Delvaux, D.

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33 The Western Rift branchA REGION OF GEOLOGICAL, MORPHOLOGICALAND BIOLOGICAL RECORDSDAMI EN DELVAUXI•fz~~1-la ke Kivu, bordering Rwanda and t he Democratic Repub lic of the Con go, lies in the weste rn branch of the East African Rift system.I,T HEWESTERN RIFT VALLEY is an arcua te zone ofI ~ontrasted relief with large deep lakes, fert ile valleys,steep mountains an d h igh plateaus affected by volcanism,earthquak es and landslides. Th is 2,600 km-l ong geographicalfeatu re stretches from Lake Albert between Uganda and theDemo cratic Republi c of th e Congo (DRC) in th e north, toth e Indian Ocean at Beira in Moza mbique in the south ,with lakes Edward, Kivu, Tanga nyika, Rukwa and Malawi inbetween . It separates East Africa from t he DRC, Zambia an dMalawi and played an important role during early Europeanexplora tion of Cen tral Africa, such as th e qu est for thesource of the Nile and the snow-capped 'Mounta ins of theMoon' (Ruwenzori Mountains).The large lakes promoted exchange (such as th e Arabicin fluence an d the Swahili language), and travelling fromthe Indian Ocean coast extended westwards across the greatJakes, up to the nor th- south stretc h of the upper part of theCongo River, beyon d whic h starts the rain forest. The regionalso played a role in the developmen t of early hominids.Geological developmentThe East African Rift System is a ma jor tecton ic zone tha tsepara tes the Nubian plate ('stable' Africa) from the Somalianplate, with the eastern an d western rift branch es isolatin gth e sma ller Victoria and Rovuma crusta l blocks betweenthem (figure 33.1). The rift is spreading in a gen eral east-west direction at a rate of a few millimetres pe r year. Thewestern branch developed in a series of n arrow and deepfault-cont rolled basins with limited volcanism restricted toth e junctions and accommodation zones between th e majorbasins. This was largely contro lled by old basement structuresin a series of successive tectonic stages that reactivated th eNeoprote rozoic basement. The internal structu re of the basinsis characte rized by an assemblage of grabens or half-grabens,which are the building blocks of the rift arch itecture.It is difficult to define a starti ng date for the beginningof rifting in East Africa as there have been severa l periodsof rifti ng since early Permian times, about 300Ma, TheTheWestern Rift branch EASTERN AFRICA 225Rift architectureFigure 33.1 The African RiftSyste m. The S-shaped Western Rift isa narro w zo ne of strongly con trasting relief with large, deep lakesbord ered by high mountains and plateaus.A rift system is a typical product of extens ional tec toni cs. Itconsists of a depressed linear axial zone (rift valley), borderedby up lifted margins (rift shoulders). The East African Riftis cons idered to be th e type example of a contine ntal riftbecause it is almost en tirely int racontinent al and still active,an d thus accessible for observation .The me ch ani sm of for mation of th e rift system hasintr igued m any geologists an d geophysicists. During thefirst ha lf of the 20th century, it was believed that the riftbasins in the Great Lakes region of Cen tral Africa formedas a result east- west hori zontal shorten ing. Th e presen ceof a negative gravim etric anomaly in th e depr essed axialzon e of the rift was interpreted as reflec ting the presenceof a lighter crust there than on the rift sho ulders. Accordingto the se research ers, suc h a lighter cru st sho uld be upliftedrelative to the surround ing s due to buoyancy forces. Insteadit is depressed, requ irin g a mech anism to force it down .It was proposed that the rift bor der fa ults were dippingawa y fro m the rift axis and, un de r th e action of h orizontalcompression, the rift sho u lders were ov erriding th e axialzone, forcing it down by th eir m ass. This was kn own as a'pop-do wn basin', whi ch typically form s in a compression alenv ironment. It was only after the in stallat ion of theIRSAC (In stitu t pour la Recherche Scientifiqu e en AfriqueCen trale) seism ic net work in the Kivu Rift Region that itwas demonstrated that th e rift forme d in an exten sionalrather than a compressiona l context.In the 1990s it