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Age of Lake Malawi (Nyasa) and Water Level Fluctuations
Abstract and Figures
Lake Malawi (Nyasa) is one of the few deep-water long-lived lakes that presently exist on earth. It lies in an uncompensated tectonic graben, controlled by differential vertical movements between the rift basin who subsides and the rift shoulders who are uplifted. Active tectonics is the major factor controlling the development and persistence ofthose deep lakes. For Lake Malawi (Nyasa), climatic influence is also important, due to the small dimensions ofthe hydrological catchment area and the subtropical climate with long dry seasons. The Malawi rift basin, hosting the lake, started to develop in the late Miocene (since 8.6 Me), but deep water conditions were acquired only by 4.5 Ma. The lake then dried out almost completely at the beginning of the Pleistocene (from 1.6 to 1.0 Me), as a consequence of stable tectonic conditions and dry climate. A new regression started at about 0.42 Ma until 0.25 Me, which is well documented by high-resolution seismic stratigraphy. The tectonic lowering of the overflow sill, through subsidence of the rift floor, combined with erosional incision have lowered the water level by 40m since the late Pleistocene. Short-term, small amplitude lake-level fluctuations are documented by direct observations since 1915. Resume.-Le lac Malawi (Nyasa), dans la branche occidentale du rift est-africain, est l'un des rareslacs profonds ayant une ttes longue duree de vie. I1 est localise dans un graben tectonique non compense, controle par les mouvements diiierentiels des epeules, montantes, et du sol, subsident. La tectonique active est le facteur principal regisssnt Ie developpement et la persistance de ce type de lac. L'influence climatique est aussi importante pour le lac Malawi, suite aux faibles dimensions de son bassin-versant et des conditions climatiques relativement seches. Le bassin tectonique du lac Malawi a commence a se developpet au Miocene tardif (8,6 Ma). Le lac n'est cependant devetiu profond, que vers 4,5 Ma. I1 s'est essech« presque cotnplctement au debut du Pleistocene (1,6-1,0 Me), pendant une petiode de stebilite tectonique sous un climat plus sec. Une nouvelle regression, docutnentec par la stratigraphie sismique de haute resolution, a eu lieu entre 0,42 et 0,25 Ma. Depuis lafin du Pleistocene, le niveau du lac s'est ebeisse de pres de 40 metres, suite al'abaissement tectonique et al'erosioti de l'exutoire. L 'observation directe, depuis 1915, montre l'existence de fluctuations de plus courte duree et d 'amplitude reduite. Semeavetting.-Het Malawimeer (Nyasa) is een van de weinige reeds lang bestaande diepwater meren op aarde. Het is gelegen in een niet-gecompenseerde graben, gecontroleerd door diiierentiele vertikale bewegingen tussen het inzakkende slenkbekken en de slenkschouders die opgeheven worden. Actieve tektoniek controleert als belangrijkste factor de ontwikkeling en het behoud van deze diepe meren. Voor het Mala wimeer is de klimatologische in vloed ook van belang ingevolge de kleine dimensie van het hydrologisch vergaarbekken en het subtropisch klimaat met zijn lange droge seizoenen. De ontwikkeling van het Malawislenkbekken begon in het Laat-Mioceen (8.6 Ma). De echte verdieping van het meer startte slechts rond 4.5 Ma. Het meer droogde bijna volledig uit bij het begin van het Pleistoceen (1.6-1.0 Ma) tijdens een petiode van tektonische stabiliteit onder droog klimaat. Een nieuwe regressie, goed gedocumenteerd door hoge resolutie seismische stratigrafie, vond plaats tussen 0.42 en 0.25 Ma. Sinds het einde van het Pleistoceen is het meerniveau ongeveer 40 metergedaald door tektonische subsidentie en uitvloei-erosie. Directe observatie sinds 1915 toont het bestaan aan van waterpeil fluctuaties met een korte periode en een kleine amplitude. MALAWI There is no direct measurement of the age of formation of Lake Malawi, and also there is no general agreement on its estimated age of forma-branch has a general NW-SE orientation and is occupied by the Tanganyika, Rukwa and Malawi (Nyasa) rift valleys. In these rifts, the