The Tana basin, Ethiopia: intra-plateau uplift, rifting and subsidence

ELSEVIER Tectonophysics 295 (1998) 351–367

The Tana basin, Ethiopia: intra-plateau uplift, rifting and subsidence

J. Chorowicz a,Ł, B. Collet a, F.F. Bonavia a, P. Mohr b, J.F. Parrot a, T. Korme c

a Departement de Geotectonique (URA 1759), Universite Pierre-et-Marie Curie, Case 129, 4 Place Jussieu,75252 Paris Cedex 05, France

b Department of Geology, University of Asmara, P.O. Box 1220, Asmara, Eritreac Department of Geology, University of Addis Ababa, P.O. Box 1176, Addis Ababa, Ethiopia

Received 29 May 1997; accepted 20 May 1998

Abstract

The Tana basin is situated on the northwestern plateau of Ethiopia, west of the Afar depression. The basin is perched ona topographic high. New data from digital elevation modelling and satellite imagery analysis confirm the basin’s locationat the junction of three grabens: the Dengel Ber (buried), Gondar (exposed by erosion) and Debre Tabor (reactivated).This structural complex was notably active during the build-up of the mid-Tertiary flood basalt pile, into which the Tanabasin is impressed. Fault reactivation occurred in the Late Miocene–Quaternary, accompanied locally by predominantlybasaltic volcanism. Fault-slip indicators are consistent with crustal subsidence centred on the present morphologic basin.Concentric and radial dike patterns in the Tana region indicate that diking and basin formation were contemporary. Tanarifting and magmatism occurred above the inferred western side of the Afar mantle plume-head. 1998 Elsevier ScienceB.V. All rights reserved.

Keywords: Ethiopia; Afar plume; Lake Tana; triple junction

1. Introduction

The Ethiopian plateau is not a simple, undeformedstructural block. In the northwest, it contains theTana basin, a faulted depression located between theerosional escarpment overlooking the Sudan plainsto the west and, to the east, the tectonic escarpmentof the plateau margin overlooking the Afar depres-sion (Figs. 1 and 2a). Few data and descriptions arepresently available concerning the Tana basin, yetthe importance of this area in the context of theAfrican Rift System and its flood basalt magmatismwarrants the present contribution.

Ł Corresponding author. Tel.: C33 (1) 4326-8246; Fax: C33 (1)4427-5085; E-mail: [email protected]

The structural complex in which the Tana basinparticipates (Fig. 2b) has a dominant meridionaltrend that parallels the plateau–Afar margin far-ther east, but no connecting structural link is evi-dent (Mohr, 1967a, 1975; Kazmin, 1972). The newdata presented here, from digital elevation modelling(DEM), satellite imagery and fault-slip analysis ofthe Tana area, seek to focus more sharply on this in-tra-plateau locus of volcanism and faulting (Fig. 1).

2. Structural and tectonic setting

The Tana basin is perched within a plateau setting(Fig. 2a) that averages ¾2000 m in elevation (Grab-ham and Black, 1925; Jepsen and Athearn, 1961).

0040-1951/98/$19.00 1998 Elsevier Science B.V. All rights reserved.PII: S 0 0 4 0 - 1 9 5 1 ( 9 8 ) 0 0 1 2 8 - 0

352 J. Chorowicz et al. / Tectonophysics 295 (1998) 351–367

Fig. 1. (a) Location of the Tana basin in the Afar triple-junction system, showing the extent of Cenozoic volcanism. The E–W line is thelocation of cross-section of (b). (b) Geologic cross-section (located on a) drawn along profile 2 of Fig. 2a, using an isohypsal map of thetop surface of the Precambrian basement compiled by Beyth (1991).

East of the basin, the plateau supports several de-nuded shield volcanoes, the largest being the 4620-m-high Simen Mountain (Fig. 2b) (Minucci, 1938a;USCGS, 1963; Mohr, 1967b). West of the basin,where denudation has removed the original volcaniccover, the Belaya massif preserves the core of anisolated mid-Tertiary volcanic centre (Grabham andBlack, 1925). The watershed between Tana-directeddrainage to the east and Sudan-directed drainage to

the west is formed by a persistent, indented, west-facing escarpment, the West Tana escarpment.

