Strike-slip faults in a rift area: a transect in the Afar Triangle, East Africa

TECTONOPHYSICS ELSEVIER

Tectonophysics 241 (1995) 67-97

Strike-slip faults in a rift area: a transect in the Afar Triangle, East Africa Ernesto Abbate a,* Pietro Passerini a Leonardo Zan b a Dipartimento Scienze della Terra, Universitfl di Firenze, Via La Pira 4, 50121 Firenze, Italy b Aquater, San Lorenzo in Campo, Pesaro, Italy

Received 6 July 1993; revised version accepted 29 June 1994

Abstract The Afar Triangle, a diffuse triple junction where the Red Sea, Ethiopian and Gulf of Aden rifts converge, is examined along an E - W cross section in order to recognize traces of strike-slip faulting summarily known from earlier studies. Both field evidences from slickensides and airphotograph or satellite image data indicate that strike-slip faults, although less numerous than normal ones, occur throughout this area. These faults mainly strike parallel or at small angles relative to rifting axes, rather than transversal to them as would be expected if they were transforms. Strike slip subparailel to rifts is explained through lateral displacement between the major lithospheric plates around the junction or, subordinately, by a domino fault mechanism in zones of diffuse transform deformation. Faults at small angles with the rift axes often constitute conjugate systems suggesting along-axis compression, which is considered to be frequently induced by the lateral intraplate shift mentioned above. In other cases, this compression may develop in intervals between mantle plumes wedging up along a rift, or at the head of a propagating rift. The main lateral displacements among the boundary lithospheric plates during the last million of years are supposed to have been sinistral. This does not challenge the notion of mainly divergent plate movements, but adds to this divergence an anticlockwise shift of the plates around the junction.

1. Statement of the topic T h e A f a r T r i a n g l e is an a r e a o f active extensional t e c t o n i c s a n d b a s a l t i c m a g m a t i s m f r o m which t h e G u l f o f A d e n , t h e R e d S e a a n d t h e E t h i o p i a n rift systems r a d i a t e . N o r m a l faults a n d o p e n fissures a r e t h e p r i n c i p a l e l e m e n t s o f t h e A f a r tectonics. H o w e v e r , strike-slip faults a r e also

* Corresponding author.

r e c o g n i z a b l e in this area. Q u i t e surprisingly, t h e s e faults strike p a r a l l e l to, o r at small angles with, t h e rifting axes m o r e t h a n t r a n s v e r s a l to them. Strike-slip m o v e m e n t s p a r a l l e l to t h e A f a r rifts w e r e first i n f e r r e d by M o h r (1967), a n d seismological a n d / o r field e v i d e n c e s for t h e i r occurr e n c e have b e e n r e p o r t e d by M c K e n z i e et al. (1970), G o u i n (1979), B o u c a r u t a n d Clin (1980), P a s s e r i n i et al. (1988, 1991), P a s s e r i n i a n d Z a n (1989), Z a n et al. (1990) a n d L ~ p i n e a n d H i r n (1992). T a p p o n n i e r et al. (1990) p o s t u l a t e d t h e

0040-1951/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0040-1951(94)00136-7

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E. /lbbate et al. / Tectonophysics 241 (1995) 67-97

existence of such movements in the Afar region on the ground of a model incorporating paleomagnetic and tectonic data. Strike slip has been also inferred by Christiansen et al. (1975), although with rift-oblique strike. South of the Afar region, a strike-slip component of movement has been recognized by Gibson and Tazieff (1970) and Boccaletti et al. (1992) along the Ethiopian Rift. The present study re-assesses this item providing new evidences for strike-slip movements along a transect of the Afar Triangle.

2. Geological setting The Afar Triangle is not a ruler-drawn triple junction. Only two out of its three rift systems (Red Sea and Ethiopian) meet together, whereas the third (Gulf of Aden) has no clear connection with them. The rift system along the Red Sea bifurcates southward with one branch (eastern) which continues along the southern Red Sea, while the other (western) penetrates on land at the northern apex of the Afar Triangle and produces an almost uninterrupted row of volcanic centers. Tectonics and volcanism are active mainly in the latter ramification. The Ethiopian Rift also splits in slightly diverging branches as it approaches the Afar region, where it joins the Red Sea rift system (Tendaho Rift) near Lake Abhe. Here some of the major lineaments of the T e n d a h o Rift and of the main branch of the Ethiopian Rift define an arcuate, rather than angular, connection indicating some kind of continuity. As mentioned, the third rift system (Gulf of Aden) does not directly join the other two arms of the junction. It penetrates landwards with the rifts of Asal, Inakir and Manda which roughly parallel the Red Sea system for one hundred of kilometres, and then branch out as a NW-trending horsetail merging into the complex exten-

sional structure of the northern part of the Afar Triangle. The junction is thus incomplete and results in a scattered spreading covering approximately 60,000 km 2. Most of the Afar Triangle is covered by a Pliocene to Early Pleistocene (approximately 4-1 Ma) basalt suite with minor rhyolite centres, which is labelled the Afar Stratoid Series (Varet, 1975), and constitutes the Middle Extrusive Complex in the map of Fig. 1 together with other minor units as the basalts of the Gulf of Tadjura margins (Gasse et al., 1985) and the Dat'Ali Basalts (Vellutini, 1990). Younger basalts and subordinate rhyolites (the U p p e r Extrusive Complex, in the same map) were produced along rifts and at volcanic centres. Pre-Stratoid formations crop out mainly at the margins of the Triangle. Directly beneath the Afar Stratoid Series lie the Miocene to Pliocene Dalha Basalts (Varet, 1975; 8.9-3.8 Ma, Gasse et al., 1987) which are the main component of the Lower Extrusive Complex of Fig. 1. They are common towards the borders of the Triangle, and locally occur at its interior (west and south of Lake Asal). Earlier products of rifting include Miocene granites and rhyolites. A Paleozoic crystalline basement, a Mesozoic sedimentary cover and Paleogene flood basalts constitute the shoulders of the Afar depression. Crustal thinning in Afar was brought about through magmatic and tectonic processes from the Oligocene to the Present (see Merla et al., 1979) through phases of prevailingly magmatic or tectonic activity (Courtillot et al., 1984). Two peaks of flood basalt extrusions (Dalha Basalts and Afar Stratoid Series) can be recognized between 9 and 1 Ma, whereas a significant decrease in extrusion rates occurred in post-Stratoid times (the last million of years) with magmatic activity limited to localized rifts and volcanic centres. In this time interval, extensional tectonics produced depressions filled with sediments alternating with volcanites. The tectonic style in this phase is

Fig. 1. Map of the geologicalstructures of the Central Afar region. (See colour fold-out, pages 69-72)

MAP OF THE G E O L O G I C A L STRUCTURES OF T H E CENTRAL AFAR REGION L. Z a n , A q u a t e r S.p.A., S. L o r e n z o in C a m p o (PS) Italy E. A b b a t e , P. Passerini, D i p a r t i m e n t o di S o i e n z e d e l l a Terra, Universit~t di F i r e n z e , F l o r e n c e Italy

Legend Sedimentary filling of young basins : alluvial lacustrine, evaporite and aeolian deposits ( ~ 300: thlekness,in metres in some major basins). These sediments may be partly overlain by the Uppar Extrusive Complex. Pleistocene to Present, Upper Extrusive Complex:active rift basatts and subordinate rhyolitas ( e. g, Asal series ). Pleistocene volcanites o1 the Mou. sa'Ali area. Pleistocene to Present.

Middle Extrusive Complex : basalis o1 the Afar Stratoicl Series with associated rhyOlitas. Basalts of the Gulf of Tadjura margins, and of the Dat'Ali range. P/lecene to Pleletocene ( mainly early ).

Lower Extrusive Complex : Mabla Rhyolites, Dalha ancl Adolei basalts. Miocene to Early Pliccene.

Pre-Miocene substratum : Mesozoic sedimentary cover and Patsogene flood basalts. Early Miocene granites.

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Varat. J.. 1975. Geological Map Of Central and Southern Afar. Scale 1:503.000. Centre National de la Recherche Scientifique, France - Consiglio Nazionale delle Ricerche, Italy. F. Gasse. 1983-1987, Carte G~ologique de la Rdpublique de Djibouti 1:100.000. ISERST, Djibouti - Republique de DjibOuti: Orstom, Paris - France. Stieltjes, L, 1974. Carte gdologique du rift d'Asal, ISERST, Djibouti, R(}publique de Djibouti. Vellutini. P.. 1990, The Manda - Inakir Rift, Republic of Djibouti: a comparison with the Asal Rift and its geodynarnic interpretatlen. Tectonophysics. 172,141-153. Aquater, 1980, 1981, 1986, unpublished reports

Geothermal Resources Exploration Projects.

