crustal structure of the main ethiopian rift from gravity ... · fig. 2. geological sketch map of...

ELSEVIER Tectonophysics 313 (1999) 363–382www.elsevier.com/locate/tecto

Crustal structure of the Main Ethiopian Rift from gravity data:3-dimensional modeling

R. Mahatsente a, G. Jentzsch b,Ł, T. Jahr b

a Formerly at Institut fur Geophysik, Technische Universitat Clausthal, Arnold-Sommerfeld Straße 1,D-38678 Clausthal-Zellerfeld, Germany

b Institut fur Geowissenschaften, Friedrich-Schiller Universitat, Burgweg 11, D-07749 Jena, Germany

Received 4 September 1998; accepted 9 July 1999

Abstract

A three-dimensional interpretation of the newly compiled Bouguer anomaly map of the Main Ethiopian Rift ispresented. A high-resolution 3-D model constrained with the seismic results reveals a possible crustal thickness and densitydistribution beneath the graben. The Bouguer anomalies along the axial portion of the rift floor, as deduced from the resultsof the regional and residual separation, are mainly caused by the deep-seated structures. The inferred zone of intrusion,which is the main subject of the present study, coincides with the maximum gravity anomaly of the rift floor. The intrusionis displaced at several sectors along the east–west direction, and the two major displacements coincide with the locationsof the major rift offsets on the surface. Because of the asthenospheric uplift, the crust under the Main Ethiopian Rift isslightly thinned. The zone of crustal thinning (�31 km) coincides with the location of the intrusion beneath the rift floor,and the maximum of which is attained in the northern and central sectors of the graben. The trend of the crustal thinningzone, which is from south to north, is the same as the one obtained in the Afar depression. The southeastern and westernplateaus, on the other hand, show by far the largest crustal thickness in the region (38–51 km). In contrast to the Afardepression, where the crust is partly oceanized, the thickness and density of the crust suggest that the Main Ethiopian Riftis underlain by a purely continental crust. The deep and relatively large nature of the intrusion leads to the conclusionthat a large-scale asthenospheric upwelling might be responsible for the thinning of the crust and subsequent rifting of thegraben. 1999 Elsevier Science B.V. All rights reserved.

Keywords: Main Ethiopian Rift; intrusion; Wonji Fault Belt; Bouguer anomaly; 3-dimensional gravity modeling

1. Introduction

The unique geological setting of the Ethiopian riftsystem including the Main Ethiopian Rift and theAfar depression, where the inception of plate bound-aries within the continental rift is clearly observed,

Ł Corresponding author. Fax: C49 3641 630280; E-mail:[email protected]

makes it the main focus of interest for geoscien-tific researchers. The three major rifts, namely theEthiopian rift system, the Red Sea rift, and the Gulfof Aden rift converge in the northern part of theAfar depression (Fig. 1 and inset map of Fig. 2). Thenature of the crust underlying the northern segmentof the East African rift system (the Afar depression)has been controversial among researchers for thepast three decades. Consequently, the crustal struc-

0040-1951/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved.PII: S 0 0 4 0 - 1 9 5 1 ( 9 9 ) 0 0 2 1 3 - 9

364 R. Mahatsente et al. / Tectonophysics 313 (1999) 363–382

Fig. 1. The central sector of the Main Ethiopian Rift and adjacent areas. Thick line segments represent rift margin. The Wonji Fault Belt(WFB) and Siliti Debrezit Fault Zone (SDZFZ) are shown as tightly defined lines. Pointed stars represent rift-shoulder volcanoes, andasterisks are Quaternary peralkaline rhyolite centers of the rift axis (from Woldegabriel et al., 1990).

ture of the depression has been well studied anddocumented (Mohr, 1962a,b, 1971, 1992; Berckhe-mer et al., 1975; Makris and Ginzburg, 1987). Thecrust of Afar was considered to be of essentiallyoceanic type (Barberi and Varet, 1977). Contrary tothe oceanic nature of the crust, however, argumentsbased on the results of the seismic refraction experi-ments lead to the idea of a thinned continental crust(Berckhemer et al., 1975; Makris and Ginzburg,1987). The thinning of the crust is attributed to theexistence of an anomalous upper mantle (7.4 km=s)and, hence, a large intrusion beneath the depression(Makris and Ginzburg, 1987). Mohr (1989, 1992)proposed a newly generated igneous crust under theAfar depression whose physical characteristics re-flect more the continental than the oceanic nature ofthe crust. The thinned crust beneath Afar is the result

of both magmatic and tectonic processes (Courtillotet al., 1984).

Similarly, the anomalous nature of the uppermostmantle (7.5–7.7 km=s) beneath the central segmentof the East African rift system (Kenyan Rift) is wellknown from the results of the seismic refraction–wide angle reflection experiments (Mechie et al.,1994, 1997). The crustal thickness beneath theKenyan Rift, as obtained from the results of theKenya Rift International Seismic Project (KRISP-85and KRISP-90), decreases from south to the northernsector of the graben (Mechie et al., 1994, 1997).The results of the teleseismic studies in the south-ern Kenyan Rift also reveal that the lithosphere issignificantly thinned (Achauer et al., 1994).

In contrast to the Afar depression and the KenyanRift, the Main Ethiopian Rift, which is the subject of

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this study, is relatively less known with respect to itscrustal structure. The Main Ethiopian Rift connectsthe Afar depression, in the north, and the KenyanRift in the south. Despite its tectonic importancealong the East African rift system, the nature andstructure of the rift floor are not well established.Furthermore, the southward extension of the high-temperature upper mantle material underlying theQuaternary tensional fault belt is not delineated.