Cenozoic basins and major faults are partly superimposed on an older Permo-Triassic (Karoo) rift system (Dypvik et al., 1990; Morley et al., 1992). The Malawi rift extends over 900 km from the Rungwe volcanic province in the north to the Urema graben in the south (Fig. 2). The rift sedi-mentary basin is about 750 km long and 75 km wide. Much of the rift valley floor is occupied by Lake Malawi, 570 km long and on average 60 km wide. The surface of the lake is 474 m above sea level and its floor descends to 225 m below the sea level. Maximum depth is close to 700 m and the flanks of the rift rise 400-2000 m above the surface of the lake. To the north, the Malawi rift is blocked by the development of the Rungwe volcanic provinces, with three major active volcanic centres, culminating at 2940 m (Harkin, 1960). To the south, the Malawi rift ends in Shire plain. The rift structures of Lake Malawi are assumed to be younger than mid-Miocene (Ebinger et al., 1989). The East African rift system lies atop the East African plateau, and consists of two branches, the eastern one, running from the Gulf of Aden to the Kenya (Gregory) rift and the western one, from Lake Albert to the Indian Ocean,through the Tanganyika and Malawi (or Nyasa) rift valleys (Fig. 1). It developed mainly in the Protero-zoic mobilebelts, between the Archean cratons. The Western Rift and Kenya Rift follow Protero-zoic mobile belts and avoid the Central Tanga-nyika craton which acted as resistant block (for example, McConnell, 1972). The Western Rift is developed mainly along the Ubende mobile belt characterised by northwest-trending mylonites and shear-zones. The central part of the western 2. GEOGRAPHY AND MORPHOLOGY The age of formation of Lake Malawi, the fluctuation of water level and its eventual complete drying are essential questions regarding its biological evolution and degree of endemicity. Lake Malawi (Nyasa) is, together with Lakes Tanganyika and Baikal (Siberia), one of the few deep-water long-lived lakes that presently exist on earth. The common characteristics of these lakes is that they lie in uncompensated tectonic grabens whose subsidence is faster than their filling by sediments. This requires specific and highly active tectonic movements that are typical for active continental rift systems: differential vertical movements between the rift basin who subsides and the rift shoulders who are uplifted. These movements are accommodated by faulting , which generated important seismic activity, controlled the location of hot springs, and probably also favoured the volcanic activity. Active tectonics is therefore the major factor controlling the development and persistence of those deep lakes. In consequence, they are relatively unstable on geological timescale , because the intensity and the regime of tectonic activity fluctuate with time. The small dimensions of the hydrological catchment area and the subtropical climate with long dry seasons result in a relative instability of the water level in Lake Malawi, very sensitive to climatic changes
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... Shifts in sedimentation rate due to land use changes and decreases in oxygen levels due to a warming climate are both predicted to be key factors in future carbon cycling in aquatic systems (Munthali et al., 2011;Jane et al., 2021) and thus trends seen in anthropogenically impacted, warm, anoxic Lake Malawi may become more prevalent in the near future. Additional motivation for studying carbon cycling in Lake Malawi comes from its unique age and sedimentary record, where up to 4 km of underlying sediment records information from 4.5 million years of past environmental conditions (Delvaux, 1995;Rosendahl, 1987). In this study we identified the relative sources and ages of organic carbon in lake surface sediments using bulk sediment parameters, biomarker information, stable and radiocarbon measurements on bulk sediment and isolated organic matter fractions and comparison with isotopic values in water column organic matter. ...
... Uncited references Healey and Hendzel, 1980;Leng and Marshall, 2004;Liu and Liu, 2017;Delvaux, 1995. ...
