IH·

r ' Tectonic development of the western branch of the East African rift system

C. J. EBINGER* Massachuselts Institute of Technology/ Woods Hole Oceanographic Institution Joint Program, Department of Earth, Atmospheric, & Planetary Sciences, Massachuse//s Institute of Technology, Cambridge, Massachusetts 02139

ABSTRACT segmentation generally is not inherited. faulting across the broad uplifted rift flanks, The Western rift, the western branch of the Western rift border-fault segments propa­ however, were poorly understood prior to this East African rift system,-is bordered by high­ gated to the north and south to link originally study (Fig. I b). Earlier compilations of East Af­ I angle normal fault systems bounding one side isolated basins, and this along-axis propaga­ rican rift structures commonly have included in­ of spoon-shaped ba'sins-'. Depth-to-detach­ tion contributes to the segmentation of the active structures shown by recent studies to be ment estimates of 20-30 km, the rollover Western rift valley. pre-Tertiary in age, and considerable variations I geometry of asymmetric basins, and seismic­ in age of volcanic sequences within and between !/ i ity throughout the depth range 0-30 km sug­ INTRODUCTION the Kenya and Western rift valleys have been I' gest that planar border faults along one side reported (for example, Baker and others, 1971; of rift basins penetrate the crust. Along the The volcanically and seismically active East Bellon and Pouclet, 1980; Crossley and Knight, length of the rift, -100-km-long en echelon African rift system lies atop a broad intraconti­ 1981; Tiercel in and others, 1988). In order to border-fault segments are linked by oblique- nental swell, the East African Plateau, and estimate crustal extension and to evaluate tec­ slip transfer faults, ramps, and monoclines consists of two branches, the Western and tonic models of the East African rift system, a within comparatively high-strain accommo- Kenya (Gregory) rift valleys (Fig. I). In earlier detailed chronology of subsidence, rift flank up­ dation zones. These transfer faults accommo- tectonic models of the East African rift system, lift, and volcanism along the length of the lesser date significant along-axis variations in the both the Kenya and Western rift systems have known Western rift is needed. When evaluated elevation of basins and uplifted rift nanks but been interpreted as incipient plate boundaries on a basin-by-basin basis, spatial and temporal do not appear to extend outside the 40- to linked to the Afar-Red Sea-Gulf of Aden rift relations repeated along the length of the West­ 70-km-wide rift basins across the uplifted systems to the northeast (for example, Gregory, ern rift provide constraints on the nature and llanks. Kinematic constraints from several 1896; Fairhead and Girdler, 1969; Degens and cause of the along-axis segmentation of this ac­ basins indicate a regional east-west extension others, 1971; McConnell, 1972; Chorowicz, tive continental rift. direction that is consistent with existing scis- 1983; Courtillot and others, 1987). No connec­ The objectives of this study are (I) to use con­ micity data. Estimates of crustal extension tion between the two approximately north­ sistent structural and stratigraphic criteria to de· based on fault geometries interpreted from south-striking Western and Kenya rifts, how­ lineate individual extensional basins along the seismic reflection data and field observations ever, is apparent in structural, morphologic, or length of the Western rift, (2) to summarize within 13 basins arc less than 15% (<10 km). seismicity patterns. Tectonic interpretations geometric and kinematic comtraints on the tim­ I Volcanism began at -12 Main the nortil and : differ in the timing, direction, and amount of·· ing and amount of vertical and horizontal crustal I at -7 Ma in the south prior to or concurrent crustal extension within the East African rift sys­ movements within the Western rift system, and I with the initial development of Western rift tern, largely owing to lack of observational con­ (3) to use these structural and stratigraphic ob- · l sedimentary basins. During Pleistocene time, straints within the Western rift (for example, servations to evaluate mechanisms for the along­ asymmetric basins have narrowed through Fairhead and Girdler, 1969; Chorowicz, I983; axis segmentation of the rift. The results of this progressive hanging-wall collapse and with Shudofsky, 1985; Rosendahl and others, 1986; regional synthesis are compared to interpreta­ Ii uplift of the rift flanks. These geometric and Ebinger and others, 1987; Morley, 1988a; Tier­ tions of the better-known Kenya rift located temporal patterns of development are similar celin and others, 1988). along the eastern side of the East African Pla­ to those of the more .magmatically active Seismic, gravity, and bathymetric data from teau. Detailed field observations within several Kenya rift, where volcanic activity began -11 the Western and Kenya rifts show that the two representative rift basins supplement structural m.y. prior to initial volcanism in the Western rift valleys are segmented along their length into interpretations of 30-m-resolution Landsat im­ rift. Both the poor correlation between Mio- a series of generally asymmetric basins approx­ agery, reports, and geologic maps and extend cene-Recent border and transfer faults and imately 100 km in length (for example, King, interpretations of seismic reflection profiles from pre-Miocene shear zones, and the repetitive 1978; Crossley and Crow, 1980; Chorowicz, Western rift lake basins across the uplifted rift basin geometries indicate that the along-axis 1983; Ebinger and others, 1984; Bosworth, flanks. Field studies in the Western rift also were 1985; Rosendahl and others, 1986; Baker, 1986; designed to enhance and check existing struc­ tural and stratigraphic interpretations, and to *Present address: Geodynamics Branch, National Morley, 1988a). The three-dimensional geome­ Aeronautics and Space Administration/Goddard try and continuity of Tertiary faults bounding obtain kinematic constraints on the timing and Space Flight Center, Greenbelt, Maryland 20771. and linking Western rift basins and the extent of direction of extension.

Geological Society of America Bulletin, v. 101, p. 885-903, 14 figs., 2 tables, July 1989.

885 ~,,-----

C. J. EBINGER

3000 A '\I\ {::{ / me1ers , / Mobut~ WESTERN':.... RIFT ~ I. ' 1000 ~

0 800 0 A A' KM

EAST AFRICAN PLATEAU b

r~-~J LAKE

L.CENOZOIC VOLCANICS

,,

30° 380 a ·, Figure 1. a. Tertiary volcanic provinces of East African rift system; lakes filling parts of the Western rift valley labeled. Boxes show coverage ' provided by high-resolution Thematic Mapper (5) and Multi-Spectral Scanner (MSS) (1-4) images; small squares show approximate centers of black-and-white MSS imagery used in analyses. Arrows point to regions outside scenes 1-5 where field studies were conducted. Elevations greater than 800 m within uplifted East African Plateau enclosed by light line. Dashed double line shows the approximate position of the Western and Kenya rift valley floors. b. Topographic profile A-A' across East African Plateau. Shading emphasizes approximately 400-km­ wide zone of uplift flanking rift valleys.

BACKGROUND profiles average crustal structure over distances 1985). Recent analyses of gravity data from East of 200-800 km, however, crustal thickness be­ Africa suggest that thermal processes and fault­ Geophysical evidence for crustal thinning neath individual rift basins may be less. If an ing have weakened the lithosphere beneath the across the 1,300-km-wide East African Plateau approximately east-west extension direction is Western rift valley relative to ~hat beneath the is restricted to 40- to 75-km-wide zones beneath assumed, estimates of crustal thinning based on uplifted craton (Ebinger and others, I989). the Western and Kenya rift valleys (Rykounov reconstructions of surface fault geometries in the The earliest volcanism in the East African and others, I 972; Bram and Schmeling, I 975; Tanganyika rift range from 2%-15%, and iso­ Plateau region occurred at 23 Ma in the central Maguire and Long, I 976; Hebert and Langston, static models of subsidence patterns within the plateau region and near the future site of the 1985; KRISP, I 987). On the basis of seismic Malawi rift indicate that the crust has been Kenya rift system (for example, Baker and oth­ refraction data, crustal thinning beneath the thinned by less than 25% (Ebinger and others, ers, 1971) (Table 1). Initi·al uplift of at least the northern part of the Western rift system is less 1987, I 989; Morley, I 988a). Along the length eastern part of the plateau began in early Mio­ than 25%; crustal thickness estimates from the of the Western rift system, numerous small­ cene time, and sediments shed from the uplifted Edward-Kivu rift systems and the Kivu-Tangan­ magnitude earthquakes generally with tensional plateau were deposited rapidly within the ma­ yika rifts are approximately 30 km, or 5-1 I km focal mechanisms oc.:ur throughout the depth rine basin along the Indian Ocean margin of less than those found beneath the East African range 0-30 km with no apparent vertical gap in Africa (Saggcrson and Baker, 1965; Shackleton, Plateau outside the rift valleys (Bram and seismicity (Wohlenberg, 1968; Rodrigues, 1970; 1978; King, 1978; Coffin and Rabinowitz, Schmeling, I 975; Maguire and Long, 1976; Zana and Hamaguchi, 1978; Brown and Girdler, 1983). Although Paleogene 4°K-40Ar ages have Hebert and Langston, I 985). Because refraction 1980; Fairhead and Stuart, 1982; Shudofsky, been reported (for example, Bellon and Pouclet, I