Relief within the Tana basin is subdued. Lowmesas and intervening gently incised valleys circum-scribe a broad, saucer-shaped depocentre. The mor-phologic basin containing Lake Tana occupies anarea of 16,500 km2, of which the lake covers 3156km2 at an elevation of 1786 m (USCGS, 1963). Max-imum water depth in this ‘velo di acqua’ is a mere

J. Chorowicz et al. / Tectonophysics 295 (1998) 351–367 353

14 m (Morandini, 1938; Dainelli, 1943). The lake isconsidered to owe its present form to damming by a50-km-long Quaternary basalt flow, which filled theexit channel of the Abbay river to a possible depth of100 m (Grabham and Black, 1925; Minucci, 1938b;Jepsen and Athearn, 1961). The age of this lava flowis estimated to be some 10,000 years B.P. (Grabhamand Black, 1925).

The Tertiary flood basalt pile in northwesternEthiopia averages 1000 m to 1500 m in thickness(Minucci, 1938b; Jepsen and Athearn, 1961). Thestratoid succession is not yet mapped in the Tanabasin sector. Along the eastern side of the plateau,it comprises four major formations, all diachronous:from bottom to top, Ashangi (an ascribed Eoceneage remains disputed), Aiba (32–25 Ma), Alaji (32–15 Ma), and Termaber (30–13 Ma) (Zanettin et al.,1974; Mohr, 1983; Mohr and Zanettin, 1988). Withinthe Tana basin, attribution of the pile to the Aiba andAlaji fms. is tentative (Merla et al., 1979), althougha thick succession of trachytic lavas and tuffs east ofthe basin (Zanettin et al., 1974) suggests the pres-ence there of the Alaji Fm. New geochronological(40Ar=39Ar) and magnetostratigraphic data along theescarpment, east of the Tana basin, indicate that thebulk of Ethiopian traps erupted at about 30 Ma in twopulses, each possibly only a few hundred thousandyears long (Hofmann et al., 1997).

The stratoid pile rests in turn with unconformityon Mesozoic marine strata. Those strata crop outas a 200-m-thick sequence on the Tana escarpment130 km to the west, and as a 1500-m-thick sequencein the Abbay canyon to the south (Grabham andBlack, 1925; Dainelli, 1943; Jepsen and Athearn,1961). A basement of Neoproterozoic metamorphicintrusive rocks, not exposed within the basin itself,is considered to be a northward continuation of thesutured terrains mapped by Davidson (1983) andAyalew et al. (1990) along the western side of thecentral Ethiopian plateau.

Quaternary olivine basalt and subordinate phono-litic lavas cover much of the southern Tana basin(Jepsen and Athearn, 1961). Quaternary lacustrineand alluvial deposits have been observed to rest dis-conformably on a palaeosurface, over the Tertiaryflood basalts that underlie the Tana basin (Grabhamand Black, 1925; Comucci, 1950).

3. Morphotectonic analysis

3.1. Regional uplift

Uplift at close to 2000 m of the Tana area (Fig. 2)is inferred from several observations:

(a) The drainage systems outside the Tana basinare radially divergent. They comprise the Beles,directed southwestward, the Dinder, westward, theTekeze, northeastward, and the Abbay (Blue Nile),southeastward.

(b) The Abbay river provides the outflow from thelake. It falls into a deeply incised canyon (Fig. 2b)initially bearing southeastward, but after 100 kmit commences a gradual clockwise turn through ahuge semicircular loop across the plateau, deflectingaround the Chokay volcanic shield. The Abbay fi-nally leaves the plateau to emerge westward onto theSudan plains (Fig. 2a). This pattern suggests a focusof uplift in the Tana region.