Little is known of modern carbon cycling in East African rift lakes. These large tropical systems with anoxic bottom waters can act as test cases for carbon cycling under future climate change scenarios as well as key sediment repositories for paleolimnology studies and fossil fuel generation. This study is the first to combine stable and radiocarbon isotopic information on bulk sediment and biomarkers (n-alcohols) in an African rift lake by investigating surface sediments collected on a nearshore to offshore transect in northern Lake Malawi. This study is also the first to report radiocarbon content in water column particulate organic, dissolved organic and inorganic carbon from an African rift lake. General surface-sediment trends with increasing water depth were an increase in %TOC, and decreases in grain size, δ¹³C and C/N. Carbon radioisotope analyses showed that bulk sedimentary organic matter was modern across the transect. Biomarker data indicated a primarily aquatic origin for sedimentary organic matter at offshore sites; terrestrial material dominated the n-alcohol signature closest to the shore. Protokerogen stable isotope signatures were similar, and radiocarbon was somewhat depleted relative to bulk sediment organic matter. As compared to bulk organic matter, lipids were ¹³C-depleted throughout the sampling transect and ¹⁴C-depleted at the most nearshore location. This indicates an aged lipid carbon source and likely significant diagenetic alteration of modern sources prior to deposition in nearshore sediments, perhaps a combination of soil erosion, in-river processing, sediment resuspension and mobilization, and preferential degradation of younger organic matter in the oxic nearshore region.
... Nicholson (1998) extended the water-level chronology for Lake Malawi back to 800 AD (Figure 4) based on a synthesis of published archaeological, geological and historical information, and updated climate chronologies for the region. Nicholson's (1998) record provided an indication of fluctuations over six centuries and suggested that fluctuations of >14 m have occurred over the last two millennia; Delvaux (1995) cited fluctuations of 250 to 400 m further back in time (4.5-8.6 million years) that are attributed to, inter alia, natural climate change. ...
The Elephant Marsh lies on the floodplain of the lower Shire River, in southern Malawi. It is both a cultivated, seasonal floodplain and a wetland, characterised by a complex mosaic of meandering channels, marshland and shallow lakes. In 2016, the Marsh was granted RAMSAR status based on a series of supporting studies, including the modelling of eco-social options for managing its ecological condition. This paper describes two-dimensional (2-D) hydrodynamic modelling using RMA2, in support of this eco-social assessment. The hydraulic model was parameterised using mostly existing data but augmented with bathymetric channel and lake surveys. Calibration and verification used water level data from the hydrometric network for the period 1999 to 2009. Generally, observed water level time-series are well replicated in the model, but there are large discrepancies prior to this period. These are due to temporal changes in hydraulic controls, mainly sedimentation, but also breaching of an embankment at the downstream end of the Marsh. Selected hydraulic-habitat variables (based on daily hydrological time-series 1976 to 2009) provide the indictors of hydrological and hydraulic change used to inform a DRIFT eco-social assessment. The Marsh displays moderate flood attenuation characteristics, but the importance of this will likely increase under continued sedimentation and predicted climate futures, which include more severe storms. The 2-D modelling contributes to an improved understanding of the Marsh’s hydraulic behaviour, particularly regarding anthropogenic influences since the early 1900s.
... Nicholson (1998) extended the water-level chronology for Lake Malawi back to 800 AD (Figure 4) based on a synthesis of published archaeological, geological and historical information, and updated climate chronologies for the region. Nicholson's (1998) record provided an indication of fluctuations over six centuries and suggested that fluctuations of >14 m have occurred over the last two millennia; Delvaux (1995) cited fluctuations of 250 to 400 m further back in time (4.5-8.6 million years) that are attributed to, inter alia, natural climate change. ...
The Elephant Marsh lies on the floodplain of the lower Shire River, in southern Malawi. It is both a cultivated, seasonal floodplain and an area of permanent shallow lakes. The marsh is highly biodiverse, has a productive fishery and supports the livelihoods of a large human population. Chronicles and hand-drawn sketches by 19th-century explorers and subsequent travellers describing the marsh landscape were used to develop an account of the marsh’s historic hydromorphology. These anecdotal accounts are supported by Shire River outflows from Lake Malawi, inferred from observed water levels in the lake from c. 1900. Records show that the marsh has experienced regular periods of drying out, followed by periods of greater inundation than it currently experiences. Long-term hydrological trends have been accompanied by anthropogenic influences such as catchment degradation and embankment construction, which have altered sedimentation patterns in the marsh. The marsh exhibits traits of resilience to natural long-term climate change through its ability to return to a similar state during wet periods, even following prolonged dry periods. Over relatively short time frames, however, the ecology of the marsh, as described when it was declared a RAMSAR site in 2017, likely has low resistance to hydrological regimes that are sensitive to climate change. An appreciation of long-term climate and recent human-induced changes afforded a unique opportunity for contextualising its hydromorphological trends within the Anthropocene.