I I TECTONIC DEVELOPMENT OF EAST AFRICAN RIFT SYSTEM 887

I TABLE I. CHRONOLOGIC CONSTRAINTS FIWM EAST AFRICAN DOME REGION, USED IN FIGU1{~14", within accommodation zones has been sug­ gested, en echelon border-fault segments are Site Description Age (m.y.) Rckrcno: connected by oblique-slip transfer faults and

A Samburu basalts, K/ Ar n.o:!: 0.2 Baker and others. 1971 relay ramps in parts of East Africa (for example,

Mount Elgon m:phclinitc. K/ Ar 22.0.: 0.2* Baker and o1hcrs. 1971 King, 1978; Bosworth, 1985; Rosendahl and c Rusinga Island tufT, K/ Ar 17.9' 0.1 Drake and others. 1988 others, 1986; Ebinger, 1989). Existing geologic

J) Kishalduga ncphdinite, K/ Ar 15.1 Crossley and Knight, 1981 maps from the Western rift region, however, are

E Elgcyo phonolite, K/ Ar 13.6 = 0.6 Crossley and Knight. 1981 at a scale of 1:2,000,000, and more detailed geo­

Kapiti phonolite, K/ Ar 13.1 = 0.5 Crossley and Knight, 1981 logic reports generally have focused on mineral

G Virunga alkali basah, K/ Ar 12.6 = 0.7, Bellon and Pouclct, 1980 resources and PrecambFi:l'n·structures. Thus, late

H S. Kivu tholeiite, K/ Ar 100,0.6 Pastecls and others. 1989 Cenozoic structures were unreported or undif­

S. Kivu nlkali basalt, K/ Ar 8.5 = 0.5 Pastccls and others., 1989 ferentiated from Precambrian and Mesozoic

Mobutu basin, fauna -8 M. Pickford. 1989. personal commun. structures in much of the Western rift.

K Rung we phonolite, K/ Ar 7.2 = 0.6 Ebinger and others, 19H9 Field studies and enhanced color Landsat-5 01 f:.s.ayeiti trachyte, K/ Ar 6.7 Baker and othc:-rs.. 1971 Thematic Mapper (TM) and Multi-Spectral

M Rusi~i basin s.cdiments, fauna -5.0 D. Stone, 1987. personal commun. Scanner (MSS) imagery supplemented by

N Chiwondo beds, fauna 4-5 Kaufulu and othc:-r'.i, 1981 I :50,000 aerial photographs were used as a

0 Mount Sodiman ncphdinite, K/ Ar 4.5 '0.4 Bagda).aryan and others. 1973 primary data base to examine fault patterns be­ Edward basin scdimcnlS, fauna t 4.1 P. Williams.on, 1987, personal commun. tween discrete rift basins and along the uplifted

Q Ngorongoro b3s.alt, K/ Ar 3.7 ~ O.R P. William~)n, 1987. pt:r~nal oommun. flanks of the Western rift valley and to integrate

ll Mount Kenya syenite, K I Ar 3.1 B~kcr and others, 1971 existing maps from the region (see App. I; Fig.

Usangu {Mhcya) picritc, K/ Ar 3.04 = 0.04 Ebinger and others, 19X9 I). Transects of 14 rift flanks along -east-west

T Kilinumjuro nc:phdinitc, K/Ar 2.3 '0.4 ll:tgJ:~~ry~n And other~. 197~ continuations of Project PROBE seismic reflec­

lJ Songwc truchytc, K/Ar 0.55, O.oJ Ehingrr 11nd other\, 19~9 tion profiles of lake basins and detailed studies in

I' Torn-1\nkok mafuritr:, K/Ar 0..15! 0.15 U!LH~oill~rynn lind other), 1973 5 rift basins were made in 1986 and 1987. In order to extend these structural interpretations •Faun!tl ni~oknl.'c ~~~~c~b tlu11 Mount Clg1Jtl h:1-.:Lit) :m• npp19>.irn:ttdy I H m.y. uld IM. PkU1~1d. 1')~9. p;-rwnal ~;ummun.), 1tt:.'>l.· of l>C'I.'1Ltll\ lltJI ~en. between the specific field study locales shown in Figure I, a mosaic of I: I ,000,000 MSS imagery provided by Musce Royale de !'Afrique Cen­ trale was assembled. Where faults and linea- 1980; Kampunzu and others, 1983; Tiercelin McConnell, 1972). Metamorphic basement be- ments had not been noted previously, the and others, 1988), recent radiometric analyses of neath the Western rift is characterized by following criteria were used to differentiate volcanic rocks indicate that initial volcanism northwest-trending, northeast-trending, and ap- Neogene faults from older structures in imagery: within the Western rift commenced at approxi­ proximately north-south-trending mylonites and amount of topographic relief, appearance of mately 12 Main the north, and volcanic activity shear zones (for example, Reeves, 1960; Cahen fault scarps, occurrence of hot springs, and hori- continues to historic times within the isolated and Snelling, 1984; Theunissen, 1986; Daly, zontal oiTseL~ of pre-rift basement faults and Western rift volcanic provinces (Harkin, 1960; 1986; Klerkx and Nanyaro, 1988) (Fig. 2), geologic contacts. The scale of topographic Bagdasaryan and others, 1973; Bellon and Within parts of the East African rift system, • maps used in the field and to interpret Landsat Pouclet, 1980; Pasteels and others, 1989; upper Paleozoic-Mesozoic sedimentary se- imagery was generally 1:50,00Q. Ebinger and others, 1989), Mid-Miocene:-age cc··quences deposited in rift basins underlie Tertiary .. ,", Sediment thickness, fault geometries, and the fauna found in fluvial sequences of the Mobutu sequences (for example, Arambourg, 1933; sense of asymmetry within rift basins are derived (Albert) basin and an extrapolation of Holocene Spence, 1954; Dixey, 1956; Williamson and from interpretations of single-channel and multi- sedimentation rates have been cited as evidence Savage, 1986). channel seismic reflection data, aeromagnetic for initial faulting during early Miocene time data, and gravity data from Lake Tanganyika (Hopwood and Lepersonne, 1953; Patterson, WESTERN RIFT BASINS (Degens and others, 1971; patterson, 1983; 1983). In the southern part of the Western rift Lorber, 1984; Rosendahl and others, 1986; Bur­ system, the oldest reported lacustrine sediments Approximately 100-km-long normal fault gess and others, 1989), Lake Kivu (Degens and contain fossils dated at .6-5 Ma (Kaufulu and systems with 1- to 6-km throws bound the others, 1973), the Rusizi basin (D. Stone, 1987, others, 1981; C. Morley, 1988, personal deeper side of asymmetric basins (border-fault personal commun.), the Rukwa rift (Peirce and commun.). segments), and the sense of basinal asymmetry Lipkov, 1988; C, Morley, 1988, personal com­ The East African rift system formed largely commonly alternates along the length of the rift mun.), and the Malawi rift (Ebinger and others, within Proterozoic orogenic belts surrounding valley (Crossley and Crow, 1980; Ebinger and 1987). Seismicity patterns and depth to detach­ the Archean cratons of central and eastern others, 1984; Rosendahl and others, 1986; ment estimates (for example, Gibbs, 1983) pro­ Africa, although faults bounding the central rift Peirce and Lipkov, 1988; Burgess and others, vide information on border-fault geometries at fracture unmetamorphosed cratonic rocks (Fig. 1989). The broad flanks of the Western rift have depth. 2). In a regional sense, the location of the two been uplifted 1-4 km above the surrounding Generalized Three-Dimensional Geometry rift valleys follows Proterozoic orogenic belts topography of the East African Plateau, and of Western Basins and avoids the central craton, possibly reflecting metamorphic basement lies below sea level pre-existing heterogeneity in lithospheric strength beneath many basins (for example, Figs, 1b, 3). Structural relations within the Moba and Ma­ across the I ,300-krn-wide plateau (for example, Although basin linkage by ductile deformation rungu basins of Lake Tanganyika illustrate the -..-..... ~

888 C. J. EBINGER

. ~-

.. ··.i::ns:·.-· ·.: . .. ·.