(c) Topographic cross-sections made from digitalelevation modelling (Fig. 2) emphasise the singularelevation of the Tana region (Fig. 3). Profile 1 is thatof a typical rift shoulder resulting from tectono-ther-mal evolution of the border of a rift system. Profiles2 and 3 can be regarded as resulting from threecomponents: (1) tectono-thermal uplift of the riftshoulders (shaded in Fig. 3); (2) a swell centred onLake Tana (dotted line); and (3) the shield volcanoesto the east (white in Fig. 3). These patterns are alsowell expressed on a geologic cross-section whichwe have drawn along profile 2 (Fig. 1b), across thenorthwestern plateau of Ethiopia, using a map of thetop surface of the Precambrian basement compiledby Beyth (1991). Subsidence occurs at the centre ofthe Lake Tana swell.

(d) The Tana escarpment, passing west of the Tanabasin (Fig. 2), represents an erosion of at least 1000 mof stratoid lavas. This erosion was necessarily inducedby uplift. In the north, the N–S-trending escarpmentturns to a NE–SW trend across the Gondar graben,and then intensifies into the 1500-m-high, west-fac-ing cliffs of the Simen massif (Mohr, 1967b). Theage of initiation of the escarpment has yet to be es-tablished, but the degree of excavation of the Simenand Belaya massifs would not be inconsistent witha Late Miocene (or even earlier) individualisation ofthe Ethiopian plateau from the Sudan peneplain.



Fig. 3. Automatic topographic cross-sections made from the DEM of Fig. 2. Profiles 1 to 3 are located on Fig. 2a. The thick line is theidealised topography of a typical tectono-thermal rift shoulder. In profiles 2 and 3, the shaded component expresses the tectono-thermaluplift of the Afar and Ethiopian rift shoulders, while the white component expresses a swell centred on the Tana basin, depicted bydotted lines. The shield volcanoes are confined within this line.

McDougall et al. (1975) have argued for a ¾23.5Ma (Early Miocene) initiation of the Abbay drainagesystem, and thereby for the initial uplift of theplateau. Renewed uplift, since 8 Ma, of the Tanabasin region through some 1000 m is indicated frompalaeofloral evidence (Yemane et al., 1985). Thisprovides a mean uplift rate of 0.1 mm=year, whichhowever must incorporate a relatively abrupt andrapid uplift phase in Plio–Pleistocene time (Mohr,1967b).

3.2. Dikes and plugs

The Tana area is characterised by widespread dikeand pipe feeders which contributed to the mid-Ter-tiary flood basalt flows of this sector of the Ethiopianplateau (Mohr and Zanettin, 1988). Especially dur-ing the later stages of volcanism, however, magmaticegress became focused and built-up large shield vol-canoes. The centre of the Simen shield has beenexposed by deep Pleistocene glacial excavation toreveal a N–S-biased nexus of short dikes (Mohr,1967b). Even more severe denudation, caused by re-cession of the West Tana escarpment, has revealedthe 20-km-diameter Angareb Ring Complex (Hahnet al., 1977) north of the Tana basin (Fig. 4). Withinthe Tana basin itself, small basaltic feeder plugs are a

Fig. 2. (a) Digital elevation model of north-central Ethiopia and adjacent regions, based on 1 : 2,000,000 topographic map and 200-mcontour intervals. Insets show locations of Fig. 1b (solid line) and Fig. 3 (dotted lines). A D Abbay (Blue Nile) river canyon; ER DEthiopian rift. 1, 2, 3 represent the locations of automatic topographic cross-sections of Fig. 3. (b) Digital elevation model of the Tanabasin and adjacent areas. A D Abbay (Blue Nile) river canyon; B D Belaya volcano; C D Chokay volcano; DBG D Dengel Ber graben;DTG D Debre Tabor graben; GG D Gondar graben; G D Guna volcanic centre; S D Simen Mountain; T D Tekeze river valley; TE DWest Tana escarpment (facing west and northwest).

common feature north of the lake, while plugs (‘am-bas’) of trachyte and phonolite east and southeast ofthe lake mark terminal parasitic activity on the flanksof the Guna shield volcano (Jepsen and Athearn,1961).