was proposed that th e central part ofth e Western Rift (South Tanganyika, Rukwa and NorthMalawi rift segment) acts as an intracontinen tal transform,accommodating dextral strike-slip movement along nort hwest-trending reactivated Precamb rian discontinuities. This m odelimplies a gene ral ope ning of th e East African Rift system in anorthwest-southeast direction . It was based on remote-sensinginte rpretations, coupled with morphological observations,The two branch es of the East Africa n Rift Syst emdiffe r in th eir arch itec ture. Th e western branch iscomposed of a n ar row ch ain of lon g and deep rift ba sin s(grabe ns) bordered on on e or both sides by a single .'border fault' tha t co ncentrates much of the tec tonicmovements. Magmat ism is relativel y scarce, restricted toaccommodation zones between different rift segments. Inco mpariso n , th e eastern branc h con tains m uch smallerrift bas in s, but volcanism is far more extens ive withinthe ri ft and on the eastern platform . The Western Rift hasundergone more exten sion (possibly as much as 30kmin the last 7Ma ) t han the Grego ry (East ern) Rift (8- 0kmfro m n orth to south over 5Ma).In vestigation of th e Malaw i and Tangan yika rift ba sin sby seismic reflectio n profiles in the 1980s enabled th einternal architecture of rift ba sin s to be evaluated. Itsh owed that th e larg e rift ba sins, suc h as th ose of Mala wian d Tanganyika, are co mm on ly com posed of a seriesof asymmet ric down -fau lted depression s (half-graben s,fig ure 33.2), wh ich are bounded on on ly one side by anormal fault and relat ed rift-flank uplift . These 'build ingblocks' are grouped in variou s ways, oppos tng or facin geach other, and arranged ~ n an alternating, en echelon orline ar way to form long rift segmen ts.Kinematics of rift opening300 kNAINDIANOCEANI'"\ ' -.\\Io...i/most recen t per iod of rifting started in the Mid-Miocene ,at about 13M a, It likely initiated as iso lated rift basinsthat progressively expanded longitudinally an d conn ectedto each ot he r at accom modation zones, which h ost m ostof the volcanism (9- 8Ma). A strong accelera tion of basindown -fau lting, mountain growth and volcanism occurredin the Plio-Pleistocen e (3-2Ma), produci ng much of thetopography we see today, wh ich is therefore geologica llyvery youn g.•DRCONGOLEG ENDD Younger sediments• Rift valley volcanic rocksD Baseme nt- - Rift valley fau lt / edge- ··- Int ernational boundaryI226 EASTERN AFRICA TheWestern Rift branchA\\ _ ----- --:..-:::::--Bord er fault)0Figure 33.2 (A) Half-graben as a building block of the rift system, with rift sed iments inclined toward s a major border fault on oneside and uplifted on the ot her. (8) The Karo nga/livingstone basin at the no rthern end of lake Malaw i illust rates this featur e. On theMalawian side, in th e region of Karonga, Pleistocen e Ch itirnwe sands to nes are upl ifted above lake level and slight ly t ilted towardsLake Ma lawi. (After Rutgers Univers ity 1996)Figu re 33. 3 (A) Shell-rich bioclastic laye r, Karonga regio n,Malawi. (8 ) Small fau lt a ffect ing Permian sedimen ts,deposited during an ea rlier rift stage, Karon ga region,Malawi.Figure 33.4 (A) Frontal view of the 2km -h igh LivingstoneFau lt scarp bordering the Livingstone Range plateau on theTanzanian side . (8) Edge of the liv ingstone Range plateau,Tanzani a, etched by erosion owing to its strong uplift alongthe liv ingstone fau lt.TheWestern Rift branch EASTERN AFRICA 227Figure 33.5 The Virunga volca nic massif viewe d from Go ma (foregro und) at the northern extremity of lake Kivu. O n th e left is theMikeno volca no (4,437m) and in th e ce ntre is the Karisimibi volcano (4,S07m).and the first ava ilable fault-kinematic analyses. With theacquisition of earthquake, GPS geodetic and geological fielddata, it became clear that the overa ll extension across thesyst em occurs in a general east-west direction. At the scaleof the rift basins, extension tends to occur at a high angleto thetrend of the basin. Ob lique movements appear to berestricted to the area of interaction between different basinsof the rift system (accommodation zones).Geomorphology and natural hazardsA rift system is not just a large zon e of extension formedbetween two blocks