The same intrusion, and hence an anomalous up-per mantle, is supposed to continue southward fromAfar into the Main Ethiopian Rift (Searle and Gouin,1972). But this has not yet been proved. However,the lithological data obtained from the deep bore-hole (LA-1) drilled in the Main Ethiopian Rift floorindicate a basaltic intrusion at a depth of 1.3 km sim-ilar to the one found in the Kenyan Rift (Johnstone,1983). Moreover, the detailed gravity survey, con-ducted in the central part of the Main Ethiopian Rift,revealed the association of the Bouguer anomalyin the rift floor with the inferred zone of intrusion(Searle and Gouin, 1972).

In the present work an attempt is made to pre-pare a high-resolution 3-dimensional gravity modelof the Main Ethiopian Rift from existing gravity databases. The results obtained from the 3-dimensionalmodeling are supposed to fill the gap between theAfar depression, in the north, and the Kenyan Riftin the south. The results of the seismic refractionexperiment in the southern Afar, and the one whichcrosses the western Ethiopian plateau (Makris andGinzburg, 1987), have been used to constrain the ini-tial 3-dimensional gravity model of the graben. Thegravity data were collected by the joint collabora-tion of the Ethiopian Institute of Geological Surveyswith the Swedish Agency for Research Cooperationwith Developing Countries. The present study in-corporates additional gravity data sources from thesouthern part of the rift (Ebinger, 1991). The spacingof the gravity stations along lines, for both gravitydata sets, ranges from 2 to 5 km, and the averageaccuracy of the gravity data is in the order of 1mGal. The two data sets were reduced using a den-sity value of 2670 kg=m3. Moreover, the gravity dataare terrain-corrected up to zone I (160 km) usingtopographic charts and the Hammer table (Hammer,1939).

2. Geological setting

The general geological background of the studyarea has been incorporated both in the qualita-tive and quantitative interpretation stage of the ob-served Bouguer gravity anomaly. In particular, the1 : 2,000,000 scale geological map of Ethiopia, com-piled by the Ethiopian Institute of Geological Surveys(Kazmin, 1975), was used in determining the lateraldimension of the major rock units outcropping on thesurface of the rift floor and the adjoining plateaus.A clear understanding of the regional geology of theMain Ethiopian Rift and the adjoining region is, there-fore, necessary for the modeling and interpretation ofthe gravity data. A short account of the regional geol-ogy of the Main Ethiopian Rift is given below.

The Main Ethiopian Rift forms a part of thelargest Tertiary–Quaternary rift system which ex-tends from Mozambique in the south to Israel, Jordanand Syria in the north. The rift, like the rest of theEast African rift system, has undergone a very com-plicated geological evolution and tectonic history.The regional geology of the Ethiopian rift systemhas been extensively described and well documented(Dainelli, 1943; Mohr, 1962b, 1971). A simplifiedgeological sketch map of the Main Ethiopian Rift,illustrating the major structural trends and locationsof the rhyolite centers, is shown in Fig. 2. The riftvalley was the site of extensive volcanic activitiesduring the Tertiary. Volcanic rocks of Pliocene andPleistocene age such as pantelleritic rhyolites, tra-chytes and ignimbrites are abundant within the riftfloor and on the adjoining plateau (Kazmin, 1975).The pre-Cambrian rocks in the Ethiopian rift system,except in the northern Afar and at the extreme southof the Main Ethiopian Rift, are mostly covered bymore recent Tertiary volcanic rocks and Mesozoicsediments (Mohr, 1962b). However, an older and ex-tensive group of volcanic rocks of early and middleTertiary age, normally called the Trap series, is wellexposed on the southeastern and western plateaus(Mohr, 1971). This group consists predominantlyof alkaline basalt with interbedded pyroclastics andrare rhyolites erupted from the fissures. The old-est sedimentary sequence in Ethiopia is masked bysediments of middle to Upper Pleistocene age. La-custrine deposits of Pleistocene to Holocene age arecommon within the rift valley.

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Fig. 2. Geological sketch map of the Main Ethiopian Rift. Also shown are the names of the main localities and the area for which the 3-dimensional gravity modeling wasdone (the framed area on the map). The letters (A to H) represent the locations of the lakes mentioned in the text (after Kazmin, 1975).


A number of major centers of silicic volcanismand obsidian flows, related to the zone of intensefaulting, occur in the rift. The centers have pro-duced lava of more variable composition includingtrachytes and andesites (Kazmin, 1975).

The main trend of the tectonic structures in thegraben is the same as that of the Main EthiopianRift, which is dominantly of NNE–SSW direction(Mohr, 1962b). In the area of Lake Ziway (Fig. 1 andlocation A in Fig. 2) and at Asela, however, this gen-eral trend of the rift changes to a NE–SW direction(Meyer et al., 1975). Within the rift floor, two majorstructural trends are also recognizable. The first, theWonji Fault Belt (WFB), is a Quaternary tensionalfault belt and connects the peralkaline silicic centers(Fig. 2). The fault belt forms the eastern margin ofthe rift floor (Mohr, 1960). A deep borehole drilled inthe rift floor encountered an intrusion along this Qua-ternary tensional fault belt similar to the one foundin the Kenyan Rift (Johnstone, 1983). The generaltrend of the volcano-tectonically active WFB is inthe NNE–SSW direction. The second, which runsNNE–SSW, is the active western marginal graben orthe Siliti Debrezit Fault Zone (SDZFZ in Fig. 1).The SDZFZ mostly follows the western escarpmentof the rift (Mohr, 1962a,b; Woldegabriel et al., 1990).

