3.4.2.1 Fault distribution in the Magadi-Natron trough
In the SKR under study, intra-rift faults are restricted to the Magadi-Natron inner floor, as being totally absent in the OOB, except the abovementioned Sanjan fault. For simplicity, the volcanic inner floor is here considered to be uniformly composed of the Magadi trachyte series, since its subdivision into poorly-defined volcanic formations (Muirhead et al., 2016) does not appear to supply any further insights onto the spatio-temporal development of inner strain. In the same way, the thin sediments that locally overlie the volcanic floor (Baker, 1958) are assumed not to have recorded any significant fault-related strain history. The inner fault array is not homogeneously distributed over the volcanic axial floor. Fault density is maximum in the ~15
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wide axial belt extending continuously from the Magadi Lake (N) to the Gelai volcano (S). Unfaulted, lense-shaped regions are observed around a number of axial volcanoes (Olorgesaile and Gelai), indicating that the latter likely acted as resistant nuclei with respect to brittle strain. The flexural hinge zone to the east is much less faulted, with a 15 km-wide monoclinal volcanic surface, dipping regularly westwards (Figs. 3.2, 3.5).
The relatively high density of inner faults is also expressed by a mode (maximum frequency) for spacing < 3 km along the 12 seriated cross-sections in Fig. 3.2A. One of the most striking structural features of the intra-rift fault population is the NE/SW alignment of the southern tip points of the easternmost structures (x 19 data), parallel to the NW edge of the ENS basement salient, while keeping a nearly constant submeridian orientation (Fig. 3.7A). The origin of this specific fault map-arrangement, as resulting from the barrier effect exerted by the ENS salient on fault propagation, is tested below from fault displacement/length profiles.
Fig.3.7. The Engare Nairobi transverse basement salient (ENS) on the eastern flank of the Natron trough. A. Map-trace of the inner fault array, immediately N of the ENS, on the SRTM-30 DEM.
Note the NE/SW alignment of their southern tip points, parallel to the NW edge of the ENS. B.
Topographic section showing the tilted-fault block structure of the ENS in relation to a transverse normal fault network, facing inwards to the NW. Vertical exaggeration = 20. Location in Fig. 7A.
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Displacement and length values of 140 inner faults have been analyzed in an attempt to provide insights into the propagation and slip history of the intra-rift fault array in the Magadi-Natron axial trough. Fault lengths range over three orders of magnitude from 2 km up to >50 km, with a peak value in the class 5-10 km (Fig. 3.6B).
When plotted on the log-log diagram in Fig. 3.6C, their Dmax/L ratios define a field that fits within the linear distribution established from published data (Kim & Sanderson, 2005).
Most of displacement profiles display second-order corrugations that coincide with higher displacement gradients at overlapping segment tips, as examplified by faults A6 and A71 in Fig. 3.8A. Such fault patterns are classically attributed to the shift of the slip peak towards interacting segment tips in the stress shadow of adjacent faults (Walsh
& Watterson, 1988; Peacock & Sanderson, 1991). More interestingly for our purpose is the asymmetrical versus symmetrical shape of fault displacement profiles, known to define specific fault histories. Three main slip profiles are usually distinguished (Pollard
& Segall, 1987; Scholz & Cowie, 1990). Elliptical or bell-shaped profiles, with a linear decrease in displacement from the fault center to tip lines, characterize isolated, or unrestricted faults (Figs. 3.8B, C) (Dawers et al., 1993). In this ideal, but rare case, growth of a given fault is not disturbed by any adjacent structures. But, most of natural examples display higher displacement gradients adjacent to either one (half-restricted) or both (double-restricted) tip-lines (Figs. 3.8B, C), in response to impeded fault growth by interaction with confining features (Peacock, 1991; Burgmann et al., 1994; Nicol et al., 1996). Abrupt terminations of slip at fault ends are attributed to various mechanisms, including either interferences with adjacent faults (Dawers & Anders, 1995), or the mechanical effect of heterogeneities or stronger material acting as barriers to fault propagation (Aki, 1979).
In the present study, we are a priori concerned by the latter process, given the plausible barrier effect of the triangle-shaped ENS on fault propagation, south of 2°S. One way to test this hypothesis is first to verify the expected southerly-restricted profiles of these faults, with steep displacement gradients southwards, and then to compare these results to those obtained about the remaining inner fault population
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which should not be necessarily restricted structures. Among the 19 analyzed fault profiles in the eastern zone, the majority of them (x 18, i.e. 94%) are restricted, hence in agreement with the working hypothesis above (Fig. 3 . 8B, Table 1). However, the weak number of expected southerly-restricted tips (x 1, i.e. 6%, Fault A47 in Fig.
3.8B), which furthermore does not occur specifically close to the ENS, does not give any support to the presumed model. Displacement profiles of the remaining inner fault population (x 121), which should have propagated more freely away from the ENS indenter (Table 3.1), still show a majority of restricted profiles (x 100, i.e. 82%), most of them are restricted on both tips (x 72, i.e. 59%), whereas the same amount of faults (x 14) are restricted on either northern or southern tips (Fig. 3.8C). Again, southerly-restricted faults are evenly distributed within the Magadi-Natron axial fault belt.
Comparing the two analyzed fault populations (140 data) shows that: i) they both display a high ratio of restricted/unrestricted faults, as similarly documented about most fault systems elsewhere (Manighetti et al., 2004), ii) southerly-restricted faults are very few, and iii) not spatially confined north of the hypothetical ENS mechanical barrier.
Our results contradict those of Muirhead et al. (2016) who inversely assessed that most of their analyzed inner faults (40 data) display symmetric (bell-shaped) displacement profiles, typical of unrestricted fault growth. Based on the exhaustive analysis of 140 inner fault displacement profiles, we come to the conclusion that the restricted nature of most intra-rift normal faults in the Magadi-Natron half-graben is not an unambiguous diagnostic criteria for inhibition of tip propagation against a mechanical barrier. Without any further detailed analysis of the inner fault population, its prominent restricted nature should be rather related to strong interactions of closely-spaced faults in a structurally mature rift, as documented elsewhere (Dawers & Anders, 1995). This is in agreement with the low average fault spacing (<3 km, see above) which does not seem to conform to the stress-reduction shadow commonly documented around normal faults (Ackermann & Schlische, 1997) and that likely indicates that the entire inner fault system in the SKR has attained saturation (Wu & Pollard, 1995).
The only conceivable structural impact of the ENS triangle-shaped basement salient on the SKR rift pattern is to have provoked the gradual narrowing of its axial trough southwards over a >30 km-long rift parallel section, and thus to have caused a deficit of extension, accordingly. Extension estimates are presented below, not only
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about the Magadi-Natron half-graben as previously performed by Muirhead et al. (2016), but also about the OOB offset rift block to the west.
Fig.3.8. Map traces and displacement profiles for inner normal faults in the Magadi-Natron rift trough. A. Faults traces and corresponding slip distribution along two highly-segmented faults (A6, A71), facing eastwards. Location in Fig. 5A. Segment tip point (full circle). Overlapping segments in black. B. Examples of displacement profiles for unrestricted (A48), double-tip restricted (A70), half-restricted (to the north, A24, and to the south, A47) faults, north of the ENS basement salient. Location in Fig. 7A. C. Examples of displacement profiles for unrestricted (A63), double-tip restricted (A54), half-restricted (to the north, A32, and to the south, A46) faults in the axial part of the trough. Location in Fig. 5A.
Table 3.1. Restricted versus unrestricted fault populations in the Magadi-Natron half-graben.