RI FT LAKES OF EAST AFRI CA Ne w Sei s mi c Dat a an d I mpl i c at i on s f or Fut ur e Resear ch b y B. R. Ros en dahl an d D. A. L i v i n gs t on e Cu r r e nt s ei s mi c s t udi es o厂E as t A厂 r i c an Ri 厂 t l ake s ar e l e adi ng t o new i de as on t h e ear l y s t ages o厂r i 厂 t b as i n dev el opme nt . Ver t i c al r e 厂 l ec t i on pr of i l i ng o 厂Lake Ta nga nyi ka i ndi cat e s t he pr e s e nce o厂> 3 00 0 i n of s edi ment an d s ug ge s t s an age加r t he t a ke no yo u nger t han Lower Mi oce ne . Some par t s o f Lake Mal awi may al s o be t hat ol d. Bot h l ake s ar e st r u ct ur al l y c ompl e x, c on si s t i n g o厂s ev er al di s t i nct b as i ns e ach compos ed o厂man y i ndi vi dual b l o ck s . 角 e b a si n s ar e t he户价dame nt al st r uc t ur al u ni t s o厂t he Af r i c an Ri 厂 t , but t he y ar e nei t her s y nc hr on ou s nor c hr o nol o gi ca l l y s ys t e mat i c i n t hei r dev el o pment . Se di me nt s o f oce ani c t hi c k nes s h ave b ee n de po s i t e d i n b a si n s l e s s t h an 80 km wi de , r es ul t i ng i n和ci e s p ack a ges wi t h as pect r at i os o f 1 : 1. Se i s mi c s t u di e s ar e co nt i n ui n g, an d wi l l be e xt e nde d t o ot her maj or l a kes of t h e A介i c an Ri f t . The s e s ur vey s pr o vi de a new i ns i ght i nt o t he pr oce s s o 厂co nt i ne nt al br ea k- up, a nd may s er v e as a g al v ani zi n g向r ce i n t he r ec ov er y of a ver y det ai l ed 20 Ma r ecor d厂r om one o厂East A介i ca' s ri 厂t l akes. Ba ckg r o un d EAST AFRI CAN RI FT ZONE 曰曰日// s ys t em whi ch ex t e nds ov er a l en gt h of 1200 km ( Fi g. 0. / / 多李 洲宁共 了 蕊 10* 〔 一 ! n r a vel l i n g t he g eol o gi ca l s t or y of t he Eas t Af r i can Ri f t h as b een di f f i cul t b ec aus e l ar ge ex pan s es of t he Ri f t ar e co ver ed by l akes . Two of t h es e, Mal awi and Tan gan yi ka, compr i s e a Compar ed t o ot he r bo di e s of wat er wi t h eq ui v al ent geo l o gi cal s i gni f i c an ce, t h es e t a kes ha ve b een s t udi ed by o nl y a f e w 35 \ \ 日口 州曰 ti on and t he ear l y st ages of ocean basi n evol uti on. 30 /护 月 厂尹 I nt er pr et a t i ons of t h e ar cha eol o gi ca l , cl i ma t ol ogi ca l an d pa i a eo eco l o gi cal r e cor ds o f Eas t Af r i ca ar e i nt r i c at el y l i nk ed t o t he ge ol ogi cal hi s t o r y of t he Eas t Af r i can Ri f t . Thi s hi s t or y al s o h as an i mpor t a nt b ear i ng on many geol o g i c co nc ep t s , es p ec i al l y t hos e i n vol vi n g con t i nen t al f r ag ment a- 10 》 5 5 0* 0 r es ea r ch er s ( e. g. Andr ew, 1 974 ; Ch or owi c z et al . , 1 979 ; 5 ye ar s co nd uc ti ng ge op hys i ca l s ur vey s o f t he g r e at r i f t - v al l ey l ak es of Eas t Af r i ca. A mer e 11 m i n l eng t h an d di s pl aci n g onl y 1 0 t ons , t he bo at h ous es a mod er n 48 - c ha nnel s ei s mi c s ys t em wi t h a 10 00 m- l on g hy dr op hon e s t r e amer ( Fi g. 2) , h i gh - r es o l ut i on s ei s mi c s y s t ems , s i d e- s c an s on ar , ec ho s o und - er , mar i n e magn et o met er an d a pl et h or a of na vi gat i ona l de vi ces s u ch as s i n gl e an d du al ch ann el s at el l i t e n avi gat or s , r ad ar an d an au t omat e d co ur s e p l ot t er . NYANJA h as been 107 3 e 0 飞 飞 、{ 巧 / 1 4 5 / l eff ort s i n East Af ri ca. NYANJA wi l l spend the next three 嘴六 曳 /7// Cr os s l e y an d Cr o w, 1 98 0; Cr ow, 1 980; De ge ns et al . , 1 971 ; Di xey , 1 92 6, 1 95 6; Heck y, 1 978 ; Li v i ng s t o ne, 1 96 5; Sut t on an d Ber g, 1 95 8; vo n } Her z en an d Vac qui er , 1 9 67; Won g a nd v on Her z en, 1 97 4 ) . I n a s e ar c h f or d ee p t ar get s f or s ci ent i f i c dr i l l i ng we hav e be gun a s y s t ema ti c g eo phy s i cal i n ve s t i gat i on of t he mai n r i f t - val l ey l ake s of Af r i ca ( Cr omi e, 1 98 2) . The cov er of t hi s i s s ue s h ows t h e R/ V NYANJ A, t h e nu cl eus o f our g eop hy s i ca l 1 0 s peci al l v bui l t to fi t i nt o t he carao