Estratigrafía

This sector is located in the north of the study area, it is marked by an interesting topography that includes Loita Hills that are located west of Nguruman border faults (fig.4.2). Its topography is as high as ~ 2500 m a.s.l (fig. 4.5a). It lies on top of two major geological formations, the Proterozoic metamorphic basements and the syn-rift trachytes of the Lengitoto (Kirikiti) origin (7 - 1.7 Ma) (Baker, 1958; Crosley, 1979; Birt et al., 1997) (fig. 4.5b).The eastern end of this area is bordered by east facing, N-S striking Nguruman fault system which is composed of two parallel faults that have been active at different times. The oldest is the west Nguruman fault (NgW) whose age is estimated to be 7 Ma while the east Nguruman (NgE) which is dated at 1.7 Ma. Spacing between these faults decreases southward from 8 km in the north to 3 km in their southern tips. They are both segmented making relay zones whose length vary between 1.5 and 8 km (fig. 4.5a).

Drainage networks in this sector correspond to W1-3 catchments and Rw1-3 river systems (figs. 4.5a and 4.5b). Values of surface area, HI and AF in W1 are 721 km², 0.51, and 21, respectively. This indicates that this landscape is relatively incised and tilted (fig 4.5c and 4.5d respectively). Rw1 river system (Ewaso Ngiro) is a dendritic drainage pattern flowing from the uplifted metamorphic basements in the Loita hills, then becomes parallel crossing NgW, before turning eastward and cutting through NgE at right angle into the hanging-wall.

W2 is located to the south of W1, its high (0.65) HI values indicates that incision is high. It is a weakly tilted water catchment due to its low AF value (6). The main river network Rw2, is a trellis drainage pattern whereby its low order streams follow the shape of metamorphic foliation (circular flowing river networks) joining the high order stream at right angles which flow parallel to one of segments of NgW then passes in the southern tip of NgW segment and then flow South eastward for about 6 km before incising the hanging wall of NgE at 50° (fig.4.5).

W3 is located south of this sector and has HI 0.69 which can be interpreted as representing a water catchment with high incision rate its AF (4) shows that is weakly tilted.

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Fig. 4.5. Main topographical and geological units of the Nguruman-Loita domain. A. DEM showing main topographical variation and river steepness indices for main drainage networks. B.

Simplified geological map incorporated with drainage networks (legend as in Fig.s 4.1 and 4.3).

Main river streams are shown in thick lines. C, D and E. Main topographical and structural blocks.

Vertical exaggeration X10. F. Along river profiles showing drainage shapes and main drainage anomalies marked as knickpoint (KPf in red and KPd in black arrows).

The main stream Rw3 is a dendritic drainage pattern that flows southward from Loita hills, depositing recent sediments into depression within Proterozoic terrain (fig. 4.5b) and then it is diverted to follow the morphology of metamorphic foliation before crossing both NgE and NgW at right angles and into the hanging wall of the NgE. The shape and the direction which this stream takes indicates the role of both lithology and structure on the control of the stream flow.

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Long-river profiles of Rw1-3 system present some clear knickpoints either pinned to major faults (named KPf) or diffused in the drainage system (named KPd) (Figs. 4.5f).

Ksn values obtained in this area range between 13 and more than 800. We observe a high correlation between high Ksn values and the presence of KPf pinned to NgW and NgE faults in the eastern part of this domain.

EW topographic profiles (fig. 4.5c and 4.5e) shows three major topographic observations i) paleo-topography dipping E, surface dipping N and S (these changes of tip direction are observed from the most elevated surface of the area); ii) first (older) tilted block due to NgW escarpment; iii) second (young) tilted block due to NgE. Topographic profiles above and fig. 4.5d show that i) Loita hills act as drainage divide due to its highest elevation leading into changes of water course direction either eastward, or southward;

ii) there is a contrast of competence between rocks marked by the presence of folds and iii) in the eastern side of the study area, slope decreases southward making rivers run parallel to faults.

We also note that the deflection of Rw1 and Rw2 rivers (flowing E-W in the upstream part, from N-S in the middle part) crossing NgW escarpment seems to be controlled by the hanging wall motion of this normal fault. This pattern is classically described in the evolution of rift topography (e.g., Cowie et al., 2006).