... The late Oligocene/early Miocene age of the Rungwe Volcanic Province in southern Tanzania provides an upper estimate for the onset of EAR activity in northern Malawi (Mesko, 2020;Mortimer et al., 2016;Rasskazov et al., 2001;Roberts et al., 2012). To the south, the age of the EAR is poorly constrained with a Late Miocene-Pliocene age proposed for the central and southern basins of Lake Malawi from extrapolating modern depositional rates (Delvaux, 1995;McCartney & Scholz, 2016;Scholz et al., 2020). A southward propagation of the EAR in Malawi is also consistent with the thinner sedimentary cover and smaller escarpment heights in southern Malawi (Laõ-Dávila et al., 2015;. ...
We present the Malawi Active Fault Database (MAFD), an open‐access (https://doi.org/10.5281/zenodo.5507190) geospatial database of 113 fault traces in Malawi and neighboring Tanzania and Mozambique. Malawi is located within the East African Rift's (EAR) Western Branch where active fault identification is challenging because chronostratigraphic data are rare, and/or faults are buried and so do not have a surface expression. The MAFD therefore includes any fault that has evidence for displacement during Cenozoic East African rifting or is buried beneath the rift valley and is favorably oriented to the regional stresses. To identify such faults, we consider a multidisciplinary data set: high‐resolution digital elevation models, previous geological mapping, field observations, seismic reflection surveys from offshore Lake Malawi, and aeromagnetic and gravity data. The MAFD includes faults throughout Malawi, where seismic risk is increasing because of population growth and its seismically vulnerable building stock. We also investigate the database as a sample of the normal fault population in an incipient continental rift. We cannot reject the null hypothesis that the distribution of fault lengths in the MAFD is described by a power law, which is consistent with Malawi's relatively thick seismogenic layer (30–40 km), low (<8%) regional extensional strain, and regional deformation localization (50%–75%) across relatively long hard‐linked border faults. Cumulatively, we highlight the importance of integrating onshore and offshore geological and geophysical data to develop active fault databases along the EAR and similar continental settings both to understand the regional seismic hazard and tectonic evolution.
... Above all, between the three East African Great Lakes, the haplochromine cichlids are especially interesting, as they share common ancestry and comprise the Tropheini at LT and the entire the LV and LM haplochromine radiations [53,54]. As the lakes have all very different geological histories, with Lake Tanganyika being the oldest [55], LM the intermediate [56] and LV the youngest of the three [57], they also depict three extensive radiations at different time points. Thus, depending on the evolutionary age of the different lakes, species (and their morphologies) had more or less time to diverge, despite sharing parallels. ...
Background
Elasmoid scales are one of the most common dermal appendages and can be found in almost all species of bony fish differing greatly in their shape. Whilst the genetic underpinnings behind elasmoid scale development have been investigated, not much is known about the mechanisms involved in moulding of scales. To investigate the links between gene expression differences and morphological divergence, we inferred shape variation of scales from two different areas of the body (anterior and posterior) stemming from ten haplochromine cichlid species from different origins (Lake Tanganyika, Lake Malawi, Lake Victoria and riverine). Additionally, we investigated transcriptional differences of a set of genes known to be involved in scale development and morphogenesis in fish.
Results
We found that scales from the anterior and posterior part of the body strongly differ in their overall shape, and a separate look on scales from each body part revealed similar trajectories of shape differences considering the lake origin of single investigated species. Above all, nine as well as 11 out of 16 target genes showed expression differences between the lakes for the anterior and posterior dataset, respectively. Whereas in posterior scales four genes ( dlx5 , eda , rankl and shh ) revealed significant correlations between expression and morphological differentiation, in anterior scales only one gene ( eda ) showed such a correlation. Furthermore, eda displayed the most significant expression difference between species of Lake Tanganyika and species of the other two younger lakes. Finally, we found genetic differences in downstream regions of eda gene (e.g., in the eda - tnfsf13b inter-genic region) that are associated with observed expression differences. This is reminiscent of a genetic difference in the eda - tnfsf13b inter-genic region which leads to gain or loss of armour plates in stickleback.
