Bouguer anomaly

3-21 | P a g e Figure 3.7: Bouguer density by Comparison with Variance of the Upward-Continuation (CVUR).

3-22 | P a g e minimum-curvature constraint by introducing some tension into the surface. This technique tries to force a thin elastic plate to go through randomly-spaced (x,y,z) dataset and generates a binary grid file of gridded values z(x, y) by solving:

(1 − 𝑇) × 𝐿(𝐿(𝑧)) + 𝑇 × 𝐿(𝑧) = 0 [3.24]

where 𝑻 is a tension factor between 0 and 1, 𝑳 indicates the Laplacian operator, and 𝒛 is small displacements a thin elastic plate of constant flexural rigidity.

The Surface module achieves this by finding a function that satisfies the following constraints (Smith & Wessel, 1990):

𝑍(𝑥_𝑘, 𝑦_𝑘 ) = 𝑧_𝑘, for all data (𝑥_𝑘, 𝑦_𝑘, 𝑧_𝑘), 𝑘 = 1, 𝑛 (1 − 𝑡)∇⁴𝑧 − 𝑡∇²𝑧 = 0 elsewhere

[3.25]

where 𝒕 is the tension, 𝟎 ≤ 𝒕 ≤ 𝟏.

3-23 | P a g e Figure 3.8: Bouguer anomaly map calculated for a Bouguer density of 2.23 × 10³ kg m⁻³. The Bouguer anomaly observed has high values trending NNW-SSE that changes from -189 mGal to -134 mGal, with a dominant two-fold anomaly located to the north-northwest of the caldera and within the caldera (see Figure 3.8). The low Bouguer anomaly is associated with Solai graben and Rongai plain located to the north-east and south-west areas, respectively. As seen from local geology, these low anomaly zones are covered by volcanic soil and sediments.

The high Bouguer anomaly appears to be confined within the caldera and to the immediate north-west of the caldera (Olrongai hill) and is associated with the occurrence of trachytic rocks as well as geothermal surface manifestations as evidenced by fumarolic activities. The caldera rim and the extensive faulting seem to control the extent of the hydrothermal system.

3-24 | P a g e 3.6.3 3-D gravity modeling

The planar slices at elevations of 1000 m asl (Figure 3.9), sea level (

Figure 3.10) and 1000 m bsl (Figure 3.11) were extracted from the inversion model, then overlain on a shaded relief map and plotted alongside faults. These anomalous areas are viewed in shallower levels as discrete bodies but converge with depth to form two main bodies; lying within the caldera and the Olrongai area. The flow of magma through and along the faults in the shallow parts of the system seems to have caused the differences of density distributions with depth.

These two main bodies coincide with the presence of trachytes while the dense bodies north of Olbanita and also north and east of Solai seem to be associated with the presence of phonolites.

A striking relationship is also observed between the occurrence of the high-density anomalous structure and the existence of geological faults. The two appear to coexist, and this association becomes even more distinct with increasing depth, implying that these structures acted as

3-25 | P a g e conduits during the ascent and emplacement of at least the phonolites and trachytes. The high-density anomalous structure beneath the caldera appears to be confined mainly within the caldera walls while NNE-SSE and N-S trending structures enclose the anomaly beneath Olrongai. This relationship underscores the significance of these structures (i.e., ring structures and faults) in guiding and restraining the up flow of magma and also the hydrothermal system.

Figure 3.9: A planar view of density distribution at 1000 m asl elevation.

3-26 | P a g e Figure 3.10: A planar view of density distribution at sea level.

3-27 | P a g e Figure 3.11: A planar view of density distribution at 1000 m b.s.l elevation.

The isosurface of 2.45 × 10³ kg m⁻³ in Figure 3.12 infers the geometry and positions of shallow-crustal high-density volumes located beneath the caldera and Olrongai hill. The isosurface displays the geometry of the high-density structure of the study area, and similar isosurface was also used in Figure 3.13 and Figure 3.14. The two masses appear as discrete bodies, but they probably may be connected at depth. These two high-density features underlie Menengai caldera and Olrongai areas that have been explored for geothermal power generation. Menengai caldera and Olrongai are known by their surface manifestation of the high-temperature geothermal system, and the presence of dense bodies suggest the heat source of this area.

3-28 | P a g e Figure 3.12: A planar profile and isosurface (2.45 × 10³ kg m⁻³) inferred from the density model.

The orientation of tectonic faults shown in Figure 3.13 appears to delimit the lateral extent and intensity in distributions of the dense body beneath Olrongai hill. The relationship is still apparently visible despite the depth difference between surface structures and the dense body.