Field sun/eys, Djibouti 1979. 1988, 1989, 1990, Ethiopia 1989. 1990. Satellite images. Landsat Thematic Mapper, images processed by Aquater; Errs, Spot. Airphotographs.

E. Abbate et al. / Tectonophysics 241 (1995) 67-97

characterized by closely faulted belts and intervening plateaus where the fault network is much looser (stable blocks in the map of Fig. 1). The long-lasting and abundant volcanic activity in the Afar region is commonly related to the presence of a broad mantle plume (Schilling, 1973; White and McKenzie, 1989). On the other hand, there is not a general agreement on the nature of the Afar crust. This has been considered to be oceanic on a large extent (Ruegg, 1975; Barberi and Varet, 1977) or thinned continental (Berckhemer et al., 1975; Makris et al., 1975; Makris and Ginzburg, 1987). Mohr (1978, 1989, 1992) believes that the Afar Triangle crust was newly generated by magmatic processes, yet its physical characters would liken it to continental rather than to oceanic crust. It seems likely that both remnants of continental crust and late Tertiary to Quaternary magmatics share in the composition of the Afar crust.

3. Description of the studied area

Evidences for strike-slip faulting are examined in a transect across the Afar Triangle (Fig. 2) from its western sectors near Tendaho to its eastern end at the Bay of Ghoubbet. Along this section, we have measured slickensides, examined features from airphotographs and satellite images, and considered earlier seismological data. The location of selected satellite images, airphotographs and particular maps is shown in Fig. 3. These analyses have been carried out in: (1) the Karrayyu Rift, a lateral branch of the Ethiopian Rift; (2) the Tendaho Rift, which is the southward prolongation of the Red Sea rift system; (3) the Dobi-Hanle rifts; (4) the Gaggade Rift; (5) the Awwadou-Dat'ali range south of Lake Asal; (6) the Alol-Gahannawal'i fracture zone; (7) the Rift of Asal, which is part of the rift system of the Gulf of Aden; and (8) the northwestern shoulder of the Asal-Bay of Ghoubbet rifts. In the structures (2) and (7) of the above list, tectonic activity is accompanied by historical and prehistorical volcanism, whereas in the others equally recent magmatic activity is not known. The transect cuts through two parts of the

73

Afar Triangle with different tectonic style: the central part, where tabular structures alternate with variously dipping flexures, and the southeastern part with prevalent homoclinal structures dipping southwest. The transect crosses the area with block-and-flexure style in its western portion running from Karrayyu to Dobi and including the Tendaho Rift. In its eastern portion, from Hanle to the Gulf of Tadjura, it is characterized by the homoclinal style, only interrupted by the recent Rift of Asal.

3.1. The Karrayyu Rift This rift is a western branch of the Ethiopian rift system at its northward termination. It is floored by the alluvial deposits of rivers Awash and Mille, and its shoulders are in the Dalha Basalts and the Afar Stratoid Series. Unlike the main branch of the Ethiopian Rift (see below), the Karrayyu Rift is sharply interrupted by the Tendaho Rift, and in its northern portion faults with Ethiopian (NNE) and Red Sea (NW) strike cross mutually. Aerial photographs and satellite images show stepwise offsets in NNE-trending fault scarps, possibly due to sinistral movement in crosscutting NW-trending faults (Figs. 2 and 4). Slickensides in the Karrayyu Rift are rarely observable due to the mature erosion of the fault scarps. Scant slickensides recognized along NWtrending fault scarps in the area southwest of Tendaho (Fig. 2, site a; Fig. 5A) principally record dip-slip movements, overprinting an earlier, much less evident (sinistral?) strike slip.

3.2. The Tendaho Rift The NW-trending Tendaho Rift (Figs. 1 and 6) is a southern portion of the Erta A l e - H a r a r o Manda rift system, which extends the active tectonics of the Red Sea to the south. Its width around 50 km is comparable with that of many continental rifts. This rift joins the Ethiopian Rift close to the volcano Dama Ali, west of Lake Abhe. As stated above, the two rifts, rather than crossing, merge together, one bending toward the other, and could be regarded as one single, knee-shaped rift. The

74

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Tendaho Rift, however, does not merely swerve into the Ethiopian Rift, but is prolonged east of the latter by the Goba Ad structure. This is a half-graben of limited length devoid of recent magmatic activity which shrinks and dies out eastward. The flanks of the Tendaho Rift are in the Afar Stratoid Series; the rift is filled with lacustrine and alluvial deposits and with post-Stratoid basalt flows. This filling is topped by recent volcanoes, among which the historically active Kurub and the Dama Ali. Hydrothermal vents are common at Alalo Bad and near Dubti (northeast of Tendaho). The difference in height between the rift floor and its shoulders is few hundreds of metres, but geoelectric prospecting between Tendaho and Sardo indicates a sedimentary filling locally thicker than 1600 m (Fig. 1). The real thickness of the sediments may be locally greater, since near Dubti a geothermal well drilled down to 2100 m in 1994 found 1500 m of sediments against only 1000 indicated by geoelectric prospecting. Five samples of volcanites have been dated by the K / A r method (Istituto di Geocronologia e

Fig. 4. Stepwise sinistral offsets in NNE-trending fault scarps in the rift of Karrayyu. Arrows give the inferred strike slip. Area A in Fig. 3, Landsat T M image.

Fig. 3. Location m a p for satellite images, airphotographs and detailed maps presented as text figures. A = Fig. 4; B = Fig. 8; C = F i g . 16; D = Fig. 12; E = F i g . 13. Stippled = sedimentary basins; white = volcanites; horizontal rules = sea and lakes; thick lines = major rift boundaries with riftward barbs.

Geochimica Isotopica, C.N.R., Pisa) and are located in the profiles of Fig. 6: sample S1, Afar Stratoid Series at the rift shoulders: groundmass in a basalt at Alalo Bad, 2.11 + 0.12 Ma; sample $2, Upper Extrusive Complex: basalt, 16 km southeast of Sardo, near the rift margin, 0.90 + 0.32 Ma; sample $3, Upper Extrusive Complex: basalt at Gum'Atmali, 22 km west of the volcano Kurub, in

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C Fig. 5. Mesoscopic faults in various outcrops from Karrayyu to Dobi and location m a p of m e a s u r e m e n t sites. Wulff projection m lower hemisphere. Degree values denote the pitch of the striae represented in each plot. For locations B and D strike-slip (0-30 ° pitch), oblique-slip (31-60 ° pitch) and dip-slip (61-90 ° pitch) faults are plotted separately. Dots within the plots represent striae. Dextral or sinistral polarity of strike-slip and oblique-slip faults, when known, is indicated at the upper margin of the plot: black pointers correspond to sinistral faults and barred circles to dextral faults. Stippled areas in the m a p represent volcanites. Quaternary deposits in white. (A) Karrayyu Rift, 7 km southwest of Tendaho. Basalts of the Afar Stratoid Series. (B) Boundary between the inner floor and the southwestern slope of the T e n d a h o Rift, about 1 km north of Tendaho. Quaternary siliceous veins in lacustrine deposits. (C) Miscellaneous locations in the T e n d a h o Rift: near G u m ' Atmali, northeast of Tendaho, in the post-Stratoid basalts of the active rift floor; near Alalo Bad, southeast of Tendaho, and near R~so, southeast of Sardo, in the Afar Stratoid Series. (D) in the Dobi graben, Afar Stratoid Series.

E. Abbate et al. / Tectonophysics 241 (1995) 67-97

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the inner floor of the rift, 0.89 ± 0.4 Ma, groundmass, and 0.38 +_ 0.27 Ma, plagioclase; samples $4 and $5, rhyolites at Gablaytu, 18 km west-southwest of the volcano Kurub, close to the rift axis, 0.51 _+ 0.08 Ma, whole rock, and 0.16 +_ 0.04 Ma, groundmass. If we trust relatively old ages for the basalts of the Upper Extrusive Complex which floor the rift at Gum'Atmali, much of the rift subsidence in this area should be even older. The Tendaho Rift shows open fissures, and active faults which define a pattern of elongate blocks. At the rift floor, these fault blocks may be tilted with homoclinal setting, or subhorizontal with a small-scale horst-and-graben array. At the rift margins, the blocks are mildly tilted toward the rift in the form of more or less continuous flexures broken by antithetic (outward-dipping) faults. The elongate fault blocks at both the rift floor and margins show a typical wavelength of some hundreds of metres to one kilometre as in many other areas of the Afar Triangle (Fig. 6; Mohr, 1978). The vertical throw of the faults is currently few tens of metres with the only exception of some larger border faults. The wavelength of the faulted blocks, interestingly, is approximately the same in the thick basalts of the Afar Stratoid Series at the rift borders, and in the recent basalts alternating with abundant sedimentary levels in the inner floor of the rift. Their tectonic style is thus poorly sensitive to the superficial lithologies and presumably rooted at depth beneath the rift. This pattern of closely spaced, small-throw faults suggests the existence of a relatively thin brittle lid, underlain by a shallow soft layer with downward increasing density which damps the vertical offsets. This applies to many sections of the Afar Triangle with similar tectonic style. The rift structure outlined above can be accounted for by extension. Evidences for strike-slip movements at rift margins and inner floor are described below.

