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Evolution of Eastern Afar and the Gulf of Tadjura
355
Tectonophysics, 198 (1991) 355-368
Elsevier Science Publishers B.V., Amsterdam
Evolution of Eastern Afar and the Gulf of Tadjura M. Clin Institut de GPodynamique, avenue des FacultPs, F 334OSTalence
Cedex, France
(Received July 25,1989; revised version accepted April 23,199O)
ABSTRACT Clin, M., 1991. Evolution of Eastern Afar and the Gulf of Tadjura. In: J. Makris, P. Mohr and R. Rihm (Editors), Red Sea: Birth and Early History of a New Ckeanic Basin. Tectonophysics, 198: 355-368. The structure of the Gulf of Tadjura has previously been interpreted in conformity with a model derived chiefly from seismic and magnetic data. This model comprises an en-echelon axial valley of ENE-WSW, Gulf of Aden trend, offset by NE-SW transform faults. Such a structure implies a transition between the Gulf of Aden oceanic ridge and an emerged rift system in Central Afar. This model is now questioned. It is suggested, based on tectono-volcanic features which have been mapped and dated onshore, and which extend offshore into the Gulf of Tadjoura, that continental crust underlies the Gulf as well as the surrounding area of Eastern Afar. Bach of the main events in the geodynamic evolution of this area has created a set of brittle structures with a specific trend. NNW-SSE (Red Sea) and associated NE-SW trends have a Miocene age. Some of the NE-SW trending fractures were reactivated by shearing during Lower Pleistocene time, creating a NW-SE horst-and-graben block system. An E-ENE (Aden rift) trend controls the Middle Pleistocene to Recent geodynamics, both on and offshore in the Gulf of Tadjura. This new analysis of the structural evolution of Eastern Afar provides further evidence for a 3-stage evolutionary model of the region, with the Pleistocene events now more clearly defined.
Introduction
In Eastern Afar and the Gulf of Tadjura, most of the tectono-volcanic structures have resulted from activity at the Afar triple junction. Their trends are dominated by those of the Red Sea, Aden Gulf and Ethiopian rifts. A detailed chronological study of these structures builds up a Cenozoic geodynamic evolution of the region. Based on field data from the Republic of Djibouti and on a new structural map of the Gulf, we propose the major events that have contributed to the tectonics of this region. Tectonic activity peaked during the Miocene and Pleistocene, a chronology in agreement with the widely accepted two-stage history of Afar (Mohr, 1971a; Lowell and Genik, 1972; Barberi et al., 1972; Girdler and Styles, 1974, 1978; Juch, 1975; Mohr, 1978; Merla.et al., 1979), which assumes a first pulse during late Oligocene-Miocene time, and a later, possibly stronger episode during
Pliocene the later parts, so region is
to Recent times. Our data indicate that episode itself can be separated into two that a 3-stage evolutionary model of the obtained.
General geology
According to the data summarized by Beyth (1989), based on Beyth (1972), Black et al. (1974) Kazmin (1975) Beauchamp (1977), Merla et al. (1979) and other authors, most of the present tectonic trends are likely to have originated in pre-existing major structural elements of the Precambrian basement and pre-Tertiary Phanerozoic cover. In Djibouti, the crustal structure is similar to the rest of the Afar area, with a 6.0-6.1 km/s upper crust and a 6.7-6.8 km/s lower crust (Ruegg, 1975; Makris and Ginzburg, 1987; Egloff et al., 1991). Crustal thickness is reduced to less than 14 km immediately west of the Gulf of Aden.
0040-1951/91/%03.50 6 1991 - Blsevier Science Publishers B.V. All rights reserved
356
:eRB3CfinPHii 3’MBOLS
VOLCANIC FORMATIONS
Ras Siyysn
Sawabi
cOne
HOUSA’ALI
D,R-1
VOLCANO
6% iK,‘Ar) Remain Cobble, Coarse
of madrrpore (+ 90 m, + 100 pebble, gravel. sand. silt alluvial deposits
m>
mw OUEt?lOL?NA FOKMATION
P9
SANKAL FORMATIMI LOUCAC’ALE FOWATIDN
BPX
BALHO
Kte
:
-
GA”ARRI
Rhyolites MIXED FO~TION
I
FORMATION
OF MARYAN’AD
I
-
BARSA
Fig. 1. The Mesowic and Cenozoic volcanic and sedimentary sequences of the Republic of Djibouti, according to the legend of 1 : 100,000gevlogical maps by Boucarut et al. (1974). Barrere et al. (1975) and Boucarut et al. (1978a).
EVOLUTION
OF E,ASlERN
AFAR
AND
THE
GULF
357
OF TAQJURA
Precambrian rocks outcrop near Aisha, 40 km south of Djibouti (Dainelli, 1943; Mohr, 1962). In Djibouti itself, the exposed stratigraphic SWXSsion commences with Mesozoic sediments, unconformably overlain by Cenozoic volcanic and sedimentary sequences (Figs. 1 and 2). Volcanism started relatively abruptly with basaltic outpourings and the spreading of massive ignimbrite sheets during late Oligocene-early Miocene and Middle Miocene times. Differentiated lavas and plateau basalts flows subsequently predominated during Upper Miocene to Holocene times. The palaeogeographic distribution of the late Pliocene-Holocene “rift series” flows was controlled by successively developed morphotectonic features (Boucarut and Clin, 1980; Boucarut et al., 1980a) Inter-eruptive erosion produced unconformable contacts between the Lower and Middle Miocene, between the Middle and Upper Miocene (with rifting), within the Upper Miocene and, after regional tilting, within the Pleistocene sequences (Figs. 1 and 3). The main tectonic events are linked to the activities of the rift zones converging on the Afar triple junction, as follows (Fig. 2): (1) NNW-SSE rifting (Red Sea trend) during the Lower and Middle Miocene. Opening was accompanied by a sinistral shear along NE-trending faults. Some of these faults subsequently exerted an element of control on the structure of the Gulf of Tadjura. During the Upper Miocene and Pliocene, further regional extension permitted eruption of the plateau series which covered all of Central and Eastern Afar (Bannert et al., 1970; Mohr, 1971a). (2) Regional sinistral shearing along NE faults (Ethiopian rift trend) during the Lower Pleistocene, which was accompanied by transverse NWSE faulting. A NW-SE horst-and-graben and tilted block system was then formed and the central region of Djibouti was uplifted. (3) Westward unzipping of the Gulf of Tadjura (Aden Gulf trend) during Middle Pleistocene-Holocene time. This opening reactivated the earlier NW- and NE-trending fault systems. These data provide a basis for revision of the Afar tectonic history given by CNRS-CNR Afar Team (1973) and Varet and Gasse (1978). Inter-
pretation of the Tadjura structure as an E-W en-echelon axial valley offset by NE-SW transforms (Lepine and Ruegg, 1976; Lepine et al., 1976; Abdallah et al., 1979; Richard and Varet, 1980; Arthaud et al., 1980a) requires modifying in the light of the forms of the axial deeps, the presence of NW-SE faulting, and the existence of compressive structures. More recent geomagnetic studies (Courtillot et al., 1980) and the 1984 Cyana dives (Choukroune et al., 1986, 1988) provide further reasons to question the adequacy of the previous model. ~wer-~~e
h%mne cm&d anatexis
Rocks of Miocene age are chiefly exposed in the eastern part of the Republic of Djibouti, on the Mabla range (Tadjura High) and Aisha horst (Ali Sabih area). These bards to day are connected topographically with the DanakiI and Somali platelets, respectively (Laughton, 1966; Mohr, 1967; Roberts, 1970). The major magmatic event of this time was eruption of voluminous rhyolitic ignimbrites, as massive sheets more than 200 m thick. Both mafic and silicic dike swarms were emplaced. Some dikes in the Aisha horst carry xenoliths of Jurassic marble and Precambrian granite. Two periods (Fig. 1) of paroxysmal rhyolitic activity have been identified: late Oligocene-early Miocene and Middle Miocene. Previous syntheses (e.g. Varet and Gasse, 1978) do not distinguish the first episode, but interpret the Oligo-Miocene basaltic rhyolitic series of Djibouti as part of a wider, regional, single ma~atic cycle. Based on chronological data, Gadalia and Varet (1983) relate the entire Miocene volcanic sequence of eastem Djibouti with the Affara-Dara and other Afar alkaline-peralkaline plutons. Such a simultaneity would more accurately comect these granitoids (26 to 22 m.y.) solely with the Lower Miocene volcanics in Djibouti (subalkaline basalts, 26.7 to 22.6 may.; rhyolitic ignimbrites, 23.8 to 19.1 m.y.) and thus excluding the Middle Miocene volcanics (16 to 13 m.y). These Middle Miocene rocks are preserved at several localities, both north and south of the Gulf (Chessex et al., 1975, 1980; Boucarut et al., 198Ob), resting unconformably on the Lower
358
EVOLUTION
OF EASTERN
AFAR
SW
AND
THE
GULF
359
OF TADJURA
NE
0 B
Fig. 3. Non-parallel diagrammatic cross sections illustrating the main tectonic events of the Eastern Afar (Republic of Djibouti). A. Lower to Middle Miocene unconformity (RRdB,R,), Middle Miocene fracturing and rifting, subsequent Upper Miocene and Pliocene plateau flows (Mi, B,, ha). B. Lower Pleistocene fracturing, tilting and bl~k-up~ft according to a NW-SE trend, unconformable subsequent lavas (&b, 2.1 to 0.8 my.} and sediments. C. Middle Pleistocene to Present E-W deformation and basaltic flows (&, Bad and Asal-Ardoukoba volcanics, h).
