Bio

DR. EDWIN DINDI - BIO

Dr. Edwin Dindi Ndubi is a Senior Lecturer in the Department of Geology, University of Nairobi. His areas of specialization are:  Exploration Geophysics, Seismology and Structural Geology. Prior to joining the University of Nairobi as a Tutorial Fellow in 1985, he worked as an Exploration Geophysicist in the Mines and Geological Department (Kenya) for 3 years. At the University of Nairobi he rose through the ranks to Lecturer in 1987 and Senior Lecturer in 1995.

DR. EDWIN DINDI - CV

Publications


2019

Kanoti, JR, Olago D, Opiyo N, Nyamai C, Dindi E, Kuria Z.  2019.  Characterization of Major Ion Chemistry and Hydro-Geochemical Processes in Mt. Elgon Trans-Boundary Aquifer and Their Impacts on Public Health. Journal of Environmental Earth Science.. 9(4) Abstract47529-51080-1-pb2.pdfWebsite

There is a gradual paradox shift from the utilization of surface water to groundwater in both urban and rural Kenya. This is because surface water is both diminishing in quantity due to climate variability and deteriorating in quality due to high levels of anthropogenic contamination. In the quest to attain the Sustainable Development Goal number 6 that aim at ensuring access to safe water by all by 2030, the Government of Kenya is encouraging the development of groundwater resources whose potential is enormous though it has not been quantified. The Inter-governmental Authority on Development (IGAD) supported this research on the shared Mt. Elgon trans-boundary aquifer between Kenya and Uganda aimed at understanding its dynamics. Mt. Elgon is a Tertiary age mountain that straddles the Kenya-Uganda border and has a trans-boundary aquifer. This study investigated the groundwater chemistry and its implication on water management and human health. Physico-chemical parameters of water that included electrical conductivity, pH, and temperature were measured in the field and the major cations and anions were measured at the Central Laboratories of the State Department for Water. Geological mapping and identification of sanitary risks were undertaken during the field work. The study revealed that the concentration of cations and anions in the groundwater varied spatially and temporally. Abundance of these ions were in the order Ca²⁺ > Na⁺ > Mg²⁺ > K⁺ for most samples and HCO₃⁻ > Cl⁻ > SO₄²⁻ >NO₃⁻. Interpretation of hydro-chemical data suggests that calcium carbonate dissolution, halite dissolution, Ca/Na ion exchange and Mg/Na ion exchange are the major processes that control the ground-water chemistry. Chemical results indicate further that the groundwater is suitable for domestic use but is threatened by both anthropogenic and geological factors. Extensive use of fertilizer and the destruction of the catchment area coupled with low permeability and rock-water interactions in the metamorphic rock terrains are the main threats to groundwater quality in the region. A few water points had water with some ionic composition exceeding WHO and the local KEBS maximum limits for drinking water. Such water pose a risk to human health.

2016

Dindi, EW, J.J. M.  2016.  Geological and geophysical characteristics of massive sulphide deposits: A case study of the Lirhanda massive sulphide deposit of Western Kenya. Journal of African Earth Sciences. 120:89-101. AbstractWebsite

An integrated geophysical ground survey was conducted on an airborne electromagnetic (EM) anomaly located in Kakamega forest of Western Kenya. The purpose of the study was to establish the existence of massive sulphides and identify suitable optimal geophysical method(s) for the investigation of similar anomalies. The study was also expected to provide information on the geological and geophysical characteristics of the deposit.

Field work involved electromagnetic methods: Vertical Loop (VLEM), Horizontal Loop (HLEM), TURAM EM and potential field methods: gravity and magnetics. Geochemical sampling was carried out concurrently with the geophysical survey.

All the geophysical methods used yielded good responses. Several conductors conforming to the strike of the geology were identified. TURAM EM provided a higher resolution of the conductors compared to VLEM and HLEM. The conductors were found to be associated with positive gravity anomalies supporting the presence of bodies of higher density than the horst rock. Only the western section (west of 625W) of the grid is associated with strong magnetic anomalies. East of 625W strong EM and gravity anomalies persist but magnetic anomalies are weak. This may reflect variation in the mineral composition of the conductors from magnetic to non-magnetic. Geochemical data indicates strong copper anomalies (upto 300 ppm) over sections of the grid and relatively strong zinc (upto 200 ppm) and lead (upto 100 ppm) anomalies. There is a positive correlation between the location of the conductors as predicted by TURAM EM and the copper and zinc anomalies.