The southwestern margin The marginal grabens and flexures at the southwestern margin are illustrated in Fig. 6, sections A - C . Abundant hydrothermal activity,

both present and fossil, is recognizable along this margin, in particular along a master border fault at Alalo Bad. No evidences of strike-slip faulting have been detected along the rift shoulder, where the rare observed slickensides (southwest of Alalo Bad, Fig. 5C) denote dip-slip displacement. Strike-slip faults have been rather observed at the boundary between the rift floor and its southwestern slope about 1 km north of Tendaho. Well exposed slickensides are here shown (Fig. 2, site b; Figs. 5B and 7) in siliceous veins produced by hydrothermal circulation along NW- to NNWtrending subvertical fractures cutting marls and sandstones of the fluvio-lacustrine rift deposits (C.T.I., 1938). These rocks emerge from the relatively flat topographic surface as sharp "dykes" denudated by selective erosion (Fig. 7). In these veins hydrothermalism acted until 12 _+ 6 ky ( U / Th dating of late hydrothermal calcite in chalcedony, CISE Laboratories, Turin). The slickensides obviously postdate the former fracturing and silicification and some of them can be more recent than the mentioned late hydrothermal vents. The strike of slickensides is mainly northnorthwest. Movements along them consist of a well-marked principal dip slip, and a subordinate strike slip shown by more delicate and less frequent striae. A sinistral polarity can be detected in some NNW-trending strike-slip fault planes. In a limited number of cases where striae with different pitch coexist in the same slickenside, strike slip predates dip slip. Slickensides generally have high angles of dip, yet some low-angle slickensides are present and their movements are variously oriented relative to the principal fault trend. Multiple phases of movement are recorded in this fault pattern, although it reflects only the tectonics of young lacustrine sediments.

The inner floor At the interior of the Tendaho Rift, lacustrine and alluvial plains alternate with zones where basalts crop out as NW-oriented elongate fault blocks. Evidences for active NW-striking faults are present also in the sediments (e.g. 10 km

E. Abbate et al. / Tectonophysics 24l (1995) 67-97

79

Fig. 7. Left: siliceous veins cutting the lacustrine sediments at the southwestern border of the Tendaho Rift, approximately 1 km north of Tendaho. The veins are due to the hydrothermal replacement along fractures and fracture filling. Abundant slickensides are beautifully exposed in these veins. Right: mesoscopic strike-slip faults in the veins. The strike of the fault plane is north-northwest. Sight from west-southwest. The mesoscopic fault data from these veins are given in Fig. 5B.

southwest of volcano Kurub). These consist of aligned steaming grounds, fumaroles, hydrothermal deposits and fault steps. In the G u m ' A t m a l i area the basalts of the fault blocks (see dating of sample $3) are locally carved by NNW-trending extensional fractures with en echelon array. This can be interpreted as the product of a WNW-oriented dextral shear. Scant slickensides in a site corresponding to the middle portion of section D (Figs. 2 and 6) only record dip-slip faulting along the overall northwest trend (Fig. 5C). The northeastern m a r g i n

This margin shows a discontinuous flexural style, which is well developed for instance between Sardo and Gargori Bad (50 km southeast of Sardo, Fig. 6, sections F and G). The flexure involves the Afar Stratoid Series (along the rift

margins), and later basalts (inward the rift). Its complex history presumably includes early stages of deformation predating the post-Stratoid flows. This flexure makes transition toward the inner part of the rift to untilted, although still blockfaulted structures (Fig. 6, section G). Slickensides are very rare in this region and only record dip-slip movement (about 16 km southeast of Sardo, Fig. 5C). In aerial photos and satellite images, at several places of the northeastern margin the NW-oriented elongate blocks appear as if they were horizontally offset along planes forming small (15-30 °) angles with their trend (Fig. 2). If these offsets are interpreted as strike-slip faults, they can be seen as a conjugate system bisected by the direction of the rift axis. A stress field with maximum compressional stress parallel to the rift (NW) and minimum compressional stress lying horizontal could be assumed.

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E. Abbate et al. / Tectonophysics 241 (1995) 67-97

Regarding present-day movements, P wave first motion of the 1969 Sardo earthquake has been interpreted in terms of NW-oriented (rift-parallel) sinistral shear (McKenzie et al., 1970; Gouin, 1979) or NE-oriented (rift transversal) dextral shear (Keebe et al., 1989). Field observations reported by Dakin et al. (1971) and Gouin (1979) support the NW-trending fault plane solution (Fig. 2). In summation, evidences for strike-slip faulting, although subordinate to those for dip slip, are scattered throughout different elements of the Tendaho structure. The strike-slip episodes likely span from early rifting times to tectonic phases subsequent to the sedimentary filling of the rift.

3.3. The Dobi-Hanle rifts What we refer to as Dobi-Hanle rifts (Fig. 1) is a system of NW-trending structures including

the Dobi graben (to the northwest) and the Hanle half-graben (to the southeast). These are among several tectonic depressions devoid of recent volcanism in the Central Afar region. The two rifts are roughly aligned, yet with some en echelon arrangement. One of them (Dobi) lies in the block-and-flexure zone of the Central Afar region, the other (Hanle) is part of the homoclinal SW-dipping structure (see above). The Dobi and Hanle rifts are cut in the Afar Stratoid Series, and floored by alluvial and evaporite deposits. As the Tendaho Rift, the Dobi graben is bounded both by inward-dipping (synthetic) and outward-dipping (antithetic) normal faults, frequently associated with flexures tilted toward the rift. The two rifts are also comparable in the typical wavelength of their faulted blocks, yet the Dobi rift differs from Tendaho in its lesser size, steeper flanks and the absence of recent extrusive activity.

Fig. 8. Oblique faults suggesting rift-parallel strike-slip shear (arrows) in the D o b i - H a n l e graben and southwest of it. Area B in Fig. 3, Landsat TM image.

E. Abbate et al. / Tectonophysics 241 (1995) 67-97

The fault pattern in the Dobi graben was mainly produced by NE-SW-oriented extension, but it also denotes a sinistral shear as recognized since long time by Mohr (1971). In particular, faults in the Dobi-Hanle structure, which follow the NW regional trend, often bend to the westnorthwest and west with an oblique array suggesting a NW-trending sinistral strike-slip parallel to the rift. Note the impressive fault bundle which extends west-northwest of Hanle, bounds the Gamarri stable block to the north and impinges on the eastern flank of the Tendaho Rift (Figs. 1, 2 and 8). Besides the diffuse effects of NW-oriented shear, airphotographs locally indicate episodes of rift-parallel maximum compression; this is shown by some conjugate NNW- and EW-trending faults which offset elongate blocks limited by earlier faults (Fig. 2). Slickensides observed at several points in the Dobi rift and at its shoulders record mainly normal faulting, but strike-slip faults also are present with NNE to ENE (rift transversal) strike, and WNW strike forming a small angle with the rift

Fig. 9. Mesoscopic strike-slip fault in the basalts of the Afar Stratoid Series at the southwestern shoulder of the Dobi graben. Site c in Fig. 2.

81

axis (Fig. 2, sites c-e; Figs. 5D and 9). Sinistral displacements prevail in these faults. Precise geodetic measurements have been carried out in this area in 1974 and 1992 (P. Mohr, pers. commun., 1994). In the Guma graben (about 20 km north of Dobi) these measurements indicate that a minor longitudinal sinistral shear component was probably associated with the dominant transversal extension. In the Dobi graben they show a NW-SE-oriented shortening. These results are in accordance with both rift-parallel shear and rift-parallel compression suggested above. An independent line of evidence for strike-slip movements in this area has been given by LEpine and Hirn (1992). According to these authors earthquake focal mechanisms in the Hanle to Gaggade area denote dominantly sinistral strikeslip faulting along NNW-striking planes (Fig. 2). These seismological data assign to this kind of faulting even greater importance than indicated by field and airphotographs data.