Fig. 2. Geological map of the Republic of Djibouti, 1: 900,000. The nomenclature of the formations is the same as in Fig 1. 23 = boundary of the Republic of Djibouti; 22 = main cliffs, aerial and undersea, usually associated with faults; 2I= fractural network: the fracture pattern is the result of reactivation of some previous fractures; 20 = fractural network: the fracture pattern is the result of successive additions; 19 = lava flows younger than 9000 years, Asal area; 18 = Upper-Pleistocene to Present scatter-cones and other volcanic vents; 17 = Upper-Pleistocene to Holocene aa or pahoehoe iava flows; 16 = hyaloclastic seamounts: Ghubbat-al-Kbarab (the western one is emerged) and Bab-el-Mandeb (emerged crescents); 15 = late to Middle Pleistocene aligned aerial scatter- and lava-cones and mounts; 14 = Tadjoura deep (800 m), Obock deep (1100 m); 13 = alluvial deposits and other sediments, coralhan buildings, Lower Pleistocene to Present; I2 = Middle Pleistocene We’a-Rarou (B2c) valley flows; Ii = volcanic vents made of B2b basalts, aligned on some feeding fractures; 10 = Lower Pleistocene Gummouna (B2bR2b) formation: mainly flood basalts; 9 = Ado Ale rhyolitic central volcano; 8 = Lower Pleistocene Lougag Ale&u&al (B2x) formation: low alkalinity flood basalts; 7 = Balbo-Gamarri (B2a) formation overlain by Lougag Ale-Sankal (B2x) formation: basalts capping residual hills; 6 = Pliocene Balbo-Gamarri (B2aR2a) formation: mainly flood basalts; 5 = Upper Miocene to Pliocene Damayyi-Galemi (Bl) formation: transitional plateau basalts without ~ffer~tiat~ terms; 4 = (Mi) iavas filling the axial trough of a Lower-Miocene rift structure (Ado&Falka rift structure}; 3 = Upper Miocene Maryan Ad-Harm (Mi) formation: plateau lavas, mainly trachybasalts, trachytes, scarce rhyolites and ignimbrites; 2 = Lower Miocene basic dykes and lodes (x), 19.6 m.y., Ali Sabih area; I = Jurassic and Cretaceous sediments (Ali Sabih area), Galile basalts and sills (B,,), Upper Oligocene to Middle Miocene Golwa-Chirrnile (B,R,) and D~~-~y~ D&da (l&R,) formations: mainly K-rhyolites of cm&al origin. A = Ardoukoba; Ab - I,& Abhe; Af= lakes Afambo and Gamarri; As-L.&e Asat, AS = Ali Sabih, D = Dacca volcanic range; l3e = depression of Der Ela; G = Guissi volcanic range; Gu - Garbi Mountain; Gg m Gaggade depression; Gm = Gamarri cliff; GK = Ghub~t-~-~~a~ H = Hanle depmasion; Ib - Ibira rauge; Mk = Mak’arrasou area; Mu - Muaa Alli vokxtno; MR = Mabla Range; Ok = O&k town; S = Sawabi archipelago; Td = Tadjura town; Y = Yager Mountain.
360
Irl
(‘I II\.
The bimodal proportions become reversed in the end Miocene-Pliocene volcanics, where basaltic lavas predominate over felsic rocks in a volume ratio of 100 : 1. In this sequence, two thin ignimbrites have similar geochemical characteristics to the older ignimbrites. However, the bulk of the younger rhyolites comprise lava domes; they lack quartz phenocrysts, SiO, is lower than 72%, Al,O, averages 12.5%, and Na,O (av. 4.4%) exceeds K,O (av. 3.9%). Boucarut and Seyler (1980) have compared Kuno solidification index (S.I.) plots for the Lower-Middle Miocene rhyolitic rocks with those for Upper Miocene and younger rocks in Djibouti. Based on major element chemistry and S.I. trends, and considering the enormous preponderance of silicic over the mafic rocks, the absence of differentiated rocks and the abundance of ignimbrites in the older suite, Boucarut and Seyler conclude
Miocene volcanics (Barrere et al., 1975; Boucarut et al., 1978a). The Lower and Middle Miocene rhyolites have a similar major element geochemistry (Boucarut and Seyler, 1980). Both suites are persilicic and alkaline. Quartz rhyolite lavas and ignimbrites in the Lower Miocene suite predominate over basalts (200 km3, and 2 to 3 km3, respectively). The rhyolites have high SiO, (> 72%) low Al,O, ( < 10.6%) and K,O (av. 4.4%) greater than Na,O (av. 3.2%). In the Middle Miocene suite, the volume ratio rhyolite:basalt is similarly 150 km3:few km3. The ignimbritic facies of the silicic volcanics again have high SiO, (70 to 77%) and Al,O, and Na,O values similar to those of the Lower Miocene silicics. On the alkali-silica diagram, basalts and rhyolites are located along common trends (Figs. 2a, 2b), marked by the absence of intermediate composition rocks.
IO
IO -=3
sz.3
:-
I (c)
1,’
1
50
70
60
50
(4
I
I 60
1
I 70
_UI.. (W
5102
L
SlO2
1
I (d)
50
’
I 50
70
60
I
1 60
1
I 70
SlO2
1
I
SiO7
Fig. 4. Alkali-silica diagrams for the volcanic formations of the Republic of Djibouti, according to Boucarut and Seyler (198O),based on 210 analyses. The trends concentrate along the boundary between the subalkaline and calcalkaline series (u = 3, Rittman, 1958) from subalkaline transitional basalts via alkaline rhyolites to commditcsand relatively rare pantellerites. Four magmatic sequences have been distinguished: (a) Late Oligocene-Early Miocene; (b) Middle Miocene to Pliocene; (c) Lower Pleistocene; (d) Middle Pleistocene to Recent.
EVOLUTION
OF EASTERN
AFAR
AND
THE
GULF
OF TADJURA
that the Lower-Middle Miocene rhyolitic ignimbrites and lavas of Djibouti are the result of deep crustal melting, and not of fractional crystallization (cf. Varet and Gasse Team, 1978). This endorses the far-sighted but then unfashionable argument of Holmes (1931). The southern Danakil block is taken as a zone of Miocene crustal attenuation (Marinelli and Varet, 1973). Based on 1 :‘100,000 mapping (Barrere et al., 1975; Boucarut et al., 1978a), the Lower to Middle Miocene magmatic activity can be related to coeval distensive tectonism of NNW (Red Sea) trend. Compare for example the NNW Adori-Falka rift structure, north of the Gulf (Figs. 2 and 3A), and the NE trending doleritic fissures of the Ali Sabih area dated at 19 m-y. (see x, Figs. 1, 2 and 4a). This tectono-volcanic combination of NNW and NE faults and fissures suggests a “pull apart” model for the Lower Miocene organization in eastern Afar, to be compared to that of Makris and R&m (1991) for the northwestern Red Sea. In addition to the NNW- and NE-trending structures, a N-S fault set is developed on both sides of the Gulf. That to the north may express an internal deformation of the southern Danakil block into an intensely fractured, bow-shaped structural zone (Boucarut and Clin, 1980). This deformation could in turn be related to an eastward propagation of early Cenozoic rifting in the northern Ethiopian plateau (Zanettin et al., 1980). During Upper Miocene-Pliocene (13 to 3.5 m.y.), extensive flood basalt flows covered all of the Republic of Djibouti, excepting the Ali Sabih block. The lower part (13 to 8 m.y.) of this pile contains mainly trachybasalts, trachytes, and sparse rhyolites and ignimbrites (Fig. 4b). The upper part (7.9 to 4.5 m.y.) is composed of transitional flood basalts without differentiated terms. The last flows (3.5 m.y.) are again differentiated (Fig. 4b). Lower Pleistocene uplift associated with regional shearing
The Pliocene basaltic outpouring was followed by intense fracturing of the Afar floor, with development of a complex horst and graben system, and zones of tilted blocks. In Djibouti, the system
361
trends either NW or NNW (Figs. 2 and 3B). Stratigraphic and geochronologic data indicate that NW faulting occurred during the early Pleistocene. Some NW-SE depressions were progressively individualized during this time, as proved by the increasingly dispersed distribution of the sediments which filled them (Roger et al., 1974, 1975; Thibault, 1980). Most of the NNW faults are located to the north of Djibouti (Mak’arrasou area) and appear as reactivated Miocene Red Sea trending fractures. North to ll”N, the faulted and tilted area in Djibouti is more or less exactly located along a continuation of the Ethiopian rift. The margins of the Ethiopian rift and the Wonji fault belt were impressed at this time during the “Nazret phase” of faulting (Baker et al., 1972) dated at about 1.6 to 1.8 m.y. (Meyer et al., 1975). In central Djibouti, an angular unconformity clearly separates, as a result of the Nazret phase, the faulted and tilted Mio-Pliocene lavas and sediments from overlying Lower Pleistocene lavas and sediments 2.1 to 0.8 m.y. old (Boucarut et al., 1974, 1985; Boucarut and Clin, 1980; Thibault, 1980). The Afar horst-graben system is crossed by NE-SW trending lineaments. In Djibouti, a prominent NE-SW fault extends from the Obock deep in the eastern Gulf of Tadjoura, onshore to about ll”30 (Fig. 5), and then relays with a reactivated Miocene NE-SW fault in Ali Sabih. Regionally, the NW-SE fractures terminate against these faults, not abruptly but with a curved S-shape adjustment suggesting a sinistral strike-slip component (Fig. 2). Another NE-SW lineament may cross Musa Alli volcano and the Dobi Graben, again in association with S-shaped fractures. Associated with this graben, northwest of Djibouti, a sinistral transtensional zone has been mapped (Mohr, 1971b, 1974), superimposed by some NE lineaments. NE-SW trending lines offset E-W regional magnetic anomalies in the Gulf of Aden (Hall, 1970; Laughton et al., 1970) as well as in Eastern Afar (Le Pichon and Francheteau, 1978). In Eastern Afar, Bouguer gravity anomalies reveal the same NE-SW trend (Makris and Ginzburg, 1987) crosscutting the NW-SE axial-range trend. The palaeogeographic distribution of Lower Pleistocene lava flows indicates that the Afar
362
M. C‘l,llu
horst-graben system was broadly uplifted at this time along a NE-SW ridge (Boucarut and Clin, 1980; Boucarut et al., 1985). The Lower Pleistocene volcanic series onlapped this ridge only at its edges, where an unconformity can be observed (Fig. 3B). Based on these chronological and tectonic data, the NNE-trending “Mak’arrasou transform fault” (Tapponnier and Varet, 1973), important in the formulation of the Afar small-scale plate tectonic model (Varet and Barberi, 1976; Barberi and Varet, 1977), does not exist as a Recent structure (Boucarut and Clin, 1976). Tapponnier and Varet propose that the transform fault is masked by younger volcanic formations, and underlies a conjugate set of NW-NNW faults. The hidden fault is thus discordant with surface structures. The NW-trending Pleistocene and the reactivated NNW Miocene fractures of Central Afar cannot be considered as conjugate owing to disparate ages. Moreover, as these fractures cut only Plio-
Fig. 5. Tectonic map of the Gulf of Tadjura and surrounding
cene and older lavas, which were tilted during the Nazret phase, the proposed transform fault must be related to this aider phase of deformation, and not to the more recent evolution of the Afar such as, for example, that which would link Asal to Manda-Inakir. Similar doubts concern a proposed transform fault from Manda Harraro to Manda Inakir (Varet and Barberi, 1976). Nevertheless, the tectonic activity of visible or hidden NE-trending lineaments is useful to explain structures formed during the Nazret phase. In Djibouti, the general pattern of the NE trending belt of faulted and tilted blocks can be explained by a model of cmstal shearing along NE strike-slip faults, indirectly projected from Ethiopian rift structures, and producing secondarily strike-slip and tilting movements along the NW structures (Boucarut and Clin, 1976, 1980; Boucarut et al., 1977, 1978b; Lombard, 1978). In that model, the “‘Mak’arrasou strike-slip zone” (Vellutini, 1990) appears as a combination with
areas. Field data after 1 : 100,000 geological
1974, Barrere et al., 1975; Boucarut et al., 1978a) and other unpublished. 1977, with complements)
and tire Charcot cruise (Clin and Pelissier-Hermitte,
1977) near the southern coastline. 3 = Ardoukoba NW-SE
volcanic
range; 4 = volcanic
graben near Djibouti;
10 = landslides
I = Submarine volcanoes, 6 = faults (on land);
on the present shore; T = Tadjura;
1981), and from the Tudjoura cutter cruises (1973,
Tadjura and Obock deeps (T and 0);
vents, associated
maps (Boucarut et al.,
Submarine data from the Thalussa cruise (Boucarut et al.,
to BZb formation,
2 = E-W Guissi volcanic range;
early Pleistocene:
5 = Br,b lava flows, filling a
7= faults (submarine); 8= fault scarps; 9= continental shelf boundary; DZ = Cape DabaDj = Djibouti; As = Lake Asai; Gk = Ghubbat-al-Kharab; Libah; Rd = Ras Duan.