A test drill hole proposed on the basis of the geophysical results of this study struck massive sulphides at a depth of 30m still within the weathered rock zone. Unfortunately, the drilling was stopped before the sulphides could be penetrated. The drill core revealed massive sulphide rich in pyrite and pyrrhotite.

An attempt has been made to compare characteristics of the Lirhanda massive sulphide deposit with those of better documented massive sulphides. Despite the fact that very little is known about Lirhanda, there are several similarities on the characteristics compared. These include evidence of back arc regional environment, calc-alkaline volcanic associations, conformity of anomalies to the structural trend of the host rock, proximity of synvolcanic rift, dispersive anomalies of copper and zinc in soils, presence of gossan and association of the deposit with strong EM anomalies.

J.D, M, S.A K, H. L, M. S, B.D F, G. K, E.W D, S. S.  2016.  Evolution of upper crustal faulting assisted by magmatic volatile release during early-stage continental rift development in the East African Rift. Geosphere. 12(6):1670-1700. Abstract1670.pdfWebsite

During the development of continental rifts, strain accommodation shifts from border faults to intra-rift faults. This transition represents a critical process in the evolution of rift basins in the East African Rift, resulting in the focusing of strain and, ultimately, continental breakup. An analysis of fault and fluid systems in the younger than 7 Ma Natron and Magadi basins (Kenya-Tanzania border) reveals the transition as a complex interaction between plate flexure, magma emplacement, and magmatic volatile release. Rift basin development was investigated by analyzing fault systems, lava chronology, and geochemistry of spring systems. Results show that extensional strain in the 3 Ma Natron basin is primarily accommodated along the border fault, whereas results from the 7 Ma Magadi basin reveal a transition to intra-rift fault-dominated strain accommodation. The focusing of strain into a system of intra-rift faults in Magadi also occurred without oblique-style rifting, as is observed in Ethiopia, and border fault hanging-wall flexure can account for only a minor portion of faulting along the central rift axis (~12% or less). Instead, areas of high upper crustal strain coincide with the presence of hydrothermal springs that exhibit carbon isotopes and N2-He-Ar abundances indicating mixing between mantle-derived (magmatic) fluids and air saturated water. By comparing the distribution of fault-related strain and zones of magmatic fluid release in the 3 Ma Natron and 7 Ma Magadi basins, we present a conceptual model for the evolution of early-stage rifting. In the first 3 m.y., border faults accommodate the majority of regional extension (1.24-1.78 mm yr⁻¹ in Natron at a slip rate ranging 1.93-3.56 mm yr⁻¹), with a significant portion of intra-rift faulting (38%-96%) driven by flexure of the border fault hanging wall. Fluids released from magma bodies ascend along the border fault and then outward into nearby faults forming in the flexing hanging wall. By 7 m.y., there is a reduction in the amount of extension accommodated along the border fault (0.40-0.66 mm yr⁻¹ in Magadi at a slip rate ranging from 0.62 to 1.32 mm yr⁻¹), and regional extension is primarily accommodated in the intra-rift fault population (1.34-1.60 mm yr⁻¹), with an accompanying transition of magmatic volatile release into the rift center. The focusing of magma toward the rift center and concomitant release of magmatic fluids into the flexing hanging wall provides a previously unrecognized mechanism that may help to weaken crust and assist the transition to intra-rift dominated strain accommodation. We conclude that the flow of magmatic fluids within fault systems plays an important role in weakening lithosphere and focusing upper crustal strain in early-stage continental rift basins prior to the establishment of magmatic segments.