3. 4. The Gaggade Rift Also the Gaggade depression is a half-graben devoid of recent extrusive centres. It lies parallel to the Hanle half-graben seated to the southwest, and provides a typical instance of the homoclinal style of the southeastern part of the Afar depression. It is constituted by the Afar Stratoid Series with its acidic differentiates (Babba'olou). A horsetail fault pattern indicating NW-trending strike slip is recognized at the northwestern termination of the Gaggade depression by Tapponnier et al. (1990). The very scarce slickenside data available from the Gaggade depression (northeastern flank, two slickensides in the Afar Stratoid Series cumulated with Kalou slickensides in Fig. l l A - - F i g . 2, site D indicate strike- and oblique-slip sinistral shear along NW-striking planes, combined with the dominant NNE-oriented extension described by Gaulier and Huchon (1991). The occurrence in this area of earthquakes, whose focal mechanism is consistent with NNWtrending sinistral strike-slip faults, is mentioned in the foregoing paragraph (Dobi Rift).

82

E. Abbate et aL /Tectonophysics 24l (1995) 67-97

3.5. The A w w a d o u - D a t ' a l i range

This mountain range separates the Gaggade depression from Lake Asal. It is an ensemble of tilted fault blocks which includes the Dalha Basalts, the Afar Stratoid Series and the Dat'ali Basalts (Vellutini, 1990). Abundant slickensides are exposed in the Dalha Basalts at Kalou (Fig. 10), a gorge cutting some 10 km across the range. These slickensides show a prevalently NW strike with both dip-slip and strike-slip movements (Passerini et al., 1991). A significant set of NE-trending faults can be related to the downfaulting of the Dat'Ali Basalts (Vellutini, 1990). Dip-slip faults are mainly normal, yet reverse faults are also present; strike-slip faults are prevalently sinistral (Fig. 2, sites f and g; Fig. l l A ) . Strike-slip striation in slickensides is generally less marked than dip-slip one, by which it may be often overprinted on the same fault plane indicating its earlier origin. The age of the faults at Kalou is post-Dalha. The Dalha Basalts are more densely faulted than the Afar Stratoid Series and therefore they certainly contain pre-Stratoid faults. However, the northernmost of the Kalou faults involve the lacustrine deposits of the Rift of Asal, and belong to the recent tectonics which shaped this rift. A significant number of slickensides at Kalou are engraved in calcite veins; this distinguishes them from slickensides described above from the Afar Stratoid Series and younger formations, which are directly carved in the rock walls in a large majority of cases. Prior or during faulting the Dalha Basalts were thus affected by stronger fluid circulation than that recorded in the Afar Stratoid Series. This also applies to what is observed at the northeastern shoulder of the Rift of Asal (see later). The alternation of strike-slip faulting with dip-slip faulting left in this area the best record observed in the Afar slickensides. 3.6. The A l o l - G a h a n n a w a l ' i fracture zone

Northward from Lake Asal the fault pattern in the Afar Stratoid Series splits into two major

Fig. 10. Large fault in the Dalha Basalts at Kalou (northern section). The fault plane strikes northwest and dips northeast. It records principally dip-slip (normal) and subordinately strike-slip movements. About 20 mesoscopic fault planes represented in Fig. 11 have been measured along this large fault. The exposed fault plane, well preserved in the lower part of the cliff, is several tens of metres in height.

diverging structures, the Mak'Arrasou and the Alol-Gahannawal'i fracture zones (Fig. 1). Bifurcations of this type are widespread in the northeastern Afar Triangle, and in particular are apparent in the bundle of diverging rifts and fracture zones which radiate further northwestward from the Mak'Arrasou fracture zone and again from the Manda active rift. The Mak'Arrasou fracture zone obliquely connects the spreading Rift of Asal (to the south) with the M a n d a - I n a k i r rifts (to the north) (Figs. 1 and 2). The Alol-Gahannawal'i fracture zone shows a trend roughly similar to that of the Rift of Asal, yet it is not exactly aligned along the

83

E. Abbate et al. / Tectonophysics 241 (1995) 6 7 - 9 7

N

0°-3 pitch

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~

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~

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~ ~

31°-60° ~ / pitch ~

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Fig. 11. Mesoscopic faults in various outcrops from the Gaggade rift to the Gulf of Tadjura and location map of measurement sites. Wulff projection in lower hemisphere. Degree values denote the pitch of the striae represented in each plot. Strike-slip (0-30° pitch), oblique-slip (31-60 ° pitch) and dip-slip (61-90 ° pitch) faults are plotted separately. Dots within the plots represent striae. Dextral or sinistral polarity of strike-slip and oblique-slip faults, when known, is indicated at the upper margin of the plot: black pointers correspond to sinistral faults and barred circles to dextral faults. Stippled areas in the map represent volcanites; Quaternary deposits in white. (A) From north of Gaggade (very few measurements in the basalts of the Afar Stratoid Series) to Kalou (numerous measurements in the Dalha Basalts). A l includes data from site f and the southern half of site g in Fig. 2. A 2 gives data from the northern half of site g in the same figure. (B) Rift of Asal, in the basalts of the Asal Series. (C) Ra'isa, northeast of the Rift of Asal, Dalha Basalts.

E. Abbate et al. / Tectonophysics 241 (1995) 67-97

84

same northwest axis, being slightly rotated to the north-northwest. Both the M a k ' A r r a s o u and the A l o l - G a h a n nawal'i fracture zones are characterized by southwestward tilted blocks bounded by antithetic faults. The M a k ' A r r a s o u represents a flexure-like transition from the Dalha plateau (which is a southern prolongation of the Danakil horst) to the Afar depression. The A l o l - G a h a n n a w a l ' i fracture zone can be described as a composite

r--

half-graben, which merges with Mak'Arrasou to the southeast. Among its dominant tectonic features, is an important fault system ( I m m i n o - A s a l , see later) which bounds it to the southwest. This fault system is marked by an escarpment which is over 1000 m in height, and brings the Dalha Basalts in contact with the younger Afar Stratoid Series (Stieltjes, 1973). It extends beyond the A l o l - G a h a n n a w a l ' i structure, running from the Rift of Asal to a sigmoidal graben (Immino) in

r

74

x+e

i--] 2

,.;;'/4

*

6

Fig. 12. S t r u c t u r a l s k e t c h e s of the A l o l - G a h a n n a w a l ' i region, a r e a D in Fig. 3. A f t e r S P O T satellite image. Left: total fault p a t t e r n . F a u l t s are m a i n l y normal. 1 = s e d i m e n t a r y basins; 2 = volcanites; 3 = fault with a s s u m e d strike slip in the I m m i n o - A s a l system s o u t h w e s t of A l o l - G a h a n n a w a l ' i ; 4 - m a j o r faults (barbs on d o w n t h r o w n block); 5 = m i n o r faults (barbs on d o w n t h r o w n block); 6 = inclined and h o r i z o n t a l beds. Scale and o r i e n t a t i o n as to the right. Right: selection of faults with i n d i c a t i o n s of strike-slip m o v e m e n t . 1 - 3 , see left; 4 = sinistral faults; 5 = dextral faults; 6 = faults for which the e v i d e n c e of strike slip is m o r e compelling.

E. Abbate et al. / Tectonophysics 241 (1995) 67-97

the central part of the Afar Triangle (Figs. 1 and 2). The Alol-Gahannawal'i structure is devoid of post-Stratoid extrusions and predates the recent tectonics of the Rift of Asal (see below), in respect of which it shows more mature erosion and weaker seismic and hydrothermal activity. Satellite images of this structure show oblique offsets in elongate faulted blocks which suggest strike-slip faulting slightly oblique to the dominant fault system (Figs. 2 and 12). These strike-slip faults display two conjugate trends, WNW and NNW, with dextral and sinistral movement, respectively. These faults can be due to an event of horizontal compression subparallel to the regional rift system. A similar small-angle lozenge fault pattern appears in some sections of the Mak'Arrasou fracture zone, where, however, geomorphic evidences for strike slip are not equally compelling. Strike-slip movement, combined with dip slip, is also inferrable in the Immino-Asal fault sys-

85

tem. A large dip slip is here shown by the mentioned 1000 m high scarp as well as by a strong vertical offset of the Dalha Basalts against the Afar Stratoid Series. Strike slip is suggested by the unusually rectilinear path of the longest fault line in this system. The supposedly strike-slip fault runs subparallel to some of the sinistral faults in the conjugate system mentioned above at Alol-Gahannawa'li. Sinistral strike-slip movement along the Immino-Asal fault system may have been transmitted to the oblique Immino graben lying to the northwest (Fig. 2), whose sigmoidal shape and obliqueness to the regional tectonic trend may be compatible with a transtensional origin.