3. Gravity data analysis

In order to interpret the gross crustal structure ofthe Main Ethiopian Rift, a Bouguer anomaly map, ata constant contour interval of 5 mGal, has been pre-pared (Fig. 3). The newly compiled Bouguer gravitymap is based mainly on the regional gravity datacollected by the Ethiopian Institute of GeologicalSurveys and partly by Ebinger (1991). The gen-eral trend of the Bouguer gravity anomalies in therift floor is NE–SW. However, the trend changes toNW–SE toward the southern sector of the graben.The map reveals a broad regional negative Bougueranomaly over the Ethiopian Dome. The anomaly in-creases in magnitude with a decrease in the reliefof the topography and attains its maximum of �157mGal along the axis of the rift floor. Closer obser-vation of the axis of the maximum anomaly in therift floor suggests that its general trend follows theinferred zone of intrusion along the Wonji Fault Belt,

forming a patchwork in the north and broadeningtowards the south.

The quite different nature of the Bouguer gravitymap on the two opposite facing plateaus is markedby a steep gravity gradient along the eastern es-carpment of the rift floor and a relatively moderategravity gradient along the western escarpment. TheBouguer gravity on the southeastern plateau reachesa minimum value of �237 mGal near Kofele.Whereas the gravity over the western plateau firstdecreases to a value of �207 mGal near Indibir andthen gradually attains its maximum value of �150mGal south of the western plateau. To the south, thecenter of the maximum anomaly in the rift floor isterminated at two places by NW–SE-trending grav-ity gradients. The first gravity gradient coincideswith the major offset of the Quaternary tensionalfault belt and the two opposite facing escarpments.Mohr (1967) interpreted the rift offset as a transformfault. This weak zone is characterized by strong seis-micity (Asfaw, 1992). The second gradient marksthe termination of the Main Ethiopian Rift trend atthe southern end of Mount Amaro (Fig. 2) and thebeginning of the Lake Chewbahir Rift (location Hin Fig. 2). The Bouguer anomaly south of Gidole(at 37.2ºE and 4.5ºN) is shifted towards the westand gradually becomes stronger (�90 mGal) andopen along the eastern shoulder of the Lake Chew-bahir Rift (location H in Fig. 2). This indicates thatthe intrusion might have been displaced westward,southwest of Mt. Amaro, and ultimately taken up inChewbahir Rift. However, more gravity data cover-age is required to delineate the continuation of theintrusion within the Chewbahir Rift and towards thenorthern segment of the Kenyan Rift.

In an attempt to estimate the depths of the causativebodies, the observed Bouguer gravity anomaly hasbeen analytically separated using a grid residualizingtechnique (Griffin, 1949). The resulting residual andregional fields depend on the number of points usedto average the regional field and the distances fromthe point whose expected regional value is to be de-termined. The choice of the two parameters dependson the scale of the survey. As the present survey isregional (the spacing of the gravity station along linesranges from 2 to 5 km), a simple four-value templatewith a 3-km grid spacing was used to determine thesmooth regional trend in the vicinity of the stations.


Fig. 3. Bouguer anomaly map of the Main Ethiopian Rift. The framed area shows part of the rift for which the 3-dimensional gravitymodeling was done. Also shown are the gravity observation points (dots) and the names of the localities mentioned in the text (contourinterval 5 mGal).


Fig. 4. The residual gravity anomaly map of the Main Ethiopian Rift. The letters (A to G) represent the main residual anomalies of therift mentioned in the text (contour interval 3 mGal).

As shown in Fig. 4, there are two distinct featuresthat can be identified from the residual gravity map:the patchy anomalies along the rift axis and the rel-atively uniform residual values characterizing both

the western and southeastern plateaus. The residualanomalies in the rift floor are the surface manifesta-tion of the upper-mantle-derived material (intrusion),and are mostly correlated with the main Quaternary


peralkaline rhyolite centers of the rift floor along theWonji Fault Belt and the Siliti Debrezit Fault Zone.Within the rift floor, the anomalies are mainly concen-trated in the northern and central sectors of the graben.The patchy anomaly at the northeastern corner of themap (anomaly A on the map) is correlated on the sur-face with Mt. Bora and Gademsa (refer to Figs. 1 and2 for the locations of the rhyolite centers mentioned inthe text). Mt. Bora and Gademsa, which are rhyolitecenters of the rift floor, constitute the northern seg-ment of the Wonji Fault Belt. The closed anomaly B,on the residual map, occurs over the Aluto volcaniccomplex along the Wonji Fault Belt. Farther south-west along the Siliti Debrezit Fault Zone, the residualanomaly C on the map is associated with the peralka-line rhyolite center of Mt. Duguna. The anomaly E,on the other hand, is attributed to the sediments onthe rift floor. The gravity data coverage on the adjoin-ing plateaus is relatively minimum. Consequently, theinterpolation effect resulted in some fictitious anoma-lies on the plateaus (anomalies F and D on the map).Although the magnitude of the residual anomaly de-creases towards the south, the surface manifestationof the intrusion is still remarkable in the southernsector of the rift floor (anomaly G on the map). Thesouthward decrease of the magnitude of the residual isprobably attributed to the deep nature of the intrusionbeneath the southern sector of the graben.

The regional gravity field, which is primarilyascribed to variation in the deep-seated crustal struc-ture, is shown in Fig. 5. Most of the features ob-served on this map, except those caused by thenear-surface causative bodies, are similar to thoseof the Bouguer gravity map (Fig. 3). In particular,the gravity gradient along the eastern escarpment ofthe rift floor and the two NW–SE-trending gravitygradients are prominent.