G., WG, D. O, Dindi E, Owor M.  2016.  Genesis of the East African Rift System. Genesis of the East African Rift System. : Springer Abstract

The East African Rift System (EARS) started in Late Oligocene to Early Miocene time and gradually propagated southwards from the Afar Depression, beginning in the Middle Miocene. The hot, low-density mantle material of the Afar Plume heated the overlying lithosphere, causing thinning, regional doming, and the earliest basaltic volcanism in southern Ethiopia. In Ethiopia, the Afar Depression, the Main Ethiopian Rift, and the broadly rifted zone of southwestern Ethiopia represent the northern segment of the EARS. In the Kenyan sector of the EARS, uplift and doming also gave rise to the Kenya Dome. The radial flow patterns of the initial phonolites provide evidence for doming. Another important observation is that the rift geometry was greatly influenced by pre-existing structures of the underlying Mozambique Mobile Belt. Rifting proceeded through alternating episodes of volcanism and tectonics. Crossing into Tanzania, the influence of the neighbouring Tanzania Craton becomes evident. Here, the rift is expressed only in the northern part, splaying out in diverging half-graben valleys that are outside the Kenya Dome. Large boundary faults and opposing flexural margins, producing mobile asymmetrical full and half-graben basins that are individually linked along the rift axis, mark the Western Rift Valley. These basins are frequently occupied by elongate and narrow lakes (largely freshwater) separated by accommodation zones and containing significant hydrocarbon resources especially in the Albertine Graben. Small to large lakes existed in the EARS during the Plio–Pleistocene. Lakes in the Western Rift are large and deep, whereas those in the Kenya, Main Ethiopian, and Afar Rifts are generally small and shallow. Geological records indicate that the lakes sensitively responded to orbital forcing as well as to local, regional, and global climatic, environmental, and tectonic changes, resulting in fluctuating lake sizes and even desiccation.

2015

Dindi, E.  2015.  An assessment of the performance of the geophysical methods as a tool for the detection of zones of potential subsidence in the area southwest of Nakuru town, Kenya. Environmental Earth Sciences. 73(7):3643–3653. AbstractWebsite

The area to the southwest of Nakuru town in Kenya located within the Kenya Rift Valley is prone to incidences of ground subsidence especially toward the end of the heavy rain season. The zones affected by subsidence are typically linear, trending approximately north–south conforming to the structural trend of the Kenya Rift Valley. These ground subsidence incidences have in the past led to collapse of houses and damage to roads that happen to be located above the affected linear zones. This study set out to investigate the potential of geophysical methods to delineate these linear zones. Two profiles of lengths 2,430 and 2,850 m, respectively, were selected for this purpose. The study was conducted immediately after one major incidence of subsidence when the zones affected were still fresh and observable. Magnetics, very low-frequency electromagnetics (VLF-EM), and gravity methods were used in the study. The choice of these methods was dictated by their widely known applicability in the detection of linear structures, especially faults. The results of the study have shown that the linear zones affected yield good response to magnetic and VLF-EM methods. Anomalies similar to those detected across fresh subsidence zones were also detected at locations where there was no surface expression of subsidence, suggesting that the latter locations are potential zones of future subsidence. Further, there were locations along the profile that were without any anomalies. The relatively low sensitivity of the zones to the gravity method is attributed to the very narrow nature of these zone and hence limited low-density material to generate appreciable negative gravity anomalies. It is concluded from this study that geophysics permits identifying the potential areas to develop surficial collapse and to identify sectors that are potentially dangerous because they present similar signatures as the sectors with surficial evidence and sectors without anomalies. The sectors without anomalies are interpreted as having a different subsoil structure to that with sinkholes.

2013

Katternhorn, SA, Muirhead JD, W. DE, Fischer TP, Lee H, Ebinger CJ.  2013.  Contribution of transverse structures, magma, and crustal fluids to continental rift evolution: the East African rift in southern Kenya, December 2013. AGU FALL 2013. , San Francisco, California Abstract