. .. ·.

a·s

SEDIMENTARY ROCKS l::q~~~ Tertiary- Quaternary

· ~ Permo - Triassic ~ (Karroo) Precambrian METAMORPHIC ROCKS p£s I I (undifferentiated) ~ Pan-African . VOLCANIC ROCKS L:.!J (900- 600 Ma) !I ~ Tertiary - Quaternary r---;(1 Proterozoic ~ basalts L..:._j (undifferentiated) Bangweulu volcanics ( 1850Ma) [}}ft\J Archaean

Figure 2. Summary of East African geology, emphasizing Permian-Triassic rift basins (dark shading). Regional structural trends within Precambrian orogenic belts indicated by short lines. Note that several Western rift basins have developed within Archean basement. Geologic information from Afonso (1976), Baker (1986), Cahen and Snelling (1984), Carter and Bennett (1973), Kent and others (1971), Lepersonne (1977), Quennell and others (1956), Reeves (1960). typical three-dimensional geometry of Western two border segments along the southwestern western margins of the Moba and Marungu ba­ rift border faults and basins. On the basis of the margin of Lake Tanganyika (Rosendahl and sins, and these faults arc characterized by distinct asymmetry in topographic and bathy­ others, 1986) (Fig. 4). High-angle (45°-75°) triangular scarps in Landsat scene 3 and aerial metric relief and sediment thickness variations normal faults with throws of I 00 m or more photographs (Fig. 5). Faults bounding the Moba apparent in seismic reflection data, r interpret occur in a I 0- to 15-km-wide zone along the basin strike northwest; those bounding the Ma- TECTONIC DEVELOPMENT OF EAST AFRICAN RIFT SYSTEM 889

w w 0 0 co 0 N r<> oo + + oo

L. Edward TORO-ANKOLE PROVINCE

VIRUNGA Rutshuru basin b PROVINCE SENSE OF Virunga basin INTRABASINAL .... HORST ASYMMETRY

NEOGENE BORDER FAULT []. SEDIMENTS ,/ SEGMENT NEOGENE NORMAL FAULT VOLCANICS / t'\" MONOCLINE ... VOLCANIC CENTER EARTHQUAKE • EPICENTER HYDROTHERMAL • SPRING a 0 100 KM

Figure 3. a. Tectonic interpretation ·or· rift basins bounded by border-fault segments (BFS's) 1...,10 described in text .. Focal mechanism_ solutions of earthquakes with epicentrallocations denoted by squares shown as lower-hemisphere projections; hypocentral depths as indicated (from Shudofsky, 1985). Sense of basin asymmetry indicated where known. Structural interpretations from Holmes (1951), Davies (1951), Pouclet (1977), Degens and others (1973), this study. b. Smoothed contours of topographic relief, showing rift valley topography and regional elevation variations. Contour interval, 200 m; elevations above 1,400 m (Albert-Edward) and 1,800 m (Kivu) shown shaded to illustrate asymmetry of rift flank uplift.

rungu basin strike approximately north-south valley, and no evidence for faulting is found out­ inadequate to place tight constraints on the (Figs. 5, 6). Northwest-striking faults interpreted side the inner-facing normal faults (Figs. 5, 6, geometry of border faults at depth (for example, as oblique-slip faults commonly exhibit subhori­ and 7). The eastern margins of both basins ate Rosendahl and others, 1986). zontal slickenside striations (Figs. 5, 6) (Choro­ structural monoclines, and metamorphic base­ The Moba basin is characterized by 5- to 10- wicz, 1983). The geometrical arrangement of ment dips gently to the west beneath sedimen­ km-wide tilted blocks that generally trend generally linear faults along these -1 00-km-long tary sequences contained in these two basins northwest parallel to normal faults bounding the border-fault segments produces a curvilinear (Fig. 7). Thus, sedimentary basins bounded by Moba basin. Faults within the Marungu basin basin margin (Fig. 6). these 10- to 15-km-wide systems of high-angle generally strike north-south parallel to Marungu . The approximately 2,000-m-high plateau on normal faults (border-fault segments) have the border faults (Fig. 6). Synsedimentary faults the western side of the Moba and Marungu ba­ characteristic cross-sectional form of half-graben penetrate to all stratigraphic levels of the sedi­ sins dips gently to the west away from the rift (Fig. 7). Seismic velocity control, however, is mentary sequence within both basins, although . -" ·~ ~ ~ .. ~ - ...... ," _..,...... - .. -·. -- -_.- -- ~-""- ..-. -~~- .. _. ..,_ ___ . - ~; ,>

a INTRABASINAL ... SENSE OF -- HORST ASYMMETRY !:-'

Kalemie

w 0 (\J -'ff""'""c..;..-'11 Moba r<) + + 7°S 4°S . .' basin

0 100 basin w KM 0 N r<) +10°5

b TECTONIC DEVELOPMENT OF EAST AFRICAN RIFT SYSTEM 891 the magnitude of subsidence is greater in the Marungu basin than in the Moba basin (Morley, 1988a) (Fig. 7; Table 2). Sedimentary accumu­ lations arc grea test near the central part of border-fault segments and decrease toward the tips, producing spoon-shaped basin morpholo­ gies (Figs. 6, 7). Estimates of extension based on the geometry of faults offselling metamorphic and/or acoustic basement are 4-5 km (- 10%), or slightly more than the_l km estimated by Morley ( I 988a) based on seismic data alone (Table 2). On the basis of balanced cross sec­ tions, depth to detachment for faults bounding these basins is - 25 km. Dip-slip and oblique-slip transfer faults bounding the western margin of Lake Tangan­ u yika intersect at the tips of the two border-fault segments, an interbasinal accommodation zone (Figs. 5, 6, and 8). Within the accommodation zone, a basement ridge separates the two basins, but sei~mic coverage is insufficient to constrain the geometry of fau lts linking the two ex ten­ sional basins beneath the lake (for example, Rosendahl and others, 1986; Morley, 1988a). On the eastern side of the rift, normal faults strike N20°W and Nl0°E (rig. 8b). Subhori- 7.tlntal slickcnside striations indicate that the most recent displacement along cast-west-striking faults was strike sl ip, but no lithologic or struc­ tural indic.1tors were found to document the sense and amount of offset (Figs. 8b, 9). Faults bounding the Moba and Marungu ba­ sins cut unmctamorphosed and homogeneous Archean and Precambrian basement and, there­ fore, should be unaffected by pre-existing crustal weaknesses (Figs. 2, 6). The northern part of the Moba border-faul t segment extends across a fault conlact between lower Proterozoic and Ar­ chean rocks with no change in fault orienlation {Z, Fig. 6). Faults on the eastern side of the basins arc oblique to metamorphic foliations and mylonites striking N60°W (Fig. 8b). Figure 5. Part of Landsat MSS image 3 (Fig. 1), covering the southwestern section of the Tanganyika rift (Moba and Marungu basins). Black-and-white photograph of false-color com­ Basinal Descriptions posite image (see App. 1). A-A' and B-B' refer to profil es shown in Figures 7a and 7b, respectively. Regional tectonic maps of rift basins bounded by border-fault segments (BFS's) 1-23 shown in Figures 3 and 4 and briefly described from north depth to pre-rift basement is not known beneath region, and throws decrease toward the north to south arc based on similar analyses of topog­ many Western rift basins; however, seismic stud­ and south (Rodrigues, 1970; Shudofsky, 1985) raphy, surface and subsurface structures, and ies within Western rift lake basins show that (Fig. 3b). Ilot salt-water springs and oil seeps analogy to border-fault geometries illustrated depth to pre-rift basement generally follows bath­ have been reported along the base of the above. New and existing constraints on crustal ymetric contours (for example, Degens and Mobutu escarpment (Davies, 195 I). The fault­ extension, border-fault geometries, and the tim­ others, 1971; Wong and Von Herzen, 1974; plane mechanism determined by Shudofsky ing of subsidence, uplift, and volcanism are de­ Ebinger and others, 1984; Rosendahl and oth­ ( 1985) for an earthquake at I0 km depth be­ scribed briefly and in Table 2. Accommodation ers, 1986). neath the Mobutu basin indicates pure normal zones are identified, but the variable geometry of faulting along a north-south striking fault plane transfer faults within specific accommodation Mobutu Basin (BFSl) (Fig. 3a). On the eastern side of the basin, the zones requires more detailed analyses (for ex­ eastward-tilted surface of westward-dipping ample, Kmg, 1978; Ebinger, 1989). For com­ The northwestern margin of the Mobutu (Al­ faults form a stair-step pallcrn (step faults) rising parative purposes, maximum amounts of subsi­ bert) basin is bounded by BFS I (Fig. 3a). At its to the elevation of the uplifted East Afncan Pla­ dence within basins arc listed as present central part, the seismically active Mobutu es­ teau (Davies, ·195 I). Borehole data indicate that clc\ at ion' of the rift valley or lake floor because carpment nscs 1.300 m al"love the surroundmg at lc.1st 1,250 m of fluvio-lacu\trinc \Cdimenwy n I! i 892 C. J. EBINGER -. . \ ...... , . Y.:' ··· ...... ··' \ :' . ',>. ! Oo .... J