General remarks on fault pattern and drainage organization is that, there are two main faults in which NgW is the oldest followed by NgE. Drainage network in the northern (Rw1) flows and cuts both faults changing its direction along, because to the far W of the NgW the main direction of the river is towards E, but once it cuts NgW it changes its direction and flow southward before changing its course in the tip of one of synthetic fault segments where it turns its direction toward E cutting through NgE. Other two drainages in this sector show strong control of both, lithology and tectonic where by rivers flow by following the shape of metamorphic foliation and also the effect of faults whereby rivers follow the fault direction e.g., Rw3 or Rw2 which flows passing through relay zones NgW segments. Also the flow direction of courses of rivers are controlled by differences of slopes of block, in the northern Rw1 starts as a N-S river then it is directed towards E by the influence of topography, while Rw2 and Rw3 start as N-S drainages before being diverted as discussed above.

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121 4.4.2 Sonjo transfer fault zone (STFZ).

Sonjo transfer fault zone marks the transition zone from narrow N-S rift to a wide double border fault rift system which lies in a heterogeneous formations mainly composed of Proterozoic metamorphic basements and syn-rift volcanic rocks of the Lengitoto (7 Ma) and Oldoinyo Sambu (3.5 Ma) (Foster 1997) (fig 4.6a). Its topography is as high as

~ 2500 m a.s.l (fig. 4.6b). This sector is marked by two major fault systems: i) the N-S striking faults composed of the southern and northern tips of Nguruman and Oldoinyo Sambu faults respectively, and ii) a cluster of NE striking Sonjo transfer faults (STFZ).

The STFZ is a transverse NE-SW fault system (striking between 40°-70°) juxtaposed between two major N-S faults namely the Nguruman-Sambu fault system in the East and the Oldoinyo Ogol fault in the West (fig. 4.6a). The STFZ is a major transitional area connecting the classical N-S South Kenya rift trough to a more diffuse wide rift. In this system faults are closely spaced between 0.5 to 4 km, forming horst and graben systems (e.g., fig. 4.6c).

Three water catchments W4-6 (rivers Rw4-6 respectively) are found in this in this part of the study area. W4 has, surface area 78 km², HI and AF, 0.62 and 11, respectively.

Both HI and AF values correspond to a highly incised and relatively asymmetric catchment. Rw4 originates from the elevated block which is comprised of metamorphic foliations (fig.4.6a), flowing eastward through the southern tip of NgW and into the hanging wall of NgE.

W5 has an HI of 0.5 indicating that this drainage network is moderately incised.

AF analysis indicates that this water catchment is weakly tilted with AF value of 4. The main river channel Rw5 originates from the uplifted block like Rw4, it flows south eastward passing through the north eastern end of the SFTZ then crossing the Oldoinyo Sambu fault at 50°, later it crosses the southern tip of NgE, so this river passes through both systems marking the beginning of the transfer systems from N-S to NE-SW faults.

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Fig. 4.6. Main topographical and geological units of the STFZ. A. Simplified geological map integrated with river networks (main rivers are shown as thick lines). B. DEM showing main topographical variation and river steepness indices for main drainage networks (Ksn legend as in Fig. 4.5a). C. NW-SE Topographical profile showing main STFZ faults. Vertical exaggeration X10. D. N-S topographical profile showing N-S structural trends. E. Along river profiles showing drainage shapes and main drainage anomalies marked as knickpoint (KPf in red and KPd in black arrows). F. DEM showing variations of KPf and KPd along river networks.

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The largest water catchment in this domain is W6 covering the area with all transfer faults. This catchment has HI 0.58 meaning that it is a highly incised drainage network. AF value is 32 indicating that this drainage is strongly tilted. Drainage network is dendritic drainage pattern whose main river Rw6 has two main streams each representing different surficial characteristic and response to the SFTZ. The first main tributary (Rw6a) originates from northern uplifted block (fig. 4.6a and b) of the sector in which high order tributaries from all sides join each other into the low order main stream then flow southward at the crest of metamorphic foliations and bent into hemi-graben and join the other main tributary at the southern tip of the SFTZ. The second main tributary (Rw6b) flows southward from the western tip of the SFTZ bending eastward through the foliation and then flow parallel to one of NE-SW fault joining the other major tributary.

At the confluence, the two main tributaries are joined by tributary that passes through the centre of the SFTZ and then flow at 30° towards the Natron hanging wall where they deposit sediments into the Lake Natron basin.