that are pulled apart and in which apiece of lan d is downfaulted. In the Western Rift , the riftfloor, although sometimes strongly depressed relative toits surroundings, generally lies at altitudes much h igherthan the rest of the African continent (400rn for theMalawi depression and up to 11600rn for th e Lake Kivudepression) , It is also bou n ded by m ountain ranges thatoften reach 2,500- 31000rn in height. Th e RuwenzoriMountains, which actively rise between two ov erlappin grift segments, are composed of basement rocks that rea ch ,h eights of Sl109m. The Virunga volcanoes (figure 33.5Lwhich erupted in the m iddle of the ri ft valley, rea ch <..height of 4,507m (Karisim bi). A lava lake began fo rm ingin the summit of the Nyragongo volcano (3,4 70m) in 1927(figure 33. 6). Sin ce then it has erupted catastrophicall y, in1977 and 2002 .This rugged and elevated region acts as a climaticbarrier, separating the East African savanna from the rain-forests of Cen tral Africa. The region also acts as a refuge foranimal spe cies such as gorillas and ch impanzees and m ayhave played a role in the development and speciation ofearly hominids .The Western Rift hosts some of t he longest, largest,deepest an d most vo luminous lakes in the world (LakeIIIIIFigure 33.6 Helicop ter view of the Nyamulagira lava lake, west of Nyragongo volcano in the ORe , taken in 2011.228 EASTERN AFRICA The Western Rift branchFigu re 33.7 Aerial view of the l ake Tanganyika outlet into the Lukuga River at Kalemie on its western (ORC) side. The river wasfollowed by Sir Henry Morton Stanley on his trip towards the Atlantic coast along the Congo River in 1874-1877.Tanganyika, the longest at 677 km and th e secon d dee pestat l ,433m, and Lake Malawi, the th ird largest and thirddeepest in Africa).Since hi sto rical times, the region has attracted a largeh uman population and, rather tha n acting as a barr ier , t helarge lakes promoted exc han ge an d travel from the India nOcean coast westwards . The rela tive ly high -altit ude reliefand abundan t surface wa ters ma ke the region more fertilean d h ospitable than the surro undin g dr y savanna and deeprainforest (figure 33.8). Un for tun ately, it is al so prone to aFigure 33.8 Canyon of the Ruzizi River in the ORe , nea r theborder with Burundi. Most of the steep flanks are cultivatedbut are also affected by large landslides.spect rum of natural geolo gical and h ydro logical h azard s,such as earth quakes, volcani c eruptions, landslides, mudflows, Inundations an d soil eros io n . The region is alsoone of t he most populated regions of Africa , and t he riskcau sed by the se n atural h azards to the population andinfrastructure is pa rti cu larly h igh .Figure 33.9 Soft-sediment deformat ion induced by ancientearthquakes in 20,OOO~year~old sediments of the Rukwa basin,Tanzania. These depos its were introdu ced when the lake levelwas much higher than today.TheWestern Riftbranch EASTERN AFRICA 229Natural resourcesRift systems generally host a wide range of natu ral reso urces(figure 33.10), an d th is is particul ar ly true in th e case ofthe Western Rift bran ch. Large quanti ties of oil have beenfound in th e northern end of the Albertine graben inUganda and ma y also be presen t in other rift basins. LakeKivu con tains methan e, which is being used in a pilo t plan tto prod uce electricity. Coa l was fou nd in early co lon ialtimes and supported industrial de ve lopmen t and railways.Hydroelect ric power plan ts h ave been in sta lled or arc underdevelopment in many locations.Numerous hot springs occu r th roughou t the region (figure33. '11 ) an d the first hydrothermal plant in East Africa beganoperating as early as 1952 near Manana in the Upem ba Rift ofsoutheastern ORe , producing electr icity for tin mining. Thesearch for profitable hydrotherm al energy sites is cu rrentlyunder way. Precambri an carbonatites are m ined for variousminerals and for cement facto ries. Travert ine, produced byhydrothermal activity, is also used for making chalk, cementand orna m en ta l marbles (figure 33.12).Figure 33.10 lvuna salt pan in the floor of the Rukwadepression, Tanzania. The salt is introd uced by a thermalspring. It is harvested using traditional met hods.Figu re 33.11 Sempaya