Conclusion
These findings provide evidence for cross-species transcriptional differences of an important morphogenetic factor, eda , which is involved in formation of ectodermal appendages. These expression differences appeared to be associated with morphological differences observed in the scales of haplochromine cichlids indicating potential role of eda mediated signal in divergent scale morphogenesis in fish.
... Nevertheless, the much lower freshwater crab diversity of Lake Malawi compared to Lake Tanganyika is puzzling. The estimated age of the Lake Malawi basin is between 8 and 4 Ma (Delvaux, 1996) which means that Lake Malawi formed roughly at the same time as Lake Tanganyika (8-5 Ma; Lezzar et al., 1996;Cohen et al., 1997). One potential explanation for different evolutionary diversification rates between these two ancient lakes is attributed to the Pleistocene drying of the major lakes of the East African Rift Valley (Van Damme & Gautier, 2013), i.e. ...
Climatic and geomorphological changes during the Quaternary period impacted global patterns of speciation and diversification across a wide range of taxa, but few studies have examined these effects on African riverine fishes. The lower Congo River is an excellent natural laboratory for understanding complex speciation and population diversification processes as it is hydrologically extremely dynamic and recognized as a continental hotspot of diversity harboring many narrowly endemic species. A previous study using genome-wide SNP data highlighted the importance of dynamic hydrological regimes to the diversification and speciation in lower Congo River cichlids. However, historical climate and hydrological changes (e.g., reduced river discharge during extended dry periods) have likely also influenced ichthyofaunal diversification processes in this system. The lower Congo River offers a unique opportunity to study climate-driven changes in river discharge, given the massive volume of water from the entire Congo basin flowing through this short stretch of the river. Here, we, for the first time, investigate the impacts of paleoclimatic factors on ichthyofaunal diversification in this system by inferring divergence times and modeling patterns of gene flow in four endemic lamprologine cichlids, including the blind cichlid, Lamprologus lethops. Our results suggest that Quaternary climate changes associated with river discharge fluctuations may have impacted the diversification of species along the system. Our study, using reduced representation sequencing (2RADseq), indicates that the lower Congo River lamprologines emerged during the Early-Middle Pleistocene transition, characterized as one of the earth's major climatic transformation periods. Modeling results suggest that gene flow across populations and between species was not constant but occurred in temporally constrained pulses. We show that these results correlate with glacial-interglacial fluctuations. The current hyper-diverse fish assemblages of the lower Congo River riverscape likely reflect the synergistic effects of multiple drivers fueling complex evolutionary processes through time.
Preserved assemblages of invertebrate remains in lacustrine sediment reveal temporal variations of community composition and environmental conditions. However, records for large tropical lakes are scarce. Lake Victoria, the largest tropical lake, has a dynamic history of changes in water level, biogeochemistry and fish community composition over the past ~17,000 cal yr BP.
In order to quantify changes in the invertebrate assemblage of Lake Victoria from the late Pleistocene throughout the Holocene, we examined chitinous remains of Cladocera and larval dipterans (Chironomidae and Chaoboridae) from a sediment core (37 m water depth) dated from ~13,700 cal yr BP to present.
We identified four major phases in the invertebrate assemblage throughout this period of lake history. Firstly, Chironomidae and Chaoboridae appeared at low abundances during the earliest stages of lake inundation in the late Pleistocene, at a time when Cladocera were notably absent. Secondly, chaoborids and chironomids increased in abundance during the mid‐Holocene, which coincided with high diatom production toward the end of the Holocene African Humid Period. Thirdly, starting ~4,700 cal yr BP, Alona , a predominantly littoral cladoceran genus, consistently appeared in the invertebrate assemblage alongside changes in mixing regimes and persisted throughout the late Holocene to the present. Fourthly, the arrival of both Chydorus and Bosmina longirostris marked the establishment of an abundant cladoceran assemblage at ~1,350 cal yr BP. The assemblage then gradually shifted toward the increasing dominance of B. longirostris , a planktonic cladoceran.
