The Afar Depression

The Afar Depression is an area of active extensional deformation and basaltic volcanism from which the Red Sea, the Gulf of Aden and the MER radiate (Abbate et al., 1995). The depression is characterized by three geometrically distinct structural trends (Fig. 2.2): (1) a NNW trend that parallels the Red Sea rift; (2) a nearly E-W trend that runs through the Aden Gulf and continues onto land at the Tadjoura Gulf in the east-coast of Djibouti; and (3) a NNE trend of the MER (Thurmond et al., 2006). The Red Sea and Gulf of Aden are highly evolved and active rift segments currently propagating into the Afar Depression through a series of en-echelon arranged emerging spreading centers (Courtillot et al., 1984).

The Afar Depression, core of the Oligocene-Recent Afro-Arabian continental breakup, is one of the few active tectonic zones in the globe where one can easily study the activites of mantle plume, rift-rift-rift triple junction and Microplate formation on land (Eagles et al., 2002). Moreover, it is probably the only active intracontinental rift in the globe where both the incipient continental breakup history and the current seafloor spreading can be observed on the surface (Almond, 1986; Tesfaye et al., 2003). The early Oligocene–Present volcano-tectonic development of the Afar Depression has been deduced to have been causally influenced by the influx of the Ethio-Afar hot mantle plume, resulting in extensive dome-shaped elevated plateaus between ~40 Ma and ~30 Ma (Redfield et al., 2003, and references therein). The Ethio-Afar mantle plume interpretation is largely coincident with outpouring of the ~30 Ma massive Ethiopian flood basalt and the 31–25 Ma Yemen continental flood volcanism (Baker et al., 1996; Hofmann et al., 1997; Redfield et al., 2003).

The progressive NE movement of the Arabian plate for the last 30 My produced the 300–400-km-wide Afar triangle, which comprises the triple junction (Wright et al., 2006).

Since the late Oligocene, the Ethio-Afar mantle plume head has influenced an extensive area centered on the present day Afar triple junction (Hofmann et al., 1997; Audin et al., 2004). When both the Red Sea and Gulf of Aden spreading centers approached the triple junction area at

~20 Ma (Courtillot et al., 1987), the Afar Depression began to open and detached two continental microplates, the Danakil and Ali Sabieh Microplates (Fig. 2.2), from the Nubian and Somalian plates, respectively (Audin et al., 2004).

Figure 2.2 Outline of the Red Sea, Gulf of Aden and MER tectonic trends and the position of Danakil and Ali Sabieh blocks with respect to the Nubian, Somalian and Arabian plates (modified after Collet et al., 2000).

Nature of the Afar Depression crust: The nature of the crust or lithosphere in the Afar Depression is controversial (Makris and Ginsburg, 1987). The Afar Depression lithosphere has been described differently by various authors as oceanic, continental, and transitional (Beyene and Abdelsalam, 2005). Most of the axial zones of the depression (e.g., Erta’Ale Range, Asal–Manda Inakir, Manda Hararo–Goba’ad, and the Sabure-Hertale–Adado rifts) are characterized by transitional to tholeiitic basalt flows accreted along open fractures similar to a young oceanic-type lithosphere (Fig. 2.3; Barberi and Varet, 1977). According to these authors, the entire Afar Depression lithosphere could be an oceanic-type that was formed during the onland propagation of the Aden Gulf and the Red Sea (i.e., Pliocene–Pleistocene) or a thin continental lithosphere injected by basaltic dykes and covered by the Stratoid flood basalts.

Geophysical and structural studies regarding the center of the depression pointed out that the Afar is progressing to an oceanic type of lithosphere, similar to the lithosphere observed in Iceland or Mid-Atlantic Ridge, with anomalously low asthenospheric velocities found at fairly shallow depth (Barberi et al., 1980, and references therein). The combined effect of extension and volcanism and/or diffused faulting could have resulted in the evolution of the Afar lithosphere that is transitional between continental and oceanic where crustal thinning is compensated by accumulation of new magmatic material (Beyene and Abdelsalam, 2005). The nature of the Afar Depression lithosphere has been described as oceanic along its axial zones and becomes progressively continental towards the marginal areas. From this perspective, Morton and Black (1975) believed the western Afar margin to be a zone of highly stretched continental crust induced by series of normal faulting, block rotation, and magma injections along weak zones (Fig. 1.4a).

The lithospheric thickness in Afar is not uniform throughout the Depression. Makris and Ginzburg (1987), Maguire et al. (2006), and Stuart et al. (2006) estimated the thickness of the lithosphere to be ~26 km in the southern and central Afar and gradually decreases towards the north reaching a minimum of 14 km in the Dallol area, northern Afar Depression.

Bonatti and Emiliani (1971) supposed that separation of the Danakil Microplate from the Nubian plate by concentrated tensional and rotational movements created the depression. As a consequence of these concentrated extensional deformation, continental lithosphere is very thin below the axial zone of the northern Afar Depression.

The extended and intruded crust beneath the central Afar Depression is 18–22 km thick with strong similarities to crust beneath Iceland (Hirn et al., 1993; Dugda and Nyblade, 2006). The existence of abnormally thinned crust in this active tectonic zone is the main reason for the assumption that the Afar Depression is in a state of oceanic-type crust. From volumetric considerations, Redfield et al. (2003) pointed out that either the base of the lithosphere underlying the Afar Depression is not well constrained or there has been massive removal of the lithospheric material by the upwelling hot mantle plume since the onset of the extension. From the elastic strength measurements, the crustal thickness of the Afar Depression decreases from 17 to 5 km in the highly stretched portions of the region (Ebinger and Hayward, 1996). The NW-SE–trending Asal–Manda Inakir rift (Figs. 2.3 and 2.4) on the eastcentral tectonic zone of the Afar Depression is widely cited as a location where incipient seafloor spreading can be observed on land (Gupta and Scholz, 2000).

Figure 2.3 Simplified geological map and major tectonic structures (segments) of the Afar Depression.

ER, Erta’Ale; TA, Tat’Ali; AL, Alyata; MH, Manda Hararo; MI, Manda Inakir; MH-G, Manda Hararo–

Goba’ad; AG, Asal Ghoubett (after Hofstetter and Beyth, 2003).