The Magadi rift in southern Kenya formed at ~7 Ma within Proterozoic rocks of the Mozambique orogenic belt, parallel to its contact with the Archean Tanzania craton. The rift is bounded to the west by the ~1600-m-high Nguruman border fault. The rift center is intensely dissected by normal faults, most of which offset ~1.4-0.8 Ma lavas. Current E-W extensional velocities are ~2-4 mm/yr. Published crustal tomography models from the rift center show narrow high velocity zones in the upper crust, interpreted as cooled magma intrusions. Local, surface-wave, and SKS-splitting measurements show a rift-parallel anisotropy interpreted to be the result of aligned melt zones in the lithosphere. Our field observations suggest that recent fault activity is concentrated at the rift center, consistent with the location of the 1998 seismic swarm that was associated with an inferred diking event. Fault zones are pervasively mineralized by calcite, likely from CO2-rich fluids. A system of fault-fed springs provides the sole fluid input for Lake Magadi in the deepest part of the basin. Many of these springs emanate from the Kordjya fault, a 50-km-long, NW-SE striking, transverse structure connecting a portion of the border fault system (the NW-oriented Lengitoto fault) to the current locus of strain and magmatism at the rift center. Sampled springs are warm (44.4°C) and alkaline (pH=10). Dissolved gas data (mainly N2-Ar-He) suggests two-component mixing (mantle and air), possibly indicating that fluids are delivered into the fault zone from deep sources, consistent with a dominant role of magmatism to the focusing of strain at the rift center. The Kordjya fault has developed prominent fault scarps (~150 m high) despite being oblique to the dominant ~N-S fault fabric, and has utilized an en echelon alignment of N-S faults to accommodate its motion. These N-S faults show evidence of sinistral-oblique motion and imply a bookshelf style of faulting to accommodate dextral-oblique motion along the Kordjya fault. Fault relationships imply that the NW-SE transverse structures represent recent activity in the rift, and have locally tilted Late Pleistocene sediments. Given the abundance of N-S striking faults in the rift, the tendency for fault activity along transverse features suggests a change in the rifting driving forces that are likely the result of an interplay between strain localization at the rift center, inherited crustal fabric (NW structures in the Mozambique belt), a possible counterclockwise rotation of stress related to interacting rift segments in southern Kenya, and an active hydrothermal fluid regime that facilitates faulting. By connecting the Lengitoto fault to the rift center, the Kordjya fault has effectively caused the Magadi rift to bypass the Nguruman border fault, which has been rendered inactive and thus no longer a contributor to the rifting process

Muirhead, JD, Kattenhorn SA, Dindi EW, Gama R.  2013.  The evolving contribution of border faults and intra-rift faults in early-stage East African rifts: insights from the Natron (Tanzania) and Magadi (Kenya) basins, December 2013. AGU FALL 2013. , San Francisco, California Abstract

In the early stages of continental rifting, East African Rift (EAR) basins are conventionally depicted as asymmetric basins bounded on one side by a ~100 km-long border fault. As rifting progresses, strain concentrates into the rift center, producing intra-rift faults. The timing and nature of the transition from border fault to intra-rift-dominated strain accommodation is unclear. Our study focuses on this transitional phase of continental rifting by exploring the spatial and temporal evolution of faulting in the Natron (border fault initiation at ~3 Ma) and Magadi (~7 Ma) basins of northern Tanzania and southern Kenya, respectively. We compare the morphologies and activity histories of faults in each basin using field observations and remote sensing in order to address the relative contributions of border faults and intra-rift faults to crustal strain accommodation as rifting progresses. The ~500 m-high border fault along the western margin of the Natron basin is steep compared to many border faults in the eastern branch of the EAR, indicating limited scarp degradation by mass wasting. Locally, the escarpment shows open fissures and young scarps 10s of meters high and a few kilometers long, implying ongoing border fault activity in this young rift. However, intra-rift faults within ~1 Ma lavas are greatly eroded and fresh scarps are typically absent, implying long recurrence intervals between slip events. Rift-normal topographic profiles across the Natron basin show the lowest elevations in the lake-filled basin adjacent to the border fault, where a number of hydrothermal springs along the border fault system expel water into the lake. In contrast to Natron, a ~1600 m high, densely vegetated, border fault escarpment along the western edge of the Magadi basin is highly degraded; we were unable to identify evidence of recent rupturing. Rift-normal elevation profiles indicate the focus of strain has migrated away from the border fault into the rift center, where faults pervasively dissect 1.2-0.8 Ma trachyte lavas. Unlike Natron, intra-rift faults in the Magadi basin exhibit primarily steep, little-degraded fault scarps, implying greater activity than Natron intra-rift faults. Numerous fault-associated springs feed water into perennial Lake Magadi, which has no surface drainage input, yet survives despite a high evaporation rate that has created economically viable evaporite deposits. Calcite vein-filled joints are common along fault zones around Lake Magadi, as well as several cm veins around columnar joints that imply isotropic expansion of the fracture network under high pressures of CO2-rich fluids. Our work indicates that the locus of strain in this portion of the EAR transfers from the border fault to the center of the rift basin some time between 3 and 7 million years after rift initiation. This transition likely reflects the evolving respective roles of crustal flexure and magma budget in focusing strain, as well as the hydrothermal fluid budget along evolving fault zones.