4. Three-dimensional gravity modeling

4.1. Methodology

As the general geological strike along the rift flooris variable, a 3-dimensional modeling, covering anarea of 105ð 103 km2 (360 km ð 290 km), was ap-plied. The 3-dimensional forward gravity modelingpackage IGAS (Interactive Gravity Analyzing Sys-

tem), developed by Gotze and Lahmeyer (1988), wasused to establish the geometry and density distribu-tion of the graben. The method is based mainly ontransforming the volume integral involved in the ver-tical attraction of a homogeneous polyhedron into asum of line integrals suitable for computer program-ming. The program requires an input data structurealong a definite number of vertical modeling planesperpendicular to the general geological strike. Ineach plane, the vertices of the assumed subsurfacestructures are interconnected to form a line separat-ing two media with different densities. Lines of theneighboring planes separating identical medium den-sities are then joined to form triangles and, hence,layer boundary surfaces. The model so developedgives a more realistic approximation of the geologicstructures (Gotze, 1976, 1984; Gotze and Lahmeyer,1988). One of the advantages of this method is itsability to approximate a body with a small number ofpolyhedrons. Consequently, any change in the shapeof the body can be achieved with only few data ma-nipulations. Besides the simplicity of approximationin data handling and manipulation pertaining to thechanges of the shape of the body, the method takescare of the earth curvature during the computationof the effects of very large structures. The results ofthe gravity model studies of the deep structure of theAlpine and Harz mountains are the best case histo-ries (among others) that show the applicability andeffectiveness of the method (Gotze, 1984; Gabriel etal., 1996, 1997).

The location and orientation of the vertical mod-eling planes and site for the deep exploratory wells(star) are shown in Fig. 6. The orientation of the ver-tical planes is perpendicular to the general geologicalstrike of the rift. The vertical planes are parallel toeach other, and the distances between the planes arevariable along the rift floor depending on the locationof the anomalies, on the Bouguer map, and their cor-responding causative bodies, on the geological map.The data distribution (dots) has also been taken intoconsideration during the selection and orientation ofthe modeling planes along the investigated area.

4.2. Results and discussion

The sources of a-priori information pertainingto the geometry and density of the initial gravity


Fig. 5. The regional gravity map of the Main Ethiopian Rift. Also shown are the names of the localities mentioned in the text (contourinterval 5 mGal).

model are various published and unpublished studies(Kazmin, 1975; Berckhemer et al., 1975; Makris andGinzburg, 1987). In particular, the determination ofthe initial crustal thickness and density values for

the deep structure of the rift are based on the resultsof the seismic refraction experiments in the south-ern Afar and western Ethiopian plateau (Berckhemeret al., 1975; Makris and Ginzburg, 1987). Table 1


Fig. 6. Location map of the 3-dimensional gravity modeling planes. Also shown are the gravity observation points (dots) and site for thedeep exploratory wells (star).

shows the P-wave velocities of the structural unitsfrom the southwestern Afar (Makris and Ginzburg,1987). Also shown are the estimated density valuesused for the 3-dimensional gravity modeling of theMain Ethiopian Rift. The density values were esti-mated from the seismic velocities using the Nafe–Drake relation (Nafe and Drake, 1957).

The density measurements from boreholes, drilledfor geothermal investigation, have also been takeninto consideration (Belaineh, 1983). Most of theboreholes are located along the Quaternary tensional

fault belt, where the volcanic complexes are visible.Although the boreholes are too shallow (the maxi-mum depth reached in borehole LA-7 is 2448.5 m)to furnish density information on the large and deepcrustal structure of the rift, the measured densityvalues have been used to control the densities ofthe shallow rock units incorporated in the model.Measurements made on samples from the drill-holesin the rift floor between 1.5 and 2.1 km depthsproduced density values of 2850 kg=m3 and 2560kg=m3 for basaltic and siliceous rocks, respectively


Table 1The P-wave velocities of the geological units from the south-western Afar (after Makris and Ginzburg, 1987)

Geological units Velocity VP Density(km=s) (kg=m3)

Sediments 2.8–3.75 2500Upper crust 6.1–6.2 2700–2780Lower crust 6.7 2900Low velocity upper mantle 7.4 3200

(Belaineh, 1983). Table 2 depicts the different rockunits used in the 3-dimensional gravity modelingand their respective densities. The geological unitsconsidered in the gravity model are identified withnumbers.

A section from the central sector of the MainEthiopian Rift is shown in Fig. 7 (refer to Fig. 6for the locations of the modeling planes). The modelportrays a possible crustal thickness and density dis-tribution beneath the graben. The long-wavelengthgravity anomaly along the axial portion of the riftfloor is best explained in terms of an intrusion ofdeep origin (Nos. 5 and 7 of Fig. 7). The intru-sion exhibits two distinct densities at different depths(3000 to 3100 kg=m3). The cause for the existenceof the intrusion under the rift floor could be partialmelting in the lithosphere. There is also evidenceof an anomalous upper mantle material (VP D 7:4km=s), north of the present study area, beneath thesouthern Afar depression (Berckhemer et al., 1975;

Table 2The different rock units used in the 3-dimensional gravity modeling and their respective densities

No. Geological units Description Density(kg=m3)

1 Magdala group (upper crystalline basement) Rhyolites, trachytes, rhyolitic and trachytic tuffs, ignimbrites,agglomerates, basalts

2700

2 Ashangi group Alkali olivine basalt and tuffs, rare rhyolites 28003 Undifferntiated Sediments 25004 Lower crystalline basement – 27805 Solidified intrusion Crust mixed material 30006 Lower crust – 29007 Solidified intrusion Mantle material 31008 Anomalous upper mantle – 32009 Siliceous domes and flows Pantellerites, obsidians, complex volcanoes of

andesite–trachyte–rhyolite composition2500

10 Basaltic flows and related spatter cones Alkaline olivin basalt 2860

Makris and Ginzburg, 1987). Consequently, the grav-ity anomaly from the Afar depression was interpretedin terms of the same intrusion (Makris et al., 1972).The variability of the density within the intrusionmight then be attributed to the different stages ofdifferentiation of the hot upper mantle material on itsway to the lower crust as well as the upper part of thecrystalline basement. The intrusive body, as deducedfrom the density values (3000 to 3100 kg=m3), ismost probably of mantle or crust–mantle origin. Thedensity of the upper part of the intrusive body (No.5 of Fig. 8) is not much larger than the lower crust(2900 kg=m3). Such a situation, however, could ariseas a result of the petrologic differentiation of themantle-derived material.