'I' 'I

~ CONTACT t HOT SPRING , .. NORMAL FAULT DEPTH TO •' \ ,· ACOUSTK: P€s \ PRE-RIFT FAULT BASEMENT "< (m) ;I LINEAMENT

p€s 0 KM

Figure 6. Interpretation of lineaments observed in part of MSS image 3. Faults have formed in undeformed Archean volcanic sequence (Wv); flat-lying sedimentary rocks (p£s) along the uplifted flank roughly correspond to highest elevations. Geologic legend same as in Figure 2; XK, Kibaran belt (Late Proterozoic). Z denotes structural contact referred to in text. Box encloses region shown in Figure 5; fault patterns in upper right corner shown in Figure 8b. A-a-A', B-b-B' refer to cross-sectional profiles shown in Figures 7a and 7b, respectively. Location of hydrothermal springs from Tshimanga and Kabengele (1981); lithologic units and basement structures from Lepersonne (1977) and McConnell (1950). Depths to basement corrected for sediment loading calculated from sediment isopach maps shown in Burgess and others (1989) and using sediment loading correction described in Crough (1983).

sequences have accumulated within the Mobutu western side of the 5,200-m-high Ruwenzori formed by the intersection of the monocline and basin (Davies, 1951 ). Early Miocene fauna mountains, and a faulted monocline bounds the BFS2 (Wayland, 1934; Holmes, 1951). Numer­ within basal fluvial sequences have been re­ northwestern margin of the Semliki basin (Fig. ous hot springs are found along the escarpment, ported, but the earliest lacustrine sequences were 3). Fault planes inferred from focal mechanisms and hanging valleys and sediment terraces attest dated at -8 Ma (Hopwood and Lepersonne, of earthquakes that occurred at depths of 7-29 to Holocene faulting within the Semliki basin 1953; Pickford, 1989, personal commun.). km beneath the Semliki basin strike N20°E, or (Davies, 1951). parallel to the strike of normal faults bounding Sernliki Basin (BFS2) the Semliki basin (Shudofsky, 1985; Fig. 3a). At Lake George Basin (BFS3) the northern end of the Scmliki basin, Pliocene­ The Semliki basin is bounded by a series of Pleistocene units correlative with those found in Faults with large throws and triangular scarps normal fau!Ls dipping N60°-80°W along the the Mobutu basin crop out along the nose bound the eastern margin of the Ruwenzori

~------~----- ~ -- f .

TECTONIC DEVELOPMENT OF EAST AFRICAN RIFT SYSTEM 893

MOBA BASIN"" w A a A. E 4

p£s 0 Xu KM -4 VE 2:1 a 20 60 120 140 - -~,- KM

MARUNGU BASIN , w B b B E 4 p£s

0 Wv KM -4 VE 2:1 ~·~ b 20 40 60 KM 80 100 120 140 Figure 7. Cross-sectional profiles of Moba and Marungu basins vertically exaggerated (VE = 2:1) to illustrate uplifted flanks and basinal morphology. a. Cross-sectional profile of Moba basin along line A-a-A' (Figs. 5 and 6). Cross-sectional profile of Marungu basin along line B-b-B' (Figs. 5 and 6). Structural patterns beneath lake from interpretation of seismic profiles 90 and 220, respectively (after Rosendahl and others, 1986).

l',IIII.E 2. SIJMMAHY OF IIOIU)ER·FIIVI.l' GEOMETRIES. sm

Mobutu N30"E/NW 2.500/570 -8 (n) strike of BFS3 (Fig. 3a). Subdued topographic Scmliki N20"E/SE7 5.200/760 ..::4.1 (b) L. George N20'E/NW7 5.200/760 relief along the eastern margin suggests that the Toro·/\nlolc mlanic province CJ lc:vel; maximum subsidence: within basins li~ as c:k:vation oft he rirt \"Biley or lake: noor becau~ depth to pre-rih ba~ment i~ not known in many ba~ifl5,, Maximum !>Cdirnentthickncu bene-ath T:tnganyika lak.c: basins in Rosendahl and othc~ ( J9H6), Morley ( 1988a}; quences in the northern part of the Kenya rift L:tke Kivu. in Wong and Von Hcn.en ( 1974); an~ Lal:c Malawi, in Ebinger and oth<-~(1987). Qs tcrraa:s indicu~ Quaternary lake !>Cquenco found along uplifted basin margin~ system (Table 2). Periods of basinal desiccation (a} M. PidJord ( 1989, pcrwnal commun.) (f) Ebinger and othe1~ ( 1989) (b) P. Willi.:~mson (1987, pcr!>onal <."'mmun.) (g) Pa_~tcch and othen ( 1989) correlated with major faunal radiations at 4 Ma (r) Br~gJ:u..ai}OHl and othrrs (1973) (h) C. Morky ( \9HB. p:r~n.at cornmun,) and between 2 and 3 Ma may be related to {d) f\,uclct {1975) (i) K:tufu!u and others ( 1981) . !e) lkllon Bnd Poudn (1980) episodes of faulting and uplift along BFS4 •Jndicato cucmion o.timatc: from Mollcy ( 196Sa). (P. Williamson, 1987, personal commun.). 894 C. J. EBINGER

W .Moba BFS

-- STRIKE DIP OF FOLIATION ...... STRIKE OF VERTICAL FOLIATION

b

\.\ PLUNGE OF SLICKENSIDE '\. STRIATIONS Figure 8. a. Schematic diagram of border-fault segments and morphology of Moba and Marungu basins. Intersection of Moba 0 5 ( 30°\V strike) and Marungu (north-south strike) border-fault systems KM produces zig-z.ag lake outline (for example, Fig. 5). Strike-slip faults d~------~7" 3f23 32" (dotted lines) interpreted from fau lt patterns in region shown in Figure Bb. b. Late Cenozoic faults and metamorphic foliations along eastern margin of Moba basin. Faull surfaces oriented approximately east-west commonly show subhori7.0ntal slickenside striations. Note gneissic foliation (N30°-40°W) oblique to normal and strike-slip faults. Legend same as in Figure 2. Xu, Ubendian belt; Qs, Quaternary sedimentary rocks.

Figure 9. Slickenside striations plunge 15° to N80°E along surface of fault striking N85°E along eastern margin of Moba basin (sec Fig. 6). Foliation of augen gneisses N35°W / 40°SW.

Rutshuru Basin (BFSS)

The eastward-tilted Rutshuru basin is bound­ ed by a north-south-striking border-fault seg­ ment (BFS5; Fig. 3b). Basalts from the Virunga province to the south cover lake sedimentary sequences that crop out along terraces found at 1,225, 1,300, 1,400, 1,450, and 1,600 m above the Rutshuru valley noor and at least 500 m above the highest reported lake level (de Ia Val­ lee Poussin, 1933; Pouclet , 1977).

Virunga Basin (BFS6)

High-angle (50°-70°) east-dipping faults bor­ der the western margin of the Virunga basin (Fig. 3b) (Pouclet, 1977). Within the accom­ modation zone between the Virunga basin and the Kivu basin, northwest-striking fissures marked by active shield volcanoes crosscut the rift valley (Fig. 3). The spatial distribution of basa lt Oows within the Virunga basin suggests that initial volcanism at 12.6 Ma preceded or was concurrent with initial faulting and basinal subsidence (Bellon and Pouclct, 1980). Along the length of 13r-S6, step faults arc capped by I l [

ZAIRE

0 30 ~------' km

f f f I

SEDIMENTS

liL~g&;l HOLOCENE

PLIO-PLEISTOCENE

" STRIKE AND DIP I NORMAL FAULT PLUNGE OF SLICKENSIDE STRIATIONS

HYDROTHERMAL SPRING

N

NYANZA- LAC

Figure I 0. Fault patterns along the northern part of the Tanganyika rift; bathymetric contours beneath Lake Tanganyika in meters below lake level (774 m). Fault patterns based on field mapping and interpretation of aerial photographs. Lithologic contacts and basement structures from Lepersonne (1977), Theunissen (1986, 1989a, I989b), Ilunga (1984). Structural information beneath Lake Tanganyika from multi-channel seismic reflection profiles 204 and 206 (dotted lines) provided by B. Rosendahl (1986, personal commun.); single-channel data from Patterson (I 983). Bold line N-N' indicates location of cross section shown in Figure 11. W, Archean; XR, Rusizian; X8 , Burundian. 1 ' 896 C J. EBINGER l -. J "' W. Nyanza -lac Basin 4 1 BFSII i ·' KM 0 I

-4 0 8 KM

Figure 11. Geologic cross section of the Nyanza-lac basin including the northern part of the Nyanza-lac border-fault segment that crosscuts Archean basement (N-N'; Fig. 10). Dashed lines are Precambrian thrust faults (from Theunissen, 1986). Estimate of crustal extension, < 15%; depth to detachment, 20-25 km. Note rollover geometry of basin, with zone of maximum subsidence near the base of BFS11. (Note slight overlap in center.)