2005

Dindi, EW.  2005.  Suggested guidelines for the NDC Evaluation of the first System-Wide Performance Test (SPT1)., 2005. , Vienna, Austria: Internal PTS Report of the CTBTO

2000

Opiyo-Akech, N, Olago DO, Dindi EW, Ndege MM, Njue F.  2000.  Investigation of the impact of salt and sand harvesting activities on the Timboni well field, Gongoni, Malindi District, Kenya. Environmental Geology. 40(1-2):99-110. Abstractinvestigation_of_the_impact_of_salt_-_e_dindi.pdfWebsite

The area studied is a north–south oriented, V-shaped, clayey lagoonal depression bordered by Pleistocene sands on the west and Recent dune sands on the east. The freshwater aquifer in the area is the main source of potable water for the urban centres of Gongoni and Timboni and the main Mombasa Salt Works processing plant. The aim of the study was to look at the impact of sand harvesting, and possible saline contamination of the aquifer by activities at the nearby salt harvesting plant and by seawater intrusion. The major factor abetting pollution of the freshwater aquifer is the sand harvesting activity, which exposes the aquifer to the atmosphere. Extension of the salt works closer to the aquifer field, combined with high abstraction rates, may also lead to saline water intrusion and contamination of the aquifer.

1998

Dindi, E.  1998.  Review of metrics for the evaluation of the waveform technologies data products, 1998. , Vienna, Austria: Internal PTS Report of the CTBTO

1997

Byrne, GF, Jacob AWB, Mechie J, Dindi E.  1997.  Seismic Structure of the Upper mantle beneath the Southern Kenya Rift from wide-angle data. Tectonophysics. Abstract

In February l994, the Kenya Rift International Seismic Project carried out two wide-angle reflection and refraction seismic profiles between Lake Victoria and Mombasa across southern Kenya. Our investigation of the data has revealed evidence for the presence of two upper mantle reflectors beneath southwestem Kenya, sometimes at short range, from seven shotpoints. Two-dimensional forward modelling of these reflectors using a pre-existing two-dimensional velocity—depth model for the crust [Birt, C.S., Maguire, P.H.K., Khan, M.A., Thybo, H., Keller, G.R., Patel, J., l997. The influence of pre-existing structures on the evolution of the Southern Kenya Rift Valley — evidence from seismic and gravity studies.
Tectonophysics 278, 211—242], has shown them to lie at depths of approximately 51 and 63 km. The upper reflector, denoted d1, shallows by about 5-10 km in the area beneath Lake Magadi, situated in the rift itself. Correlations for the deeper reflector, denoted d2, are sparse and more dificult to determine, so it was not possible to define any shallowing corresponding to the surface expression of the rift. Only limited control exists over the upper mantle velocities used in the modelling. Immediately beneath the Moho we use a value of Pn calculated from the crustal model, and constraints from previous refraction, teleseismic and gravity studies, to determine the velocity at depth. At the d2 reflector a reasonable velocity contrast was introduced to produce a reflector for modelling purposes. Beneath the d1 reflector the velocity decreases to the average value over 3 km. Beneath the rift the velocity also rises across d1 and again, decreases to the average value over the next 3 km. At the d2 reflector a similar model is used. This model accounts for the presence of the mantle reflectors seen in the data by using layers of thin higher velocity in a lower background velocity. Due to the uncertainty in the velocities the absolute position of both dl and d; could vary, but the relative upwelling beneath the rift is reasonably well constrained and data from four different shotpoints which indicate the shallowing show good agreement. A significant result of this study is that the continuity of the d, reflector indicates that the sub-Moho lithosphere has not been substantially disrupted by mantle upwelling, even though probably thinned and stretched.
Keywords: rifting; upper mantle; Kenya; wide-angle reflections; tectonics; P-wave velocities