The depth to the top of the intrusion varies alongthe rift floor. The shallowest depth to the top of theintrusion is under the northern sector of the graben (4km; Fig. 9), and the bottom of the intrusion is locatedwithin the upper mantle. The east–west width of theintrusion, on the other hand, ranges from 38 km inthe central and northern parts of the rift (Figs. 7and 9) to 125 km in the southern sector of thegraben (Fig. 8). Thus the dimension of the intrusionindicates that the material derived from the uppermantle or crust–mantle is relatively large and deep inorigin. However, farther south along the East Africanrift system, the density models indicate that the depthto the top of the low-density mantle material underthe Kenya Rift is below the lower crust (Birt et al.,1997; Simiyu and Keller, 1997). This shows that


Fig. 7. A vertical cross-section of the 3-dimensional model from the central sector of the Main Ethiopian Rift (plane 9). The densityvalues of the geologic units are given in Table 2. The intrusion (3000 to 3100 kg=m3) arises from the deep upper mantle.

the degree of the associated crustal thinning andextension decreases towards the southern segment ofthe East African rift system.

The gravity model also incorporates the sedimentsof the rift floor (No. 3 of Fig. 7), mostly surroundedby the basalts of the Magdala complex (No. 1 ofFig. 7) and partly by the basaltic flows in the graben(No. 10 of Fig. 9). As discussed above, the negativeresidual anomaly is attributed to the sediments of therift floor. The anomaly is well explained with a den-sity value of 2500 kg=m3 attached to the Pleistocenesediments of the graben. So far, there is no infor-mation pertaining to the thickness of the sedimentsfrom some other independent geophysical methods.However, the results of the seismic refraction exper-iments in the southwestern Afar, which is north ofthe present study area, indicate a P-wave velocity of2.8 km=s for the Pleistocene sediments of the graben(Makris and Ginzburg, 1987). The total thickness ofthe Pleistocene sediments in the Main Ethiopian Rift,

as obtained from the gravity modeling, is over 6 km(Fig. 7). The sediment, though sparsely deposited,thickens towards the central part of the graben.

The basaltic flows of the rift floor (No. 10 ofFig. 9), which generally follow the western and east-ern escarpments of the rift, have been modeled herewith high density (2860 kg=m3) compared to theunderlying top crystalline basement (No. 1 of Fig. 9)and the siliceous flows of the rift floor (No. 9 ofFig. 9). The use of large density for the basalticflows is based on measurements made on samplesfrom the drill-holes in the rift floor (Belaineh, 1983).The relatively high density of this unit compared tothe surrounding implies that the basalt is young andmight have reached the surface as a result of volcaniceruptions from the fissures of the rift floor. The cen-tral part of the rift floor, where the basaltic flows areobserved, is highly dominated by positive gradientsof the Bouguer anomalies. This is mainly attributedto the existence of the intrusion. However, the best


Fig. 8. A vertical cross-section of the 3-dimensional model from the southern sector of the Main Ethiopian Rift (plane 3). The densityvalues of the geologic units are given in Table 2. The intrusion (3000 to 3100 kg=m3) arises from the deep upper mantle.

fit has been obtained with the basaltic flows incor-porated in the model. Moreover, the relatively smalldensities of the surrounding sediments (2500 kg=m3)and the siliceous flows (2500 kg=m3) could notchange the general ascending trend of the Bougueranomalies in the rift floor (Fig. 9). Although thebasaltic flows are small in size, they are, therefore,significant to explain the observed Bouguer anoma-lies of the rift floor. However, this interpretationis more plausible if one considers a 3-dimensionalmodel rather than a section along the model.

Because of the uncertainty of the depth of the unit,it was difficult to place a better limit on the thicknessof the basalt in the initial model. However, the presentgravity model indicates a significant thickness (4 km)of the basalt in the rift floor (No. 10 of Fig. 9). Theeast–west width of the unit is ca. 12 km.

The best fit for the high-frequency anomalies,besides the density of the sediments, was achieved

using a density value of 2500 kg=m3 assigned to thesiliceous flows of the graben (No. 9 of Fig. 9). Theuse of this density value in the model is based onmeasurements made on samples from the drill-holesin the rift floor (Belaineh, 1983). The maximumthickness of the siliceous flows, as derived from thegravity modeling, is over 8 km (Fig. 9). The centersof the siliceous flows, which are closely associatedwith the major fault belts of the rift floor, might bethe surface manifestation of the intrusion.

The crystalline basement (Nos. 1 and 4 of Fig. 7),which is considered here between ca. 25 km and thebottom of the sedimentary layer, has been modeledwith two density values (2700 and 2780 kg=m3). Theuse of a density value of 2780 kg=m3, for the lowerpart of the crystalline basement (No. 4) between 10and ca. 26 km, is based on the seismic velocitydetermination made for the upper crust of the region(VP D 6:1 km=s; Makris and Ginzburg, 1987). In


Fig. 9. A vertical cross-section of the 3-dimensional model from the northern sector of the Main Ethiopian Rift (plane 16). The densityvalues of the geologic units are given in Table 2. The intrusion (3000 to 3100 kg=m3) arises from the deep upper mantle.

order to accommodate the transition from low to highP-wave velocity within the upper crust, a densityvalue of 2700 kg=m3 has been assigned for the upperpart of the crystalline basement (No. 1). This value isconsistent with the borehole determination and closeto the density of reduction used in preparing theBouguer gravity anomaly map of the study area.