Persistent dikes identified and mapped from satel-lite imagery form intersecting concentric and radialpatterns (Fig. 5). Dikes gently curved in plan view,concave to the lake at the focus of the basin, are anotable feature of the western rim of the Tana basin(Fig. 5a) and east of the lake (Fig. 6a). However,there is no field evidence for any volcanic focuswithin Lake Tana, to support the concept that anAngareb-type centre underlies the stratoid pile here(see also Chadwick and Dieterich, 1995).

3.3. Faulting and grabens

Lake Tana has previously been indicated by Mohrand Rogers (1966), Kazmin (1972) and Merla et al.(1979) to be located in fault-bounded grabens, withthe Gonder and Debre Tabor grabens (Fig. 2) clearlyindicated. According to Jepsen and Athearn (1961)and Berhe et al. (1987), the Gondar graben runsdiscontinuously far south across the Tana basin. Weshall argue that the three grabens together form atriple junction at the centre of the Tana basin: (1)



the Gondar graben is evident on satellite images;(2) the Dengel Ber graben is also present, but ismorphologically inverted and its southern prolonga-tion is hidden; (3) the Debre Tabor graben is welldocumented in the field and on our images.

(1) The northern sector of the Tana basin is anasymmetric graben (the Gondar graben), the westernboundary fault zone being framed by clearly definedNNW–SSE- to N–S-trending faults (Fig. 4). Farthernorth, the Gondar graben is cut by the West Tanaescarpment. In its southern sector the Gondar grabenfloor preserves olivine basalt flows and overlyinglignitiferous lacustrine sediments (Fig. 7). Togetherthese form a thin cover upon faulted mid-Tertiarybasalts (Usoni, 1945; Tezera and Heeman, 1983).The younger basalts have yielded an 8–10 Ma ra-diometric apparent-age range (Yemane et al., 1985).They mantle older faults that now lack morpho-logic expression, whereas the youngest faults cutacross these mid-Tertiary flows and retain identifi-able scarps.

(2) The southern sector of the Tana basin com-prises the Dengel Ber graben (Figs. 4 and 5). Itswestern border is framed by NE–SW- to NNE–SSW-striking faults. The graben is largely maskedby the profuse volcanism that has built up the Dan-ghila plateau. Our mapping proves the exposure ofPrecambrian basement in the upper Beles valley,reaching as close as 40 km southwest of Lake Tana(Fig. 5). Grabham and Black (1925) report outcropof probable Mesozoic sandstone on the eastern flanksof Mt. Belaya, consistent with this discovery. The in-dication is therefore one of shoulder uplift along thewestern side of the Dengel Ber graben, in concertwith observed block faulting (Fig. 5b). This shoulderuplift favoured location of the West Tana erosionalescarpment along the western border of the graben,resulting in inversion of the graben topography. Thesouthward continuation of the Dengel Ber grabenremains to be elucidated.

(3) Over the eastern side of the Tana basin, al-luvial cover partly masks the Debre Tabor graben

Fig. 4. Stress tensors of striated faults, located on inset map. Stereoplots on lower hemisphere Schmidt diagrams; percentages refer todata falling in the extensional dihedron. The inset map is derived from four Landsat Multi-Spectral Scanner (MSS) images enlargedto 1 : 500,000. Areas of Fig. 5a and Fig. 6a are outlined. Area in grey is the inferred three-graben junction (Gondar, Debre Tabor andDengel Ber).

(Mohr and Rogers, 1966) (Figs. 4 and 6a). Neverthe-less, some of the bounding faults of the graben showtopographic reactivation. North of Debre Tabor, E–W-striking faults with a morphologic expression in-dicating southerly downthrows, can be traced farthereast, where they turn to ESE–WNW before termina-tion against the Guna shield volcano (Fig. 6a). Thefaulting then reappears on the southeastern flanks ofthe shield. On the opposing, southern side of theDebre Tabor graben, a complementary set of E–Wfaults veers to the east-southeast farther east, and isdownthrown to the north (Fig. 6). On skirting thesouthern fringe of the Guna volcanic massif it turnsback to the east-northeast, and again testifies to alater faulting episode.