EVOLUTION
OF EASTERN
AFAR
AND
THE
GULF
OF TADJURA
reactivated previous faults and merely represents the border of the Danakil platelet. Quaternary
continental tearing of the Gulf of
Tadjura After Lower Pleistocene time, the entire tectonic pattern of Djibouti changed, as E-W arching and faulting began to predominate in Central Afar. A new set of E-W trending fractures developed during the Middle Pleistocene, both north and south of the Gulf in the relatively stable areas of Tadjura and Ali Sabih, as well as in some other parts of Djibouti including the Gulf itself. E-W basaltic valley flows were emplaced. The E-W fractures were intruded by dikes which were feeders for Upper Pleistocene and Holocene basalt flows. Some of them are marked at the surface by aligned spatter cones. Also formed were hyaloclastites, in the Ghubbat-al-Kharab and Bab-el-Mandeb areas. West of the Gulf of Tadjura, an E-W morphotectonic arching is observed. The Tertiary lavas and sediments have been uplifted to an elevation of 1600-1700 m only a few kilometres from the Gulf. The relief of the Garbi and Yager mountains and the Gamarri cliff are due to this arching. Moreover, 75 km west of the Gulf of Tadjura, near the Awash valley, this E-W arching has produced a cambered structural surface which can be directly observed as the top of the NNW-SSE scarp dominating the Afambo and Gamarri lakes (1: ~~,~ French I.G.N. topographic map, sheets Abhe Bad and Gamarri). From Upper Pleistocene to Holocene time, pre-existing NW-SE faults were reactivated, thus facilitating NW-trending volcanic feeders such as Asal-Ardo~oba (Fig. 3C). However, the active deeper structural elements are E-W trending. This trend is expressed in the geomagnetic data (Girdler, 1970; Roeser, 1975; Courtillot, 1980). Prominent E-W volcanic alignments include (Figs. 2 and 5): at ll’lO’N, Dacca volcanic range; at 11*20/N, Guissi volcanic range; at 11’35’N, Asal-Ardoukoba and Der Ela-Gaggade basaltic areas; at 11”45’N, E-W axial faults in the Gulf of Tadjura; at 11’50’N, submarine volcanoes in the eastern Gulf; at 12”20’N, Sawabi volcanic archipelago, Ibira range, Manda-In~r system, and
363
Musa Alli volcano; possibly at 12”58’N (in Ethiopia), the Assab volcanic range. Manda and Assab belong to the “ transverse ranges” of Barberi et al. (1974). We note that the area between 11”OO’N and 12”20’N coincides with an anomalous E-W zone of Central Afar, identified on the basis of geomagnetic anomalies (Girdler and Styles, 1976) and of recent epicentres (Gouin, 1970; Fairhead and Girdler, 1970). Some arguments for the probable westward extension of this structure into the Ethiopian Plateau are given by Zanettin et al. (1980). The new E-W fault set in eastern Djibouti, together with the morphotectonic arching in the west, are geologic indicators of a progressive westward opening along the Gulf (Boucarut et al., 1978b). Geophysical data (Courtillot et al., 1980; Courtillot, 1980, 1982) support the concept of a westward “tearing” of the Gulf. The Gulf began to open 4.5 to 5 m.y. ago (Girdler et al., 1978; Styles and Hall, 1980) as a Pliocene re-activation of Afar geodynamics. Westward tearing along the Gulf has taken place from 2.7 m.y. to Present (Gasse and Fournier, 1983; Boucarut et al., 1985) These regional and local data may relate to the E-ENE system of major transverse discontinuities considered by Beyth (1989) to account for the southern Red Sea evolution. A tectonic map of the Gulf of Tadjura and surrounding areas West to 43”2O’E, the Gulf marks an important tectonic zone separating the Danakil and Somali platelets. The structure of the Gulf is not, as previously considered (Lepine et al., 1976; Richard and Varet, 1980; Arthaud et al., 1980a), the result of an en-echelon arrangement of ENE-WSW rifts and NE-SW transforms, and admitting an additional NNE transform (Athaud et al., 1980b), but is considerably more complex. Observations made during dives with the Cyana submersible (Chou~oune et al., 1986, 1988) confirmed the NW-SE (140”) zones of active faulting and recent volcanism and the NE-SW (60”) faults, partly mapped previously (Boucarut et al., 1977; Clin and Pelissier-Hermitte, 1981). The 140’ fractures interact with the 60’ fault set, usually inactive and
364
considered to be older. According to Choukroune et al. (1988),“there is no active faulting trending at high angle to the 130”-140” normal faults. This invalidates all previous kinematic modeis of the Gulf of Tadjura”. When developing tectonic models for the Gulf, it is imperative to consider data from the surrounding terrain. The Plio-Pleistocene deformation of the Gulf should not be seen as highly localized and restricted within this area only (Backer et al., 1973; Boucarut et al., 1977, 1980b, 1985). A similarly complex geodynamic history characterizes much of the onshore territory of the Republic of Djibouti. Figure 5 shows the general geologic structure of the Gulf, based on the Thalassa (Boucarut et al., 1977) and Charcot (Clin and Pelissier-Hermitte, 1981) cruise data. The Gulf ,contains a major NE-SW trending fault system between 43’20’E (Obock deep) and 43”OO’E (Tadjura deep), continued southwestward onto land. This fault system originated during Lower-Middle Miocene, without implication that the Gulf opened at this time. West of 43”OO’E, the NE-SW fault system is crosscut by a NW-SE faulted ridge (42”52’43“OO’E). This ridge traverses the Gulf and the faults outcrop onto the southern shore. Thus near Djibouti, the southeastern segment of the submarine fault system forms a graben 15 km long and about 4 km wide, filled by Lower Pleistocene basaltic flows (Hauquin, 1978). The presence of these lavas indicates a probable overprinting of the NW-SE faults on an earlier Gulf structure, 2.1 m.y ago or later. Many other NW trending fault zones have been mapped east of 43”OO’E (Clin and Pelissier-Hermitte, 1981; Choukroune et al., 1986, 1988). In this eastern part of the Gulf, the NW structures are only known as parts of the axial zone. However, they are likely of the same origin as the fractures controlling the western faulted ridge as well as the active Asal-Ardoukoba rift and other, parallel structures. In the western Gulf of Tadjura, the offset of the “axial zone” by NW-SE trending structures was described as early as 1969 by Roberts and Whitmarsh (1969) based on a magnetic survey interpretation. These authors postulated a submarine continuation of the onshore NW-trending
M
horst-and-graben terrain. They also discussed a possible extension onto land of the ENE-WSW active zone, supposing that “in this area, tensional separation is not sufficiently advanced to lead to the development of WSW trending fissures and associated volcanism”. Some E-W structures are important as limits of the axial deeps. Whereas the Tadjoura deep (Fig. 5) is bounded north by an E-W faulted scarp, the Obock deep is bounded south by the E-W Maskali scarp. The two scarps lie along a common strike with a switch of polarity. A set of NW-SE fractures on the southern rim just marks the junction (~houkroune et al., 1988). In the western Gulf, NW-SE faults do not extend continuously across the northern side of the Gulf, but are truncated by younger NE-ENE faults. Vertical fractures of the latter trend occur west of the Tadjura deep, and cut the shore-line up to the 100 m coastal platform. Between individual faults, as indicated from echograms, Recent sediments have been intensely folded and compressed, and this is considered related to dextral strike-slip motion along the E-W fault zone bordering the Tadjura deep. Due to this motion, ENE-WSW structures, located both southwest and southeast of Tadjura town, are probably inherited from pm-existing major structures of NE-SW trend in the Gulf, and determining also the coastlines between 43%) and 43”lO’E. The ENE-WSW Aden rift zone, which is well defined eastwards, cannot be identified west of 43”25’E. There, E-W trending fractures are only present in axial position, not ENE ones. Another example of the marine continuation of major onshore structures is provided by the topographic high at 11”38’N, 42”55’E, bounded by NW- and NE-trending faults. This relief probably comprises Miocene rhyolites, an i~te~reta~on (Fig. 6) supported from magnetic surveys by Pouchan et al. (196S), and by the proximity of Lower-Middle Miocene rhyolitic volcanics of the Sabih range, and most proximal in the SE part of the Ghubbat-~-~~ab (Fig. 2). It is of interest to compare the distribution of active structural elements in the Gulf of Tadjura (Fig. 5) with the seismicity map (Lepine and
EVOLUTION
OF EASTERN
AFAR
AND
THE
GULF
\
36 000 a -
TI
I
365
OF TADJURA
I
Fig. 6. A profile of the total magnetic field across the Gulf of Tadjura. After Pouchan et al. (1965). The anomaly A (J =10(J) was attributed by the authors either to a trot&r or to a mass of rocks of lower magnetic subtility such as some rhyohtic lavas. The bathymetry supports the second interpretation.