The top interface of the crystalline basement be-neath the rift floor (No. 1) is generally marked by themaximum thickness of the sedimentary layer (over 6km; Fig. 7). Because of the asthenospheric uplift and,hence the intrusion, the crystalline basement underthe Main Ethiopian Rift floor exhibits variable thick-ness along the graben. The minimum thickness hasbeen observed beneath the rift floor where the mantleor crust–mantle material exists in the form of an in-trusion (Fig. 7). The unit generally thickens towardsthe adjoining plateaus (eastward and westward fromthe rift floor). The thickness of the crystalline base-

ment above the intrusive body (No. 1), at the extremesouth of the rift floor, is ca. 12 km (Fig. 8). However,the same crystalline basement has a thickness of 8km in the central sector of the graben (Fig. 7). Thisunit attains further its minimum thickness under thenorthern part of the study area (Fig. 9). This impliesthat the crust under the northern and central partsof the Main Ethiopian Rift floor is slightly thinned.Compared to the Afar depression, however, the crustbeneath the Main Ethiopian Rift floor is relativelythick. The trend of the crustal thinning under therift floor, which is from south to north, is the sameas the one obtained in the Afar depression. Despitethe remarkable crustal thinning and extension in thecentral and northern sectors of the graben, no indi-cation of crustal separation is observed. In contrastto Afar, where the crust is partly oceanized (Makriset al., 1972), the thickness and density of the unitoverlying the intrusion (No. 1) suggest that the Main


Fig. 10. The horizontal continuity of the intrusion under the rift floor. The horizontal map is taken at a depth of 38 km. The solid linesshow the locations of the offsets of the intrusion and the broken lines represent the 3-dimensional modeling planes. Also shown are thenames of the localities mentioned in the text. Local coordinate system is used.

Ethiopian Rift floor is underlain by a purely conti-nental crust. Moreover, the crust under the rift flooris still intact.

Despite the slight attenuation history of the crustalstructure of the Main Ethiopian Rift, there are somesignificant structural features that indicate the on-going extensional deformation across the rift floorand the escarpments. The extensional deformations,as shown on the horizontal continuity map of the3-dimensional gravity model (Fig. 10), are mani-fested as offsets of the intrusion under the rift floor.The trends of the extensional deformations have alsobeen mapped on the structural map of Ethiopia asrift offsets (Figs. 1 and 2). The first major offset ofthe intrusion, as obtained from the gravity modeling,is along the line connecting Irga-Alem with north ofHosaina, and the second one crosses the Lake Abayaat the northern end of Mt. Amaro between north of

Chencha and Derba (Fig. 10). The two lateral dis-placements of the intrusion coincide with the majorrift offsets observed on the surface (Figs. 1 and 2).The regions, where the Quaternary tensional faultbelt shows lateral offset, are characterized by strongseismicity (Asfaw, 1992). However, the nature of therift offsets is controversial among researchers (Mohr,1967; Gibson, 1969). Presuming that the offsets inthe Main Ethiopian Rift are really transform faults,the displacements would have implications for theon-going drifting apart of the African and Somalianplates and ultimately for the inception of new oceansin the future.

The Moho-depth map of the Main Ethiopian Riftis shown in Fig. 11. The contour lines represent thecrustal thickness distribution beneath the graben. Asexpected, the computed depths to the crust–uppermantle interface resemble the general shape of the


Fig. 11. Moho depth map of the Main Ethiopian Rift (the framed area shown in Fig. 3; contour interval 2 km).

Bouguer gravity anomaly (the framed area in Fig. 3).The thinnest part of the crust, which is shown as amorphological low (�31 km) in Fig. 11, separatesthe thickest crust beneath the western and southeast-ern plateaus. The morphological low trend of theMoho-depth map coincides with the maximum grav-ity anomaly of the rift floor (Fig. 3) and, hence, withthe location of the intrusion, whereas the morpho-logical high (½39 km) of the Moho-depth map, un-derlying the two adjoining plateaus, correlates withthe minimum gravity anomaly of the rift (Fig. 3).The two opposite facing plateaus show by far thelargest crustal thickness (38–51 km) in the region.The thickest crust is approximately 51 km and islocated under the southeastern plateau. Compared to

the crust beneath the southeastern plateau, the crust–upper mantle interface under the western plateauis relatively shallow (39–43 km), with a minimum(�39 km) at the extreme west of the investigationarea.

Closer observation of Fig. 11 shows that thecrust–upper mantle interface beneath the two oppo-site facing escarpments steepens towards the graben,and eventually attains its minimum thickness (�31km) along the rift floor. Within the rift floor, theMoho-depth map exhibits variable crustal thickness.The closed and elongated morphological anoma-lies along the rift floor, by and large, indicate thestrongest crustal thinning zone (�31 km) of thegraben (Fig. 11). The estimated Moho-depth, based


Fig. 12. The computed Bouguer anomaly map of the rift after 3-dimensional forward modeling (the framed area shown in Fig. 3; contourinterval 5 mGal).

on the results of the seismic refraction experiment,is 45 km under the western Ethiopian plateau and 30km under the rift floor (Makris and Ginzburg, 1987).This is consistent with the estimate made using the3-dimensional gravity modeling (�31 km under therift floor and 43 km under the western plateau).