Several of the observed changes in the invertebrate assemblage occurred concurrently with changes in climatic conditions in East Africa and diatom productivity that have been previously recorded in Lake Victoria. This multi‐millennial record of sedimentary invertebrate assemblages in Lake Victoria elucidates some of the temporal development of these communities throughout most of the dynamic modern history of the ecosystem.
Background: Elasmoid scales are one of the most common dermal appendages and can be found in almost all species of bony fish differing greatly in their shape. Whilst the genetic underpinnings behind elasmoid scale development have been investigated, not much is known about the mechanisms involved in the shaping of scales. To investigate the links between gene expression differences and morphological divergence, we inferred shape variation of scales from two different areas of the body (anterior and posterior) stemming from ten haplochromine cichlid species from different origins (Lake Tanganyika, Lake Malawi, Lake Victoria and riverine). Additionally, we investigated transcriptional differences of a set of genes known to be involved in scale development and morphogenesis in fish.
Results: We found that scales from the anterior and posterior part of the body strongly differ in their overall shape, and a separate look on scales from each body part revealed similar trajectories of shape differences considering the lake origin of single investigated species. Above all, nine as well as 11 out of 16 target genes showed expression differences between the lakes for the anterior and posterior dataset, respectively. Whereas in posterior scales four genes (dlx5, eda, rankl and shh) revealed significant correlations between expression and morphological differentiation, in anterior scales only one gene (eda) showed such a correlation. Furthermore, eda displayed the most significant expression difference between species of Lake Tanganyika and species of the other two younger lakes. Finally, we found genetic differences in downstream regions of eda gene (e.g. in the eda-tnfsf13b inter-genic region) that are associated with observed expression differences. This is reminiscent of a genetic difference in the eda-tnfsf13b inter-genic region which leads to gain or loss of armour plates in stickleback.
Conclusion: These findings provide evidence for cross-species transcriptional differences of an important morphogenetic factor, eda, which is involved in formation of ectodermal appendages. These expression differences appeared to be associated with morphological differences observed in the scales of haplochromine cichlids indicating potential role of eda mediated signal in divergent scale morphogenesis in fish.
Three major stratigraphic units have been recognized in the basin (from oldest to youngest): Karroo Supergroup, the Red Sandstone, and the Lake Beds. These units have traditionally been assigned the following ages, respectively, based upon lithologic correlations: Permian-Triassic, Jurassic-Cretaceous, and Tertiary. Recently, microfloral assemblages collected from sidewall cores and cuttings from two deep exploration wells drilled in the basin have provided new data on the age of these formations. The biostratigraphic data suggest the formations should be assigned the following ages: Karroo Supergroup, Late Permian; Red Sandstone, Neogene(?) (Miocene-late Pliocene); and the Lake Beds, late Pliocene-Holocene. -from Authors
This work focuses on the Cenozoic rifting period, demonstrating two major stages of paleostress state: 1) an initial near-radial extensive stress regime with two dominant extension directions (ENE-WSW and NW-SE), which generated or reactivated major rift border faults with a dominant dip-slip component; 2) a subsequent strike-slip stress regime with dominantly N-S principal compression, which mainly caused strike-slip reactivation of major rift border faults. Stratigraphic constraints fix the stress inversion from radial extensive to strike-slip regime in the Middle Pleistocene, between 0.55 My and 0.42 My.. Phase 1 represents the kinematic evolution from Late-Miocene to Pleistocene. The second kinematic phase started in Middle Pleistocene following a stress inversion caused by the permutation of the compressive and intermediate stress axis, leading to a strike-slip regime. Once established, this new regime evolved rapidly by successive clockwise rotations of the NNW-SSE maximum compression. -from Authors
