Environmental and Social Impact Assessment - Gibe III

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Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099 Finally, during the Quaternary period, the lower parts of the rift valleys were filled with alluvium poorly consolidated sediments that now make up the extensive plains of southern Ethiopia and the depressions of Lake Turkana and Chew Bahir. Numerous salic intrusions of rhyolite, trachyte, microgranite, and microsyenite occur throughout of the project area. 5.1.2.2 Tectonic Setting According to the geostructural interpretation from Landsat imagery described in the L1D Geological Report, two main structural blocks seem to characterize the region: i) a northern block, situated north of the village of Sodo, with lineament trends oriented NNE/SSW and E-W, and ii) a southern block located south of Sodo, which according to the authors seems affected by a clockwise movement of about 30°; consequently, the two main fault trends would be reoriented to NE-SW and ESE/WNW respectively. Hypoabyssal magmatic events would occur along the main fault trends. The structural photo-interpretation led to the recognition of 8 lineament sets over the area of interest; of these, the main lineament sets are directed N20°E and N120°E whereas subordinate ones have directions N50°E, N70°E, N135°E and N160°E. Two minor sets are directed N-S and E-W anyway they are represented by too few lineaments to be considered significant at the observation scale. The photo-interpreted lineaments show coherent patterns suggesting that most of them can be associated to possible regional fault or fracture systems; it must be clear that photo interpretation allows for the recognition of steep or near-to-vertical discontinuities, whereas sub-horizontal or gently dipping structures generally escape the interpretation. An interesting feature is represented by the tectonic control on the local morphology and especially on the Gibe river course. In particular, the N20 set, parallel to the MER trend, can be held responsible for the onset of the regional direction of the Gibe course. No regional structures at the scale of the whole riverbed exist. Rather, the Gibe river course set up along a series of minor N20 trending fault or fracture zones. Transversal discontinuities (e.g.: N70, N110, N135) may originate local deviations of the river course which are clearly tectonically controlled. Tentatively, the N20 and the N120+N160 sets could be associated to a same regional stress field with maximum compression axis 1 directed N-S, as the intersection angles are consistent with a conjugate discontinuity system; in this interpretation, the N120 and N160 sets are associated together as the expression of a same set. Alternatively, a N20 – N70 structural association could be the expression of a regional 1 directed approximately NE-SW (N45°E). It is opinion of the authors (300 GEO R SP 002 A) that the presence of different lineaments trends might be associated, at the reservoir scale, to variations in the regional stress field, rather than to the rotation of rigid basement blocks, as there is no clear evidence of structures allowing for such a rotation. However, field structural data are necessary to define a structural model coherent with the regional stress state. As one can see, the mean lineament length is quite low, in the order of 1.000 to 4.000 m; standard deviations are high anyway, indicating a high data dispersion around the mean value; maximum interpreted single-lineament lengths are in the order of 20.000 m or, most frequently, 10.000 m. Considering the alignments of single elements, an observed persistence in the order of 30-40 Km can be reached, especially for some lineaments intersecting the Gibe Course with NW-SE direction (i.e. N135°E to N160°E). CESI SpA - Mid-Day International Consulting Engineers Page 79
Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099 Referred to the project the study area was involved in a progressive positive movement of the plutonic rocks, trachytic flow and part of the older columnar basalts. It seems that the trachyte was extruded in a single phase episode. The differences found in the petrographic structure between the base and the top of the outcrop in the right bank seems due to magmatic differentiation. At first sight, the left bank seems less involved in the tertiary pre-rift movements, it appears more massive than the right bank. The trachytic rocks CESI SpA - Mid-Day International Consulting Engineers Page 80
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