As noted by Zanettin (1993), the Tana basintherefore lies at the convergence of three grabens:the Debre Tabor graben from the east, the Gondargraben from the north-northwest, and the DengelBer graben from the south-southwest. The grabensare impressed in the mid-Tertiary flood basalt pile;lack of exposure of the sub-volcanic terrain hidesevidence for any earlier structural history. Zanettinet al. (1980, figs. 6 and 8) propose that Eocene pre-Afar graben faulting extended northwest across whatis now the N Ethiopian plateau to the Sudan bor-der. This ‘Ashanghi graben’ is considered by theseauthors to have formed a westward projection of anEocene Gulf of Aden rift. We as yet find no structuralevidence from the Tana area to support this scenario.

3.4. Subsidence

The Tana basin is bounded between watershedsthat run 8–25 km to the west and 25–75 km tothe east of the respective Lake Tana shores, andconverge and close around the northern side of thelake (Fig. 2b). Centripetal drainage into the lake hasits main inflow from the Little Abbay river in thesouth (Fig. 8).

Along the southwestern side of the basin runs a20-km-wide zone of curvilinear faults and associated



tilted fault-blocks cut by NE–SW-striking transferfaults (Fig. 5a, Fig. 9a). It has been uncovered byrecession of the West Tana escarpment (Jepsen andAthearn, 1961; Mohr and Rogers, 1966). The faultedblocks average 1–4 km width, strike NNE–SSW,and dip east into the Tana basin. They thereforecontribute to the basin structure. Although the in-dividual tilted blocks dip at angles of 15º–20º tothe east (Fig. 9b,c=3), their antithetical arrangementexpresses an overall subsidence angle of only ¾1º.The blocks expose stratified ferro-trachytes and ign-imbrites exhumed from under a cover of horizontalaugite-phyric and aphyric basalt flows to either sideof the Beles valley (Comucci, 1950; Jepsen and At-hearn, 1961). These basalts appear continuous withthe mid-Tertiary basalt sequence on both rim andfloor of the Tana basin. The trachytes and ign-imbrites may testify to explosive volcanism linkedto a caldera, of similar size (and age?) to the An-gareb centre farther north (Hahn et al., 1977), nowhidden beneath the younger volcanics and alluviumof the Tana basin. The basin itself, with a diameterof 80 km, is too large to be regarded as a caldera.Was an episode of explosive silicic volcanism (asyet undated) associated with crustal extension andthinning, following which flood basalts covered theentire region?

Gently inward-directed dips of the Tertiary stra-toid basalts toward the centre of the Tana basin aredisplayed to the west, north and east of the lake(Fig. 8). However, near Gondar, dips as steep as 20ºto the south are observed. Lake Tana therefore oc-cupies a structural basin with narrow shoulders anda broad flat floor. The basin was occupied at leastin part by a lake some 8 Ma ago, when lignitiferoussediments were deposited within it, but it is not yetknown whether that lake survived ensuing regionaluplift (Fig. 9c=1,c=2). Sparse gravity data reveal 20–50 mGal positive Bouguer anomalies to coincide withthe axes of the Debre Tabor and Gondar grabens. Theanomalies are superimposed on a gentle gradient of

Fig. 5. (a) Structural map of the SW Tana basin, based on Landsat-TM image interpretation (path=row coordinates 170=52, acquired 10August 1988 with ¾5% cloud cover; enlarged to 1 : 250,000 for structural interpretation, and to 1 : 500,000 for stereoscopic observationusing Landsat-MSS imagery). Numbers refer to field localities where slickenside observations have been made. Small black triangles areon downthrown side of faults. White square Fig. 9a. (b) WNW–ESE section showing inferred position of flood basalt across the Belesdrainage (vertical exaggeration ð 3).

