Description:

_Journal of Structural Geology 32 (2010) 843e854_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg_ _Magma-driven antiform structures in the Afar rift: The Ali Sabieh range, Djibouti_ _Bernard Le Gall a,*, Mohamed Ahmed Daoud b, Ren? C. Maury a, Jo?l Rolet a, Herv? Guillou c, Christian Suea Universit? Europ?enne de Bretagne, Universit? de Brest, CNRS; UMR 6538 Domaines Oc?aniques, IUEM, place N. Copernic, 29280 Plouzan?, France b Institut des Sciences de la Terre, Centre d’Etudes et de Recherches de Djibouti, B.P. 486, Djibouti Ville, Djibouti c UMR 1572, LSCE CEA-CNRS, Domaine du CNRS, 12 avenue de la Terrasse, 91118 Gif-sur-Yvette, France_ _article info_ _Article history: Received 3 November 2009 Received in revised form 1 June 2010 Accepted 16 June 2010 Available online 23 June 2010_ _Keywords: Afar rift Djibouti Ali sabieh range Oligo-Miocene Ma?c laccolith Antiform Extension_ _abstract_ _The Ali Sabieh Range, SE Afar, is an antiform involving Mesozoic sedimentary rocks and synrift volcanics. Previous studies have postulated a tectonic origin for this structure, in either a contractional or extensional regime. New stratigraphic, mapping and structural data demonstrate that large-scale doming took place at an early stage of rifting, in response to a ma?c laccolithic intrusion dated between 28 and 20 Ma from new K-Ar age determinations. Our ‘laccolith’ model is chie?y supported by: (i) the geometry of the intrusion roof, (ii) the recognition of roof pendants in its axial part, and (iii) the mapping relationships between the intrusion, the associated dyke-sill network, and the upper volcanic volcaniclastic sequences. The laccolith is assumed to have in?ated with time, and to have upwardly bent its sedimentary roof rocks. From the architecture of the w1 km-thick Mesozoic overburden sequences, ca. 2 km of roof lifting are assumed to have occurred, probably in association with reactivated transverse discontinuities. Computed paleostress tensors indicate that the minimum principal stress axis is consistently horizontal and oriented E-W, with a dominance of extensional versus strike-slip regimes. The Ali Sabieh laccolith is the ?rst regional-scale magma-driven antiform structure reported so far in the Afro-Arabian rift system._ _? 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Antiform-synform structures can form in several tectonic environments, in response to either compressional or extensional stress regimes. In continental rifts, such structures usually develop over a wide range of scales. They are often associated with extensional or transfer faulting (Withjack et al., 1989; Schlische, 1995, and references therein). Alternatively, they can be connected with either salt diapirs or high-level magmatic intrusions (Rowan, 1995). Most folds described so far within the Cenozoic Afro-Arabian Rift System have been assigned to basin inversion, in response to regional and or local compression, e.g. in the Rukwa and North Kenya rifts (Ring et al., 1992; Morley et al., 1999; Le Gall et al., 2005). Active compression is also well documented further north, in parts of the Afar rift, along individual seismogenic reverse faults that cut through the <3 Ma-old Stratoid basalts ?lling the Afar depression (Fig. 1a) (Hofstetter and Beyth, 2003). Contractional strain was assumed to have operated at an earlier stage of rifting in SE Afar (Clin and Pouchan, 1970) from the antiformal structure of MioPliocene volcanics in the Ali Sabieh Range (ASR hereafter) (Fig. 1)._ _* Corresponding author. Tel.: ?33 2 98 49 87 56; fax: ?33 2 98 49 87 60. E-mail address: blegall@univ-brest.fr (B. Le Gall)._ _0191-8141 $ e see front matter ? 