terraces of Pliocene-Pleistocene lacustrine se­ servations and seismicity patterns indicate that Faults bounding the northwestern Rusizi basin quences at elevations of 1,450, 1,500, 1,650, the West Kivu border-fault segment has served formed during late Miocene time and propa­ 2,000, and 2,050 m, and the highest terrace is as the master fault for crustal extension during gated northward during Pliocene-Pleistocene now 800 m above the Quaternary lake level Quaternary time (Ebinger, 1989). Uplift along time. Terraces of lacustrine sedimentary se­ highstand (Pouclct, 1975). Holocene basalts the central part of BFS8 elevated a terrace of quences found in thin lenses between eastward­ from Karisimbi volcano have been uplifted on late Pleistocene lacustrine sequences -500 m tilted fault blocks are elevated nearly 500 m the monoclinal side of the basin (Pouclet, 1975; above the lake level highstand (Ebinger, 1989). above Lake Tanganyika (location a, Fig. I 0; De M uldcr and Past eels, 1986). This rift nank Table 1). uplift effectively narrowed the zone of subsi­ Rusizi Basin (BFS9) dence within the Virunga basin. Rumongc Basin (BFSIO) The Rusizi border-fault segment follows the Kivu Basins (BFS7, BFS8) western margin of the westward-tilted Rusizi The sense of asymmetry within the Rlimonge basin (Figs. 4, I 0). Earthquake epicenters during basin is down-to-the-east, opposite to the sense The western side of Lake Kivu is bounded by the period 1958-1965 within the Rusizi basin of asymmetry within the Rusizi basin (Figs. 4a, BFS7, and the West Kivu basin is filled with as cluster along the central Rusizi border-fault 10). The southern tip of the Rumonge border­ much as 500 m of sedimentary units and basalts segment, and epicentral depths throughout the fault segment extends beneath Lake Tanganyika from the Virunga province (Fig. 3b; Wong and range 0-30 km have been reported within the where normal faults striking Nl0°E continue to Von Herzen, 1974). Quaternary cinder cones Rusizi basin (Wohlenberg, 1968; Zana and the southwest of the Ubwari peninsula (Fig. 10). aligned along the central section of this escarp­ Hamaguchi, 1978). The eastern side of the Ru­ As along the Kivu fault systems, there is no evi­ ment are located within the inner-facing normal sizi basin is bounded by faceted step faults that dence for Neogene faults along the uplifted faults bounding the rift valley (Guibert, 1977; dip 60°-75° to the west (Fig. 10). Slickenside nanks outside the rift valley where topographic Bellon and Pouclet, 1980; Ebinger, 1989). A striations along border faults and transfer faults relief is gentle (Fig. 4b). second border-fault segment, BFS8, bounds the indicate an approximately east-west extension East Kivu basin (Fig. 3a). In this geometrical direction, with -2 km extension measured Nyanza-lac Basins (BFSll, BFS12) arrangement, a horst serves as a hinge for subsi­ across the basin and flanks (Ebinger, 1989). dence along both BFS7 and BFS8 (Fig. 3a). If Upper Miocene-Pleistocene lacustrine se­ The eastern side of the Ubwari peninsula is plane strain and extension normal to border quences are interbedded with and/or overlie ba­ bounded by a border-fault segment defining a faults are assumed, estimates of crustal extension salts dated at 6.5-5 Ma that have been narrow westward-tilted basin (Figs. I 0, II; based on surface fault geometries measured correlated with sequences within the Kivu ba­ Rosendahl and others, 1986; Morley, 1988a). along a cross section of the Kivu rift are less than sins (Tack and DePaepe, 1983; Ilunga, 1984; An intrabasinal .horst separates the West 10 km ( < 15% ), and depth to detachment is 23 Chorowicz and Thouin, 1985; Pasteels and oth­ Nyanza-lac basin bounded by BFSII from the km or more (Table 2). ers, 1989; Ebinger, 1989). Sequences thicken to East Nyanza-lac basin bounded by BFS 12, a Tholeiitic volcanism along the East Kivu more than 1,500 m in the central part of the half-graben tilted to the east. Thus, the cross­ border-fault segment began in mid-Miocene basin beneath Lake Tanganyika, with metamor­ sectional morphology of the rift valley is similar time, and alkalic volcanism within the West phic basement lying nearly ·1 ,200 m lower than to that of the Kivu basin (Fig. 4). The rift nank Kivu and East Kivu basins began in late Mio­ basement beneath the Kivu basins (Patterson, outside BFS 12 dips gently to the east with no cene time (Table 1). Tholeiitic and alkalic ba­ 1983; Morley, 1988a; Ebinger, 1989). This ele­ evidence for recent faulting outside inner-facing salts were erupted along both margins of the vation difference is accommodated by east­ faults bordering the rift valley (Figs. 11, 12). On Kivu rift and the horst in Quaternary time (Pas­ northeast-striking oblique-slip faults linking the the basis of fault geometries shown in Figure II tcels and others, 1989; Ebinger, 1989). Field ob- East Kivu and Rusizi border-fault segments. and if one assumes plane strain, crustal extension TECTONIC DEVELOPMENT OF EAST AFRICAN RIFT SYSTEM 897

E. Nyanza -Ia~ Basin "' BFSI2

Figure 11. (Continued).

is less than 4 km (15%), or slightly more than an is separated from the Moba basin by a horst (for ing the eastern side of the Rukwa trough (Fick earlier estimate of 3 km based on seismic data example, Rosendahl and others, 1986; Morley, and Vander Heyde, 1959; Peirce and Lipkov, alone (Morley, 1988a). Balanced cross-section 1988a). 1988; this study). On the basis of exploratory estimates indicate that the depth to detachment drilling (Amoco Production Company), clastic for fau)ts within the Nyanza-lac basin is 20 km Moba Basin (BFS16), Marungu Basin sequences within the Rukwa basin previously or more (for example, Morley, 1988a) (Table 2). (BFS17) assigned to Late Cretaceous time now have been shown to be late Miocene (Table 2) (Spence, Kigoma Basin (BFS13) Significant throw during Quaternary time 1954; Tiercelin and others, 1988; C. Morley, along the Marungu border-fault segment is indi­ 1988, personal commun.). A shallow northwest-striking ridge separates cated by V-shaped river valleys that are found at the Nyanza-lac basin from the westward-tilted greater than 500 m water depth at the base of Msangano Basin (BFS20) Kigoma basin (Fig. 4b; Table 2). More than 4 the Marungu BFS (Capart, 1949). km of sedimentary rocks has accumulated at the Late Cenozoic faults bound the western mar­ base of this steep escarpment (Rosendahl and Mpulungu Basin (BFS18) gin of the narrow Msangano basin (BFS20), others, 1986). The southern end of the Kigoma where< I 00 m of sedimentary sequence overlies basin border-fault segment curves to the cast and Fault systems bounding the Western rift val· metamorphic basement (Peirce and Lipkov, bounds a shallow ridge in the accommodation ley splay at the southern end of the Mpulungu 1988; Tiercclin and others, 1988). Normal fault zone between the Kigoma and Kalcmic basins basin where throws decrease to less than 1 km scarps largely are uneroded and triangular, in (Fig. 4a). (Chorowicz, 1983; Rosendahl and others, 1986; comparison to the deeply dissected fault system Morley, !988a). Along the eastern flank of bounding the eastern margin of the Msangano Kalemie Basin (BFS14) the Mpulungu basin, a mid-Miocene erosional basin. Lacustrine sedimentary sequences campo- North-south-striking faults with triangular surface has been uplifted relative to the East sitionally similar to Pleistocene sequences found scarps bound the eastern side of the Kalemie African Plateau to the east, and Pleistocene . at higher elevations ~ithin the Rukwa trough basin (Fig. 4, Table 2). Northwest-striking faults - lacustrine sequences 300m above Lake Tangan- · ··and within the adjac;ent_ Songwe basin indicate bounding a horst separating the Kigoma and yika attest to Quaternary uplift (Grantham and that faulting occurred after mid~Pie-istocene time Kalemie basins have been interpreted as strike­ others, 1958; Haldemann, 1969). (Brock, 1962; Ebinger and others, 1989). slip faults (Lorber, 1984). Tilted fault blocks within the Kalemie basin trend north-south (Ro­ Rukwa Basin (BFS19) Songwe Basin (BFS21) sendahl and others, 1986; Burgess and others, 1989). BFSI3 continues to the south along the Recent exploratory studies indicate that at The narrow Songwe basin is bounded on its eastern flank of BFS 14 outside the rift valley least 5 km of sediment has accumulated at the northwestern side by a series of high-angle (Fig. 4a). base of BFS I 9 (Spence, 1954; Brown, 1964; (60°-80°) step faults, commonly with subverti­ Peirce and Lipkov, I988; C. Morley, 1988, per­ cal slickenside striations (Grantham and others, Karema Basin (BFS15) sonal commun.). In contrast to other strongly 1958; Ebinger and others, I 989). Dip-slip and asymmetric Western rift basins (for example, oblique-slip faults become more numerous and An earthquake at 12 km depth with a Kigoma basin), the flank of BFS I 9 rises less fault-block rotations increase from less than 10° southwest-northeast tensional axis (Shudofsky, than 400 m above the East African Plateau, and to 30° near the comparatively high-strain ac­ 1985), or perpendicular to the northwest strike the western (monoclinal) side of the rift is nearly commodation zone between the Songwe and of BFS!5, occurred beneath the eastern Karema I ,000 m higher (Table 2; Fig. 4b ). Along the Karonga basins (Ebinger and others, 1989). basin (Fig. 4a). Faults within the Karema basin eastern side of the Rukwa basin, normal faults Within the accommodation zone, fissures and may reactivate Permian-Triassic normal faults in that cut Holocene lacustrine sequences of the cinder cones mark en echelon eastward-tilted the accommodation zone between the Kalemic Rukwa trough have a N0°-20°W strike that is faults of BFS21 (Ebinger and others, I989). An and Karema basins (Fig. 4a). The Karema basin oblique to N45°W-striking normal faults bound- elongate Jurassic-Cretaceous carbonatite body