bav of a Boei ne 747 ( Fi e. ))ana 1 0 De i rans porr ea over rougn r oac i s. 15} 15 To date we have col l ected about 4000 km of si ngl e- channel r efl ecti on i )r ofi l es f rom Lakes Mal awi and Tan2anvi ka (Fi e. 4) . we pr es ent ii rST some ot tne ni gr ai gnts ot tr us i nvesti ga- t i on an d t hen de s c r i b e what i s pl anne d f or t he n ext t h r ee ye ar s of f i el d s t udy . I nt er - Lak e Compar i s o ns 30 40 45 ---一一~~‘一 - Th e wat er s o f Lak es Mal awi an d Tang an yi k a co ver ar ea s of The r i 厂t val l ey syst em o厂 East A介i ca, s howi ng akes num berea as 加 l l ows: 1-Mal awi ; 2 2一 -R“ Ru北 kwa; 3- maj or l M wer u: 4- Tanganyi ka; 5-Ki vu; 6-Edwar d; 7-Mobutu SeSe great s truct ural compl exi ty. The sei smi c profi l es repr oduced Seko; 8-Tur kana; 9-Vi ct ori a: 10-Natr on. 14 F1 9ure 1: EPI SODES, Vol . 1983, No. 1. and basi nal i n s ecti on. Typi cal di mensi ons ar e 50-70 km l ong by 20- 40 km wi de. Boundar i es bet ween uni t s usual l y coi nci de wi t h ch an ges i n bo un dar y- f aul t or i en t a ti o ns an d ar e of t en ex pr e s s ed as s i l l s or r e gi ons of up l i f t ed b as ement , es p eci a l l y wh er e dog- l egs o ccu r i n t he l ak es . 撬 喻 餐舞 na Fi qur e 2: Usi 1000 m- l ong mul ti channelstream er on Lake Tanganyi ka ( Photo by C.E城nge日. I n s pi t e of thei r si mi l ar s i zes and shapes, thes e str uct ur al uni ts ar e not necess ari l y produced by t he s ame s tr ess I i el ds wi t h the s ame ori ent ati ons, nor ar e they of the s ame age or even s yst emati c i n t hei r chr onol ogi cal devel opment wi t hi n a l ake. I t appear s t hat r i f ti ng does occur wi th pr edi ct abl e and repeatabl e di mensi ons i n these l akes r egardl ess of the st r ess f i e l d or t i me o f f r act ur i ng . I nd eed , t h e l i t er at ur e on r i f t an d pul l - apart basi ns s hows t hat thi s sort of s cal i ng may be app l i cab l e t o j uve ni l e r i f t s i n gen er al , and i n p ar t i cu l ar t o those whi ch have devel oped i n ol d cr atoni c r egi ons . I f t hi s i dea i s co r r ect , t hen i t gi v es con s i der ab l e i ns i ght i nt o t he evol uti on of ri f t l akes and r i ft zones. The ki nks or dog- her e gi ve some i ndi cat i on of thi s on t he l ocal s cal e, and Fi gur e 5 s hows t he comp l ex i t y o n t he s c al e ot a wh ol e l a ke. Nei ther t ake i s a natural structural unit一 such units occur on the sm all er scal e of basins within each l ake 一 but there are some gener al si mi l ari ti es and di ff erences between the two. The maj or si mi l ar i ty i s t hat both l akes were f ormed by t ens i o nal f or ces whi ch cr e at ed mai n a xi al gr a b ens bor der ed b y l ar g e gr avi t y f aul t s , as s hown l ong a go by Gr eg or y ( 1 89 6) . Th e mai n b or d er f aul t s of bot h l akes ( Fi g. 6 ) ar e i n f l uen ce d by t h e t yp e an d t r en d of t he bas ement , i n t he s ame way t ha t t h e r i f t b i f ur cat es a r ou nd t h e Tan gan yi k a cr at on ( Fi g. 1 ) . Th e pr i mar y di f f er e nce b et wee n t h e t wo l ake s i s s hown by co nt r as t s i n t he geomet r i es of t he a xi al gr a be ns , whi ch ma y l eg s i n t h e l ak es ar e a mani f e s t a t i on of a di men s i ona l l i mi t at i o n, f or mi ng wh er e s u bp ar a l l el s e gme nt s of f u nd a- men t a l r i f t l engt h ar e cl os e e no ugh t o get her i n s pa ce t o b e c on ne ct ed by s h ear z on es ( p r e - t r an s f or ms ?) . Ri f t s f or m as a r es ul t of coal es cence of thes e segment s wi t hi n a wi dth- r est ri cted band. The success or f ail ur e of a ri ft may depend upon how cl osel y i n s pace and ti me adj acent s egment s devel op. We assume that t her e i s a di st ance beyond whi ch t he av ai l a bl e r i f t i ng e ner gy wi l l n ot p er mi t coal es cen c e wi t hi n or between r i ft basi ns, and that t he great er the ti me di f f er e nce b et ween r i f t i n g ev ent s , t he l es s l i kel