Ruegg, 1976, fig. 2). Earthquake epicentres plot precisely on the above mentioned NW, NE and E-W trending fault zones west of 43’20’E. The only presently inactive faulted structures appear to be the northern boundary of the Tadjura deep, and the set of compressive structures SW of Tadjura town. There is no ~~fi~t NNE active structure in the area west of 42*50/E, proposed by Lepine et al. (1976) and Arthaud et al. (1980b) as a transform-fault zone.
faults were formed, and older fault systems were reactivated. This explains the zig-zag fault pattern. (4) The continental nature of most of the area is supported by above-mentioned geologic, structural, petrologic and geochemical data, and appears inherited from its previous history. Continuation of the major onshore structures into the Gulf strongly suggests that the Gulf floor is composed mainly of pled-apt continental crustal blocks, separated by narrow rift zones injected with young volcanics. Recent geophysical data (Egloff et al., 1991) indicate that there is no true oceanic crust in the Gulf floor. (5) Combined deformations in the Gulf area suggest for the Central-Eastern Afar tectonics during the Quatemary a similarly composed model assuming the addition of simultaneous slight movements along the various fault sets, either strike-slip or distension according to the orientation. Thus a “non-rigid” model is obtained, using both reactivated and recent fractures and other elements. The “diffuse stretching” interpretations proposed for parts of the area (Boucarut and Clin, 1980; Courtillot et al., 1980; Choukroune et al., 1988; Vellutini, 1990) imply that sort of deformation.
Acknowledgements
(1) NNW-SSE (“Red Sea”) faults and orthogonal NE-SW ones have been active since at least early Miocene. Present seismicity in the Gulf of Tadjura suggests the r~ctivation of these structural elements today, together with the NWSE and E-W younger ones. (2) E-ENE (Aden trend) faults have been dominant from Middle Pleistocene to Recent, and have a strong presence westward to Central Afar. Figure 5 shows how the structural dominance of this system in the Gulf gradually diminishes westward, and is taken over by that of the NW-SE trend. Volcanotectonic activity along the E-W structural axis of Central-Eastern Afar is controlled by these same NW-SE fault sets. (3) As Gulf of Aden “tearing” propagated westward into the Gulf of Tadjura, new E-W
M. Boucarut had a prominent part in data collection, magma research, mapping achievement and discussion of the model. J.Z. De Boer gave appreciated suggestions. The author is greatly indebted to P.A. Mohr for critical reading and improvement of the manuscript.
References AbdaRah, A., CourtiRot, V., Rasser, M., Le Dain, A.Y., Lepine, J.C., Robineau, B., Kue88, J.C., Tappomuer, P. and Tarantola, A., 1979. Relevance of Afar seismicity and volcanism to the meohanics of accreting plate boundaries. Nature, 282: 17-23. Arthaud, F., Choukroune, P. and Robmeau, B., 1980a Teotouique, microtectonique et &ohttion struoturafe du golfe de Tadjoura et du Sud de la depression Afar (R&p. de Djibouti). Bull. Sot. G&l. Fr., 22: Wl-!Xt$.
366 Arthaud, F., Choukroune, P. and Robineau, B., 198Ob. Evolution structurale de la zone transformante d’Arta (Rep. de Djibouti). Bull. Sot. GeOl. Fr., 22: 909-916. Baker, B.H., Mohr, P.A. and Williams, L.A.J., 1972. Geology of the Eastern Rift System of Africa. Geol. Sot. Am. Spec. Pap., 136: l-67. Backer, H., Clin, M. and Lange, K., 1973. Tectonics in the Gulf of Tadjura. Mar. Geol., 15: 309-327. Bannert, D., Brinckmann, J., Kading, K.Ch., Knetszch, G., Kursten, M. and Mayhofer, H., 1970. Zur Geologie der Danakil Senke. Geol. Rundsch., 59: 409-443. Barberi, F. and Varet, J., 1977. Volcanism in Afar: small-scale plate tectonics implications. Geol. Sot. Am. Bull., 88: 1251-1266. Barberi, F., Borsi, S., Ferrara, G., Marinelli, G., Santacroce, R., Tazieff, H. and Varet, J., 1972. Evolution of the Danakil Depression (Afar, Ethiopia) in light of radiometric age determination. J. Geol., 80 (6): 720-729. Barberi, F., Bonatti, E., Marinehi, G. and Varet, J., 1974. Transverse tectonics during the split of a continent: data from the Afar rift. Tectonophysics: 23, 17-29. Barrere, P., Boucarut, M., Clin, M., Moussie, C., Muller, J., Pouchan, P., Thibault, C. and Weidmann, M., 1975. Carte geologique au 1: 100.000 du Territoire Fran+s des Afars et des Issas, feuille Ah Sabih, 1975. Universitd de Bordeaux 3-CEGD Djibouti, pp. l-8. Beauchamp, J., 1977. La s&ie sedimentaire en Ethiopie centrale et orientale. MCm. These SC., Univ. Claude Bernard, Lyon, 417 pp. Beyth, M., 1972. Paleozoic-Mesozoic sedimentary basin of Mekele Outlier, northern Ethiopia. Am. Assoc. Pet. Geol. Bull., 56: 2426-2439. Black, R., Morton, W. and Hailu, T., 1974. Early structures around the Afar triple junction. Nature, 248: 496-497. Boucarut, M. and Clin, M., 1976. Conceptions nouvelles sur les relations Mer Rouge-Golfe d’Aden a partir du Territoire Franqais des Afars et des Issas. C.R. Acad. Sci., Paris, D, 282: 1145-1148. Boucarut, M. and Clin, M., 1980. Structural history of the Republic of Djibouti. A summary. Geodynamic Evolution of the Afro-Arabian Rift System Int. Meeting. Atti Convegni Lincei, Roma, 47: 515-526. Boucarut, M. and Clin, M., 1986. Is the Gulf of Tadjura a platelet boundary or complex rift zone? Nature, London, 324: 394. Boucarut, M. and Seyler, M., 1980. Volcanic formations of the Republic of Djibouti: geochemical and petrological data. Geodynamic Evolution of the Afro-Arabian Rift System Int. Meeting. Atti Convegni Lincei, Roma, 47: 527-540. Boucarut, M., Burri, M., Clin, M., Debon, F., Roger, P., Thibault, C., Weidmann, M., Hauquin, J.P., Langguth, R. and Pouchan, P., 1974. Carte g6ologique au 1: 100.000 du Territoire Fran9ais des Afars et des Issas, feuihe Asal. Universite de Bordeaux 3-CEGD Djibouti, pp. l-12. Boucamt, M., Clin, M, and Dam, R., 1977. Les relations entre le rift de Tadjoura et la structure d’Arta (Repttblique de
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Djibouti). Refraction d’une frontitre de plaques dam une crogte continentale. CR. Acad. Sci., Paris, D, 285: 491-494. Boucarut, M.. Chessex, R., Clin, M., Dars, R., Pouchan, P.. Seyler, M. and Thibault, C., 1978a. Carte geologique au 1 : 100.000 de la Republique de Djibouti, feuille Khor Angar. Universite de Bordeaux 3-ISERST Djibouti, pp. l-14. Boucarut, M., Clin, M. and Thibault, C., 1978b. Prolongement du rift d’Aden en Republique de Djibouti. Structures superposees dans l’ttude des points tectoniques triples. CR. Acad. Sci., Paris, D, 287: 679-682. Boucarut, M., Clin, M. and Thibault, C., 1980a. Etude stratigraphique de la Republique de Djibouti. Universite de Bordeaux 3, U.E.R. de Gtologie, pp. l-49. Boucarut, M., Chessex, R., Clin, M. and Delaloye, M., 1980b. Donnees gdochronologiques K-Ar de roches volcaniques de l’Afar, partie Nord de la Republique de Djibouti. Schweiz. Mineral. Petrogr. Mitt., 60: 263-269. Boucarut, M., Clin, M., Pouchan, P. and Thibault, C., 1985. Impact des Bvtnements tectono-volcaniques pliopleistocenes sur la sedimentation en RBpublique de Djibouti (Afar Central). Geol. Rundsch., 71: 123-137. Chessex, R., Delaloye, M., Muller, J. and Weidmann, M., 1975. Evolution of the volcanic region of Ali Sabieh (T.F.A.I.), in the light of K-Ar age determinations. Afar Depression of Ethiopia, IUCG Rept., 14 (1): 221-227. Chessex, R., Delaloye, M. and Fontignie, D., 1980. K-Ar datations on volcanic rocks of the Republic of Djibouti. Geodynamic Evolution of the Afro-Arabian Rift System Int. Meeting. Atti Convegni Lincei, Roma, 47: 505-514. Choukroune, P., Auvray, B., Francheteau, J., Lepine, J.C., Arthaud, F., Brun, J.P., Auzende, J.M., Sichler, B. and Khobar, Y., 1986. Tectonics of the westernmost Gulf of Tadjura from submersible observations. Nature, 319: 396399. Choukroune, P., Francheteau, J., Auvray, B., Auzende, J.M., Brun, J.P., Sichler, B., Arthaud, F. and Lepine, J.C., 1988. Tectonics of an Incipient Oceanic Rift. Mar. Geophys. Res., 9: 147-163. Clin, M. and Pelissier-Hermitte, G., 1981. Le cadre structural de la campagne Orgon IV. In: Geochimie organique des sediments marins profonds. C.E.P.M.C.N.E.X.O., Ed. du C.N.R.S., Paris, pp. 183-188. CNRS-CNR Afar Team, 1973. Geology of northern Afar (Ethiopia). Rev. Geogr. Phys. Geol. Dyn., (2), 15: 443-490. Courtillot, V., 1980. Opening of the Gulf of Aden and Afar by progressive tearing. Phys. Earth Planet. Inter., 21: 343-350. Courtillot, V., 1982. Propagating rifts and continental break-up. Tectonics, 1: 239-250. Courtillot, V., Galdeano, A. and Le Mouel, J.L., 1980. Propagation of an accreting plate boundary: a discussion of new aeromagnetic data in the Gulf of Tadjurah and Southern Afar. Earth Planet. Sci. Lett., 47: 144-160. Dainelll, G., 1943. Geologia dell’ Africa Orientale. R. Act d’Italia, Centro Studi per l’Africa Orientale Indiana, Roma. Egloff, F., Rihm, R., Makris, J., Izzeldin, Y.A., Bobsien, M., Meier, K., Junge, P., Noman, T. and Warsi, W., 1991.