f t('':.·~w.MMTmA~~I\'ll!.,m'l.w:'~$i~~>1;:~~~:;:,m~~~~~«t'l~~;· =r------

4°N

'J

,,

:; { WESTERN oos oos

R/ F T

Mt. KARISIMBI

'. ' 5. KIVU '!' Mwengo·Komitugo

4°S 4° s BRANCH v ~INDIAN

OCEAN

8°S

Explanation ~'

I2°S [J. Sediment TERTIARY D Volcon1cs • Major 'ilolt.anic Center

y· Tertiary Normal Fault .,.., Monocline I I6°S I6°S ·I

36° ! I Figure 12. Major structures of the Tertiary East African rift system, summarin~d from Figures 3 and 4. Structural interpretation of Kenya rift from Baker (1986) and Bosworth (1985). I [

TECTONIC DEVELOPMENT OF EAST AFRICAN RIFT SYSTEM 899

has been exhumed by recent faulting, suggesting oldest known sedimentary sequences in the Ka­ and others, 1986). If the topographic relief of the that the Songwe border-fault segment has reac­ ronga basin (Kaufulu and others, I ~8 1 ; Ebinger rift is partly or wholly flexurally compensated, tivated a pre-rift shear zone (Fick and Van der and others, 1989). Upper Pleistocene basalt these elevation differences may be due to vary­ Heyde, 1959; Brown, 1964). Pleistocene-Recent flows have been offset 50-70 min the past 0.10 ing amounts of crustal extension and the geo­ volcanic flows from eruptive centers within the m.y., or, extrapolating 2.5-3.5 km in the past metrical arrangement of border faults that Songwe basin are interbedded with and overlie 5 m.y. (Ebinger and others, 1989). effectively fracture the elastic lithosphere be­ - 200 m of volcaniclastic lake units and Cre­ neath the rifts (for example, Weisse! and others, taceous (?) sequences (Grantham and others, Malawi (Nyasa) Rifl Basins 1987; Ebinger and others, 1989). 1958; Ebinger and others, 1989). Contours of poles to planes of border-fault The Malawi rift continues to the south of the segments 1-32 (surface dips) and earthquake Usangu Basin (BFS22) llplifted East African Plateau and consists of foca l mechanisms show the average north-south nine border-fault segments (excluding BFS23) strike of border faults (Fig. 13a). Both earth­ The Usangu basin is bounded by a system of that have been delineated using these same crite­ quake focal mechanisms and slips interpreted north-south-striking step faults rising to the level ria (Ebinger and others, 1987; Table 2). Throws from slickenside striations within Western rift of the uplifted Usangu flank (Fig. 4). The faulted along Malawi rift border-fault segments vary basins support an east-west extension direction southeastern margin of the Usangu basin is the from more than 4 km in the Karonga and (Fig. 13b). Balanced cross sections and fault uplifted flank of the Karonga basin to the south Nkhata Bay basins to less than I km in the Shire geometries within 13 lake basins and their (Fig. 4a). Field observations and gravity data basin at the southern end of the Malawi rift. faulted margins indicate that the crust beneath from the Usangu basin indicate that a < 1-km Seismic stratigraphic sequences vary between Western rift basins has been thinned by less than sequence of basalt flows and volcaniclastic units basins and only the uppermost sequence occurs 15% (< I 0 km), assuming plane strain and an overlies metamorphic basement within the lake-wide, suggesting that rift segments have approximately cast-west extension direction. Usangu basin. Pliocene-Recent volcaniclastic developed diachronously (Ebinger and others, These estimates are slightly more than those sequences tilted - 10° have accumulated along 1984). No constraints on the age of basal sedi­ based on interpretations of seismic reflection the downthrown sides of 1- to 5-km-wide tilted mentary sequences are available from the Ma­ profiles alone (Morley, 1988a; sec Table 2). blocks bounded by N30°-40°E-striking normal lawi rift, however. Depth-to-detachment calculations within sev­ faults (Teale and others, 1962; this study). As in eral rift basins indicate that normal faults extend other parts of the Western rift, eruptive volcanic EXTENSIONAL BASIN GEOMETRY to depths greater than 20 km (for example, Fig. centers coincide with closely spaced faults II ; Morley, 1988a). If deformation internal to within the high-strain accommodation zone be­ The discontinuous Western rift, including the fault blocks is common, these estimates of tween the Usangu and Karonga basins (Ebinger Malawi rift, comprises at least 32 basins crustal extension and depth to detachment rep­ and others. 1989). The Usangu border-fault bounded by approximately 100-km-long sys­ resent minimum estimates. The rollover geome­ segment developed in early Pliocene time, or tems of dip-slip and oblique-slip faults (border­ try characteristic of Western rift basins (for after initial volcanism within the Rungwe prov­ fault segments; fig. 12). The flanks of the rift example, Fig. II) is consistent with planar nor­ ince (Ebinger and others, 1989). outside basins are uplifted 1-4 km above the mal faults that extend to lower crustal levels (for elevation of the East African Plateau, and the example, Gibbs, 1984). Karonga Basin (BFS23) magnitude of uplift is generally greater along the On the basis of detailed field studies supple­ side of the basin bounded by the border-fault mented by geophysical data, en echelon border­ Throws along BFS23 are greatest near the segment (for example, Figs. 3, 4, 7, and 1 1). fault segments are linked by relay ramps and central part of the Karonga basin where the rift Along the length of the rift, depth to pre-rift transfer faults within comparatively high-strain valley is filled by Lake Malawi, and I find little basement beneath basins varies by several ki­ accommodation zones. Roughly comparable evidence for late Cenowic faulting along the lometers in part owing to the regional uplift of amounts of extension (2-10 km) and throws uplifted flanks (Fig. 4). The opposite side of the the East African Plateau (Table 2; Rosendahl (1-6 km) along border-fault segments lead to a Karonga basin is bordered by a regional mono­ cline of dissected Mesozoic sequences and metamorphic basement (Stockley, I 948; Ha rkin and Harpum, I 978; Crossley and Crow, 1980). Cinder cones commonly mark cast-northeast­ striking oblique-slip transfer faults connecting BFS23 and BFS21, and slickenside striations within the accommodation zone suggest an ap­ proximately cast-northeast extension direction (El>inger and others, 1989). If plane strain is assumed, late Cenowic crustal extension is 2.7- 3.5 km (5%-9%), and depth to detachment estimated from a balanced cross section is 20 km or more. Late Miocene volcanic centers dis­ placed by border faults indicate that BFS23 Figure 13. Equal-area lower-hemisphere projections. a. Contours of poles to fault planes of developed after 7.2 Ma (Table I) (Ebinger and border-fault segments listed in Table 2 (surface dips plotted) and earthquake focal planes listed others, 1989). Alkali basalt flows dated at -5 in Shudofsky (1985). Contour interval, 2o. b. Slip vectors and planes of slickenside striations Ma derived from centers along intrabasinal (light lines) within the Western rift (bold dots). Slips along focal planes (bold lines) were faults were erupted at about the same time as the converted to plunge (open squares) (after Shudofsky, 1985). 900 C. J. EBINGER