y i t i s t hat spati al proxi mi t y can be mai ntai ned. Th er e ar e t wo p os s i bl e ex pl ana t i o ns f or t he f und ame nt al r i f t par amet er . Th e f i r s t co nt e nd s t hat t he r i f t i n g f or ce s ar e rep - i onal and uni f orm and at tri but es spati al r epeat abi l i t y t o t he i nf l ue nc e an d ef f ect s of a pr e- e xi s t i n g gr ai n i mpo s e d by res ul t from di ff er ences i n the ori ent ati on of the tensi onal di ff er ent ki nds of bas ement. The s econd ar gues that t he regim e. cau s at i v e age nt s of r i f t i n g ar e l oca l i zed an d t h r e e- di r r i en - s i onal ( e. g . ma nt l e pl umes ) d ur i ng t he i ni t i al s t ages of r i f t i ng an d b eco me pr ogr es s i v el y mor e r eg i on al as co al es ce n ce of La ke Mal awi s e ems t o be ch ar a ct er i z ed b y h al f - g r ab en s l i k e t h os e s h own i n Fi gur e 7, wi t h t he mai n b or der f au l t s de vel - op ed on t h e wes t er n s i d e. Al t h oug h t hi s geomet r y d es cr i be s much of La ke Mal awi , a r ev er s al ( i . e. dee p s i de on t he e as t ) occurs i n the nor ther n quart er of the l ake. I n contr ast , Lake Ta ng anyi k a i s char a ct er i z ed by f ul l gr a b ens wi t h we l l - de vel - oped bor der f aul ts on both si des. Pr ofi l es of the mai n nor th ba s i n of t hi s l ak e ( Fi g. 8) al s o s h ow an ax i al dome wi t hi n t he f ul l gr ab en. Th e f u l l g r ab en f or m t en ds t o b eco me bowl - s h ape d i n s e ct i on as s e di men t a t i on p r o gr e s s es , co ve r i n g t h e dom e. Hal f gr abe ns oc cur i n t he mai n ki n k ( dog - l eg) r egi on of Ta ng anyi ka a nd f ul l g r ab ens may oc cur i n t h e s o ut her n en d of Mal awi , b ut t her e i s a gen er al t en den cy t o war d on e s t y l e or the other i n each l ake. Thi s di fference could be a refl ecti on of t he i mp or t an ce of upl i f t r el a t i v e t o p ur e ext ens i on i n t he de vel o pment of r i f t gr ab ens , wi t h upl i f t p r e domi n at i ng i n Mal awi and e xt ens i on i n Tang an yi k a. Upl i f t may ev ol ve i nt o e x t en s i o n as a r i f t ag es , as i mpl i ed b y Di x ey ( 1 956 ) , an d t he axi al dome i n Ta ng any i k a may b e a r e mna nt of i t s j uv eni l e u pl i f t pha s e. An ot h er e xp l an at i on no w b ei n g co ns i der ed by our r es ear ch t eam r el ates t hi s di f f erence to t he proporti on of s h ear i n vol ved i n t he r i f t i n g pr o ce s s ( i . e. amou nt of obl i q ue r i f t i ng) ; t he gr eat er t h e compon ent of s he ar t he hi g her t he f undament al uni t s conti nues. We f avour the s econd expl ana- t i o n b ut ut i l i z e t h e f i r s t t o exp l ai n s ome of t h e s pat i al var i abi l i t y between f undamental ri ft uni ts. Even wi thi n basi ns of fundament al si ze, great str uct ur al compl exi ty ca n occur. Thi s i s es peci al l y appar ent i n Lake Malawi. F shows a profi l e that i s near l y perpendi cul ar i gu r e9 to that show n in f aul t i ng a nd Fi gur e 7. Ther e i s asmuch f ol di ng i n t hi s l on gi t udi nal or i e nt a t i o n asthere is across the b as i n. Mu ch of t hi s de f or mat i on i s ap par ent l y a con s e qu en ce of s he ar i ng p r od uc ed by ext r emel y obl i qu e r i f t i n g. As t h e end r es ul t , b as i ns a r e f or med comp os ed of many s mal l , p ar al l el o gr am- s h a p ed gr ab ens a nd h or s t s , de f i n ed by nor mal f aul ts on thei r east ern and western si des and st ri ke- sl i p faul t s o n t h ei r nor t h er n and s o ut h er n ed ges . Th e ex pr es s i o n " bl ock s withi n basi ns w ithin lakes within rift zones" sum m ari zes our structural results to date. S t r at 电r ap hy a nd Se di ment at i on e i nt er est i ng No p ar t of o ur s ei s mi c pr ogr am has b eenmor t ha n i t s ac ou s t i co- s t r at i gr aph i c as pe ct , a ndnone m ore dif- f i cul t. The r eas on f or thi s di f fi cul t y i s so i ntr i gui ng i n i t s own ri ght t hat we present i t here. pr op en si t y t o war ds ha l f g r ab ens . Our axi al t r an s e ct s i n Mal awi s how muc h mor e f aul t i n g and f ol di ng t han t h ei r T an gany i k a e qui val en t s , s u pp or t i n g t h e con t e nt i on t hat s he ar - i n g i s ge ner al l y mor e per va s i ve i n Lak e Ma l awi . 