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AND
THE GULF
OF TADJURA
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Le Pichon, X. and Fran&e&au, J., 1978. A plate tectonic analysis of the Red Sea-Gulf of Aden area. Tectonophysics, 46: 369-406. Lepine, J.C. and Ruegg, J.C., 1976. La sismicit6 du T.F.A.I. BuB. Sot. Geol. Fr., 18: 841-846. Lepine, J.C., Richard, O., Ruegg, J.C., TreuiI, M. and Varet, J., 1976. Mise en evidence dune zone de faiBe tr~fo~~te reliant la vaB&eaxiaie de la dorsaIe de Tadjourah a ceIIe de Ghoubbet-AsaI (T.F.A.I.). C.R. Acad. Sci., Paris, D, 283: 9-12. Lombard, Ch., 1978. Etude g6ologique et volcanologique de la feuiIIe au lOO.OC@ede Dorra (R6pubIique de Djibouti). M&m. th. 3t cycle Univ. de Bordeaux 3,148 pp. Lowell, J.D. and Genik, G.J., 1972. Sea-floor spreading and structural evolution of southern Red Sea. Am. Assoc. Pet. Geol. Bull., 56: 247-259. Makris, J. and Ginzburg, A., 1987. The Afar Depressiontransition between continental rifting and sea floor spreading. T~tonoph~~, 141: 199-214. Makris, J. and R&m, R., 1991. Shear-controlled evolution of the Red Sea: pull apart model. In: 3. Makris, P. Mohr and R. Ribm (Editors), Red Sea: Birth and Early History of a New oceanic Basin. Tectonophysics, 198: 441-466. Marine& G. and Varet, J., 1973. Structure et 6volution du Sud du “horst Dam&I” (TFAI et Ethiopie}. C.R. Acad. Sci., Paris, D, 276: 1119-1122. Merla, G., Abbate, E., AzzaroIi, A., Bruni, P., Canuti, P., Fazzuoh, M., Sagri, M. and Tacconi, P., 1979. A Geological Map of Ethiopia and Somalia (1973), 1: 2,000,000. Cons. Naz. delle Rieerche Italy, Centro Stampa, Fiienre, pp. l-95, 11 pl. Meyer, W., Piiger, A., R&&r, A. and Stets, J., 1975. Tectonic evolution of the northern part of the Main Ethiopian Rift in southern Ethiopia. Afar Depression of Ethiopia, IUCG Rept., 14 (1): 352-362. Mohr, P.A., 1962. The Geology of Ethiopia. University College Addis Ababa Press, reprinted 1971,268 pp. Mohr, P.A., 1967. The Ethiopian rift system. BuB. Geophys. Obs. Addis Ababa, 12: l-65. Mohr, P.A., 1971a. Ethiopian Rift and plateaus: some volcanic petrochemical differences. J. Geophys. Res., 76: 1967-1984. Mohr, P.A., 1971b. Tectonics of the Dobi graben Regi&, Central Afar, Ethiopia. Bull. Geophys. Obs. Addis Ababa, 13: 73-89. Mohr, P.A., 1974. ENE trending Iineaments of the African rift system. Proc. First Int. Conf. New Basement Tectonics. Utah. Geol. Ass. Publ., 5: 327-336. Mohr, P.A., 1978. Afar. Arm. Rev. Earth Planet. sci,, 6: 145-172. Pouchan, P., Duprat, A. and Vanier, J., 1965. Univ. of Bordeaux 3, Inst. Geodyn. Intern. Report. Richard, 0. and Varet, J., 1980. Study of the transition from deep oceanic to emerged rift zone: Gulf of Tadjura (Republic of Djibuti). Geodynamic Evolution of the Afro-Arabian Rift System Int. Meeting Atti Convegni Lincei, Roma, 47: 569-582.
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M. <‘I Ih
Tapponnier, P. and Varet. J., 1973. La zone de Mak’arrasou en Afar: un equivalent emerge des “failles transformantes” octaniques. CR. Acad. Sci., Paris, D, 278: 209-212. Tszieff, H., Varet, J., Barberi, F. and Giglia, G., 1972. Tectonic significance of the Afar (or Danakil) Depression. Nature, 235: 144147. Thibault, C., 1980. Dating of Quatemary tectonic movements in the Republic of Djibouti. Geodynamic Evolution of the Afro-Arabian Rift System Int. Meeting. Atti Convegni Lincei, Roma, 47: 5.55-568. Varet, J. and Barberi, F., 1976. Trace des front&es de plaques en Afar: discussion a partir de donnees volcanologiques. Bull. Sot. Geol. Fr., 7 (18): 831-846. Varet, J. and Gasse, F., 1978. Geological map of the Central and Southern Afar (Ethiopia and Republic of Djibouti). Ed. CNRS, Paris, pp. l-125. Vellutini, P., 1990. The Manda-Inakir Rift, Republic of Djibouti: a comparison with the Asal Rift and its geodynamic interpretation. Tectonophysics, 172: 141-153. Zanettin, B., Justin-Visentin, E. and Piccirillo, E.M., 1980. Migration of the Ethiopian continental rifts in the course of the Tertiary evolution of the Afro-Arabian Rift System. Geodynamic Evolution of the Afro-Arabian Rift System Int. Mtg., Atti Convegni Lincei, Roma, 47: 253-265.
Tectonophysics, 198 (1991) 355-368
Elsevier Science Publishers B.V., Amsterdam
Evolution of Eastern Afar and the Gulf of Tadjura M. Clin Institut de GPodynamique, avenue des FacultPs, F 334OSTalence
Cedex, France
(Received July 25,1989; revised version accepted April 23,199O)
ABSTRACT Clin, M., 1991. Evolution of Eastern Afar and the Gulf of Tadjura. In: J. Makris, P. Mohr and R. Rihm (Editors), Red Sea: Birth and Early History of a New Ckeanic Basin. Tectonophysics, 198: 355-368. The structure of the Gulf of Tadjura has previously been interpreted in conformity with a model derived chiefly from seismic and magnetic data. This model comprises an en-echelon axial valley of ENE-WSW, Gulf of Aden trend, offset by NE-SW transform faults. Such a structure implies a transition between the Gulf of Aden oceanic ridge and an emerged rift system in Central Afar. This model is now questioned. It is suggested, based on tectono-volcanic features which have been mapped and dated onshore, and which extend offshore into the Gulf of Tadjoura, that continental crust underlies the Gulf as well as the surrounding area of Eastern Afar. Bach of the main events in the geodynamic evolution of this area has created a set of brittle structures with a specific trend. NNW-SSE (Red Sea) and associated NE-SW trends have a Miocene age. Some of the NE-SW trending fractures were reactivated by shearing during Lower Pleistocene time, creating a NW-SE horst-and-graben block system. An E-ENE (Aden rift) trend controls the Middle Pleistocene to Recent geodynamics, both on and offshore in the Gulf of Tadjura. This new analysis of the structural evolution of Eastern Afar provides further evidence for a 3-stage evolutionary model of the region, with the Pleistocene events now more clearly defined.
Introduction
In Eastern Afar and the Gulf of Tadjura, most of the tectono-volcanic structures have resulted from activity at the Afar triple junction. Their trends are dominated by those of the Red Sea, Aden Gulf and Ethiopian rifts. A detailed chronological study of these structures builds up a Cenozoic geodynamic evolution of the region. Based on field data from the Republic of Djibouti and on a new structural map of the Gulf, we propose the major events that have contributed to the tectonics of this region. Tectonic activity peaked during the Miocene and Pleistocene, a chronology in agreement with the widely accepted two-stage history of Afar (Mohr, 1971a; Lowell and Genik, 1972; Barberi et al., 1972; Girdler and Styles, 1974, 1978; Juch, 1975; Mohr, 1978; Merla.et al., 1979), which assumes a first pulse during late Oligocene-Miocene time, and a later, possibly stronger episode during
Pliocene the later parts, so region is
to Recent times. Our data indicate that episode itself can be separated into two that a 3-stage evolutionary model of the obtained.
General geology
According to the data summarized by Beyth (1989), based on Beyth (1972), Black et al. (1974) Kazmin (1975) Beauchamp (1977), Merla et al. (1979) and other authors, most of the present tectonic trends are likely to have originated in pre-existing major structural elements of the Precambrian basement and pre-Tertiary Phanerozoic cover. In Djibouti, the crustal structure is similar to the rest of the Afar area, with a 6.0-6.1 km/s upper crust and a 6.7-6.8 km/s lower crust (Ruegg, 1975; Makris and Ginzburg, 1987; Egloff et al., 1991). Crustal thickness is reduced to less than 14 km immediately west of the Gulf of Aden.