increasing gravity westward across the basin (Mohrand Rogers, 1966; Mohr and Gouin, 1967; Makris etal., 1991). Further surveys are required to seek anygravity signature related to the central subsidence ofthe basin. The geologic cross-section of Fig. 1b, madefrom the Precambrian isohypsal map of Beyth (1991),indicates important subsidence in the Tana region. Wehypothesise that asthenospheric mantle has intrudedthe lithosphere as a result of plume activity in theTana region, inducing thermal uplift (Fig. 9c=1,c=2)and formation of three grabens. Subsequent thermalsubsidence is responsible for creation of inward-dip-ping fault blocks (Fig. 9c=3).

3.5. Microtectonics

Slickenside and grooving orientations have beenmeasured at several locations around the basin(Fig. 4), in each instance confined within a 100-mradius and restricted to basaltic rocks (excepting lo-cality 10 in lacustrine sediment). Palaeostress orien-tations have been obtained from inversion of the fielddata using the right-dihedral method of Angelier andMechler (1977), which computes the probability of¦1, ¦2 and ¦3 stress component orientations. In thismethod, several fault and related striae orientationsare measured at each site. From each measurementthe plane orthogonal to both fault plane and relatedstriae is determined, forming with the fault planeright dihedra that respectively contain the ¦1 axis inthe maximum compressive dihedron and the ¦3 axisin the least compressive dihedron. Ideally, at a givensite, the computed intersections of all right-dihedradefine the orientation of the principal stress axes. Inthe Tana basin (Fig. 4), the ¦1 orientation is found tobe always vertical; the ¦3 orientation is defined onlyat localities 3 (Debre Tabor), 6 and 10 (Gondar),and localities 15 and 20 (Dengel Ber). The overallstress pattern therefore conforms with basin subsi-dence at the locus of three converging grabens, withno preferred regional extension direction.



Fig. 7. Drawing from a photograph in the field in the southern sector of the Gondar graben, looking north, showing olivine basalt flowsand disconformable overlying lignitiferous lacustrine sediments preserved on the graben floor.

The palaeostress pattern can be further quantifiedusing the 4-D method of Angelier (1984). This com-putes slip vectors and '-ratio .¦2 � ¦3/=.¦1 � ¦3/

which defines the stress ellipsoid: ' takes up val-ues between 0 (¦2 D ¦3, prolate uniaxial ellipsoid)and 1 (¦1 D ¦2, oblate uniaxial ellipsoid), whileintervening values where ¦1 > ¦2 > ¦3 indicate atriaxial ellipsoid. The stereoplot data confirm thevertical orientation of ¦1 at all localities (Fig. 10).Four locations (1, 5, 12, 14) yield '-values closeto 0.5 (triaxial ellipsoid), with respective horizontal¦3 orientations aligned NW–SE (locations 1 and 5),WNW–ESE (location 14) and N–S (location 12). Ineach instance, the ¦3 orientation is radial to the Tanabasin (Fig. 10, inset). However, a large set of thestereoplots (locations 3, 4, 6, 7, 10, 11, 13, 15–18)yield '-values close to 0 (prolate uniaxial ellipsoid,with ¦2 ³ ¦3), consistent with subsidence due topurely vertical forces.

Location 20, 50 km from the Tana basin in basaltsof the Dengel Ber graben, is the only samplingsite close to exposed Proterozoic basement. Analysisof striated fault planes here offers two interpreta-tions. Single-stage tectonism would yield a '-valueof ¾0.2 consistent with pure subsidence, while atwo-stage tectonism yields a '-value of >0.4 foreach of the stages. In the second interpretation, one

Fig. 6. (a) Structural map of eastern side of Tana basin. Interpretation from Landsat-TM images, path=row coordinates 169=52, acquired10 August 1988, cloud-free. Enlargements made to 1 : 250,000 for structural interpretation, and from Landsat-MSS imagery to 1 : 500,000for stereoscopic study. Numbers refer to field localities where slip measurements have been made. (b) NE–SW section through the Gunashield volcano and its peripheral fault belts (vertical exaggeration ð 3).

set of fault planes (20a) yields a NNE ¦3 orien-tation radial to the Tana basin, as for locations 1,5, 12 and 14 (Fig. 10, inset), again consistent withbasin subsidence centred on Lake Tana. The secondset (20b) yields a NW–SE orientation which mayspecifically be related to extension across the DengelBer graben.