臀。 . Concept of t he Fundament al Ri ft Paramet er Per h ap s t h e mos t exci t i n g and f ar - r e ach i n g r es ul t of our s u r ve ys i s t h e emer gi n g co n cept of what we ha ve t er me d t he "f undament al ri ft par amet er. " Thi s i dea has evol ved out of t h e ob s er v at i o n t h at Mal a wi an d Tan gan yi ka ar e di vi ded i nt o natur al str uctur al uni t s wi th a cer tai n di mensi onal repeat a- bi l i t y. The uni ts ar e general l y par al l el ogram- shaped i n pl an EPI SODES, Vol. 1983, No. I. Fi gur e 3 : NYANJ A a厂 t er o 厂 厂 - l o adi ng at He at hr ow ai r p or t ( Photo by C. Ebi nge时・ 15 习脚 Ot her f act ors compl i cat e t he pi ctur e f ur ther. The fi r st i s t he eff ect of mor phol ogy on deposi ti onal patt er ns, parti cul ar l y the i nfl uence on l at er al conti nui ty of t he "bl ock wi t hi n basi n" morphol ogy di s cussed earl i er. Second are t he enormous 溉 50 k印 、、 \/产 卫0 M, 冷 泊 洲》 尹勺 Und er s t an di ng h ow a l l t h es e f act or s c ome t o get her t o p r o- d uce t he ac ous t i c s i gn at u r e s ob s er ved i s ch an gi ng t he way we i nt erpret s ei smi c data fr om the l acus tri ne por ti ons of ri f t M ALAW I STRUCTURAL GEOLOGY OF LAKE MALAW I . . ... 12, , ‘产. 11 l 凡 !方1. 、es. , / BURI ED FAULTS DIAPIRS oceans , f or t hey contai n most of the components as s oci ated wi t h c on t i n ent al mar gi ns , i nc l u di ng s hel f - s edi ment wed ges , MESOZOI C FAULTS * 日OT SPRI NGS 与 N Fr om t he s t r at i g r aph i c an d s edi ment ol ogi cal p er s pec t i ves , Lak es Mal a wi and T ang any i k a c an b e vi ewed as mi n i at ur e 1人 \\叭 CENOZOI C FAULTS 五.甲.! SiEAR ZONES 口卜 , . 卜 , ; , Fi gure 4: Sei smi c re厂l ect i on pr o厂i l er cover age on Lakes Tanganyi ka ( A) and Mal awi ( B) . 卜卜卜*‘ M ODERN FAULTS 留 / I 皱橇1 雌 、 \ \ 玛气 \\ B I / 、 \, , 日DUNDARY FAULTS 、 ) 沪 l 、 妙 卜~ f 、 , 11 『 彻刀 一 房 飞 、飞 "\ 飞 ,\补 丫 、 、丫 叹 成,广/ 旧 \ 卜 卜 门 啾训 \铆 比卜 | 一 / 谧 少 ‘勺 、 洲 入 丫 ! I 6 , 1 } 13 } 、 L叫 . I2。 城 I I 愁尸 , 盈 }、\ 份 T A N Z A N IA } ‘ T A N Z A N IA 火招 **饭 豫、 份 1‘ 、 \ 主I factor i s t he l i kel y presence of hydr other mal eff ects i n the sedi ments, es peci al l y around f aul ts. A f ourt h i s the occur- rence of unconform iti es in the secti ons. 队 拼肠』 七 火 ch an ges i n ampl i t ude of r ef l ec t i on , pr es uma bl y d ue t o v er y hi gh and var i a bl e or gani c cont en t s of t he s ed i ment s and p os s i b l y t o t h e l o cal o ccur r e nc e of hy dr oc ar b ons . The t hi r d 七 MONOCL I N ES del t as , submari ne fans, tur bi di t es and sl umps. They al so s how many of the f eatures associ at ed wi th open ocean s edi r nenta- t i on, s u ch as dee p bas i ns , b i a geni c ooz es , ano xi c en vi r on - ment s and even contour i tes. The di ffi cul ty ari ses because al l the, e component s are s queezed i nto wi dt hs of l ess t han 80 km. Fac i es ch an ges t h at or di n ar i l y o cc ur on t h e s cal e of hundreds or thousands of kil om eters i n the oceans occur over di stances of a few kil om eters to tens of kilom eters in these l a kes . A compl i ca t i ng f act or i s t h at t h e t hi ckn es s es of s e di ment ar y p ack a ges can be comp ar abl e t o t hat i n t h e o cea ns , and i n s ome pl ace s t h e t hi ckn es s of a si n gl e f ac i es ex ce ed s i t s l e ng t h i n cr os s s ec t i o n. The en d r es ul t can b e f a ci es pa ck ag es wi t h as p ec t r at i os of 1 : 1. I6 Fi gur e 5 : St r uct ur al map o厂Lak e Mal awi and s ur r o un di ng ar e a, compi l ed f r om many s ou r ce s by C. Ebi nge r . Lak e st r uct ures tr endi ng obl i que to t he mai n N-S ori ent ati ons ar e i nt erpret ed as s hear加 ul t s. The modern NE- SW faul ts s hown have st r ong st ri ke- sl i p components . Many ot her s t oo numer - ous to s how ar e present but are di 厅i cul t t o or i ent ate between pro厂i l e l i nes. EPW DES, Vol . 