predominance of oblique-slip transfer faults, in flanks away from the rift (for example~ Usangu volcanic centers along the rift flanks reflects a contrast to strike-slip faults linking shallow basin). Within these volcanic provi"nces,' chains lack of faulting along the rift flanks. crustal detachments in other continental rifts of volcanic centers are aligned along the tips of In mechanical models proposed to explain al­ (for example, Burchfiel and Stewart, I 966; border-fault segments and along oblique-slip ternating basinal asymmetries, it is assumed. that Gibbs, 1983, 1984). Because Western rift transfer faults crosscutting the rift valley (for ex­ border-fault segments serve as detachments for transfer faults accommodate large differences in ample, Pouclet, I 975; Ebinger and others, crustal and/ or lithospheric extension; differences elevation between adjoining basins and their up­ I 989). The general occurrence of volcanic cen­ among the models are related to the geometry of lifted flanks, as well as regional variations in ters along the tips of border-fault segments and the detachment faults and their along-axis topographic relief related to the 1,300-km-wide along oblique-slip faults linking basins reflects linkage (for example, Bally, 1982; Gibbs, 1984; East African Plateau, half-graben commonly are the higher densitY...Qfjaul!s within accommoda­ Wernicke, I 985; Mohr, I 987; Bosworth, 1987). separated along the length of the rift valley by tion •zones. This coincidence also suggests that In order to explain alternating basin asymme­ horsts or sills (for example, Kivu/Rusizi ac­ transfer faults are steeper at lower crustal levels tries in the East African rift system, Bosworth commodation zone). than are the central parts of border-fault (1987) and Mohr (I 987) suggested that sub­ The location of volcanic provinces and vol­ segments. crustal lithospheric-scale detachments beneath canic centers within these provinces shows sev­ Three structural relations consistently ob­ the rift flanks are linked by -300-km-wide ac­ eral consistent correlations with rift basin served along the length of the Western rift valley commodation zones. There is, however, no evi­ development. In a regional sense, the Toro­ strongly suggest that zones of crustal thinning dence for extension or continuation of transfer Ankole, Virunga, South Kivu, and Rungwe vol­ are limited to rift basins and that the crust be­ faults outside 40- to 70-km-wide Western rift canic provinces coincide with interbasinal neath the rift flanks is unthinned. First, few basins. Likewise, spatial patterns of uplift and accommodation zones (Figs. 3, 4; Table 2). faults occur on the uplifted flanks outside inner­ volcanism predicted by thermo-mechanical Eruptive centers within Western rift volcanic facing normal faults bounding the rift valley. models of asymmetric lithospheric detachments provinces are restricted to rift basins within the Second, detailed field studies in four accommo­ are in poor agreement with observations in the inner-facing normal faults, although in some in­ dation zones indicate that oblique-slip faults Western rift (for example, Buck and others, stances, basalts from centers located along linking basins do not extend outside the rift val­ I 988). Instead, earthquake epicentral depths border faults have flowed down outward-tilted ley across the rift flanks. Third, the absence of throughout the range of 0-30 km, the deep

Western Rift System Kenya Rift System I Central Plateau Time (Ma) Tectonic Magmatic Tectonic Magmatic rlelotoc•n• I' I I' .. Songwe.tt. t b.·1 Ed lld bPt ToroAntlev \ Flank~ 1x~ centers ..c 1 volcanoes 1 0 _LUsangu 1 , ? Flafl.tkW" I Flank uplift 0 j_J R,T ~ I Nupl 7 5- I 1 0 a: L...'-'Karo~a b. 1-1--1-1- erosion --1H Aberdare ? ~~ K L ~uslzlj_ ~?urumanL,o Rungwe I L 1 Nguruman b. Kivu b. 1 2 I Mobutu b: W. Trachytes, basalts 10- Clb S. Kivu H J, E. Kivu b.H ? c: I = 0 (J= M D Vlrunga 0 I - D :E _ ..... Eigeyo b. 15- I 1 c Flood Phonolites o,E,F ? ? Nyanza basin (Central rift) ~ E 1,2,3,4 1 2 3 4 I' Central Plateau a,c D . ? Domlng? ' ' ' 20- ommg volcanoes

till 25- Samburu A .. =c: =(J 0 !:?} 30- 0

Doming, volcanism, and faulting In Afar region to north 5

1 Saggeraon and Baker, 1965 3 Dlxey, 1956 2 Baker, 1986 4 Shackleton, 1978 5 Mohr, 1987

Figure 14. Chronologie constraints on vertical movements, volcanic activity, and basinal subsidence within the East African Plateau region. Lines indicate approximate time span of activity (dashed,? where uncertain). Letters refer to timing constraints listed in Tables 1 and 2. Note lag between initial volcanism and development of sedimentary basins. TECTONIC DEVELOPMENT OF EAST AFRICAN RIH SYSTEM 901

depth to detachments, and the rollover geometry tion and lake level fluctuations in the Western ALONG-AXIS SEGMENTATION of basins suggest that high-angle, planar border­ rift valley, it is difficult to establish· horizontal fault systems along one side of Western rift ba­ data for crustal movements during the develop­ One proposed mechanism for the along-axis sins extend to the base of the crust (for example, ment of individual basins (for example, Living­ segmentation, along-axis shifts in basin asymme­ Zana and Hamaguchi, 1978; Shudofsky, 1985). stone, 1965; Hecky and Degens, 1973; Ebinger try, and the orientation of normal and strike-slip In the case of full-graben morphologies, observa­ and others, 1984; Johnson and others, 1987). faults in transfer fault zones is a reactivation of tions within the Kivu basin suggest that full­ Several indirect lines of evidence, however, indi­ pre-existing crustal shear zones (for example, graben morphologies may be relict features; by cate that uplift of the rift flanks superimposed on Dixey, 1956; McConnell, 1972; Villeneuve, mid-Pleistocene time, the West Kivu border­ the plateau topography was concurrent with or 1978). In a regional sense, the general outline of fault segment became the active border-fault postdated initial volcanism and faulting along the Western rift system generally avoids the per­ segment, and only minor offsets occurred along 'the length of both the Western and Kenya rifts. haps more deeply rooted cratonic areas within the East Kivu border-fault segment (Ebinger, The oldest known Western rift sequences arc the East African Plateau region, although several 1989). This shift may indicate a regional evolu­ upper Miocene (-8 Ma); changes in drainage border faults cut Archean basement (for exam­ tion to border-fault segments along the western patterns across the plateau indicate that rift flank ple, Figs. 6, 10, and II). At a shorter-length side of the rift. uplift was most rapid during late Pliocene-Pleis­ scale, a comparison of individual border and The geometrical relations noted above are not tocene time (Bishop and Posnansky, I 960; transfer faults to pre-rift structures reveals a poor unique to the Western rift system, as similar Holmes, I 978; Grove, I 983; Baker, I 986; correlation (Figs. 6, 7, 10, and II). Exceptions patterns of alternating basinal asymmetries, M. Pickford, I 989, personal commun.). During to this generalization are normal faults bounding border-fault segmentations, and geometries of late Pliocene or Pleistocene time, rates of uplift the Song we border-fault segment (BFS21) noted border-fault linkage have been described within along the northeastern flanks of the Western rift above. In most instances, faults extend across the Kenya rift system that has formed along the system exceeded rates of downcutting along sev­ contacts between tectonic units (for example, eastern side of the East African Plateau (King, eral rivers that previously had flowed north into Moba basin) or crosscut fold axes and shear 1978; Bosworth, 1985; Baker, 1986). Seismic the Nile drainage basin, and rivers that originally zones in metamorphic basement (Brown, 1964; data from the Kenya rift valley also indicate that flowed west into the Congo basin were diverted Hopwood, I 970; Haldemann, 1969; Crossley little or no crustal thinning or magmatic intru­ by uplift along the eastern flanks of the rift and Crow, 1980; Ebinger and others, 1987, sion has occurred beneath the uplifted rift flanks (Bishop and Posnansky, 1960; Holmes, I 978; 1989). In part of the Western rift where base­ (Maguire and Long, 1976; Long and Backhousc, Grove, I 983; Baker, 1986). These rivers arc ment faults and metamorphic foliations have a 1976; Bram and Schmeling, 1975; Hebert and now ponded in shallow lakes in the central up­ predominant north-south trend, or subparallel to Langston, 1985; Savage and Long, 1985). lifted plateau between the Western and Kenya the regional orientation of the Western rift sys­ rift systems (Fig. 12). Late Pleistocene uplift tem, there is little evidence for recent faulting TIMING OF CRUSTAL MOVEMENTS along the eastern margin of the Kivu and north­ along Precambrian mylonites and faults. Per­ IN EAST AFRICA ern Tanganyika rift basins produced eastward­ haps the initial location of volcanic centers tilted strand lines along the western margin of within the East African Plateau was controlled To summarize timing constraints within the Lake Victoria (Bishop and Posnansky, 1960). by pre-existing weaknesses within the continen­ East African Plateau region, initial volcanic ac­ Uplifted terraces of lake sequences indicate tal lithosphere, but late Cenozoic volcanic cen­ tivity preceded or was concurrent with initial that rift flank uplift has narrowed the initial zone ters rarely follow pre-rift faults. Thus, the faulting, and this early Miocene volcanism oc­ of subsidence within basins (Table 1). For ex­ along-axis segmentation of the Western rift and curred in isolated centers across the central part ample, terraces of Pliocene-Pleistocene lacus­ the strike of transfer faults linking en echelon of the uplifted region prior to faulting and trine sequences found along the central parts of border-fault segments generally represent there­ basinal subsidence in either the Kenya or border-fault segments were once contiguous sponse of old, cold continentl!l lithosphere to Western rift systems (Fig. 14; Table 1). Volcanic with sequences found at 500- to 800-m-lower rifting processes. ., ·i> activity in the northern and central Kenya rift elevations (for example, East Kivu basin) and on A second mechanism is changing stress orien­ commenced 11 m.y. prior to volcanic activity in the monoclinal side of basins (for example, Ru­ tations during discrete episodes of rifting. Al­ the future Western rift system (Fig. 14). During sizi basin). The morphology of step faults along though the timing of faulting in many parts of the past 10 m.y., magmatic activity generally has border-fault segments indicates that the inner­ the Western rift system is poorly constrained, been localized in the fault-bounded basins of the most triangular fault scarp is active, as higher volcanic centers aligned along faults subparallel present-day Western rift where active volcanoes scarps are deeply dissected, and basin margins to border-fault systems have been episodically are located along faults and fissures parallel to beneath the lake are steep (>40°). This narrow­ active throughout the history of the Kivu and border faults and along transfer faults crosscut­ ing of the basins with collapse of the hanging Karonga basins (Ebinger, 1989; Ebinger and ting the rift valley. Similar patterns have been wall of border faults appears to be synchronous others, 1989). Likewise, focal planes of historic noted in the Kenya rift, .although Pliocene­ with uplift along both sides of .rift basins. Both earthquakes parallel Miocene escarpments Recent volcanic centers (Kenya and Marsabit) progressive collapse of the hangi~g wall and nar­ (Shudofsky, 1985). The geometry of accommo­ are located along the eastern flanks of the Kenya rowing of basins with rift flank uplift are similar dation zones, however, may change as new rift (for example, Baker and others, 1971; King, to spatial patterns predicted by geometric and border-fault segments and extensional basins 1978; Bosworth, 1987) (Fig. 14). Thus, owing thermo-mechanical models of planar detach­ develop. to the 11-m.y.-longer history of volcanism ments (for example, Gibbs, 1983; Buck and oth­ Some authors have suggested that an along­ within the Kenya rift, differences between the ers, 1988). A similar narrowing of Kenya rift axis propagation of rifting contributes to the two rift systems largely can be interpreted as basins has been noted, and lateritized upper Pli­ segmentation of rift systems, both continental differences in the time span of volcanism and ocene-Pleistocene basalts found along uplifted and oceanic (for example, Macdonald and Fox, stage of development. flanks formed at much lower elevations than 1983; Ebinger and others, 1984; Bosworth, Because of the along-axis variations in eleva- present day (King, 1978). 1985; Bonatti, 1985). If one considers the repcti- 902 C. J. EBINGER