3.7. The Rift of Asal This is an active rift in recent basalts (Asal Series, less than 1 Ma, Stieltjes, 1973). The most recent eruption in the rift dates back to 1978 and occurred at the small volcano Ardoukoba.

Fig. 13. Aerial photograph of the northeastern flank of the rift of Asal, showing a swarm of oblique ( E - W ) fissures and small faults. This suggests sinistral strike-slip shear along the rift. Area E in Fig. 3.

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E. Abbate et al. / Tectonophysics 241 (1995) 6 7 - 9 7

The rift is the landward prosecution of the segmented spreading axis of the Gulf of Aden, and is the site of intense seismic activity. The data from a precision geodetic network operating since 1974 indicate that the rift is opening at a rate of some centimetres per year, and its floor is sinking whereas its shoulders rise with a relative velocity between floor and shoulders which is on the order of few centimetres per year (Ruegg et al., 1979; L6pine, 1987; Ruegg, 1987a,b). Faults in the Rift of Asal are apparently active as reported by Le Dain et al. (1980) after the seismic-volcanic crisis of 1978. Fault planes, examined in outcrops scattered over a 150-km 2 area, are steep (often above 80 ° angle of dip). Almost all slickensides denote dipslip movements, generally normal and, less frequently, reverse (Passerini and Zan, 1989). Marks of strike slip, however, are also present at the northeastern flank of the rift, although very rare (Passerini et al., 1991). Both sinistral (more abundant) and dextral polarities are recognizable in strike- and oblique-slip slickensides (Fig. 2 site h; Fig. 11B). The rift is streaked by numerous open fissures, reaching width of several metres. Some fissures can be traced along a distance of some kilometres. Most of these fissures, including the longest ones, are parallel to the general trend and denote extension normal to the rift. Other fissures show an E - W orientation and are consistent with a sinistral strike slip parallel to the rift (Fig. 13; Passerini et al., 1991). Strike-slip movements superposed to the normal rift spreading probably have part in creating the unusually high density of open fissures and faults (average horizontal spacing of fissures and faults locally less than 10 m) which is found in some of the most active sections of the rift. In fact, such a high density is problematical in simple extension, even with a relatively shallow ductile layer (Passerini et al., 1988). The analyses of microearthquakes in the Rift of Asal give " t h r u s t faults, normal faults, strikeslip mechanisms and even faults with non-orthogonal nodal planes or completely explosive polarities" (Ldpine and Hirn, 1992). On the other hand, according to the same authors, all move-

ments obtained from focal mechanisms on the eastward prolongation of the Rift of Asal (Bay of Ghoubbet and Gulf of Tadjura) are strike slip, with preferred solutions consisting of sinistral NNW-trending faults (Fig. 2).

3.8. The northeastern shoulder of the Rift of Asal The northeastern shoulder of the Rift of Asal exposes the Dalha Basalts unconformably overlain by a local basalt unit (Initial Basalts of the Gulf of Tadjura margins, 2.8-1 Ma, Gasse et al., 1985) partly coeval with the Afar Stratoid Series. The Dalha Basalts are here tilted and fractured. Slickenside analysis in these basalts has been possible in 1988 thanks to fresh cuts along the road from Asal to Tadjura crossing the Afay valley. The data refer to limited outcrops some hundreds of metres northwest of the Raisa well (Fig. 2 site i; Fig. 11C). The slickensides record multiple deformations under different stress regimes. Strikes are dispersed, although many of them cluster around north-northeast. Dip-slip as well as strike-slip shears are present at both high and low angles of dip. Strike-slip faults span over a large strike interval, both transversal (more numerous) and subparallel to the nearby Rift of Asal. This fault pattern interestingly shows some reverse, about NNE-striking, faults. These may denote episodes of compression subparallel or slightly oblique to the trend of the present rift system and can be related to rift-parallel compressions as surmised in other areas. The chronology of different deformations cannot be exactly determined. A striated gypsum veneer on some NW-trending (rift-parallel) strike-slip faults suggests that these are relatively young. An older age can be inferred for some fault planes, also clustering around northwest, which are coated with minerals compatible with higher temperatures as calcite, quartz and, more rarely, epidote and chlorite.

4. General pattern of strike-slip faults Strike-slip faults, although subordinate to normal ones, are widespread in the Afar Triangle.

E. Abbate et al. / Tectonophysics 241 (1995) 67-97

They have been recognized in various structures as active rifts with volcanic activity (Tendaho and Asal rifts), grabens, half-grabens and fracture zones without active magmatism (Dobi-Hanle, Gaggade and Alol-Gahannawal'i), faulted ridges (Awwadou-Dat'Ali) and rift shoulders (northeastern shoulder of the Rift of Asal). Strike-slip and dip-slip faults with limited throw are scattered in a close network, but dip slip also occurs as regional, large-throw faults, currently spaced by a few tenths of kilometres. As regards strike slip with regional extent, indications are available from only one fault (Immino-Asal). Strike-slip movements have been inferred from different types of evidence and at various scales. At a mesoscopic scale strike-slip slickensides are often present besides the more common dip-slip ones. Their strike is mainly subparallel to rifts, that is about northwest or north-northwest. It is worth noting that dip-slip faults (pitch of the striae higher than 60°) and strike-slip ones (pitch of the striae lower than 30°) constitute distinct maxima in a frequency distribution, indicating separate events for these two kinds of movements. At the scale of aerial photographs and satellite images strike-slip movements are suggested by (a) offsets in previously faulted blocks (e.g. Karrayyu, Fig. 4; Alol-Gahannawal'i, Fig. 12), (b) oblique horsetail or en echelon fault swarms (e.g. Hanle-Dobi, Fig. 8; Asal, Fig. 13) and (c) unusually rectilinear fault traces (e.g. Asal-Immino, Fig. 12). Strike slip combined with normal faulting also provides a reasonable account for the frequent northwestward branching of fracture zones in the northeastern Afar Triangle. This pattern broadly extends over regions north of the area of this study, where strike slip can be also surmised, as in the case of the straight fault alignment which runs 150 km southeast-northwest connecting the Mousa'Ali and Sorcale volcanoes. As regards fault kinematics, strike-slip faults inferred from aerial photographs often form dextral and sinistral conjugate systems, whose bisectors coincide, or lie at small angle with, the NW-trending rift axes (e.g. Alol-Gahannawal'i). On the other hand, an overall prevalence of sinis-

87

tral rift-parallel displacements is supported by different lines of evidence (slickensides at Tendaho, Dobi, Gaggade and, especially, Kalou; features from aerial photographs at Karrayyu and Asal; quake mechanisms at Sardo, Hanle-Gaggade and Bay of Ghoubbet-Gulf of Tadjura). Although the degree of reliability is not the same for all strike-slip faults, their overall significance seems sufficiently compelling.

5. Age and frequency of strike-slip faulting The only available indications about fault ages are given by the dating of the faulted formations (which sets a lower limit to faulting), and the degree of erosional smoothing of fault scarps. The best slickenside record usually comes from the Dalha Basalts (Kalou and Afay, Fig. 2), which are generally more fractured than the overlying volcanic suite and where, as mentioned, fault planes more frequently show mineral fibers or striated coatings. These slickensides are partly of pre-Stratoid age and obviously postdate the Dalha Basalts (8.9-3.8 Ma). The Afar Stratoid Series provides most of the geomorphic evidences for strike slip. This strike slip, yet not exactly datable, must have occurred not much time after the Afar Stratoid Series extrusion, since the offset blocks giving evidence for them are often quite maturely eroded and do not show preserved structural surfaces (e.g. Alol-Gahannawal'i). Slickensides (both dip- and strike-slip) in the Afar Stratoid Series have been found in lesser abundance than in the Dalha Basalts, but their scattering across the Central Afar (Karrayyu, various outcrops at Dobi, Gaggade) is significant at a regional scale. In post-Stratoid formations geomorphic evidences for strike-slip movements given by sets of oblique fissures have been found only in some points of the Asal (Zan et al., 1990; Passerini et al., 1991) and Tendaho rifts. Airphotographs and satellite images show that many well-preserved volcanoes and phreato-magmatic craters (Tendaho and Asal-Manda-Inakir rifts) are cut only by dip-slip (mainly subvertical)