1 983, No. 1. The r el at i vel y gr e at s ed i men t t hi c k nes s es ar e s t ar t l i ng. Mo r e than 3 000 m of sedi ment occurs i n the nort h basi n of Lake Tan gan yi k a ( Fi g. 8 ) and pr ob abl y mor e t h an 100 0 m i n t he centr al secti ons of Lake Mal awi . Measur ed l ate一 uat er nar y a ccumu l at i on r at es ( e. g . Deg ens et al . , 1 971 ;Li vi n gs t on e, 1 96 5) a nd es t i mat es of compa ct i on, i ndi cat e t ha t t he bas al s e di me nt s i n t h e no r t h bas i n of Tang any i k a ar e u nl i k el y t o be yo ung er t ha n Lowe r Mi oc en e i n age. Cons i der i n g t h e amoun t of s ect i on mi s s i ng ab ove t he an gul ar unc onf or mi t y s ho wn i n Fi gur es 7 an d 10 , Mi o cen e ages f or Lak e Mal awi ar e al s o pl ausi bl e. De po s i t i on al b as i ns s eem t o ha ve bee n i n exi s t e nc e at l eas t i nt er mi t t e nt l y s i n ce Mes o zoi c t i me, f or we h ave s t r o ng evi de nce i n Ma l awi t h at Kar r oo t r o ug hs co nt i nue a cr o s s t he l ak e at t wo l o cat i on s ( Fi g.,) .We s us p ect t hat r eo ccu p at i on of Kar r o o bas i ns may be a r el at i vel y common ph en omen on i n t h es e gr eat r i f t - va l l ey l ak es ; i f s o, t hey do not h ave s i ng ul ar ages of f ormat i on. The Next Thr ee Years Fi g ur e 6 : Th e T anz an i an s h or e o f Lak e T an gan yi k a. The f aul t s car p s t h at compr i s e t he s hor e war d 加ce s of t h e r i f t Du r i n g t he f al l of 1 982 , NYANJ A car r i ed o ut r eco nn ai s s a nc e s ur vey s of t he s out he r n ha l f of La ke Ta nga nyi ka pl u s a gr i d s ur v ey of t he mai n nor t h bas i n. S ur ve ys ar e pl ann ed f or shoul der conti nue an additi onal 1 000 m bel ow the l ake s ur f ace ( Phot o by P. Lor b er ) . Lakes Tur kana, Ki vu, Mobut u Ses e Seko, Vi ctori a and porti ons ba si ns ( Fi g. l o ) . Bas ed on our s ei s mi c wor k t o d at e , t og et h er wi t h s o me pi s t o n co r e a nd o ut cr op i nf or mat i o n, t h e f o l l owi ng pi ctur e can be dr awn. Mai n refl ector s tend t o be bas i n- wi de of La ke Mal awi i n t he s p r i n g a nd s u mmer of 1 983 , and f or t h e r es t of La ke Mal awi pl us Lake s Ruk wa, Eya s i and Na t r o n i n 1 9 84. La kes Edwar d an d Mwer u ar e po s s i bl e addi t i ons . The ul t i mat e pur p os e of t hes e s ur ve ys i s t o de ci p h er t h e ge ol ogi c hi s t o r y of t h e Eas t Af r i can Ri f t , but t he wor k ha s many ob j e ct i ves i nv ol vi ng s ev er al di s ci pl i n es . S ome of t h es e and p r ob abl y co ns t i t ut e de po s i t i ona l pul s es ( or b r ea ks ) s t em- mi ng f r om t ec t o ni c eve nt s . Acou s t i c f a ci e s p ac kag es t en d t o be s p at i al l y var i abl e, es pec i al l y t h os e per p endi cul ar t o t he mai n a x i s of t he l ak e, a nd t hey of t en cr os s ma i n r ef l ect or s . Th e d omi n ant f a ci e s ch ang e i s i n t er pr et ed t o b e t h at b et we en are to: c o nt i nu e t he s ear ch f or dr i l l i ng s i t es whi ch ar e v er y l i kel y t o yi el d d et ai l ed a nd pr eci s e r eco r ds of cl i ma t i c ch an ge s o ver t he l as t 10 Ma or mor e of ea r t h h i s t or y; a s cer t a i n t he app l i cab i l i t y a nd us ef ul ne s s of t ect on i c a nd de pos i t i on al mo del s of t he r i f t l a kes , es pe ci a l l y i n t er ms o f o ur und er s t a ndi ng of t he b r ea k- up of co nt i - n ent s a nd t h e e ar l y evo l ut i on of p as si ve mar gi ns ; t es t t he conc ep t of t he f un dame nt a l r i f t p ar a met e r an d d ef i ne t h e s cal es t hat ap pl y t o i nd i vi du al r i f t 一a si n bi og eni c ( pe l agi c) or g ani c- r i ch s edi me nt s and cl as t i cs of t er r i gen ou