4. Discussion

The equidimensional plan of Lake Tana is notconsonant with a lava-dammed, flooded river valley.Nor is there evidence for a correspondingly aggradedsurface in the Little Abbay valley. Rather, the pre-sent-day morphology expresses a central focus ofsubsidence, despite fault reactivation and headwarddowncutting of the Abbay. In addition, severe reces-sion of the West Tana escarpment has modified theextent of the original basin.

Structural data confirm that Lake Tana occupiesa centre of subsidence and graben convergence.Initial subsidence occurred before termination ofthe mid-Tertiary flood volcanism. Evidence for thiscomes from the block-faulted terrain southwest ofthe lake, unconformably overlain by subhorizontalbasalt flows that predate construction of shields such


Fig. 8. Drainage pattern of the Tana basin. Data from 1 : 2,000,000 Flight Chart Map. Areas above ¾2200 m in grey. Dips of recentlayers are convergent to the centre of the lake basin.

Fig. 9. (a) Enlarged Landsat-TM image showing SW Tana zone of NNE–SSW-striking faults and tilted blocks. Tilts are 15º to 20ºtowards the lake. ‘V’ D site of (b). (b) N-directed composite photograph showing tilted blocks; fault scarps face west. (c) Inferred modelof doming (1), fracturing (2), and subsidence (3).




as Guna volcano (Fig. 7). Subsequent subsidencetook the form mainly of reactivation of graben faults,notably in the Late Miocene and Quaternary. Thus inthe Debre Tabor and Gondar grabens lacustrine de-posits and olivine basalt flows unconformably overliefaulted mid-Tertiary basalt (Usoni, 1945; Tezera andHeeman, 1983; Yemane et al., 1985).

The Tana basin is perched on a regional to-pographic high that is individualised within theEthiopian plateau. Interacting subsidence and up-lift were associated with intersecting concentric andradial dike patterns that fed the mid-Tertiary stra-toid basalts. Locally, uplift and concomitant erosionexceeding graben subsidence inverted the volcanic-filled Dengel Ber graben. The timing of uplift of theTana region is not yet confirmed, but Late Miocene(Yemane et al., 1985) and Quaternary episodes areindicated. According to McDougall et al. (1975) andBaker et al. (1996), initial uplift of the Ethiopian–Yemen province occurred ¾30 Ma ago when theEthiopian lithosphere moved over the Afar mantleplume (Schilling, 1975; Hart et al., 1989; Schillinget al., 1992), and has operated since that time (Mc-Dougall et al., 1975). However, according to Rogeret al. (1997), the plume-related characteristics of thelithospheric mantle in the Tana region were alreadyacquired before the Oligocene extrusion of the trapseries.

Given (a) the significant height (<1000 m) of theWest Tana escarpment, (b) the southwestward extentof a linear dike swarm exposed along the water-shed between the Beles and Dinder drainage basins(Fig. 5a), and (c) the location of the Belaya mid-Ter-tiary volcanic centre, it is clear that a large thicknessand area of stratoid basalts has been eroded from thewestern side of the Ethiopian plateau. Reconstruct-ing the original distribution of these mid-Tertiarybasalts, prior to opening of the Red Sea and Gulf ofAden (Fig. 11), shows it to have had an ovoid planwith a longitudinal axis passing closer to Tana thanAfar.

