Extensional Structures along the Western Margin and Active

Depression

Most of the Oligocene–Present extensional deformation structures constituting the EARS have been interpreted as reactivated features of the mobile Mozambique belt and the Arabian-Nubian shield (Korme et al., 2004, and references therein). The junction of the north-northwest–trending western Afar margin and the northeast–trending northern MER is an

intensely faulted and deformed zone. The arcuate accommodation zone (concave eastward opening fault zone) marks the boundary between the two tectonically distinct fault patterns.

4.6.1 The Erta’Ale Range

The Erta’Ale Range is one of the volcano-tectonically active segments of the Afar Depression, which is the onland southern termination of the Red Sea rift (Tazieff et al., 1972;

Manighetti et al., 1998) and is composed of roughly NNW-SSE aligned seven volcanic centers: Gada’Ale, Alu, Dalaffilla, Borale’Ale, Erta’Ale, AleBagu and Hayli’Gub, from north to south (Barrat et al., 1998). The axial zone of Erta’Ale Range is characterized by active normal faulting and diffuse volcanism, which is mainly basaltic in composition, but with minor felsic derivatives inter-bedded with the tholeiitic basalts (Barberi and Varet, 1970;

Barrat et al., 1998; Amelung et al., 2000).

In order to evaluate the Pleistocene–Recent rift-axis kinematics and geometry of the Erta’Ale Range, structural data have been collected along the entire western part of the range.

In the eastern part of the depression, structural data have been acquired from well-georeferenced and processed Landsat-5 ETM image, as the area is difficult to access. Most of the structures are extension fractures and elliptical caldera collapses of various sizes and, in some places, normal faults with throws not exceeding 10 m. An extension fracture is a structural feature where the separation between the two blocks is commonly the result of tectonic stresses normal to the fracture surface (Acocella and Korme, 2002). Because of the location (axial range of the northern Afar Depression), trend (parallel to the Erta’Ale Range trend), and age (Pleistocene–Present) of the open fractures, their extension directions result from the existing kinematics of the Ethiopian plateau and Danakil Microplate in the depression. A total of ~60 fracture orientations and 23 asperities were measured from the western side of the Erta’Ale Range. All measured open fractures have an opening between 0.05 and 3 m and a length between 5 and 600 m; their depth of penetration, ~700 m, is inferred from mechanical considerations (Acocella and Korme, 2002, and references therein).

The asperities along the walls of open fractures were sharp, fresh and unaffected by erosion and other secondary deformation, as for example shown in Figure 4.8a. This physical property, together with the late Pleistocene–Recent age of the rocks, confirms that the fractures are active features in the Erta’Ale Range. About 23 open fractures, which have clear asperities, were measured and all consistent with an overall ENE-WSW (066°) extension direction of the open fractures orthogonal to the mean NNW-SSE (337°) trend of the Erta’Ale Range rift axis (Fig. 4.8b–d). Moreover, the alignment of the six shield volcanoes, except the

AleBagu (Fig. 4.4a), and the trend of their elliptical calderas overlap with the mean trend of the extension fractures and normal faults of the Erta’Ale Range.

Figure 4.8 Determination of the trend and extension direction of open fractures of the Erta’Ale Range, northern Afar Depression. (a) Active open fracture whose asperities A and A’ match to give the opening direction of the Erta’Ale Range (arrow). (b) Orientation, and (c) Opening direction of the Erta’Ale Range fractures. (d) The 89° angle between the fracture strike and extension direction suggests extension orthogonal to the fractures.

4.6.2 The Tendaho–Manda Hararo Rift

The NNW– to NW–trending Tendaho–Manda Hararo graben is the southern section of the Danakil Depression, which represents the southern end of the Red Sea propagator (Figs. 4.3 and 4.7b). Tendaho graben is ~150 km long and its width varies from ~25 km near its northern and southern tips to >50 km in its center (Acocella, 2010). The Tendaho–Manda Hararo is a concave eastward graben that encompasses N-S–trending axial faults at the Manda Hararo area, NW-SE–trending central graben, and ESE-WNW–trending Goba’ad rifts (Fig. 4.11; Tesfaye et al., 2003). Because of this unique geometric setup, the Tendaho graben does not directly merge into the MER; instead it bends to the east and joins the Gulf of Aden–

trend structures. The Afar Stratoid series constitutes the edges of the Tendaho rift; the rift basin is filled with Holocene lacustrine and alluvial deposits and with post-Stratoid basalt flows (Abbate et al., 1995) as young as ~ 0.3 Ma (Acocella, 2010). The Tendaho–Manda

Hararo rift is at the center of the Afar triple junction and the influence of the other two arms of the junction (the Gulf of Aden and MER) for the geometric alignment of the Tendaho rift is significant.