s or i gi n. The bi o gen i c f aci e s t end s t o be ac ous t i - ca l l y " t r ans p ar e nt " i n t h e ooz e s t at e, b ut b ecomes hi g hl y s t r at i f i ed as i t go es t o or g ani c s hal es wi t h de pt h. The t er r i ge nous cl as t i c f aci es i n cl ud es many s ub - f aci es ( e. g. del - t a l c, s ubmar i n e f an, t ur bi di t e ) , a l l of whi ch t en d t o pr odu ce st rong and hi ghl y r efl ecti ve events at shal l ow dept hs. Wi t h bu r i al , t hes e s ub f aci e s t en d t o d i s a pp ear f r om t h e s e i s mog r ams , pr o ba bl y du e t o t h ei r s e i s mi c i mpe ne t r a bi l i t y. I f we now i nt r o duc e s ome o f t he compl i ca t i o ns men t i on ed ab ov e, s uch as h y dr ot he r mal ci r c ul a t i o n cel l s , bl oc k- wi t hi n- ba s i n mo r ph ol ogy and un co nf or mi t i es , t he n i t i s no t h ar d t o ap pr eci at e wh y t h e s i mp l e t wo- f a ci es mod el r es ul t s i n i nt er - pr et at i o ns as comp l e x as t hat i n Fi gur e 1 0. model s; 聆一 of hom i ni d evol uti on. 月隽 擎一 嗜j 书 熟 少 、 弋 _ 分熟 簇令 矍一 黔 臀彗 ・ ‘ 一‘ 灌 一 蟹 八‘ 一 泞 黔 穿夕 洲r卜r 二卜 喀 电 靓歼线抓认} 失 摇种 以要六歼 邓 了炎 训洲门 刘 」 t 物 门刁 一 热一 贪姜必二笠 协妇朴找 抓丰f把 朴 2 掩 黔 黔黔七 拳。 萝、 t』 对 唆 孰转 轰 群巍粼粼 韶一盯 彝 羚 氦 一 黔愁妻 SW 豁 腿唠岁比 取丰 一 一 签篡; 熟 NYANJA' s c ap abi l i t i es g i ve us con s i der ab l e f l exi bi l i t y i n mat c hi n g s ei s mi c met h odol ogi es t o s t u dy ar e a an d ob j e ct i ves . E C f N E W pr ov i de a ge ol ogi c f r amewor k f or ant hr o po l ogi c s t u di es I- 2 I ' a一一 Fi gu r e 7 : Se i s mi c r ef l ec t i on pr o f i l e f r om Nk hat a Bay ( N o n Fi g. 5 ) t o Li koma I s l and s , Lake Mal awi . Pr of i l e can be c on s i der ed a cr o s s - s e ct i on of t he Mal awi Ri f t i n t he vi ci ni t y of 1 2 S. V er t i c al s c al e i n s e co nd s of t wo- way r e f l ect i o n t i me . Hor i z ont al e xt ent about 70 km . EPL SODES, Vol . 1983, No. 1. 17 NORTH BASI N CROSS-LI NES NW SE 0 : 藻 1 牛乍‘,叫 黔 呷扣 恤 孙】 注勺 If 冲 『 补『} 命 犷话飞 彩幕舜带铸 王鸽 SW 0 蹦盯卜比 1 孙六 淤 一! 一 缪 缪一节 鄂 攀拜拼 p 二: ’ 飞浅 蝴 霭僻 习 耀启乎嗡 割黔鹅洲- gONOO山5 篆一 燕 豪 云蔑 茱慕感华 诵 : 攀筝 搏攀 辫攀一 愁 黔 ! } j l Fi gur e 8: S ei s mi c r e f l ec t i on pr of i l es of t he mai n nor t h ba s i n of Lake T an gan yi k a. Pr of i l es ca n b e c o ns i der ed cr o s s - s e ct i on s o f t he Ri f t bet we en ab out 40 30' S ( t o p l ef t ) and 5* 201 S ( bot t om r i g ht ) . Ver t i c al s c al e s a s f or Fi gur e 7 . Hor i z ont al ext ent a bou t 4 5 km. Gr i d s ur ve ys on t he l ar g er l akes wi l l be co ndu ct ed i n mu n o r t h b a n o f T a n g a n y i 叹a r e _ o f t h e o r d e r . o f . ¥ m l l i o n o r mo r e . Fs oi r t h i s r e a s o n , U匕七八 L p r o D a D几 y Wl l l D e3 g0 i n ni r o d e s t l our c es and a 100 0 met er l on g " mi ni - ca bl e. " Reco n- ai rgun s wi t h an a t t e mpt t o r ai s e a cor e s pan ni ng o ne mi l l i o n ye ar s channel CDP for mat i n 24一or 48-f ol d confi gurati ons . u. , from Lake Bosum t wi i n Ghana. Z I I NKHATA BAY STRI KE LI NE 砒聂芝; . ‘斗 霖 滋基 藻纂奏鬓 t i o ns o f Lake s Ta ng anyi ka and 1 981 t h i s g r o up s t ud i ed p or anned f or 1983. Rukwa, an d a r e t u r n e xp edi t i on i s pl L s t e o F n o e u r a n n i e d r ' h s i s g r c o o u - p w i o s r k n e o r w s w o i l r l k c i n o g n t o i n u L e a t k h e e i M r a p l a o w l l e i . n , L d i v a i t n o g m - and gr a s s c ut i cl e a nal ys es of pi s t o n co r es r ep r es e nt i n g t h e i ast i u uuu t o i uu uuu year s. Fi n al l y t her e i s CEGAL ( Cl i mat e, Ev ol ut i o n and Geod yn ami c s core woul d be i mmeasurabl e by sci enti f i c st andar ds一 and so n a wi l l b e t he ex pen s el Es t i mat e s of t h e cos tto obtai n the mai n co nt i nu ous co r e t hr o ugh t he s edi me nt ar y s ect i oni 18 犷 . 