The computed Bouguer gravity map of the finalmodel is shown in Fig. 12. The center of the gravityanomaly along the axial portion of the rift floor isclearly resolved. The gravity gradients, marking thetwo opposite facing escarpments of the rift, are alsowell modeled. The contour map of misfit betweenthe observed and computed Bouguer anomalies ofthe study area reveals an average modeling discrep-ancy ofš2 mGal (Fig. 13). However, in the rift floor,

where the high-frequency anomalies are prominent,the discrepancies are in the order of š3 mGal. Inparticular, in the southern segment of Lake Awasa(location A of Fig. 13) and in the southern sectorof Lake Abaya (location B of Fig. 13), the high-dis-crepancy zones coincide with the major rift offsetswhere the cross-rift faults intersect both the WonjiFault Belt and the two opposite facing escarpments.In such zones, it is difficult to get the best fit be-tween the observed and the computed gravity. Arelatively high discrepancy was also obtained in azone where the gravity coverage is very low (lo-cation C of Fig. 13). However, the data processingerror alone accounts to š1 mGal. Compared to thiserror, the achieved residual between the observed and


Fig. 13. The misfit between the observed and computed Bouguer anomaly of the study area (the framed area shown in Fig. 3; contourinterval 1 mGal). Discrepancies in the order of š3 mGal are located in the southern segment of Lake Awasa (A) and in the southernsector of Lake Abaya (B).

computed Bouguer gravity anomalies for a regionalgravity modeling is acceptable.

5. Conclusions

The newly compiled Bouguer anomaly map ofthe Main Ethiopian Rift enabled us to establish apossible density model of the graben. The high-res-olution 3-dimensional gravity model is constrainedby seismic information from the southern Afar andwestern Ethiopian plateau. The model reveals the ex-istence of hot upper mantle material, and hence, theintrusion beneath the rift floor. The intrusion, as de-

duced from the density values, is possibly of mantleor crust–mantle origin, and mostly follows the mainQuaternary rhyolite centers of the rift floor alongthe Wonji Fault Belt and Siliti Debrezit Fault Zone.The association of the intrusion with the Quaternarytensional fault belts was also reported in the Afardepression (Makris et al., 1972).

The Main Ethiopian Rift is not uniformly charac-terized by an extensional stress regime, as is evidentfrom the various widths of the intrusion. The east–west width of the intrusion in the southern sector islarger than the one beneath the northern and centralparts of the graben. The model is also in favor of adeep intrusion in the southern sector of the rift. The


shallowest depth to the top of the intrusion is locatedbeneath the northern sector of the graben. The deepand relatively large nature of the intrusion suggeststhat a large-scale asthenospheric upwelling might beresponsible for the thinning of the crust and, hence,subsequent rifting of the graben.

The tentative Moho-depth map indicates the thin-ning of the crust beneath the graben. The strongestcrustal thinning is located beneath the rift floor,where the inferred zone of intrusion coincides withthe maximum gravity anomaly of the graben. De-spite the slight thinning of the crust, the presentstudy indicates no evidence of crustal separation.The uplift of the crust–upper mantle interface un-der the rift floor could intimately be related to therifting of the graben. The western and southeasternplateaus, on the other hand, exhibit the maximumcrustal thickness in the region. This abrupt fall of theMoho interface on either side of the rift is attributedto the crustal thickening related to the formation ofthe two adjoining plateaus and to the thinning of thecrust beneath the rift floor.

Acknowledgements

The authors wish to thank the Ethiopian Insti-tute of Geological Surveys for permission to use thegravity data, and Prof. Dr. H.J. Gotze and Dr. SabineSchmidt for allowing to use the 3-dimensional grav-ity modeling package (IGAS). The immense benefitderived from the discussions with Prof. Dr. KlausSchwab and colleague Dr. Gerald Gabriel is highlyacknowledged. We are also indebted to the GermanAcademic Exchange Service (DAAD) for supportingthe principal author R. Mahatsente. This study wascarried out at the Institute of Geophysics, TechnicalUniversity of Clausthal, Germany.

References

Achauer, U. and the KRISP Teleseismic Working Group, 1994.New ideas on the Kenya rift based on the inversion of thecombined dataset of the 1985 and 1989–90 seismic tomog-raphy experiments. In: Prodehl, C., Keller, G.R., Khan, M.A(Eds.), Crustal and Upper Mantle Structure of the Kenya Rift.Tectonophysics 236, 325–329.

Asfaw, L.M., 1992. Constraining the African pole of rotation.

Tectonophysics 209, 55–63.Barberi, F., Varet, J., 1977. Volcanism of Afar: small-scale plate

tectonics implications. Bull. Geol. Soc. Am. 88, 1251–1266.Belaineh, M., 1983. Petrology, chemistry and physical properties

of core samples from well LA-3, Aluto Langano Geothermalfield. Univ. Auckland, No. 83.

Berckhemer, H., Baier, B., Bartelsen, H., Behele, A., Burkhardt,H., Gebrande, H., Makris, J., Menzel, H., Miller, H., Vees, R.,1975. Deep seismic soundings in the Afar region and on thehighlands of Ethiopia. In: Pilger, A., Roesler, A. (Eds.), AfarDepression of Ethiopia. Schweizerbart, Stuttgart, Vol. I, pp.89–107.

Birt, C.S., Maquire, P.K.H., Khan, M.A., Thybo, H., Keller, G.R.,Patel, J.P., 1997. The influence of pre-existing structures onthe evolution of the southern Kenya rift valley: evidence fromseismic and gravity studies. In: Fuchs, K., Altherr, R., Muller,B., Prodehl, C. (Eds.), Structure and Dynamic Processes in theLithosphere of the Afro–Arabian Rift System. Tectonophysics278, 211–242.