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E. Abbate et al. / Tectonophysics 241 (1995) 6 7 - 9 7

faults, and none of them provides clear indications of strike-slip offset. This is justified by the overall scarceness of strike-slip faults relative to dip-slip ones, and by the peculiar tectonics of the volcanic centres. However, this also suggests that strike-slip faults with throw recognizable at the airphotograph scale did not play after the activity of preserved volcanoes and phreato-magmatic centers and that strike-slip movements may have decreased in the latest phases of the Afar evolution. Slickensides are generally absent in postStratoid formations, unless local conditions are peculiarly favorable to their scouring and preservation (in the Rift of Asal, and in the Tendaho Rift in silicified veins). In both areas strike-slip slickensides are much more rare than dip-slip. An opposite indication is given by the regional analysis of focal mechanisms, which more frequently points to strike slip than to normal faulting (McKenzie et al., 1970; L6pine and Him, 1992). The latter authors analyze several seismic shocks in the Hanle area and in the Gulf of T a d j u r a - B a y of Ghoubbet and conclude that all of them are referable to strike-slip movements (Fig. 2). The strikes of the nodal planes in these focal mechanisms cluster around east-northeast and north-northwest, and the latter solution would be more likely. This conclusion still more radically upsets the accepted views about the tectonics of this region, where normal faulting is currently considered to be dominant. According to L6pine and Hirn (1992) the main displacements along the Hanle and Gaggade rifts on land, and those along the submarine lineaments in the Gulf of Tadjura and Bay of Ghoubbet which are currently referred to rifting, would be strike slip rather than extensional. This, by the way, seems to imply that ENE-oriented structures in the Gulf of Tadjura, which are transversal to the mentioned submarine lineaments and are commonly interpreted as transform faults, rather are the site of extensional deformation (see Passerini et al., 1991). The prevalence of strike-slip displacement along rifts goes beyond field data which indicate that strike-slip faults in the Afar Triangle, although significantly present, are generally subor-

dinate relative to normal ones. Three solutions to this contrast can be devised: (1) Normal faulting dominates over long periods, among which the last few hundreds of thousand of years (see scarceness of strike-slip faults in recent lavas). Strike-slip faulting, in contrast, occurs intermittently and concentrates in short time intervals as the present one. (2) The dominantly normal faulting in the Afar region is a superficial effect of a deep-seated strike slip. This is what L6pine and Hirn (1992) suggested, remarking that the NNW-oriented sinistral shear indicated by focal mechanism analysis is not exactly parallel to the main rifting trend, which lies about northwest. The faults observable in the rifts would represent a shallow expression of a deep-seated strike-slip shear slightly oblique to them. Actually, the slickensides data at some localities (Tendaho, Dobi) point to NNW rather than NW strike slip. (3) The prevalence of focal mechanisms with strike-slip solutions is due to the fact that strikeslip displacements give rise to a more intense seismic activity (with special reference to relatively large shocks), whereas crustal extension is largely brought about through creeping and magmatic injections. The hypothesis of point (1) cannot be dismissed, with the restriction that the supposedly vigorous strike-slip movements in present times could not be assumed to extend over the whole Afar region: in the Rift of Asal, for instance, prevailingly dip-slip displacements are recorded by studies of historical faults (Le Dain et al., 1980) and active fault monitoring (L6pine, 1987; Ruegg, 1987a,b).

ii

t Fig. 14. Diffuse transform deformation accommodated by rift-parallel domino faulting, following Tapponnier et al. (1990). Double lines denote spreading axes.

E. Abbate et aL / Tectonophysics 241 (1995) 67-97

89

6. Mechanism of strike-slip faulting

As to the hypothesis of point (2) a N N W strike slip is compatible with some of our data. However, if this were to be accommodated by NWtrending faults, these could not be prevalently dip slip, but should have a very important strike-slip c o m p o n e n t of movement. The weight of strike-slip movements is likely understated by field observations, but the latter could not be dismissed only on the ground of seismological data. The different seismic expression of strike-slip and dip-slip tectonics [point (3)] can provide a reasonable answer to the problem raised by the partial discrepancy between seismic and field geology data. It is well known that seismic activity along mid-ocean ridges often shows prevalence of strike-slip mechanisms referable to transform faults over dip-slip (normal fault) ones.

6.1. Stress origin and pattern

Several mechanisms can be devised to account for strike-slip faults parallel to, or at small angle with, rift axes. A m o n g others: (1) diffuse transform deformation resulting in rift-parallel domino faulting (Fig. 14; Tapponnier et al., 1990); (2) lateral displacement between the major boundary plates; (3) rift-parallel compression giving rise to conjugate shears bisected by the rift axis (Fig. 15a-c). We focus below on each of these issues. (1) In the Afar Triangle rift segments show a staggered array and some transform deformation

a

I%.....

I I .

.-- _ .._.:

.

.

.

,.- u_

C b

- - _

_'KT"_ _

d Fig. 15. Mechanisms of rift-parallel compression possibly giving conjugate strike-slip faults. (a) Passive spreading (vertical arrows) exceeds mantle upwelling, whose deficiency is partially compensated by rift-parallel shortening (horizontal arrows). (b) A propagating rift induces compression (small arrows) at its head. (c) Rift-parallel compression (horizontal arrows) occurs in the saddles separating local asthenospheric plumes. (d) A compression area connects rift-parallel yet staggered strike-slip faults.

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is implied thereby. Clean-cut transform faults are, however, not apparent in geological maps, airphotographs or satellite images. Published mesostructural data supporting transform faulting only refer to a Pliocene fracture zone in eastern Afar (Arta, Arthaud et al., 1980; Gaulier and Huchon, 1991). The Afar transform deformation is often believed to be accommodated by rotation of crustal blocks. A clockwise rotation (14.5 _+ 7.5 °) has been revealed on the ground of paleomagnetic studies by Courtillot et al. (1984) in the Afar Stratoid Series of a wide region between Lake Abhe and the A s a l - G h o u b b e t region in the eastern Afar Triangle. This should have taken place at about 1 Ma. A lesser amount of rotation (about 11 °) is inferred by Acton and Stein (1991). Tapponnier et al. (1990) consider the couple of forces responsible for this rotation to be generated between the overlapping rift systems of the Gulf of T a d j u r a - A s a l - M a n d a Inakir and that of Hararo M a n d a - T e n d a h o - L a k e Abhe. Courtillot et al. (1984) and Tapponnier et al. (1990) consider some basalts younger than the Afar Stratoid Series south of Lake Asal to be unrotated. Vellutini (1990) dates these basalts (Dat'Ali basalts, included in the Middle Extrusive Complex in our map of Fig. 1) at 0.9 Ma and regards this age as an upper limit to the rotation. The latter would thus have occurred at the end of the extrusion of the Afar Stratoid Series. Accordingly, it should not have been caused by the opening of the Asal Rift proper, set from 0.7 Ma to present by Vellutini (1990). Other views on the causes of rotation are held by Gaulier and Huchon (1991) who envisage a combined action of the opening of the Gulf of Tadjura and the Ethiopian Rift, and by Souriot and Brun (1992) whose picture of the Afar tectonics is based on an anticlockwise movement of the Danakil block and an externally imposed, NE-oriented dextral shear at the southern Afar border. At any rate, in the general extensional regime of the Afar region the rotation is most likely related with differential rift opening. Even if the geometry and chronology of this opening is not exactly determined, a major role must have been

played by the Hararo M a n d a - T e n d a h o - G o b a Ad Rift. So far regarding possible causes of block rotation. As to its effects, Tapponnier et al. (1990) and Sigmundsson (1992) considered it to be brought about through pervasive sinistral domino faulting with NW strike. Comparable occurrences are reported from other regions, as Iceland, where the South Iceland Seismic Zone is interpreted as a transform zone where faults develop at about right angles with the strain externally imposed by rift overlapping (Einarsson and Eiriksson, 1982; Foulger, 1988; also see J6hannesson et al., 1990). The role of domino faulting in the Afar block rotation is questioned by Acton and Stein (1991), who mainly point to extensional and compressional deformations at the boundaries of one or a few rotated blocks. Souriot and Brun (1992) also infer mainly normal faulting and limit sinistral strike slip parallel to the main fault systems to local occurrences in the southern Mak'Arrasou zone. Actually, the domino faulting model raises some problems. The trend of the fault system south of Lake Asal seems to straddle without deflection across the unrotated (Dat'Ali) and rotated (Afar Stratoid and Dalha) basalts. Moreover, strike-slip faulting at a small angle with the rifts is still being active one million of years since the discussed rotation, as indicated by earthquake focal mechanisms (L6pine and Hirn, 1992). Last but not least, NW- to NNW-trending strike-slip faults are found also outside the supposedly rotated block, at the northern flank of the Rift of Asal, at the southwestern margin of the Tendaho Rift and in the Karrayyu Rift (locations h, b and a in Fig. 2, respectively). Thus, the hypothesis of faulting with domino style generated by this rotation provides a possible clue to the understanding of some strike-slip faults between the overlapping Afar rifts, yet it could hardly account for all of them. (2) Another source of strike-slip displacement parallel to plate boundaries in the Afar Triangle can be a lateral drive remotely transmitted by the motions of the adjacent major plates (Arabian, Somali and Nubian). This is the simplest account for rift-parallel slip which is uniformly dextral or