Along river profiles show that Rw4 and Rw5 are short rivers with less than 40 km of length and both profiles have steep slopes. For both rivers have KPf and KPd where their KPf are located few meters away from NgW and one of them is located at the north eastern tip of SFTZ while KPd are located in the metamorphic foliation. The location of KPf is in correlation with the elevated Ksn values for both river networks (Rw4 and Rw5) (fig.4.6e and f). The migration of KPf away from NgW is here interpreted as the old elevated fault system in which there was enough time for its readjustment to uplift that was caused by the formation of NgW. The main river stream in this sector is the Pinyinyi (Peninj) river here denoted as Rw6. Stream Rw6a passes through the transfer faults while Rw6b avoids crossing main faults, it rather runs parallel and bends at the fault tips (fig. 4.6a). Longitudinal profile of this river is ~100 km long with gentle slope except at ~ 35-40 km and 98 km downstream where both 2 knickpoint families are observed (fig. 4.6e). These knickpoints are associated with Sonjo-transfer faults (NE-NW) and the Natron fault (N-S fault) from upstream to downstream respectively. High values of Ksn (> 500) correspond to KPf which are pinned to main scarps due to Natron and STFZ uplift. A KPd is also observed in the foliated Proterozoic metamorphic basements. High Ksn values also correspond to slopes above 28°.

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General remarks on drainage organization with respect to faults and uplifted block such as Oldoinyo Sambu volcano is that, in the northern part faults behave in a similar pattern to that of the Nguruman sector, that means faults have control on the direction of rivers in such way that Rw4 and Rw5 cross NgE at its tips. Rw4 is controlled by either lithology, faults or the uplifted block, that is in the western part, tributary Rw6b passes through metamorphic foliation before cutting across the northern tip of OOF and run parallel to one of transverse faults, and later flow approximately N-S through the control of Oldoinyo Sambu topography (fig.4.6a). These configurations of faults, drainage and faults indicate that the current drainage network is younger than both faults and Oldoinyo Sambu volcano.

4.4.3 Oldoinyo Ogol block (OOB).

Geologically this block is located between three major lithological formations as follows i) in the west of the uplifted Oldoinyo Ogol escarpment, the area is bordered by metamorphic Proterozoic basements, ii) at the centre and east there are syn-rift sedimentary and volcanic rocks marked by Oldoinyo Sambu volcanic rocks (3.5 Ma:

Foster 1997), Peninj group and iii) syn-rift volcanic rocks of the Ngorongoro crater highland (5-1 Ma: Foster 1997, Le Gall, 2008) located in the south of the sector (Fig.4.7a).

The Oldoinyo Ogol escarpment is as high as ~ 2300 m a.s.l while the most elevated sector is the Ngorongoro volcanic highland whose elevation is more than 2800 m a.s.l (fig.4.7b).

This sector is composed of three parallel N-S faults from east to west Natron, Sanjan and Oldoinyo Ogol fault (OOF), they are placed between 8 and 30 km where by Natron (NF) and Sanjan (SF) are relatively close to each other. The Natron fault (~ 60 km long) is the southern continuation of the Sambu fault, its hanging-wall borders Lake Natron which receives most of sediments that are eroded from the up-streams. The Sanjan fault (~ 30 km long) is located west of Natron fault, together with the Natron fault they mark the eastern border of the 30 x 60 km perched basin (Oldoinyo Ogol) that was probably uplifted during the uplift of Natron, Sanjan and Oldoinyo Ogol blocks. The more complex and oldest fault scarp is the Oldoinyo Ogol fault (~ 80 km long) that is dated synchronously with the Oldoinyo Sambu basalts at ~3.5 Ma (Foster 1997).

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Fig. 4.7. Main topographical and geological units of the OOB. A. Simplified geological map integrated with river networks (main rivers are shown as thick lines). B. DEM showing main topographical variation and river steepness indices for main drainage networks (Ksn legend as in Fig. 4.5a). C. N-S Topographical profile showing main N-S topographical variation. The influence of Crater highland on the topography is well noted in the southern end and the lowest topography represents both the Salei plain and OOB plain. Vertical exaggeration X20. D. DEM showing slope variation, it is marked that high slopes are either represents fault scarps or crater highland. E. Along river profiles showing drainage shapes and main drainage anomalies marked as knickpoint (KPf in red and KPd in black arrows). F. DEM showing variations of KPf and KPd along river networks.

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The north OOF is marked by closely spaced N-S segments (fig.4.7c) which die out in the Proterozoic metamorphic basement foliations. The OOF is the most uplifted scarp of the sector where the estimated elevation is 2300 m (a.s.l), it is sloping southwards attaining the minimum elevation of ~ 1300 m (a.s.l) (fig.4.7d).