Courtillot, V., Arcache, J., Landre, F., Bonhommet, N., Mon-tigny, R., Feraud, G., 1984. Episodic spreading and rift propa-gation: new paleomagnetic and geochronologic data from theAfar nascent passive margin. J. Geophys. Res. 89, 3315–3333.

Dainelli, G., 1943. Geologia dell’ Africa Orientale. Accad. Italia,Centro Studi. AOI, Vol. 4.

Ebinger, C., 1991. Gravity data acquisition, reduction and er-ror estimation. Interim Report to the Ethiopian Institute ofGeological Surveys, Univ. Leeds, 9 pp.

Gabriel, G., Jahr, T., Jentzsch, G., Melzer, J., 1996. The Harzmountains, Germany: finite element modeling of the evolutionbased on interpretation of the gravity field. Phys. Chem. Earth21 (4), 305–311.

Gabriel, G., Jahr, T., Jentzsch, G., Melzer, J., 1997. Deepstructure and evolution of the Harz mountains: results ofthe three-dimensional gravity and finite-element modeling.Tectonophysics 270, 279–299.

Gibson, I.L., 1969. The structure and volcanic geology of anaxial portion of the Main Ethiopian Rift. Tectonophysics 8,561–565.

Gotze, H.J., 1976. Ein numerisches Verfahren zur Berechnungder gravimetrischen und magnetischen Feldgrossen fur dreidi-mensionale Modelkorper. Dissertation, T.U. Clausthal.

Gotze, H.J., 1984. Uber den Einsatz interaktiver Computer-grafik in Rahmen 3-dimensionaler Interpretationstechniken inGravimetrie und Magnetik. Habiltationsschrift, T.U. Clausthal,236 pp.

Gotze, H.J., Lahmeyer, B., 1988. Application of 3-D interactivemodeling in gravity and magnetics. Geophysics 53, 1096–1108.

Griffin, W.R., 1949. Residual gravity in theory and practice.Geophysics 14, 39–56.

Hammer, S., 1939. Terrain corrections for gravimeter stations.Geophysics 4, 184–194.

Johnstone, R.D., 1983. Geological log, well LA-1, EthiopianGeothermal Exploration Project. Ministry of Mines and En-ergy, Report No. 83=15, Addis Ababa (unpubl.).


Kazmin, V., 1975. Explanation of the geological map of Ethiopia.Ethiopian Institute of Geological Surveys, Bull. No. 1.

Makris, J., Ginzburg, A., 1987. The Afar depression: transitionbetween continental rifting and sea floor spreading. Tectono-physics 141, 199–214.

Makris, J., Menzel, H., Zimmerman, J., 1972. A preliminaryinterpretation of the gravity field of Afar, northeast Ethiopia.Tectonophysics 15 (1/2), 31–39.

Mechie, J., Keller, G.R., Prodehl, C., Gaciri, S., Braile, L.W.,Mooney, W.D., Gajewski, D., Sandmeier, K.J., 1994. Crustalstructure beneath the Kenya Rift from axial profile data. In:Prodehl, C., Keller, G.R., Khan, M.A. (Eds.), Crustal andUpper Mantle Structure of the Kenya Rift. Tectonophysics236, 179–200.

Mechie, J., Keller, G.R., Prodehl, C., Khan, M.A., Gaciri, S.J.,1997. A model for the structure, composition and evolution ofthe Kenya rift. In: Fuchs, K., Altherr, R., Muller, B., Prodehl,C. (Eds.), Structure and Dynamic Processes in the Lithosphereof the Afro–Arabian Rift System. Tectonophysics 278, 95–119.

Meyer, W., Pilger, A., Roesler, A., Stets, J., 1975. Tectonicevolution of the northern part of the Main Ethiopian Rift insouthern Ethiopia. In: Pilger, A., Roesler, A. (Eds.), Afar De-pression of Ethiopia. IUGG, Sci. 14, Schweizerbart, Stuttgart,pp. 352–361.

Mohr, P.A., 1960. Report on a geological excursion throughsouthern Ethiopia. Bull. Geophys. Obs. Addis Ababa 3, 9–20.

Mohr, P.A., 1962a. The Geology of Ethiopia. Univ. Coll. AddisAbaba Press, 268 pp.

Mohr, P.A., 1962b. The Ethiopian rift system. Bull. Geophys.Obs. Addis Ababa 5, 33–62.

Mohr, P.A., 1967. The Ethiopian rift system. Bull. Geophys. Obs.Addis Ababa 11, 1–65.

Mohr, P.A., 1971. Ethiopian rift and plateaus: some volcanicpetrochemical differences. J. Geophys. Res. 76 (8), 1967–1983.

Mohr, P., 1989. Nature of crust under Afar: new igneous, notthinned continental. Tectonophysics 167, 1–11.

Mohr, P., 1992. Nature of the crust beneath magmatically activecontinental rifts. Tectonophysics 213, 269–284.

Nafe, J.E., Drake, C.L., 1957. Variations with depth in shallowand deep water marine sediments of porosity, density and thevelocities of compressional and shear waves. Geophysics 22,523–552.

Searle, R., Gouin, P., 1972. A gravity survey of the central partof the Ethiopian rift valley. Tectonophysics 15 (1/2), 41–42.

Simiyu, S.M., Keller, G.R., 1997. An integrated analysis oflithospheric structure across the East African plateau based ongravity and recent seismic studies. Tectonophysics 278, 291–313.

Woldegabriel, G., Aronson, J.L., Walter, R.C., 1990. Geology,geochronology and rift basin development in the central sectorof the Main Ethiopian Rift. Geol. Soc. Am. Bull. 102, 439–458.

crustal structure of the main ethiopian rift from gravity ... · fig. 2. geological sketch map of...

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