E, Abbate et al. / Tectonophysics 241 (1995) 67-97

sinistral at a regional scale (Passerini et al., 1991). Such a solution has been applied to comparable situations in Iceland (Passerini et al., 1990). Lateral movements among major boundary plates give a plausible explanation of many occurrences of strike-slip faults in the Afar Triangle. Between the overlapping rifts, in the central part of the Afar depression, faults caused by this mechanism are probably mingled with the domino-style faults and can be hardly discriminated. On the other hand, rift-parallel strike slip outside the area between the overlapping rifts is tenably accounted for by lateral movements of the boundary plates. Therefore, and given the difficulties raised by the domino faulting model, we incline to regard these movements as most important among several causes of rift-parallel strike-slip faulting all through the Afar region. (3) The strike-slip faults forming conjugate systems whose bisector is almost parallel to the rift strike (e.g. the northeastern margin of the Tendaho Rift, in the Dobi Rift and, especially, in the Alol-Gahannawal'i fracture zone) can be imputed to maximum compression parallel to the rift. The remarkably small angles (30 ° or even less) frequently observed between such conjugates can be due to pre-fracturing which reduces rock strength along planes with strikes close to the rift axis, as well as to a negative sign (extensional character) of the least principal stress (Brace, 1964). These conjugate strike-slip faults may combine with normal faults in the rifting process. If the maximum compression axis temporarily comes to be horizontal and rift-parallel with the least compressional stress still lying across the rift, extension transversal to the rift may go on by interfingering of rock wedges pushed parallel to its axis, rather by normal faults (Fig. 15a). Unlike normal faulting, this process alone does not give rise to crustal spreading since dilatation across the rift axis is compensated by contraction parallel to it. Compression along the rift axis is thus assumed together with extension transversal to it. The notion of compressional stress parallel to rift axes is not a new one. In situ stress measurements in vicinity of Icelandic rifts have revealed rift-parallel, as well as rift-transversal compres-

91

sions (e.g. Schaefer, 1972; Hast, 1973). Conjugate strike-slip faults indicating an axis of maximum compressional stress parallel to rifts have been described by Bergerat et al. (1988, 1990) in Iceland. Let us now consider the origin of the stress field. Acton and Stein (1991) suggested that in a microplate separating a withdrawing from a propagating rift, compression may affect the rift-transversal plate boundary at the side facing the withdrawing rift. Rift-parallel shortening due to shifting of the maximum compressional stress direction from vertical to horizontal as in Fig. 15a is conceivable in conditions of passive spreading when the compensation of lithosphere divergence by mantle uprise may be defective. This may correspond to phases in which spreading is tectonic more than magmatic. Actually, the alternation of episodes in which a rift spreading is due to wedging up of partly molten mantle material with other episodes where spreading is mainly tectonic and due to externally imposed plate divergence has been surmised by Courtillot et al. (1984). Such a mechanism possibly recurring in the normal evolution of rifts seems particularly suitable to explain a rift-parallel compression which is widespread in broad areas as the Afar region and Iceland. It can be added that the axial compression in zones of passive spreading can be enhanced by the push at the head of a propagating rift (Fig. 15b) or, within existing or nascent rifts, in the saddles separating local asthenospheric plumes (Fig. 15c) as have been postulated by Bonatti (1985, 1986) in the Red Sea and East African rifts. In quite a different way, local rift-parallel compression also can arise when rift-parallel strike slip is brought about in staggered fault segments and two fault blocks moving oppositely are pushed one against the other (Fig. 15d). This may have been the case with the conjugate strike-slip faults in the Alol-GahannawaI'i fracture zone (Figs. 2 and 12). The NW-directed compression which originated these faults can be related to a largescale sinistral displacement between the floor of the Afar Depression and its northeastern shoulder (Fig. 2). This displacement is assumed to have formerly occurred along staggered NW-trending

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E. Abbate et al. / Tectonophysics 241 (1995) 6 7 - 9 7

faults running about the present rifts of Asal and Manda-Inakir. At the offset between these faults (Alol-Gahannawal'i area) the displacement was absorbed by diffuse NW-directed compressions. They successively died out when the strike-slip displacement was taken over by the rectilinear Immino-Asal fault system. A similar interaction among crustal blocks can account for some features of the D o b i - H a n l e rift system where evidence for a regional NW strikeslip shear component is given by the mentioned swerving of the Hanle normal faults north of Gamarri. At Dobi, faults with a lozenge array suggest axial compression and are consistent with a braking of a rift-parallel sinistral slip by binding of adjacent blocks along indented margins (Fig. 2).

7. The regional context of lateral displacements In the most widely accepted reconstructions of the kinematics of East Africa and Arabia, plate movements in the Afar and adjoining regions are mainly of divergence and lateral ones play a subordinate role. These reconstructions assume: (A) A relative rotation of the Arabian Peninsula with the opening of the Red Sea and the Gulf of Aden (pole of rotation about 32°N, 22°E, Gaulier, 1990 in Gaulier and Huchon, 1991) and a divergence of Arabia relative to Nubia approximately orthogonal to the NNW- or NW-trending riffs in between (Red Sea and, partly, AsalM a n d a - I n a k i r rift systems). In this divergence, the intervening Danakil block would have remained hinged to the Nubian block at its northwestern extreme and to Arabia to the southeast, and would thus have undergone a 15 ° anticlockwise rotation (crank-arm model, Sichler, 1980). A dextral strike slip would decouple the Danakil block at it southern termination from the Somali block (Souriot and Brun, 1992). (B) A marked sinistral strike-slip along the northern tip of the rift system (Dead Sea rift). This strike slip vanishes out to the south (Dubertret, 1932; Freund et al., 1970; Garfunkel, 1981; Joffe and Garfunkel, 1987). (C) An eastward divergence of the Somali block

from Nubia, with the opening of the Ethiopian Rift. Let us see what the evidences for strike-slip movements in Afar can add to these kinematic schemes. Regarding points (A) and (B), NW-oriented sinistral shears scattered throughout the Central Afar (Fig. 2) suggest that the average direction of divergence between the Danakil (and Arabian) block and the other blocks in post-Stratoid times was not orthogonal to the intervening NW-striking rift axes, but had a sinistral strike-slip component. Some authors already considered that a sinistral component of displacement acted in the northern Red Sea, and fewer (e.g. Hempton, 1987; Camp and Robol, 1992) assumed it for the southern Red Sea. This sinistral motion is here envisaged to extend further south through the Afar Triangle. It is only eastward, where the overall direction of plate margins swerves to the northeast, that the lateral component of movement along this direction becomes dextral as in classic reconstructions. This occurs where the rift system continues in the Gulf of T a d j u r a - G u l f of Aden (see later). The role of the Ethiopian Rift (point C) needs being discussed in some detail. If a sinistral slip along the eastern arm of the junction (Asal and Gulf of Tadjura rifts) were related to the opening of its southern arm (Ethiopian Rift), a direct connection between these two structures should be inferred. The NNE-trending Ethiopian fault system should continue north of the nodal area where the Ethiopian and the Tendaho rifts meet together. This continuation is however not apparent, since north of the nodal area the Afar Stratoid Series is only affected by NW-trending faults (Barberi and Varet, 1977; Tapponnier et al., 1990). No connection between the Asal and Ethiopian rifts can be established, in post-Stratoid times at least, and the sinistral strike slip inferred along the Gulf of Tadjura-Asal rifts is not absorbed by the opening of the Ethiopian Rift. Rather, it is transmitted northwestward through the Immino-Asal, Alol-Gahannawal'i and Mak 'Arrasou fracture zones, and then scatters in the branching fracture systems of the northern Afar Triangle.