0040-1951/91/%03.50 6 1991 - Blsevier Science Publishers B.V. All rights reserved
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-
BARSA
Fig. 1. The Mesowic and Cenozoic volcanic and sedimentary sequences of the Republic of Djibouti, according to the legend of 1 : 100,000gevlogical maps by Boucarut et al. (1974). Barrere et al. (1975) and Boucarut et al. (1978a).
EVOLUTION
OF E,ASlERN
AFAR
AND
THE
GULF
357
OF TAQJURA
Precambrian rocks outcrop near Aisha, 40 km south of Djibouti (Dainelli, 1943; Mohr, 1962). In Djibouti itself, the exposed stratigraphic SWXSsion commences with Mesozoic sediments, unconformably overlain by Cenozoic volcanic and sedimentary sequences (Figs. 1 and 2). Volcanism started relatively abruptly with basaltic outpourings and the spreading of massive ignimbrite sheets during late Oligocene-early Miocene and Middle Miocene times. Differentiated lavas and plateau basalts flows subsequently predominated during Upper Miocene to Holocene times. The palaeogeographic distribution of the late Pliocene-Holocene “rift series” flows was controlled by successively developed morphotectonic features (Boucarut and Clin, 1980; Boucarut et al., 1980a) Inter-eruptive erosion produced unconformable contacts between the Lower and Middle Miocene, between the Middle and Upper Miocene (with rifting), within the Upper Miocene and, after regional tilting, within the Pleistocene sequences (Figs. 1 and 3). The main tectonic events are linked to the activities of the rift zones converging on the Afar triple junction, as follows (Fig. 2): (1) NNW-SSE rifting (Red Sea trend) during the Lower and Middle Miocene. Opening was accompanied by a sinistral shear along NE-trending faults. Some of these faults subsequently exerted an element of control on the structure of the Gulf of Tadjura. During the Upper Miocene and Pliocene, further regional extension permitted eruption of the plateau series which covered all of Central and Eastern Afar (Bannert et al., 1970; Mohr, 1971a). (2) Regional sinistral shearing along NE faults (Ethiopian rift trend) during the Lower Pleistocene, which was accompanied by transverse NWSE faulting. A NW-SE horst-and-graben and tilted block system was then formed and the central region of Djibouti was uplifted. (3) Westward unzipping of the Gulf of Tadjura (Aden Gulf trend) during Middle Pleistocene-Holocene time. This opening reactivated the earlier NW- and NE-trending fault systems. These data provide a basis for revision of the Afar tectonic history given by CNRS-CNR Afar Team (1973) and Varet and Gasse (1978). Inter-
pretation of the Tadjura structure as an E-W en-echelon axial valley offset by NE-SW transforms (Lepine and Ruegg, 1976; Lepine et al., 1976; Abdallah et al., 1979; Richard and Varet, 1980; Arthaud et al., 1980a) requires modifying in the light of the forms of the axial deeps, the presence of NW-SE faulting, and the existence of compressive structures. More recent geomagnetic studies (Courtillot et al., 1980) and the 1984 Cyana dives (Choukroune et al., 1986, 1988) provide further reasons to question the adequacy of the previous model. ~wer-~~e
h%mne cm&d anatexis
Rocks of Miocene age are chiefly exposed in the eastern part of the Republic of Djibouti, on the Mabla range (Tadjura High) and Aisha horst (Ali Sabih area). These bards to day are connected topographically with the DanakiI and Somali platelets, respectively (Laughton, 1966; Mohr, 1967; Roberts, 1970). The major magmatic event of this time was eruption of voluminous rhyolitic ignimbrites, as massive sheets more than 200 m thick. Both mafic and silicic dike swarms were emplaced. Some dikes in the Aisha horst carry xenoliths of Jurassic marble and Precambrian granite. Two periods (Fig. 1) of paroxysmal rhyolitic activity have been identified: late Oligocene-early Miocene and Middle Miocene. Previous syntheses (e.g. Varet and Gasse, 1978) do not distinguish the first episode, but interpret the Oligo-Miocene basaltic rhyolitic series of Djibouti as part of a wider, regional, single ma~atic cycle. Based on chronological data, Gadalia and Varet (1983) relate the entire Miocene volcanic sequence of eastem Djibouti with the Affara-Dara and other Afar alkaline-peralkaline plutons. Such a simultaneity would more accurately comect these granitoids (26 to 22 m.y.) solely with the Lower Miocene volcanics in Djibouti (subalkaline basalts, 26.7 to 22.6 may.; rhyolitic ignimbrites, 23.8 to 19.1 m.y.) and thus excluding the Middle Miocene volcanics (16 to 13 m.y). These Middle Miocene rocks are preserved at several localities, both north and south of the Gulf (Chessex et al., 1975, 1980; Boucarut et al., 198Ob), resting unconformably on the Lower
358
EVOLUTION
OF EASTERN
AFAR
SW
AND
THE
GULF
359
OF TADJURA
NE
0 B
Fig. 3. Non-parallel diagrammatic cross sections illustrating the main tectonic events of the Eastern Afar (Republic of Djibouti). A. Lower to Middle Miocene unconformity (RRdB,R,), Middle Miocene fracturing and rifting, subsequent Upper Miocene and Pliocene plateau flows (Mi, B,, ha). B. Lower Pleistocene fracturing, tilting and bl~k-up~ft according to a NW-SE trend, unconformable subsequent lavas (&b, 2.1 to 0.8 my.} and sediments. C. Middle Pleistocene to Present E-W deformation and basaltic flows (&, Bad and Asal-Ardoukoba volcanics, h).
Fig. 2. Geological map of the Republic of Djibouti, 1: 900,000. The nomenclature of the formations is the same as in Fig 1. 23 = boundary of the Republic of Djibouti; 22 = main cliffs, aerial and undersea, usually associated with faults; 2I= fractural network: the fracture pattern is the result of reactivation of some previous fractures; 20 = fractural network: the fracture pattern is the result of successive additions; 19 = lava flows younger than 9000 years, Asal area; 18 = Upper-Pleistocene to Present scatter-cones and other volcanic vents; 17 = Upper-Pleistocene to Holocene aa or pahoehoe iava flows; 16 = hyaloclastic seamounts: Ghubbat-al-Kbarab (the western one is emerged) and Bab-el-Mandeb (emerged crescents); 15 = late to Middle Pleistocene aligned aerial scatter- and lava-cones and mounts; 14 = Tadjoura deep (800 m), Obock deep (1100 m); 13 = alluvial deposits and other sediments, coralhan buildings, Lower Pleistocene to Present; I2 = Middle Pleistocene We’a-Rarou (B2c) valley flows; Ii = volcanic vents made of B2b basalts, aligned on some feeding fractures; 10 = Lower Pleistocene Gummouna (B2bR2b) formation: mainly flood basalts; 9 = Ado Ale rhyolitic central volcano; 8 = Lower Pleistocene Lougag Ale&u&al (B2x) formation: low alkalinity flood basalts; 7 = Balbo-Gamarri (B2a) formation overlain by Lougag Ale-Sankal (B2x) formation: basalts capping residual hills; 6 = Pliocene Balbo-Gamarri (B2aR2a) formation: mainly flood basalts; 5 = Upper Miocene to Pliocene Damayyi-Galemi (Bl) formation: transitional plateau basalts without ~ffer~tiat~ terms; 4 = (Mi) iavas filling the axial trough of a Lower-Miocene rift structure (Ado&Falka rift structure}; 3 = Upper Miocene Maryan Ad-Harm (Mi) formation: plateau lavas, mainly trachybasalts, trachytes, scarce rhyolites and ignimbrites; 2 = Lower Miocene basic dykes and lodes (x), 19.6 m.y., Ali Sabih area; I = Jurassic and Cretaceous sediments (Ali Sabih area), Galile basalts and sills (B,,), Upper Oligocene to Middle Miocene Golwa-Chirrnile (B,R,) and D~~-~y~ D&da (l&R,) formations: mainly K-rhyolites of cm&al origin. A = Ardoukoba; Ab - I,& Abhe; Af= lakes Afambo and Gamarri; As-L.&e Asat, AS = Ali Sabih, D = Dacca volcanic range; l3e = depression of Der Ela; G = Guissi volcanic range; Gu - Garbi Mountain; Gg m Gaggade depression; Gm = Gamarri cliff; GK = Ghub~t-~-~~a~ H = Hanle depmasion; Ib - Ibira rauge; Mk = Mak’arrasou area; Mu - Muaa Alli vokxtno; MR = Mabla Range; Ok = O&k town; S = Sawabi archipelago; Td = Tadjura town; Y = Yager Mountain.
360
Irl
(‘I II\.
The bimodal proportions become reversed in the end Miocene-Pliocene volcanics, where basaltic lavas predominate over felsic rocks in a volume ratio of 100 : 1. In this sequence, two thin ignimbrites have similar geochemical characteristics to the older ignimbrites. However, the bulk of the younger rhyolites comprise lava domes; they lack quartz phenocrysts, SiO, is lower than 72%, Al,O, averages 12.5%, and Na,O (av. 4.4%) exceeds K,O (av. 3.9%). Boucarut and Seyler (1980) have compared Kuno solidification index (S.I.) plots for the Lower-Middle Miocene rhyolitic rocks with those for Upper Miocene and younger rocks in Djibouti. Based on major element chemistry and S.I. trends, and considering the enormous preponderance of silicic over the mafic rocks, the absence of differentiated rocks and the abundance of ignimbrites in the older suite, Boucarut and Seyler conclude
Miocene volcanics (Barrere et al., 1975; Boucarut et al., 1978a). The Lower and Middle Miocene rhyolites have a similar major element geochemistry (Boucarut and Seyler, 1980). Both suites are persilicic and alkaline. Quartz rhyolite lavas and ignimbrites in the Lower Miocene suite predominate over basalts (200 km3, and 2 to 3 km3, respectively). The rhyolites have high SiO, (> 72%) low Al,O, ( < 10.6%) and K,O (av. 4.4%) greater than Na,O (av. 3.2%). In the Middle Miocene suite, the volume ratio rhyolite:basalt is similarly 150 km3:few km3. The ignimbritic facies of the silicic volcanics again have high SiO, (70 to 77%) and Al,O, and Na,O values similar to those of the Lower Miocene silicics. On the alkali-silica diagram, basalts and rhyolites are located along common trends (Figs. 2a, 2b), marked by the absence of intermediate composition rocks.