These Upper Pleistocene faults (McCall, 1967) are relatively younger and comprise of new faults and renewals of old fractures. The presence of fumaroles at boiling temperatures (Mungania &

Lagat, 2004), located adjacent to faults suggests that the geothermal reservoir is positioned at a relatively shallow depth and likely beneath the fumaroles.

3-29 | P a g e Figure 3.13: A display of geological faults as seen from the surface and fumaroles location superimposed against a deep planar layer at sea level and an isosurface of 2.45 × 10³ kg m⁻³ to highlight the dense body of Olrongai hill.

Unlike Olrongai area, the caldera in Menengai has no clear subsurface structures other than the E-W arcuate structure, which appears to have aided the ascend of younger trachytic magma to the surface (Figure 2.2). The pattern of this structure coincides with a high-density anomaly shown in Figure 3.14. The eastern end of this structural feature seems to be connected with a series of NNE trending normal faults. These faults consequentially render the caldera wall incomplete as it cuts through the northeastern rim by north-south trending segment graben (McCall, 1967), marking the southern end of the Solai graben. Some inferred faults are presented in Figure 3.14 based on the shape of the dense body, the caldera rim, and the pattern displayed by the locations of fumaroles.

3-30 | P a g e Figure 3.14: A display of geological faults/caldera rim ring-fracture as seen from the surface and fumaroles location superimposed against a deep planar layer at sea level and an isosurface of 2.45 × 10³ kg m⁻³ to highlight the dense body of Menengai caldera. The inferred faults are presented as broken lines.

The location of fumarolic activities in most cases describes the zones of high thermal anomalies generally located close to or on the fault structures, which offer a pathway for fluids to rise to the surface showing zones of hydrothermal upwelling. A magma emplaced at shallow levels beneath the caldera may cause localized but significant deformation that might have led to the growth of the dome-shaped forced structure and possible fracturing. The loop-shaped pattern portrayed by the locations of fumaroles inside the Menengai caldera in Figure 3.14 seems to suggest the presence of an arcuate fracture system, which can be interpreted as surface-expression of a ring-fault created during the post-caldera collapse activity. Such arcuate fracture systems normally relate with the local intensification of the tension resulting from inflation of underlying magma bodies commonly seen or inferred at volcanoes (e.g., Anderson, 1936).

( Kg m^-3)

3-31 | P a g e A closer look at the caldera wall, the distribution of fumaroles and the shape of the dense body appears to propose an alternative view of NE and NW striking network of faults (Figure 3.14), which would support a possible extension of post-caldera faults across the caldera superficially obscured by younger deposits. The fumaroles appear to be located right on top of the dense trachytic formation but on the shallower levels of ~1000 m asl (see Figure 3.9), they appear close to the edges. This is probably because the hydrothermal system occurs at lithological contact zone where the permeable host-formation allows circulation of water that interacts and is heated by the intrusives embayed within the dense formation. The generation of a network of faults or a ring fracture and subsequent uprising of fumarolic fluids along these openings could hypothesize a possible coupling of the hydrothermal-magmatic system, considering the indication of magma pockets or dikes interpreted from the borehole data to be as shallow as 2 km (i.e. at sea level).

Although the results obtained from the inversion model are encouraging, there are some limitations. First, a significant part of the caldera floor is known by its rugged terrain that is mostly inaccessible (Mungania & Lagat, 2004). Therefore, the gravity data coverage is somewhat sparse (Figure 3.8), and may not be conclusively used to infer small changes in density or explicitly define lithological contacts. Hence, this study could not image these pockets of magma since they are confined within the trachytic and syenitic formations.

Secondly, other than the E-W arcuate structure that is readily observable, the presence of faults inside the caldera floor is poorly known although some significant attempts have been made to uncover them (e.g., Riedl et al., 2015; Strecker & Melnick, 2013). The caldera floor is covered with more than 70 post-caldera lava flows, pumice deposits and strombolian cinder cones (Macdonald et al., 2011). This makes it difficult to establish any existence of geological faults that would be useful in adding value to the model. Nonetheless, the imaging of the dense body beneath the E-W arcuate structure confirms the validity of the 3-D inversion model.

Lastly, we constrained the model in this study by including density estimates of lithologies interpreted from drill hole cuttings. However, intermittent losses of circulation affected the recovery process and led to incomplete lithological logs (GDC, 2018). Consequently, the depths

3-32 | P a g e affected by these losses were avoided and this narrowed the scope of available geology. To improve the robustness and reduce any uncertainty on the inversion model, a rigorous characterization of the rock unit densities from measurements of downhole and surface samples collected from the study area is necessary