E. Abbate et al. / Tectonophysics 241 (1995) 67-97

A post-Stratoid prolongation of the Ethiopian Rift, missing in the NNE direction of the rift itself, can be rather found in the NW-trending rift of Tendaho. The structures at the knee between the two rifts record a complex history. The NW-trending Tendaho fault system does not entirely merge into the Ethiopian Rift, but partly

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continues after crossing it into a minor E - W striking appendix, the Goba Ad half-graben. The Tendaho rift and the Goba Ad half-graben were a continuous faulted structure, which was partially interrupted by the Ethiopian fault system joining the Tendaho rift from the south. N o r t h south-striking faults of the Ethiopian trend also

Fig. 16. Satellite image of the junction among the Ethiopian Rift, the Tendaho Rift and the Goba Ad half-graben. Area C in Fig. 3. Volcano Dama All and Lake Abhe are visible at the junction. Arrows indicate the trends of the three main structures radiating from the junction, a = Tendaho Rift; b = Ethiopian Rift; c = Goba Ad half-graben. Arrow b is shifted to the east relative to the axis of the Ethiopian Rift, which lies close to the image margin. Landsat TM image. See text for discussion.

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E. Abbate et aL / Tectonophysics 241 (1995) 67-97

cut into the southeastern portion of the Goba Ad structure (Gaulier and Huchon, 1991). After this phase of northward propagation of the Ethiopian rifting, the connection between the fault system of T e n d a h o and G o b a Ad was re-established as witnessed by very recent E-W-striking faults truncating the Ethiopian trend (Fig. 16). The recent tectonics of this nodal area is thus alternately d o m i n a t e d by either the T e n d a h o Ethiopian or the T e n d a h o - G o b a Ad connection. Let us consider the bearings of this varying linkage among fault systems on strike-slip components of movement. Given the continuity between the T e n d a h o and Ethiopian rifts, extension in the former must result in sinistral strike slip along the latter. The sinistral strike slip in the Ethiopian Rift was in fact recognized by Gibson and Tazieff (1970) and by Boccaletti et al. (1992) on the basis of structural and geomorphic analyses. Conversely, the spreading in the Ethiopian Rift is consistent with the dextral strike slip inferred close to the axial zone of the T e n d a h o rift ( G u m ' Atmali). The sinistral strike slip found along the southern margin of the T e n d a h o Rift and in the Karrayyu rift, on the other hand, cannot be accounted for by the Ethiopian R i f t - T e n d a h o Rift interaction and was likely transmitted through the G o b a Ad fault system. The sinistral strike slip in the NNE-trending Ethiopian Rift and the similar strike slip with NW to N N W directions in the other arms (Red Sea and Asal) of the junction centred in the Afar Triangle operated differently at its interior. Whereas the N W - N N W strike slip got right through the Triangle, the N E (Ethiopian) one died out, and the relative displacement between the Somali and Nubian blocks was absorbed by a decrease in the amount of spreading from Central to Eastern Afar region. The most apparent expression of this decrease is the southeastward vanishing of the T e n d a h o - G o b a Ad rift. Another one may be the difference between the blockand-flexure tectonic style of the central Afar region and the homocline style of the Eastern Afar (see Section 3). The former style points to local asthenospheric upwellings related with a relatively vigorous spreading, the latter suggests a more uniform and less active stretching.

::::::::::::::::::::::::

~iiiiiii~~

~

t!i

:i:::i:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ~

~ .

, , , / , , ~ ,5 e e,

!!i!ii....i.!.i.i.!ii

Fig. 17. Kinematic sketch of the Afar junction. The triplets of vectors at both sides of each block boundary indicate the relative displacement between the adjoining blocks (inner arrow) with its strike-slip (thick outer arrow) and dip-slip (thin outer arrow) components. The vectors are qualitatively drawn and give an overall account of structural data, not related to a specific stage of the Afar evolution. A general scheme of the strike-slip components in this mechanism is represented in the inset.

The general issue of our analysis is that a sinistral strike-slip component of movement has been active, although possibly discontinuous through time, along all of the three arms of the Afar junction from 1 Ma to now, and possibly also in earlier times. This gives the three major plates an anticlockwise spin around their diffuse triple junction (Passerini et al., 1991), although the asymmetry of the junction makes the rotation correspondingly crooked (Fig. 17).

8. Conclusions Data on fault displacement collected along a cross section in the Afar depression by both field (mesostructural) study and analysis of airphotographs and satellite images show that strike-slip movements parallel to, or at small angle with, the rift axes are diffusely present. The strike-slip movements appear as a distinct set and generally

E. Abbate et aL / Tectonophysics 241 (1995) 67-97

did not combine with dip-slip ones to produce oblique-slip faults. These movements may have occurred intermittently, alternating with the dominant dip-slip deformation. Two fundamental causes have been assigned to strike-slip faults: (1) rift-parallel shear giving rise to faults with uniform (dextral or sinistral) polarity of movement; and (2) rift-parallel horizontal compression resulting in conjugate faulting. Rift-parallel shear can be related to (a) diffuse transform deformation which produces faults with a domino style transversal to the externally imposed transform shear, or (b) lateral (sinistral) movements between the major lithospheric plates bounding the Afar depression. Rift-parallel compression may arise from (a) defective compensation of lithospheric divergence by mantle uprise during passive spreading, possibly coupled with episodic push parallel to rift axes due to local plumes or to rift propagation; or (b) lateral movements putting indented block margins into collision. The former hypothesis gives a general explanation for widespread axial compression, while the latter has been applied to local structures in the Afar region. Strike-slip faulting in the Afar Triangle is here interpreted as polygenetic, and we consider that all the discussed mechanisms contributed to it. A fundamental role is, however, attributed to the sinistral movement between the boundary lithospheric plates. This concerns both faults generated by rift-parallel shear and conjugate strike-slip faults bisected by the rift direction. A sinistral motion of these plates around the Afar Triangle must thus be added to their currently accepted divergence. This spin may reveal that the flow lines in the plume beneath Afar hot spot are not perfectly radial, but rather have a slightly "cyclonic" array.

Acknowledgements The authors are grateful to the Ethiopian Geological Survey, Geothermal Department, whose generous assistance has made possible field work in Ethiopia. Thanks are also due to Prof. Piero

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Bruni and Dr. Abebe Tsegaie for their active cooperation in field structural analysis in Ethiopia. Prof. Marta Marcucci, although she has not shared in drafting this work, participated into the 1990 campaign, in which have been collected the data at Kalou and measured the strike-slip faults in the Rift of Asal. Substantial improvements to this paper have been possible thanks to the constructive remarks of Prof. Paul Mohr, Prof. Philippe Huchon, Prof. Yossi Mart and an anonymous referee. Financial support has been given by Aquater, by Consiglio Nazionale delle Ricerche and by Ministero Universith e Ricerca Scientifica e Tecnologica, Italy.

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Secor, D.T., 1965. Role of fluid pressure in jointing. Am. J. Sci., 263: 633-646. Sichler, B., 1980. La biellette danakile: un module pour l'~volution g~odynamique de l'Afar. Bull. Soc. G~ol. Fr., 7, 22: 925-933. Sigmundsson, F., 1992. Tectonic implications of the 1989 Afar earthquake sequence. Geophys. Res. Lett., 19: 877-880. Souriot, T. and Brun, J.P., 1992. Faulting and block rotation in the Afar triangle, East Africa: The Danakil "krank-arm" model. Geology, 20: 911-914. Stieltjes, L., 1973. Carte g~ologique du Rift d'Asal, 6chelle 1:50,000. BRGM D~partment G6othermie, Orl6ans. Tapponnier, P. and Varet, J., 1974. La zone de Mak'arrasou en Afar: un 6quivalent 6merg6 des failles transformantes oc6aniques. C. R. Acad. Sci. Paris, Ser. D, 278: 209-212. Tapponnier, P., Armijo, R., Manighetti, 1. and Courtillot, V., 1990. Bookshelf faulting and horizontal block rotations between overlapping rifts in southern Afar. Geophys. Res. Lett., 17: 1-4. Varet, J., 1975. Carte G~ologique de l'Afar Central et Meridional. CNRS, France, CNR, Italia. Vellutini, P., 1990. The Manda-Inakir Rift, Republic of Djibouti: a comparison with the Asal Rift and its geodynamic interpretation. Tectonophysics, 172: 141-153. White, R. and McKenzie, D., 1989. Magmatism at rift zones: the generation of volcanic continental margins and flood basalts. J. Geophys. Res., 94: 7685-7729. Zan, L., Gianelli, G., Passerini, P., Troisi, C. and Abdourahman Omar Haga, 1990. Geothermal exploration in the Republic of Djibouti: thermal and geological data of the Hanl~ and Asal areas. Geothermics, 19: 561-582.