The OOB is comprised of two water catchment, W7 and W8. i) W7 has an HI 0.57 indicating that its incision is high, moreover and its drainage network is highly tilted with AF value of 20. Its river (Rw7) a parallel drainage network whose main channel runs from the metamorphic basements, it flows south eastward incising the Natron fault at 80°

depositing sediments (Peninj sediments) at the hanging wall of Lake Natron. ii) W8 with HI 0.38 and an AF values 28 which is a strongly tilted water catchment. Owing to its morphological complexity, W8 is comprised of a complex drainage system made up of six individual river tributaries that cover the entire OOB joining one another before incising the Natron escarpment at 90°. From south of the SFTZ, the geometry and flow direction of each main tributary is as follows a) the first tributary (Rw8a) originates from the southern extreme of SFTZ flowing southward before diverging into SE direction incising the Sanjan fault (fig.4.7a). b) The source of the second river (Rw8b) is from the NW of the OOB, it flows from the Proterozoic metamorphic basement foliations following the shapes of the folds and then bend eastward in the OOB plain. Both low and high order streams avoid passing through the basement, they rather find their way between structures, thus adapting their shape to the geometry of older structures. c) The tributary (Rw8c) flows through the centre of the OOB and OOF where the N-S topographic configuration is relatively low by ~ 500 m as compared to its northern and southern reliefs. High order stream of this river flows first SE and bends eastward due to influence of hard Proterozoic metamorphic foliation. d) and e) both (Rw8d and Rw8e) flow south eastward starting from the western terminal of the uplifted OOB flowing southward and parallel to the foot wall of OOF then they both bend to link up with another river stream. f) Rw7f is the longest river network of the OOB originating about 50 km off the OOB, it passes at the southern tip of the scarp and into the OOB plain, joining up with other rivers from the Ngorongoro volcanic highlands incising both Sanjan (at the northern tip of Monsonik volcano) and Natron faults to pour sediments into Lake Natron. The OOB plain is filled with Proterozoic basement sediments eroded by drainage networks flowing in this sector, the plain has high density due to the fact that it is composed of loose materials which are easily eroded and allow passage of water.

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In the OOB, along river profiles for Rw7 and Rw8 are characterized by both KPf and KPd whereby KPf are localized along the two main scarps. In the Natron, Sanjan and along the Oldoinyo Ogol main scarp (fig.4.7e), all KDfs correspond to high Ksn values (above 500) (fig.4.7f). The Sanjan river (Rw8) is a complex drainage system that flows through out the Oldoinyo Ogol domain pouring its water into Lake Natron. In this domain Ksn values can be sub-divided into three categories. i) High localized Ksn values that correspond to most elevated areas with high values of slopes (fig. 4.7d), these are associated to recent fault activities. ii) high to medium Ksn values that are diffused away from the main scarps, corresponding to older faults especially the Oldoinyo Ogol fault and Ngorongoro volcanic highlands (> 3.5 Ma) and iii) low Ksn values in the Oldoinyo Ogol plateau, this is probably due to absence of active tectonic activities taking place during the quiety period (after tectonic activities that led to the placements of current system).

Topographical configuration of faults, crater highland and directions of drainage networks in OOB lead into the following interpretation of tectonic events, i) drainage networks pass through OOF perpendiculary in E-W direction, ii) same river networks pass through the Sambu fault perpendiculary, iii) there are three main drainage directions, i.e., N-S in the northern OOB, E-W in the OOB basin and NW in the crater highland. The presence of KPd in northern OOB and in the west of OOB and its absence in the crater highland when interpreted together with the presence of high Ksn along main escarpment of OOF, and in the elevated crater highland block and the low values of Ksn in the Salei plain and OOB block indicate that drainage in the OOB is older than that which flow from the crater highland. This brings about two tectonic scenarios which are i) in the first place drainage in the OOB evolved in which E-W direction was dominant (3.5 Ma ago) then ii) drainage NW drainage from crater highland evolved (fig.4.8). This is also supported by the fact that all drainage from the crater highland act as tributaries to main sub E-W main drainages which eventually pours its water into Lake Natron.

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Fig. 4.8. Chronological arrangement of events taking place during uplift of both OOF and CH and their control on the direction of drainage networks. A. The SRTM-DEM with drainage networks and B is the proposed model. The first phase is the uplift of OOB and OOF which results into formation of main E-W drainage networks and the second phase is the uplift of CH which results into NW-networks which later join E-W drainages.

4.4.4 The NE-SW transverse structure of the Eyasi Escarpment.

The centre of Eyasi escarpment marks the limit between the Tanzanian craton and the Mozambique Proterozoic mobile belt (fig.4.9a). In NE the Eyasi scarp is bordered by the Ngorongoro crater highland which is highly marked in topographic profile of Fig. (fig.

4.9d). The most remarkable structure in this sector is a highly segmented Eyasi fault

4.9d). The most remarkable structure in this sector is a highly segmented Eyasi fault