住招下 hi s t or y and s houl d c ont ai n a r el a t i ve l y co nt i n uous a nd pr eci s e r e cor d of cl i mat i c f l uct ua t i o ns . Th e val ue of t h i s 刁 - 卜协 - 1 981 ) . Su ch a cor e mi g ht ve r y wel l s p an 1 0 or 20 Ma of ear t h 2 狱 一 洲洲 洲 瀚饭华上 一牛 nd around the ri f t- val l ey l akes. I n mappi ng pr ograms i n a of Af r i can La ke s ) . CEGAL i s a mul t i di s ci p l i n ar y and i nt er - ai s e a l ong n at i on al pr oj ect who s e ul t i mat e obj ec t i ve i s r core f rom one of t he ri f t- val l ey l akes of r i ca ( Lewi n, S 0 诊一 一 ar ger l a kes wi l l be cond u ct ed i n nai ss ance s ur veys on the l ei t her mul ti channel CDP o r s i ngl e- ch ann el , nor mal i n ci denc e f or mat , de p endi ng up on l o cat i on, o b] e ct i ve and g eol ogy . Hi gh - r es o l ut i on s ei s mi cs al on g wi t h s i d e- s can s on ar a nd ma g- net i cs d at a wi l l be co l l e ct ed al ong man y t r ack l i n es . For s ome l o cat i on s , s uch as La ke Tu r kan a, s pec i al s ur ve ys ha ve b ee n de si g ned a r ou nd t h es e met hod s . . A S e ver a l compl emen t ar y p r o gr a ms a r e pl an ned as well g r o up of geo l o gi s t s u nd er t he s up er vi s i on ofDr. Jacques ampl i ng and LeFou r ni er of t h e ELF- Aq ui t ai ne wi l l con t i n ues 黔 黔犷 鹭一 怨 苹藉 嗯 一 华_巨;一健藻 〕了_ 户 子 竺 丫一丁之一 夏 斗 叹 { 碧 丫- 厂军一爹 少乓矛 犷 _ 冬少 Sei smi c r e f l ect i o n pr of i l e of t he Nk hat a Bay Fi gure 9: Provi nce of Lak e Mal awi . V er t i cal s c al e as f or Fi gur e . 7 . Hori zont al ext ent about 25 km. EPL SODES, Vol . 1 9 83, No. 1 . ,. 二、 ,.、二吃 子耀肇赘 幸少. : . 羚盆::妙 豁汗葡即井 璐 口口羹国口 黔 UNCONFORMI TY MALAW IGRO UP NYASA G ROUP :了. . ) Ki l omet er s b 认 - . - -- ~,,J 2 NYANJA GROUP ” 谁 红沙 户‘ Fi g ur e 10 : Acou s t i co - s t r at i gr a phi c i nt er pr et at i on of r ef l ect i o n pr o f i l e s hown i n Fi gur e 7 . Bas ed on many t a pe r e pl ay s at di f f er e nt f i l t er s e t t i ng s and di f f er e nt ampl i t ude p r es er v at i on s . Th e co s t of CEGAL dr i l l i ng on Tan gan yi k a or Mal a wi i s not pr ohi bi t i v e by pe t r o l eum i n dus t r y s t and ar d s . I nd eed, we en vi s i o n achi ev ement of CEGAL' s ul t i mat e o bj ect i ve t h r ou gh a co op er at i v e ar r an geme nt b et we en s ci en ce a nd i nd us t r y. Th e gr e at e s t s ci ent i f i c i mpa ct of our g eop hy s i c al wo r k may, t h er ef or e,c ome onl y af t er t h e i n dus t r i al dr i l l i ng i n t he r i f t - val l e y l ake s n ow b ei n g cons i d er ed . Di x ey, F. , 1 95 6. Th e Eas t Af r i ca n Ri f t S y s t e m. Co l o ni al Geo l o gy and Mi ne r al Re s ou r ce s Bul l et i n , S upp l ement Se - r i es , no. 1 , v. 1 , 71 p. Gr e gor y, J . W. , 1 896 . Th e Gr eat Ri f t Val l e y. Mu r r ay , Lon don . 422 p. Hec ky, R. E. , 1 978 . The Ki v u- Tan ga ny i k a ba s i n: t he l as t 1 4, 0 00 y ear s . Po l i s h Ar c hi ves of Hydr obi ol og y, v. 25, p. Acknowl edgements Lewi n, R. , 1 981 . Lak e Bot t oms l i n ked wi t h h uman or i g i ns . S ci e nc e, v . 21 1, no . 448 2, p. 56 4- 5 66 . 159-165. Thi s paper was made pos si bl e by many gover nment s, agenci es and peopl e, es peci al - l y i n Bur undi , Kenya, Mal awi , Mozambi que, Rwanda, Tanzani a, Uganda, Zai re and Zambi a, where we have al ready worked or pl an to wor k i n t he future. We ar e s p e ci f i ca l l y i n de bt ed t o Mi n i s t e r of Mi ne s , I s ad or e Ny a bo y a a nd Dr . R6 no va t Samoya of Geomi nes, Burundi ; Dr. R. Johnson, Chi ef Geol ogi st and Dr. M. J. Crow of the Geol ogi cal Survey of Mal awi ; Mr . R. Leakey of the Nati onal Mus eum of Kenya; Pr of es sor H. Kayumbo of the Tanzani an Nati onal Sci enti fi c Res earch Counci l; Mai tr e Ngi mbi , Di rect or of the Of fi ce of t he Pr es i dent cf the Republ i c of Zai re; Dr. K. M' Baya of the I ns ti tut de Recherche Sci enti fi que, Zai re; Di rect or Dr. E. H. B. Mwa n an g' on z e an d N. Mo ne y of t h e Geo l og i ca l S ur ve y of Za mbi a . We ar e i ndebted to our shi p- boar d resear ch compani ons W. Cromi e, M. J. Crow, C. Ebi nger, K. Haber yan, J. LeFourni er , M. M. Maksazu, J. Nel son, M. Patt ers on, F. Spy-Ander