2010 Elsevier Ltd. All rights reserved. doi:10.1016 j.jsg.2010.06.007_ _However, the causal mechanisms of early compression within the overall Afar kinematic framework were not fully understood. Alternatively, the ASR was later regarded as a horst-type structure (Clin, 1991; Eagles et al., 2002)._ _These early interpretations of the ASR as a tectonic structure are challenged in the present work. Instead, we propose a magmadriven doming model from new geological mapping, ?eld structural analysis, and ASTER remote sensing interpretations, combined with K Ar radiometric dating and geochemical analyses of OligoMiocene ma?c volcanics. The growth of the ASR is here assigned to the emplacement of a ma?c laccolith in Late Oligocene-Early Miocene, under extensional stress conditions._ _2. Geological setting of the Ali Sabieh range_ _The Afar rift system was initiated during the Oligocene with the emplacement of 30 Ma-old ?ood basalts, currently exposed in the Ethiopian plateau and in Yemen (Rogers, 2006). The major downthrow of the ca. 105 km2 fan-shaped Afar depression (Fig. 1b) took place during extensional faulting, at 29e26 Ma, along the Ethiopian escarpment to the west (Wolfenden et al., 2005). Much more controversial are the causal mechanisms and timing of the uplift of the Ali Sabieh and Danakil ranges (DR) that limit the depression to the east (Fig. 1b and d). In these elevated areas, the tectonic style of_ _844_ _B. Le Gall et al. Journal of Structural Geology 32 (2010) 843e854_ _Fig. 1. Major topographical and geological structures in the Afar Triangle. (a) Plate kinematic setting of the Afar Triple Junction. Thick arrows indicate plate motion vectors and the box corresponds to the location of Fig. 1b. (b) Main geological units. 1. <3 Ma-old series in the Afar depression, 1a. Lacustrine deposits, 1b. Stratoid basalts; 2. 29e3 Ma-old synrift volcanics; 3. 30 Ma-old trap complex; 4. >30 Ma-old substratum. AH, Aisha horst; ASR, Ali Sabieh range; ATZ, Arta transverse zone; AWM, Afar west margin; NDR, North Danakil range; SDR, South Danakil range; TG, Tadjoura Gulf. The trace of the topographic pro?le of Fig. 1d is shown and the box is the location of Fig. 1c. (c) Landsat image showing the contrasted morphotectonic expression of the Ali Sabieh triangle-shaped antiform with respect to the Asal depression to the NW. (d) EW-oriented topographic pro?le across the Afar Triangle at the latitude of the Ali Sabieh range. Vertical exaggeration w30._ _Miocene synrift volcanic terranes greatly contrasts with those of younger basaltic trap-like complexes surrounding the ASR (SomalieDalha series), and the DR (Stratoid series) (Barberi et al., 1975; Barberi and Varet, 1977)._ _The ASR is a 250 km2 triangular highland that extends to the south into the Aisha horst (Fig. 1b and c) (Muller and Boucarut, 1975). Its overall map pattern is that of an eroded north-plunging antiform, cored by intrusive ma?c material, and overlain by an outward-dipping envelope which includes Jurassic-Cretaceous sedimentary sequences and Tertiary volcanics (Fig. 2). Mesozoic sedimentary units are well exposed in elevated areas, as high as 1300 m in Mt. Arrey. They comprise: (1) a >350 m-thick succession of Jurassic limestones (Gasse et al., 1986) which rest unconformably over crystalline Proterozoic basement rocks, further south in Somalia (Black et al., 1972), and (2) a >500 m-thick sequence of Cretaceous ?uviatile grits (Gasse et al., 1986). The Mesozoic substratum series in the ASR are in turn overlain by the effusive and volcaniclastic ma?c material of the so-called Ali Sabieh magmatic complex (fully described below in terms of volcano-stratigraphy_ _and age determinations). These early synrift volcanics, as old as Oligo-Miocene, are the youngest series involved into the Ali Sabieh antiform, since the overlying felsic sequences of the 20e9 Ma Mablas Fm (Chessex et al., 1975; Varet and Gasse, 1978; Zumbo et al., 1995; Audin et al., 2004) are