IO
IO -=3
sz.3
:-
I (c)
1,’
1
50
70
60
50
(4
I
I 60
1
I 70
_UI.. (W
5102
L
SlO2
1
I (d)
50
’
I 50
70
60
I
1 60
1
I 70
SlO2
1
I
SiO7
Fig. 4. Alkali-silica diagrams for the volcanic formations of the Republic of Djibouti, according to Boucarut and Seyler (198O),based on 210 analyses. The trends concentrate along the boundary between the subalkaline and calcalkaline series (u = 3, Rittman, 1958) from subalkaline transitional basalts via alkaline rhyolites to commditcsand relatively rare pantellerites. Four magmatic sequences have been distinguished: (a) Late Oligocene-Early Miocene; (b) Middle Miocene to Pliocene; (c) Lower Pleistocene; (d) Middle Pleistocene to Recent.
EVOLUTION
OF EASTERN
AFAR
AND
THE
GULF
OF TADJURA
that the Lower-Middle Miocene rhyolitic ignimbrites and lavas of Djibouti are the result of deep crustal melting, and not of fractional crystallization (cf. Varet and Gasse Team, 1978). This endorses the far-sighted but then unfashionable argument of Holmes (1931). The southern Danakil block is taken as a zone of Miocene crustal attenuation (Marinelli and Varet, 1973). Based on 1 :‘100,000 mapping (Barrere et al., 1975; Boucarut et al., 1978a), the Lower to Middle Miocene magmatic activity can be related to coeval distensive tectonism of NNW (Red Sea) trend. Compare for example the NNW Adori-Falka rift structure, north of the Gulf (Figs. 2 and 3A), and the NE trending doleritic fissures of the Ali Sabih area dated at 19 m-y. (see x, Figs. 1, 2 and 4a). This tectono-volcanic combination of NNW and NE faults and fissures suggests a “pull apart” model for the Lower Miocene organization in eastern Afar, to be compared to that of Makris and R&m (1991) for the northwestern Red Sea. In addition to the NNW- and NE-trending structures, a N-S fault set is developed on both sides of the Gulf. That to the north may express an internal deformation of the southern Danakil block into an intensely fractured, bow-shaped structural zone (Boucarut and Clin, 1980). This deformation could in turn be related to an eastward propagation of early Cenozoic rifting in the northern Ethiopian plateau (Zanettin et al., 1980). During Upper Miocene-Pliocene (13 to 3.5 m.y.), extensive flood basalt flows covered all of the Republic of Djibouti, excepting the Ali Sabih block. The lower part (13 to 8 m.y.) of this pile contains mainly trachybasalts, trachytes, and sparse rhyolites and ignimbrites (Fig. 4b). The upper part (7.9 to 4.5 m.y.) is composed of transitional flood basalts without differentiated terms. The last flows (3.5 m.y.) are again differentiated (Fig. 4b). Lower Pleistocene uplift associated with regional shearing
The Pliocene basaltic outpouring was followed by intense fracturing of the Afar floor, with development of a complex horst and graben system, and zones of tilted blocks. In Djibouti, the system
361
trends either NW or NNW (Figs. 2 and 3B). Stratigraphic and geochronologic data indicate that NW faulting occurred during the early Pleistocene. Some NW-SE depressions were progressively individualized during this time, as proved by the increasingly dispersed distribution of the sediments which filled them (Roger et al., 1974, 1975; Thibault, 1980). Most of the NNW faults are located to the north of Djibouti (Mak’arrasou area) and appear as reactivated Miocene Red Sea trending fractures. North to ll”N, the faulted and tilted area in Djibouti is more or less exactly located along a continuation of the Ethiopian rift. The margins of the Ethiopian rift and the Wonji fault belt were impressed at this time during the “Nazret phase” of faulting (Baker et al., 1972) dated at about 1.6 to 1.8 m.y. (Meyer et al., 1975). In central Djibouti, an angular unconformity clearly separates, as a result of the Nazret phase, the faulted and tilted Mio-Pliocene lavas and sediments from overlying Lower Pleistocene lavas and sediments 2.1 to 0.8 m.y. old (Boucarut et al., 1974, 1985; Boucarut and Clin, 1980; Thibault, 1980). The Afar horst-graben system is crossed by NE-SW trending lineaments. In Djibouti, a prominent NE-SW fault extends from the Obock deep in the eastern Gulf of Tadjoura, onshore to about ll”30 (Fig. 5), and then relays with a reactivated Miocene NE-SW fault in Ali Sabih. Regionally, the NW-SE fractures terminate against these faults, not abruptly but with a curved S-shape adjustment suggesting a sinistral strike-slip component (Fig. 2). Another NE-SW lineament may cross Musa Alli volcano and the Dobi Graben, again in association with S-shaped fractures. Associated with this graben, northwest of Djibouti, a sinistral transtensional zone has been mapped (Mohr, 1971b, 1974), superimposed by some NE lineaments. NE-SW trending lines offset E-W regional magnetic anomalies in the Gulf of Aden (Hall, 1970; Laughton et al., 1970) as well as in Eastern Afar (Le Pichon and Francheteau, 1978). In Eastern Afar, Bouguer gravity anomalies reveal the same NE-SW trend (Makris and Ginzburg, 1987) crosscutting the NW-SE axial-range trend. The palaeogeographic distribution of Lower Pleistocene lava flows indicates that the Afar
362
M. C‘l,llu
horst-graben system was broadly uplifted at this time along a NE-SW ridge (Boucarut and Clin, 1980; Boucarut et al., 1985). The Lower Pleistocene volcanic series onlapped this ridge only at its edges, where an unconformity can be observed (Fig. 3B). Based on these chronological and tectonic data, the NNE-trending “Mak’arrasou transform fault” (Tapponnier and Varet, 1973), important in the formulation of the Afar small-scale plate tectonic model (Varet and Barberi, 1976; Barberi and Varet, 1977), does not exist as a Recent structure (Boucarut and Clin, 1976). Tapponnier and Varet propose that the transform fault is masked by younger volcanic formations, and underlies a conjugate set of NW-NNW faults. The hidden fault is thus discordant with surface structures. The NW-trending Pleistocene and the reactivated NNW Miocene fractures of Central Afar cannot be considered as conjugate owing to disparate ages. Moreover, as these fractures cut only Plio-
Fig. 5. Tectonic map of the Gulf of Tadjura and surrounding
cene and older lavas, which were tilted during the Nazret phase, the proposed transform fault must be related to this aider phase of deformation, and not to the more recent evolution of the Afar such as, for example, that which would link Asal to Manda-Inakir. Similar doubts concern a proposed transform fault from Manda Harraro to Manda Inakir (Varet and Barberi, 1976). Nevertheless, the tectonic activity of visible or hidden NE-trending lineaments is useful to explain structures formed during the Nazret phase. In Djibouti, the general pattern of the NE trending belt of faulted and tilted blocks can be explained by a model of cmstal shearing along NE strike-slip faults, indirectly projected from Ethiopian rift structures, and producing secondarily strike-slip and tilting movements along the NW structures (Boucarut and Clin, 1976, 1980; Boucarut et al., 1977, 1978b; Lombard, 1978). In that model, the “‘Mak’arrasou strike-slip zone” (Vellutini, 1990) appears as a combination with
areas. Field data after 1 : 100,000 geological
1974, Barrere et al., 1975; Boucarut et al., 1978a) and other unpublished. 1977, with complements)
and tire Charcot cruise (Clin and Pelissier-Hermitte,
1977) near the southern coastline. 3 = Ardoukoba NW-SE
volcanic
range; 4 = volcanic
graben near Djibouti;
10 = landslides
I = Submarine volcanoes, 6 = faults (on land);
on the present shore; T = Tadjura;
1981), and from the Tudjoura cutter cruises (1973,
Tadjura and Obock deeps (T and 0);
vents, associated
maps (Boucarut et al.,
Submarine data from the Thalussa cruise (Boucarut et al.,
to BZb formation,
2 = E-W Guissi volcanic range;
early Pleistocene:
5 = Br,b lava flows, filling a
7= faults (submarine); 8= fault scarps; 9= continental shelf boundary; DZ = Cape DabaDj = Djibouti; As = Lake Asai; Gk = Ghubbat-al-Kharab; Libah; Rd = Ras Duan.