1996

Dindi, E, Havskov J, Iranga M, Jonathan E, Lombe DK, Mamo A, Turyomurugyendo G.  1996.  Potential capability of the East African seismic stations. AbstractPotential capability of the East African seismic stations

It is well known that Africa is poorly covered with seismic stations and relatively few readings reach the international data bases. In September 1993 a workshop was held in Dar es Salaam, where all available seismograms for the months November and December 1992 from Ethiopia, Kenya, Uganda, Tanzania, Zambia, and Zimbabwe were analyzed. A bulletin was prepared for the 2 months of data containing 645 events of which 222 were reported as teleseisms and the rest as regional events. Seventy events had more than three stations reporting and were located within the area, mostly in central East Africa. For the same time period, PDE has five events reported in central East Africa, and it seems that the local stations can lower the detection threshold from about magnitude 4.6 to 4.0. The existing networks in East Africa thus have a large potential for increasing the quantity and quality of data available to the seismic community, and the workshop showed that it is very important to cooperate on a regional basis to achieve this.

1995

Dindi, E, Havskov J, Iranga M, Jonathan E, Lombe DK, Mamo A, Turyomurugyendo G.  1995.  Potential Capability of the East African Seismic Stations. Bulletin of the Seismological Society of America. 85(1):354-360. Abstract

It is well known that Africa is poorly covered with seismic stations and relatively few readings reach the international data bases. In September I993 a workshop was held in Dar es Salaam, where all available seismograms for the months November and December 1992 from Ethiopia, Kenya, Uganda, Tanzania, Zambia, and Zimbabwe were analyzed. A bulletin was prepared for the 2 months of data containing 645 events of which 222 were reported as teleseisms and the rest as regional events. Seventy events had more than three stations reporting and were located within the area, mostly in central East Africa.
For the same time period, PDE has five events reported in central East Africa, and it seems that the local stations can lower the detection threshold from about magnitude 4.6 to 4.0. The existing networks in East Africa thus have a large potential for increasing the quantity and quality of data available to the seismic community, and the workshop showed that it is very important to cooperate on a regional basis to achieve this.

1994

Dindi, EW.  1994.  Crustal structure of the Anza graben from gravity and magnetic investigations. Tectonophysics. 236:359-371. Abstract

The area of the present study constitutes the northeastern sector of the Anza graben. In this sector, the graben is about 130 km wide and is characterized by a linear negative anomaly with an amplitude about - 40 mGal. Geophysical data, mainly gravity and magnetic, were analysed quantitatively, including modelling in an effort to determine the subsurface structure of the graben. The study covered a strike length of about 320 km. The results of the modelling indicate that in the area of the study, two major basins with opposite dips exist. The northern basin dips southwestwards, the southern one northeastwards. The density contrast assigned to the sediments in the final gravity model is —300 kg /m3, suggesting a maximum sediment thickness of about 8 km. The modelling of the magnetic data, however, favours a maximum thickness of over 10 km. It is proposed that intrusives occur at some depth below sections of the graben shoulders and that the graben has a gentle northwesterly regional plunge.

1993

Prodehl, C, Jacob AWB, Thybo H, Dindi E, Stangl R.  1993.  Crustal structure on the northeastern flank of the Kenya rift. Tectonophysics. 236:271-290. Abstract

The KRISP flank line E converges with the Kenya rift at an angle of about 45° and is approximately parallel to the older Anza graben to the north. The depth to the basement is almost zero along the entire onshore part of the profile with higher velocities at the southeastern end indicative of extensive Precambrian gabbroic intrusions in the upper crust. The Moho shallows steadily from about 35 km at the southeastern end of the profile to about 24 km under-Lake Turkana. Even though the Moho rises fairly steadily, there is significant heterogeneity in the crust above it. This shows that the extension is unevenly distributed between the upper and the lower crust. The Moho is laminated and variably reflective. Compared to the KRISP cross-line D further south, the crust is unexpectedly thin and shows extension increasing in a northerly direction. This extension is probably not associated with the Anza and Kenya rifting but with the profile’s position on the slope of the Kenya dome. The indications are that there is a relatively abrupt change to a 20-km Moho depth near the Lake Turkana Central shotpoint. This change to a mid-rift crustal thickness occurs not at the postulated margin at the southeastern shore of Lake Turkana but at least 50 km further to the northwest. We suggest that the position of this margin may need to be redefined. The Pn velocity is quite high at 8.1 km/s. This may indicate either a cold upper mantle or anisotropy. An upper-mantle reflector has been identified between 15 and 20 km below the Moho. It dips gently away from the rift.