assumed to post-date the onset of doming in the ASR area. The Mablas felsic volcanics are onlapped by tilted basalts of the Dalha Fm. (8.6e3.8 Ma) to the NW, and by nearly undeformed Somali basalts (7.2e3.0 Ma) to the east (Gasse et al., 1986; Deniel et al., 1994). The pre-Mablas series is intruded by a dense network of ma?c-felsic sills and dykes, and all these rocks are cut by a number of regional-scale faults, with a dominant NWeSE trend (Fig. 2)._ _3. The Ali Sabieh ma?c complex_ _The new volcano-stratigraphy proposed here for the ASR synrift ma?c series is supported by structural and petrological data, combined with new K-Ar age determinations. Three main units are identi?ed in the Ali Sabieh ma?c complex: the main intrusion, the_ _B. Le Gall et al. Journal of Structural Geology 32 (2010) 843e854_ _845_ _Fig. 2. Geological map of the Ali Sabieh antiform from published documents (Barr?re et al., 1975; Gasse et al., 1986) with revisions from ?eld observations and remote sensing data (ASTER) interpretation. The Ali Sabieh dyke swarm is simpli?ed for clarity with only the thickest dykes shown to the north as topographic ridges on ASTER data. The Asal-type fault system involving the Dalha Basalts, NW of the antiform, is not shown. A, Asamo; AA, Ali Adde; AS, Ali Sabieh; D, Dadin wadi; LP, Lougag plateau; MA, Mt. Arrey. The traces of structural cross-sections shown in Fig. 3b and c are shown, as well as those of smaller-scale sections shown in Figs. 3a, 4d, 5, and 9a (X, Z, Y, and U, respectively). Faed refer to main fault structures discussed in the text._ Ключевые слова: volcaniclastic, sedimentary, schweizerbart stuttgart, steeply-dipping attitude, tectonophysics, guillou, dipping, ethiopian, effusive, author, corry, data, mac material, gall journal, ?ows, trace, east africa, zumbo, iii, mantle source, exaggeration, arched geometry, asr area, depression, ali, upper, chiey supported, geological, arrey, evolution, laccolith model, origin, lava, tensor, emplacement, tectonics, stratum dip, western ank, angelier, cretaceous grit, map, ebinger, geophysical, basaltic, pollard, west, length, regime, event, age, sequence, close, eld observation, text, continental, annen, east, letters, structure, rock, north, compressional, total thickness, earth, ali sabieh, diagram, roof pendant, vertical exaggeration, igneous material, network, gasse, structural, cerd, sabieh, stuttgart, cretaceous, afar triangle, maximum width, basalt, geological society, causal mechanism, geophysical letters, continental rifting, ma, le, geology, intrusion roof, cruden, black, pyroclastic ows, strike-slip regime, range, ma?c, ?ank, axial, consistently horizontal, doming, structural geology, nne-ssw axis, rift, grit, stress, laccolith, extensional regime, complex, k-ar, area, extensional, early stage, intrusion, clin, asr, magmatic, antiform, constant, overburden, wa, elevated area, afro-arabian rift, geochemical analysis, thickness, dyke, mac magma, magma, basal contact, vertical, location, series, roof, log t-log, unit, zone, tectonic origin, volcanics, swarm, scale, eld evidence, rogers, mesozoic, sabieh ma?c, sample, main intrusion, djibouti, morley, determination, apparent age, journal structural, stratum, material, nature, regional, generally, sabieh antiform, afar, oligo-miocene, gall, schweizerbart, mablas, synrift, exposed, dip, eastern ank, dyke-sill network, mesozoic overburden, sill, model, intrusive, early, faulting, eld, log, danakil, carte gologique, orientation, proposed, body, rifting, tholeiitic magmatism, elsevier, felsic, contact, interpretation, patterson, journal structural geology, structural style, dunkelman, fault, journal, dating, ii, pre-mablas series, afar rift, sedimentary xenolith, mapping relationship, barberi, volcanic, observed, geometry, deposit, main, varet, sperner, ?eld