son and C. Stager. We have benefi tted especi all y from the post- cr ui s e dat a anal ys es of Ms. Ebi nger and Mr. Patt er son. Addi ti onal fi el d and l ogi s ti cal assi s tance was gi ven by A. Adr i anos, Capt ai n J. Barrett, J. Dri vas, N. B. Mukonki , J. Tur ner and the enti r e ELF-Aqui tai ne team, parti cularl y Dr. Jacques LeFour ni er. Li vi ngs t on e, D. A. , 19 65 . S edi me nt at i o n an d t h e hi s t or y of wat er l e vel ch an ge i n La ke Tan gan yi k a. Li mn ol o gy and Ocea no gr aph y, v. 1 0, p. 607 - 61 0 . S u t t o n, G. H. and B er g, E. , 1 958. Se i s mol ogi ca l s t udi es o f t he wes t er n Ri f t Val l e y of Af r i ca. Tr ans a ct i ons of t h e Amer i can Geop hy s i c al Uni on, v. 39 , p. 474 - 4 91 . v on Her zen, R. P. and Vacq ui er , V. , 1 967 . Ter r es t r i al h eat f l ow i n La ke Mal awi , Af r i ca . J ou r n al of Geo ph ys i ca l Res ear ch , v. 7 2, p. 4221 - 42 26. Th e wor k wa s f u n de d b y t h e Nat i on al Sc i e nc e Fo u nd at i on , So ci 6t 6 Nat i o na l e ELF- Aq ui t ai n e ( P r o du ct i on ) a nd Mob i l Oi l Co mpa ny . Won g, H. K. an d vo n Her z en, R. P. , 1 97 4. A geo phy s i cal s t udy o f La ke Ki vu, Eas t Af r i ca. Geop hy si cal Jo ur n al of t he Ro yal As t r ono mi cal S oci et y, v. 37 , p. 371 - 3 89. References ABOUT THE AUTHORS: Br uce R. Ros endahl i s an Ass ocj - And r e w, E. M. , 1 974. Gr a vi t y s ur vey of Mal awi : f i el d wor k at e Pr o f es s or of Geo ph ys i cs ( De - and p r oc es si ng . Re por t of t h e I ns t i t ut e of Geol o gi c al par t men t of Geo l o gy, Duk e Uni - Sci en ces , v. 74( 1 5) , 36 p. versi t y, Durham, North Carol i na 2 77 08, U. S. A. ) . Hi s r es ea r ch ac - ti vi ti es center around the st ruc- of h or i z ont al comp r e s s i on l i n ked wi t h t he op eni ng of t h e t ur e, dynami cs and evol uti on of eas t Af r i c an r i f t s ( Wes t er n b r an ch) , i nt he r i dge b et wee n cr e t i o nar y pl a t e bo un dar i es . the Kivu and t he Tan ga nyi ka l akes . Compte Rendu ac He has par t i c i pat ed i n man y Som m aire des S 6 an ces de l a S oci 6t 6 G6 ol ogi que d e Chorowi cz, J. , Mukonki , N. B. and Potti er, Y. , 1979. Evi dence 了早}、 Fr ance, f as c. 5- 6, p. 231-234. oceanographi c expedi ti ons and i s now enga ged i n dev el opi ng t he Cr omi e , W. J. , 1 98 2. Be neat h t h e Af r i c an Lakes . Mos a i c, Af r i c an La k es g eop hy s i cal pr o- U. S. Nat i on al Sci en ce Fo un dat i on, v. 1 3 , p . 2 - 6. j ect d es cr i b ed i n t h i s ar t i cl e. Cr os s l e y, R. an d Cr ow, M. J. , 1 98 0. The Mal awi Ri f t . 坦, Dani e l A. Li vi n gs t one , J ames B. Geo dy nami c Evo l u t i on of t he Af r o- Ar abi an Ri f t Sy s t em, Du ke Pr of es s or of Zo ol og y, a t Acc ade mi a Na zi o nal e De i Li nce i , Roma, p . 77 - 8 7. Duk e Uni v er s i t y, i s a bi o l o gi s t wi t h a l i f el on g i nt e r es t i n g eo - Cr ow, M. J. , 1 9 80. A r evi ew of t he ge ol o gy and mi n er al emi s t r y a nd Qua t er nar y geo l o- r es o ur ces of Lak e Mal awi . Geo l o gi cal S ur vey of Mal awi ch Rep or t No . MJC/ 1 8, 40p . gy. Af t er ear l y wor k i n embr y - o l o gy, zo og eo gr a phy an d f i s he- Dege ns , E. T. , v on Her z en, R. P. an d Won g, H. K. , 1 971 . Lake r i es h e t u r ne d t o t he p al a eo - Tan gan yi k a: Wat er che mi s t r y , s e di me nt s , g eol o gi cal eco l o gy of Al as k a and Nov a s t r u ct u r e. Nat ur wi s s en s ch af t en , v. 58 , p. 22 9- 2 41 . S co t i a. He has b een s t udyi n g Afr i can l akes and t hei r s edi - Di x ey , F. , 1 9 26. Th e Te r t i ar y an d Pos t - Te r t i ar y l acu s t r i ne ments si nce 1962 and i s now or- s edi me nt s of t he Nya s an Ri f t Val l ey. Qu ar t er l y J o ur n al of t h e Geo l o gi c al S oc i et y o f Lo ndo n, v. 83, p. 43 2- 442 . gan i zi ng Pr oj ect CEGAL. EPI SODES, Vol . 1983, No. 1. 19
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