift evolved through the episodic southward propagation of the Malawi Rift (Scholz et al, 2020), subsequently leading to its tectonic interaction with the eastern sub-basin (Lower Shire sub-basin) of the Shire Rift Zone. The North Zomba Graben is dominated by paleo-lake sedimentary deposits (Matope Beds and associated lacustrine clays) of Neogene age or younger which directly overlie the Precambrian basement unconformity surface, and are overlain by the alluvial sediments of the Shire River (Bloomfield and Garson, 1965;Thomas et al 2009;Lyons et al 2015;Dunlaya, 2017;Wedmore et al 2020).…”