EVOLUTION
OF EASTERN
AFAR
AND
THE
GULF
OF TADJURA
reactivated previous faults and merely represents the border of the Danakil platelet. Quaternary
continental tearing of the Gulf of
Tadjura After Lower Pleistocene time, the entire tectonic pattern of Djibouti changed, as E-W arching and faulting began to predominate in Central Afar. A new set of E-W trending fractures developed during the Middle Pleistocene, both north and south of the Gulf in the relatively stable areas of Tadjura and Ali Sabih, as well as in some other parts of Djibouti including the Gulf itself. E-W basaltic valley flows were emplaced. The E-W fractures were intruded by dikes which were feeders for Upper Pleistocene and Holocene basalt flows. Some of them are marked at the surface by aligned spatter cones. Also formed were hyaloclastites, in the Ghubbat-al-Kharab and Bab-el-Mandeb areas. West of the Gulf of Tadjura, an E-W morphotectonic arching is observed. The Tertiary lavas and sediments have been uplifted to an elevation of 1600-1700 m only a few kilometres from the Gulf. The relief of the Garbi and Yager mountains and the Gamarri cliff are due to this arching. Moreover, 75 km west of the Gulf of Tadjura, near the Awash valley, this E-W arching has produced a cambered structural surface which can be directly observed as the top of the NNW-SSE scarp dominating the Afambo and Gamarri lakes (1: ~~,~ French I.G.N. topographic map, sheets Abhe Bad and Gamarri). From Upper Pleistocene to Holocene time, pre-existing NW-SE faults were reactivated, thus facilitating NW-trending volcanic feeders such as Asal-Ardo~oba (Fig. 3C). However, the active deeper structural elements are E-W trending. This trend is expressed in the geomagnetic data (Girdler, 1970; Roeser, 1975; Courtillot, 1980). Prominent E-W volcanic alignments include (Figs. 2 and 5): at ll’lO’N, Dacca volcanic range; at 11*20/N, Guissi volcanic range; at 11’35’N, Asal-Ardoukoba and Der Ela-Gaggade basaltic areas; at 11”45’N, E-W axial faults in the Gulf of Tadjura; at 11’50’N, submarine volcanoes in the eastern Gulf; at 12”20’N, Sawabi volcanic archipelago, Ibira range, Manda-In~r system, and
363
Musa Alli volcano; possibly at 12”58’N (in Ethiopia), the Assab volcanic range. Manda and Assab belong to the “ transverse ranges” of Barberi et al. (1974). We note that the area between 11”OO’N and 12”20’N coincides with an anomalous E-W zone of Central Afar, identified on the basis of geomagnetic anomalies (Girdler and Styles, 1976) and of recent epicentres (Gouin, 1970; Fairhead and Girdler, 1970). Some arguments for the probable westward extension of this structure into the Ethiopian Plateau are given by Zanettin et al. (1980). The new E-W fault set in eastern Djibouti, together with the morphotectonic arching in the west, are geologic indicators of a progressive westward opening along the Gulf (Boucarut et al., 1978b). Geophysical data (Courtillot et al., 1980; Courtillot, 1980, 1982) support the concept of a westward “tearing” of the Gulf. The Gulf began to open 4.5 to 5 m.y. ago (Girdler et al., 1978; Styles and Hall, 1980) as a Pliocene re-activation of Afar geodynamics. Westward tearing along the Gulf has taken place from 2.7 m.y. to Present (Gasse and Fournier, 1983; Boucarut et al., 1985) These regional and local data may relate to the E-ENE system of major transverse discontinuities considered by Beyth (1989) to account for the southern Red Sea evolution. A tectonic map of the Gulf of Tadjura and surrounding areas West to 43”2O’E, the Gulf marks an important tectonic zone separating the Danakil and Somali platelets. The structure of the Gulf is not, as previously considered (Lepine et al., 1976; Richard and Varet, 1980; Arthaud et al., 1980a), the result of an en-echelon arrangement of ENE-WSW rifts and NE-SW transforms, and admitting an additional NNE transform (Athaud et al., 1980b), but is considerably more complex. Observations made during dives with the Cyana submersible (Chou~oune et al., 1986, 1988) confirmed the NW-SE (140”) zones of active faulting and recent volcanism and the NE-SW (60”) faults, partly mapped previously (Boucarut et al., 1977; Clin and Pelissier-Hermitte, 1981). The 140’ fractures interact with the 60’ fault set, usually inactive and
364
considered to be older. According to Choukroune et al. (1988),“there is no active faulting trending at high angle to the 130”-140” normal faults. This invalidates all previous kinematic modeis of the Gulf of Tadjura”. When developing tectonic models for the Gulf, it is imperative to consider data from the surrounding terrain. The Plio-Pleistocene deformation of the Gulf should not be seen as highly localized and restricted within this area only (Backer et al., 1973; Boucarut et al., 1977, 1980b, 1985). A similarly complex geodynamic history characterizes much of the onshore territory of the Republic of Djibouti. Figure 5 shows the general geologic structure of the Gulf, based on the Thalassa (Boucarut et al., 1977) and Charcot (Clin and Pelissier-Hermitte, 1981) cruise data. The Gulf ,contains a major NE-SW trending fault system between 43’20’E (Obock deep) and 43”OO’E (Tadjura deep), continued southwestward onto land. This fault system originated during Lower-Middle Miocene, without implication that the Gulf opened at this time. West of 43”OO’E, the NE-SW fault system is crosscut by a NW-SE faulted ridge (42”52’43“OO’E). This ridge traverses the Gulf and the faults outcrop onto the southern shore. Thus near Djibouti, the southeastern segment of the submarine fault system forms a graben 15 km long and about 4 km wide, filled by Lower Pleistocene basaltic flows (Hauquin, 1978). The presence of these lavas indicates a probable overprinting of the NW-SE faults on an earlier Gulf structure, 2.1 m.y ago or later. Many other NW trending fault zones have been mapped east of 43”OO’E (Clin and Pelissier-Hermitte, 1981; Choukroune et al., 1986, 1988). In this eastern part of the Gulf, the NW structures are only known as parts of the axial zone. However, they are likely of the same origin as the fractures controlling the western faulted ridge as well as the active Asal-Ardoukoba rift and other, parallel structures. In the western Gulf of Tadjura, the offset of the “axial zone” by NW-SE trending structures was described as early as 1969 by Roberts and Whitmarsh (1969) based on a magnetic survey interpretation. These authors postulated a submarine continuation of the onshore NW-trending
M
horst-and-graben terrain. They also discussed a possible extension onto land of the ENE-WSW active zone, supposing that “in this area, tensional separation is not sufficiently advanced to lead to the development of WSW trending fissures and associated volcanism”. Some E-W structures are important as limits of the axial deeps. Whereas the Tadjoura deep (Fig. 5) is bounded north by an E-W faulted scarp, the Obock deep is bounded south by the E-W Maskali scarp. The two scarps lie along a common strike with a switch of polarity. A set of NW-SE fractures on the southern rim just marks the junction (~houkroune et al., 1988). In the western Gulf, NW-SE faults do not extend continuously across the northern side of the Gulf, but are truncated by younger NE-ENE faults. Vertical fractures of the latter trend occur west of the Tadjura deep, and cut the shore-line up to the 100 m coastal platform. Between individual faults, as indicated from echograms, Recent sediments have been intensely folded and compressed, and this is considered related to dextral strike-slip motion along the E-W fault zone bordering the Tadjura deep. Due to this motion, ENE-WSW structures, located both southwest and southeast of Tadjura town, are probably inherited from pm-existing major structures of NE-SW trend in the Gulf, and determining also the coastlines between 43%) and 43”lO’E. The ENE-WSW Aden rift zone, which is well defined eastwards, cannot be identified west of 43”25’E. There, E-W trending fractures are only present in axial position, not ENE ones. Another example of the marine continuation of major onshore structures is provided by the topographic high at 11”38’N, 42”55’E, bounded by NW- and NE-trending faults. This relief probably comprises Miocene rhyolites, an i~te~reta~on (Fig. 6) supported from magnetic surveys by Pouchan et al. (196S), and by the proximity of Lower-Middle Miocene rhyolitic volcanics of the Sabih range, and most proximal in the SE part of the Ghubbat-~-~~ab (Fig. 2). It is of interest to compare the distribution of active structural elements in the Gulf of Tadjura (Fig. 5) with the seismicity map (Lepine and
EVOLUTION
OF EASTERN
AFAR
AND
THE
GULF
\
36 000 a -
TI
I
365
OF TADJURA
I
Fig. 6. A profile of the total magnetic field across the Gulf of Tadjura. After Pouchan et al. (1965). The anomaly A (J =10(J) was attributed by the authors either to a trot&r or to a mass of rocks of lower magnetic subtility such as some rhyohtic lavas. The bathymetry supports the second interpretation.
Ruegg, 1976, fig. 2). Earthquake epicentres plot precisely on the above mentioned NW, NE and E-W trending fault zones west of 43’20’E. The only presently inactive faulted structures appear to be the northern boundary of the Tadjura deep, and the set of compressive structures SW of Tadjura town. There is no ~~fi~t NNE active structure in the area west of 42*50/E, proposed by Lepine et al. (1976) and Arthaud et al. (1980b) as a transform-fault zone.
faults were formed, and older fault systems were reactivated. This explains the zig-zag fault pattern. (4) The continental nature of most of the area is supported by above-mentioned geologic, structural, petrologic and geochemical data, and appears inherited from its previous history. Continuation of the major onshore structures into the Gulf strongly suggests that the Gulf floor is composed mainly of pled-apt continental crustal blocks, separated by narrow rift zones injected with young volcanics. Recent geophysical data (Egloff et al., 1991) indicate that there is no true oceanic crust in the Gulf floor. (5) Combined deformations in the Gulf area suggest for the Central-Eastern Afar tectonics during the Quatemary a similarly composed model assuming the addition of simultaneous slight movements along the various fault sets, either strike-slip or distension according to the orientation. Thus a “non-rigid” model is obtained, using both reactivated and recent fractures and other elements. The “diffuse stretching” interpretations proposed for parts of the area (Boucarut and Clin, 1980; Courtillot et al., 1980; Choukroune et al., 1988; Vellutini, 1990) imply that sort of deformation.
Acknowledgements
(1) NNW-SSE (“Red Sea”) faults and orthogonal NE-SW ones have been active since at least early Miocene. Present seismicity in the Gulf of Tadjura suggests the r~ctivation of these structural elements today, together with the NWSE and E-W younger ones. (2) E-ENE (Aden trend) faults have been dominant from Middle Pleistocene to Recent, and have a strong presence westward to Central Afar. Figure 5 shows how the structural dominance of this system in the Gulf gradually diminishes westward, and is taken over by that of the NW-SE trend. Volcanotectonic activity along the E-W structural axis of Central-Eastern Afar is controlled by these same NW-SE fault sets. (3) As Gulf of Aden “tearing” propagated westward into the Gulf of Tadjura, new E-W
M. Boucarut had a prominent part in data collection, magma research, mapping achievement and discussion of the model. J.Z. De Boer gave appreciated suggestions. The author is greatly indebted to P.A. Mohr for critical reading and improvement of the manuscript.
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