In order to evaluate the Pleistocene axial zone trend of the Tendaho graben, structural data have been collected in the field along the asphalted road leading to the town of Semera, and from the Landsat-5 ETM image. The trends of the graben bounding faults are variable, from a nearly N-S trend at the northern end of the graben to NW-SE trend at the central part and WNW-ESE trend near the triple junction (Fig. 4.11). To avoid the geometric complexity all measurements were taken from the NW-SE–trending central Tendaho graben. About 44 fracture orientations and 21 fracture extensions are computed to assess the general view of the central Tendaho graben. The measured fractures range from few meters to several

kilometers and their opening ranges from 0.05 to 10 m. The asperities along the walls of fractures were still sharp, fresh and easily measurable, as for example shown in Figure 4.9a.

The average trend of the Tendaho rift is 328°, deviated by about 9° to the west from the average trend of Erta’Ale Range rift axis (Figs. 8b and 9b) and the average extension direction, 053°, is still nearly orthogonal (Fig. 4.9c and d).

Figure 4.9 Determination of the trend and extension direction of extensional fractures of the Tendaho graben, central Afar Depression. (a) A stratoid series with an active extension fracture whose asperities A and A’ match to give the best fit of the fracture (double arrow). (b) Average trend of fractures; and (c) Average extension direction of the Tendaho graben fractures. (d) The 85° angle between the fracture trend and extension direction suggesting extension still orthogonal to the trend.

4.6.3 The Western Afar Margin

The western Afar margin also called the Ethiopian escarpment is a N-S–running physiographic feature that marks the incipient breakup of the Afar Depression. The fault-plane escarpment is eroded and dissected by eastward-flowing rivers and few transverse structures (Wolfenden et al., 2005) (Fig. 4.10a). At the base of the escarpment, there are series of tectonically and seismically active marginal grabens arranged in an en-echelon

manner (Fig. 4.7a; Morton and Black, 1975; Tesfaye et al., 2003; Wolfenden et al., 2005).

Structural and remote sensing analyses revealed that the western Afar margin evolved by gradual pure extensional deformation along the border faults (Asfaw et al., 1990). In contrast, Chorowicz et al. (1999) proposed that the western Afar margin developed as a result of pure strike-slip motion. Field observation and Remote Sensing data of this study, however, revealed that in the western margin there are series of step-faults and tilted fault-blocks, which are common characteristics of dip-silp or oblique-slip deformation. But, it has been still difficult to analyze the contribution of strike-slip component for the development of the western Afar margin.

Figure 4.10 (a) Field photograph of western margin of the northern Afar Depression aligned nearly N-S. (b) Average trend of the faults and lineaments of the north-western Afar margin.

Most of the structural measurements of the western escarpment were taken from the Landsat-5 ETM image. The western Afar margin exhibits an arcuate structure in its center (Fig. 4.11) that comprises N-S, NNE, and SSW oriented border faults. More than 50 structural data (fault strike) have been collected from field measurements and landsat-5 images. Even though the structural alignment of western Afar margin is not uniform, the structural analysis reveals that this ~500-km-long margin has an average N-S trend (Fig.

4.10b). The lack of slickensides and other subsidiary structures (i.e., asperities) make the kinematics of the marginal structures still indistinct. The direction of extension in this study is extracted from previous works (e.g., Collet et al., 2000).

The structural setup of the Afar Depression and the Ethiopian escarpment is illustrated in Figure 4.11. The strike and extension direction of open fractures/normal faults of Erta’Ale

and Tendaho-Manda Hararo axial rift zones vary by about 009° and 13°, respectively. The deviation in the general orientation of the central Tendaho graben by about 009° to the west from the average trend of the Erta’Ale Range shows that the involvement of the Gulf of Aden rift (E-W–oriented ridge) is higher than the MER. Extension is orthogonal to the NW-SE–

trending fractures. The Ethiopian escarpment, on the other hand, shows distinct geometry: a N-S–trending border faults accommodating dip-slip or oblique extension (Fig. 4.10).

Figure 4.11 Outline of the structural setup of the Afar Depression with their respective extension direction. The Erta’Ale Range and the Manda-Hararo–Tendaho rifts, southern limit of the Red Sea rift, are aligned oblique to the western Afar margin. AAZ-arcuate accommodation zone. Extension direction for the western Afar margin (after Collet et al., 2000).

4.7 Reconstruction of the Nubian-Arabian-Somalian