hot spring fie ld at the nor thern tip ofthe Ruwenzori Mountains, Uganda.230 EASTERN AFRICA The Western Rift branchFigure 33.12 Fossil travertin e (Songwe dep osit), mined froma ther mal spring in the ancien t Rukwa Lake, Tanzania. Thedeposit was formed as a resu lt of hot mine ralizing water risingthrough an open fissure and spreading horizontally on the lakefloor, de positing hor izontally band ed trave rtine and verticallybanded a re.The Western Rift branch ho sts numerous natural parks ,game reserves and sites o f regional conservation inter est.Am ong others, these include the nationa l parks of No rth andSouth Luangwa and Mwe ru Walltipa in Zamhia; Lake M3la~viend Nv itcn in Malawi; Kituto, Kalavi, Maha/e and Combe illTanzalJii1; Kihira in Burundi and adjaccn t Nvungwe ForestPark in RwamJ,l; Kahuzi~Bic8a in tile rift mountains alongt.ake Kivu in the ORC; the cro ss -border sys tem of parks inthe Kivu-A lbertine rift segmen t, with tile Virunga NationalPark in the ORC; VoJcano es Park in Rwanda; the Q(JcenElizabeth, Ruwenzo ri and Sem Jiki Parks in Uganda, and theMurch ison Falls Nation al Park at the extreme northern endof the Albertine Rift Vafley. The Virunga Nationa l Park isthe olde st national park ill Africa, created in 1925, and oneof the most diversified in the world in terms of geology,landscape ilnd biology. It was designeted a UNESCO WorldHeritage Site in ·'979, but was fisted as 'World Herirc1f{C' illDanger ' since the Rwandil ge nocide in 1994 .Figure 33.13 View of the Semlikivalleyon the sout hwesternextremity of Lake Albert in the Virunga National Park, ORe.Published by Struik Natu re (an im prin t of Pen gu in Random Hou se So ut h Africa (Ply) Ltd )Reg. No. 1953/000441 /07The Estuaries No.4, O xbow Crescent, Cen tury City, 7441 South AfricaPO Box 1144, Cape Town, 8000 So uth AfricaFirst published in 201610 9 8 7 6 5 4 3 2 1Co pyright © in text, 2016: The authors of individual chaptersCo pyright © in ph otographs, 2016: The authors of ind ividual chapt e rs unless othe rwis eind icated alongsid e imagesCopyright © in illust rations, 2016: 35th Inte rn ation al Geolog ica l Co ngress Foundatio nCopy right © in maps, 2016: 35th Internation al Geolog ical Co ng re ss Fo u ndationCo py right © in published ed ition, 2016: 35th Inte rnati on al Geol ogical Co ngress Foundat ion& Pengu in Rand om Ho use South Af rica (Pty) LtdPubli sh er: Pippa ParkerPro ject man ager: He le n de Villie rsEd ito r: Mart ina M6ssme rDesign d irector: Jani ce Evan sDesign e r: Ne il Bes te rCartographer: Lynda WhitfieldProofread er : Emsi e d u Plessi sReproducti on by Hi rt & Carter Cape (Pty) Ltd6> soI<JliofI<Printed by novus printA Nevus HoldingscompanyAll righ ts reser ved. No part of this publication may be reproduced, sto re d in a retrievalsyste m, or transmitted, in an y form o r by an y mean s, elect ro nic, mech an ica l, ph otocopying,reco rdin g o r o the rwise, without the pr ior written pe rmi ss ion of the copyright ow ne r(s).Print ISBN: 9781775844488e-Pub ISBN: 9781775845386e-PDF ISBN: 9781775845393Ho w to cite th is bo ok (e ntire publicat io n):Anhaeusser, C. R., Viljoe n, M. J. & Viljoe n, R. P. (eds) 2016. Afri ca's Top Geological Sites.Struik Na tu re, Cape Town . 312p p.How to cite a chapter within this p ublicat io n (example):Moore, A. 2016. The Victori a Falls and go rges . In: Anhae usse r, C. R., Viljoen, M. J. & Viljoen,R. P. (ed s) Africa's Top Geological Sites. Strui k Nature, Ca pe Town. pp .16--2D.CO VER IMAGE: The Alo ba Arch, composed of Palaeozo ic sandsto ne , located o n the EnnediPla teau o f Chad . Sta nd ing 210m tall, it is th e seco nd -largest natu ral arc h in the world.PHOTO CRFDIT: Jacq ues TaberletTITLE PAGE IMAGE: View o f Cape Town with th e Cape Town Stad iu m in the for eground ,and with Signa l Hill and lion 's Head embracin g th e 'city bowl' th at lies at the foot of TableMo u nta in. Be h ind th e 'tab le', the mo untain s snake so uth d own th e peninsu la, a ll the wayto Cape Point. PHO TO CREDIT: Ala in Pro ustt~ 11 P'O"U'1 10f,.... ,u'lo l"o~l,mo"og. oro rel l.ono ," "I"'IIo.10""0'

The Western Rift Branch

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The Western Rift Branch

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