tive and interactive relationship between faulting complicated continental lithosphere beneath !'Afrique Centrale), L. Tack (University of ~ 'I , and magmatic processes, existing constraints East Africa. Bujumbura), A. Tesha, J. Knight, L. Willey, suggest that volcanism, and probably faulting, S. Townsend, P. Tilke, P. Eeckelers, and many propagated on both a regional and local scale CONCLUSIONS Peace Corps volunteers provided invaluable as­ within the East African Plateau region. A re­ sistance in field areas. Revisions suggested by gional north-south age progression in volcanic The discontinuous Western rift is bounded by L. Royden, K. Hodges, J. Peirce, E. Uchupi, activity has been recognized in the Kenya rift, a series of high-angle planar border faults along C. Morley, and W. Bosworth greatly improved and a similar north-south propagation of rifting one side of approximately I 00-km-long, spoon­ the text. I thank J. Klerkx, K. Theunissen, has been suggested, based on geomorphological shaped sedimentary basins. Depth-to-detach­ L. Tack, B. Rosendahl, and J. Peirce for use of evid~c_e_ in. the Western rift (Capart, 1949; ment estimates of >20 km, rollover basin unpublished information, and P. Williamson, Haldemann, 1969; Shackleton, 1978; Crossley geometries, and seismicity patterns indicate that A. Cohen, D. Livingstone, and D. Grove for and Crow, 1980; Williams and Chapman, 1986; planar, high-angle border faults may penetrate helpful discussions. I gratefully acknowledge Bosworth, 1987) (Table 1). Volcanic flows from the crust, effectively fracturing the mechanical Mobil Oil Exploration Production for photo­ the Rungwe province, the southernmost West­ lithosphere. The spatial distribution of fault sys­ graphic reproduction of Thematic Mapper im­ ern rift volcanic province, are approximately 3 tems and volcanic centers indicates that crustal agery, and Amoco Production for making m.y. older than flows in the northern part of the extension is restricted to 40- to 70-km-wide rift available proprietary data used in basinal anal­ Western rift (Table !), but additional age con­ basins bounded by border-fault segments. Struc­ yses. This project was funded by a National straints are needed to evaluate this trend. Given tural field observations and focal mechanism so­ Science Foundation (NSF) Presidential Young the temporal development of propagating rift lutions support a regional east-west extension Investigator A ward granted to L. Royden; Geo­ basins in East Africa, Pliocene-Holocene shield direction, and estimates of crustal extension are logical Society of America Student Research volcanoes along the flanks of the Kenya rift may 2%-15%. Because roughly comparable amounts Grant 3754-87; Sea Grant NA84-AA-D-00033, mark an incipient zone of crustal extension re­ of extension (2-10 km) and throws (1-6 km) RIG-II; and NSF Grant EAR84-18120. lated to a southward propagation of the Ethio­ have occurred along border faults, oblique-slip pian rift valley. transfer faults commonly link Western rift ba­ APPENDIX I. LANDSAT IMAGERY At the 100-km length scale of border-fault sins. Along the length of the Western rift, segments, isolated Western rift border-fault volcanic provinces coincide with comparatively Land~at-5 Multi-Spectral Scanner (MSS) and The­ segments have propagated to the north and high-strain accommodation zones, and individ­ matic Mapper (TM) images used in this study cover south to link discrete segments. For example, the ual eruptive centers coincide with high-angle regions of 185 km by 185 km. Standard radiometric and geometric corrections were made at the EROS Karonga and Rukwa border-fault segments transfer faults and the tips of border-fault processing center, and images arc displayed using a developed between 7 and 5 Ma, and the Songwe segments. space-oblique Mercator projection that preserves bordcr-l:wlt segment developed between 2 and Volcanic provinces within both the Western length and ungular relations. False-color composite 0.5 Ma to connect these isolated en eclzefon ba­ and Kenya rift systems arc older in the north images corresponding to scenes I, 2, 3, and 4 were generated from renectance data in band> 2 (0.5-0.6 sins (Fig. 4) (Ebinger and others, 1989). A than in the south, although initial volcanic activ­ IJm), 4 (0.6-0.7 !Jm), and 5 (0.8-1.1 I-'m), displayed progressive lengthening and coalescence of ba­ ity in the northern Western rift at -12 Ma began as blue, green, and red, respectively. Digital data from sins was interpreted from a comparison of II m.y. later than initial volcanism in the Kenya TM scene 5 (Fig. I) were processed to enhance faults seismic stratigraphic sequences within Malawi rift at -23 Ma. Initial volcanic activity preceded and lineaments and to distinguish volcanic units, using lake basins (Ebinger, 1984). Thus, a diachro­ or was concurrent with initial faulting and sub­ a variety of filtering, color-ratioing, and contrast­ stretching techniques. MSS images 1-4 shown in nous development contributes to the along-axis sidence within Western rift basins. Much of the Figure I are E-50870-07370, E-50143-07380, and segmentation of the Western rift valley border­ uplift along the flanks of basins that is superim­ E-50143-07382; TM image is Y-50850-07204. The fault system. Morley (1988b) has suggested that posed on the East African Plateau postdates in­ quality of data is excellent; cloud cover was less ·the central Kenya rift (Elgeyo basin) nucleated itial volcanism and basin subsidence in the than 20% in scene 2 and less than I 0% in scenes I, 3, 4, and 5. at -16 Ma and that volcanism and faulting Western rift. The poor correlation between the propagated to the north and south away from location and attitude of late Cenozoic border/ this magmatically active region (Fig. 14). transfer faults and pre-rift structures indicates By analogy, an along-axis propagation model that the along-axis segmentation is not inherited. REFERENCES CITED for the observed segmentation of the Western Instead, Western rift basins have developed di­ Afonso. A., 1976, A geologia de M01;:3mbiquc: Maputo, Moz.ambiquc, Dir. Scrv. Geol. 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