The natural hazards in the northern part of the Malawi rift are evaluated. In this area all types of risks related to a rift zone are present. The seismic activity is intense and concentrates along the major NW-trending fault zones which extend into the lake. Numerous explosive vol-canic eruptions occurred in the region during Pleistocene time; the Kiejo volcano, which still vents C02 from fissures, erupted in the late 1800's. The whole area is shown to expe-rience regional tectonic tilting due to subsidence along the Livingstone fault bounding the eastern side of the lake. Furthermore, extensive accumulation of gas is present under a shallow cover of sediments over large areas in the northern part of the lake. Gas erupts in the lake along a NNW tren-ding active fault: it consists of a mixture of C02 and CH 4. Resume, -Une evaluation est faite des risques naturels dans la partie nord du rift du Malawi. Tous les types de risques lies aune zone de rift sont presents dans cette region. Une intense activite sismique se concentre Ie long des failles majeures dirigees NW s'etendant dans Ie lac. De nombreuses erup-tions volcaniques explosives ont affecte la region durant Ie Pleistocene, tandis que Ie volcan Kiejo a connu sa derniere eruption au siecle dernier et continue aemettre du C02 Ie long de fissures. La region entiere est soumise aun bascule-ment tectonique du ala subsidence Ie long de la faille bor-diere de Livingstone, qui limite la partie orientale du lac. En outre, d'importantes accumulations de gaz sont presentes sous une faible couverture de sediments sur de grandes eten-dues dans la partie nord du lac. Des gaz font eruption Ie long d'une faille active dirigee NNW au fond au lac; il s'agit d'un melange de C02 et de CH 4. Samenvatting. -De natuurlijke risico's in het noordelijk deel van de Malawi rift worden opgesomd. In dit gebied komen aile soorten risico's voor die aan een riftzone kunnen gebon-den zijn. Een intense seismische aktiviteit is gebonden aan NW-gerichte hoofdbreuken die verder lopen in het meer. Talrijke explosieve vulkanische uitbarstingen hebben zich in de streek voorgedaan gedurende het Pleistoceen, terwijl een laatste uitbarsting van de Kiejo vulkaan gekend is uit de vorige eeuw; nog steeds wordt C02 uitgezonden langs breu-ken. Het ganse gebied ondergaat een region ale tektonische kanteling onder invloed van subsidentie langs de Livings-tone breuk die de oostelijke kant van het meer afbakent. Bovendien wordt gas opgehoopt onder een dunne bedekking van sedimenten over een uitgestrekt gebied in het noorden van het meer. Gas wordt uitgezonden in het meer langs een NNW gerichte aktieve breuk; het is samengesteld uit een mengsel van CO 2 en CH 4.
The Mbeya area is part of the western branch of the East African Rift System and lies on the intersection of the NW-SE trending South Rukwa-North Malawi segment and the NE-SW trending Ruaha-Usangu depression. The area was already tectonically active during earlier riftings periods (Permo-Triassic and Mesozoic), but underwent a major rift development during Cenozoic times. The neotec-tonic period, discussed in this work, corresponds to this last first-order geodynamic mechanism. In the Mbeya area this mechanism is characterized since the middle Pleistocene by a dominant strike-slip stress regime. A neotectonic map of the Mbeya area was compiled from Quaternary structural elements and volcanic centres, active hot springs and earthquake epicentres recorded by a regio-nal seismic network. An additional morphostructural map, highlighting active faulting and basin tilting in the northern extremity of Lake Malawi, was drawn using a LANDSAT-TM scene and data from a study on recent sediment distribu-tion in that part of the lake. The depth distribution of earthquakes obtained from a network of five digital three-components seismometers provides new information about the rheology and the geometry of active faults down to the lower crust. Resume. -La region de Mbeya est situee a l'intersection de la branche ouest du rift est-africain (bassins de Sud Rukwa et de Nord Malawi), orientee NO-SE, avec la depression NE-SO de Ruaha-Usangu. Cette region deja active durant de precedents episodes de formation du rift (Permo-Triasique et Mesozoique), a connu une nouvelle importante phase de riftogenese durant le Cenozoique. La periode neotectonique consideree dans la presente synthese correspond a la duree du dernier mecanisme geodynamique de premier ordre. Dans la region de Mbeya, ce mecanisme est caracterise, depuis le Pleistocene moyen, par un regime de contrainte de type strike-slip. Une carte neotectonique a ete elaboree sur base d'une compilation des elements structuraux et