Why might the Tana region have acted in sin-gular tectonomagmatic fashion within the context

Fig. 10. Fault-slip data for the Tana basin (field locality numbers as on Fig. 4; mean number of measurements at each locality ¾15).Schmidt stereograms with a lower hemisphere projection. Rotation of ¦1-axis conforms with fault block rotations; clustering of striationssuggests fault reactivations.

Fig. 11. The Ethiopian plume zone deduced from Cenozoic trapswith Nubia–Arabia reconstitution.

of the Ethiopian plateau? Several factors may havecontributed. (a) Two closely spaced lithospheric su-tures in west-central Ethiopia, dated at ¾635 Ma(Ayalew et al., 1990), project north-northeast viathe twin linear sectors of the ‘Big Bend’ (10.5ºN,36.5ºE) in the Abbay river (Jepsen and Athearn,1961), and thence north-northeast up the Beles val-ley and directly into the Tana basin. Unfortunately,the basement structure beneath the basin is com-pletely masked by the Tertiary flood basalt pile. (b)Gravity–topography relationships indicate that theEthiopian plateau is maintained at its present eleva-tion by sublithospheric mantle upwelling and out-flow from under Afar (Ebinger et al., 1989). Localdivergence from this upwelling may occur beneaththe Tana basin (Piccirillo et al., 1979; Mohr, 1983).More detailed gravity data are required from theTana region. (c) A mantle plume separate from thatof Afar may have impinged beneath Eocene Ethiopiain the Tana region (Bonavia et al., 1995).


5. Conclusions

(1) Our synthesis of previous knowledge and newstructural field and remote sensing data show thatthe Tana basin occurs perched within a large (¾1000km) dome uplifted in the Ethiopian plateau, nowconsiderably modified by erosion. The dolerite dikepattern of the Tana region shown by satellite imageryanalysis is both radial and concentric.

(2) Basin subsidence is expressed in the dip andstrike of stratoid lavas and has affected the LateOligocene flood basalt pile. Microtectonic data indi-cate a Neogene palaeostress pattern characterised byprolate uniaxial ellipsoids, with ¦2 ³ ¦3, consistentwith subsidence due to purely tensional forces. Thedevelopment of the basin is not yet terminated.

(3) Subsidence occurred at the focus of three con-verging half-grabens, confirmed by satellite imageryanalysis. At least two phases of graben faulting haveoccurred, the earlier marked by block tilting.

(4) The distribution of mid-Tertiary basalts priorto opening of the Red Sea and Gulf of Aden has anovoid plan with a longitudinal axis passing closer toTana than Afar.

(5) These evidences raise major questions con-cerning the location and regional influence of theAfar mantle plume (Hart et al., 1989; Schilling et al.,1992) and the plume-related Afar and the Ethiopianrift. In our opinion the early hot-spot (or part of it)may have been centred on the Tana lake.

(6) The importance of the Tana basin in the con-text of regional uplift, rifting and subsidence war-rants a systematic mapping of its volcanic stratigra-phy, dikes and faults, in concert with more 39Ar=40Arisotopic dating of critical samples. The role of dik-ing in basin subsidence, margin warping and pe-ripheral uplift invites further elucidation. Compre-hensive fault studies are necessary to specify thestress field(s) within and around the Tana basin.Geophysical soundings of the three grabens con-verging on the Tana basin are required to deter-mine the degrees of crustal attenuation and igneousintrusion=underplating in each graben. Not least, thelocation and tectonic nature of the original limits ofthe Tana basin require to be established. Until sucha program is fulfilled, no proper understanding ofbroader Ethiopian plateau evolution can be attained.

Acknowledgements

We acknowledge the French Embassy in Ethiopiaand the Institut National des Sciences de l’Univers(CNRS) for funding the project. An earlier phaseof the research was supported by the UniversityCollege of Addis Ababa. Reviews and suggestedimprovements by Rene Guiraud, Philippe Huchonand an anonymous referee were greatly appreciated.

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Documents

The Tana basin, Ethiopia: intra-plateau uplift, rifting and subsidence