centres volcaniques quaternaires, des sources chaudes active et de la position des epicentres de tremblements de terre enregistres a partir d'un reseau sismique local. La fracturation active et les bascule-ments de blocs a l'extremite nord du lac Malawi sont mis en evidence par une carte morphostructurale realisee a partir d'une scene Landsat-TM et d'une etude de l'evolution.de la sedimentation recente dans cette partie du lac. La distribu-tion des tremblements de terre en fonction de la profondeur, obtenue a l'aide d'un reseau de cinq seismornetres digitaux a trois composants, fournit de nouvelles informations sur la rheologie et la geometric des failles actives en profondeur, jusqu'a la base crofite. Samenvatting. -De streek van Mbeya is gelegen op de intersectie van de Westerlijke tak van de Oost-Afrikaanse rift (bekkens van Zuid-Rukwa en Noord-Malawi meren), volgens een NW-ZO richting, met de depressie, NO-ZW gericht van Ruaha-Usanga. Deze streek, reeds tectonisch aktief gedurende vorige riftperiodes (Permo-Trias en Meso-zoicum) heeft een nieuwe belangrijke fase van riftontwikke-ling gekend tijdens het Cenozoicum. De neotectonische peri ode zoals gebruikt in de huidige synthese stemt overeen met de duur van het laatste geodynamisch mechanisme van eerste orde. In de streek van Mbeya is dit mechanisme sinds het Midden-Pleistoceen gekenmerkt door een strike-slip spannings regime. Een neotectonische kaart werd opgemaakt op basis van kwartaire structurele gegevens, de localisatie van recente vulcanische centra, thermale bronnen en epicentra van aard-schokken, geregistreerd door een lokaal seismisch net. De actieve breuken en blokkantelingen op het uiterste noorden van het Malawi meer werden in het licht gesteld door een morphopstructurele kaart. Deze werd gerealiseerd op basis van een Landsat TM-beeld en een studie van de verdeling van de jongste sedimenten in dat deel van het meer. De verdeling van de aardschokken, rekening houdend met hun diepte, bekomen bij middel van een net van vijf digitale drie-componenten-seismometers, levert nieuwe informatie over de rheologische en de geometrische karakteristieken van de actieve breuken en dit tot op de basis van de korst.
Lake Malawi levels have been as much as 5m above modern peak values during the early? Holocene but, until more is known about changes in the overflow, the palaeoclimatic significance of these high levels must remain in doubt. In an attempt to view modern flooding problems in the longer term perspective, three high lake levels have been identified showing that, in the past, comparable flooding has been relatively uncommon. Only two of the past high levels exceeded modern values: one (late in the period mid-12th to mid-18th centuries) by 30cm; the other (before approximately the third century) by 100cm. Excavation along the overflow suggests that the carrying capacity of the channel during the 30cm high level was approximately similar to modern values. Lakeshore development planners might therefore be wise to allow, at the very least, for such a lake level. -from Authors
The Malawi rift is one of the youngest sections of the African Rift Valley System. The initial rift fill comprises fossiliferous Pliocene sediments. Early sediments in the extreme north were conformably overlain by pyroclastics erupted from the Rungwe volcanic field. A major phase of faulting then initiated a prolonged period of erosion and possible 'Early Stone Age' artefacts are found on the resulting gravels at the foot of the western escarpment. 'Middle Stone Age' artefacts are most abundant nearer to the modern lake. A large pyroclastic flow swept into northern Malawi from the Rungwe area just over 11 000 yr BP. -from Author
High-resolution seismic profiles, side-scan sonar records, and sediment cores collected from Lake Malawi have been analyzed to determine the nature of sedimentation in a modern rift lake. The distribution of modern sediment is quite complex. Little or no deposition occurs in most regions shallower than 100 m due to storm-generated surface-wave activity. Gravitational transport of sediment by creep, debris flows, slumping, and turbidity currents is common, particularly off deltas and border faults. Diatom-rich clays occur in the deep basins far removed from major terrigenous input. These typically have organic carbon concentrations of 3-6 wt.%. Laminated sediments are common in Lake Malawi, and their frequency increases with water depth. Neither the abundance of organic carbon nor frequency of laminations increases abruptly below the depth of the chemocline. -from Authors