1987

Dindi, EW, SWAIN CJ.  1987.   Joint three-dimensional inversion of gravity and magnetic data from Jombo Hill alkaline complex, Kenya. Abstract

ABSTRACT

Jombo alkaline complex is the largest of the alkaline complexes in Kenya. It has been the subject of several geological and geochemical studies. However, the surface geology puts few constraints on the subsurface shape of the intrusion which we here attempt to determine by simultaneous inversion of gravity and aeromagnetic data.
The major feature of the gravity map is an elliptical high >800g.u. in amplitude centred near Jombo Hill. When the filtered magnetic map is reduced to the pole and pseudo—gravity transformed, a strikingly similar anomaly is revealed, suggesting a common source. Using an iterative |east~squares technique, joint inversion of the gravity and unfiltered magnetic data for a three—dimensional model established that both data sets can be adequately modelled by a thick slab tapering upwards from c. 29 km depth and striking approximately E—W. Only the upper c. l8 km is magnetized which we interpret as an indication of the depth to the Curie isotherm. The body is predicted to be of ultramafic composition capped by the exposed syenites and ijolites. Received March 24, 1987.

1985

Dindi, EW.  1985.  GEOPHYSICAL STUDIES OF ANZA GRABEN, NORTH EASTERN KENYA. Abstract

ABSTRACT
Anza Graben is situated in the north and northeastern Kenya. It is about 120 km wide and approximately 600 km long. The area of the present study is located mainly in NE Kenya and constitutes a strike length of about 300 km. The graben is almost entirely sediment filled and is estimated to be of Cretaceous age. It is characterized by a prominent negative Bouguer anomaly reaching -40 mGal.

Several studies were made in an effort to determine its subsurface structure. These involved interpretation of a number of seismic reflection profiles, 3D interactive gravity modelling using fixed density contrasts between the sediment infill and the basement, and the analysis by filtering and modelling of the aeromagnetic data hover the graben. A crustal modelling of refraction data along the western shoulder of the Anza graben using 2D ray tracing was also performed.

Two major basins are recognizable from the results of the studies, namely the southern basin which has a northeasterly dip and the northern basin having a southwesterly one. This alternating asymmetry is also indicated for an adjoining smaller basin located to the west of the graben. The density contrast used for the final gravity model is -300 kg/m3 which indicates a maximum sediment thickness of about 8 km. The modelling of aeromagnetic data however favour a maximum thickness of at least 10km. A 2D refraction crustal P wave model of the western shoulder indicates that the crust thins northwestwards with the Moho depth decreasing from 35 km under Chanler's Falls to approximately under the southeastern shores of Lake Turkana. The abrupt shallowing of the Moho in the Lake Turkana area is attributed to the effects of the Kenya Rift Valley. However the gentle variation in the Moho depth along the remaining section of the shoulder of Anza Graben appears to depict a regional phenomenon also attributable to the formation of the Kenya Rift Valley. It is proposed from the study that sections of the graben shoulder are underlain by slightly denser rocks and that the crustal thinning associated with Anza Graben is not significant.

1982

Dindi, EW.  1982.  A GRAVITY SURVEY OF THE JOMBO HILL AREA, SOUTH COAST KENYA. , Nairobi: University of Nairobi Abstract

ABSTRACT

The aeromagnetic survey of the Jombo Hill Area conducted in l977 revealed an anomaly of 2000 gammas in amplitude in the area of the intrusive rocks. This value gtands,out as one of the highest values ever recorded in Kenya. In order to investigate the anomaly further, a gravity survey was conducted in the area within 40 km radius_of Jombo hill. After the necessary corrections to the observed data they were compiled into a Bouguer gravity map of the area.

The Bouguer gravity anomalies range from - 40 to over 50 mgals. The peak of the anomaly occurs immediately to the south of the hill and most of the contours close on this peak. It is noticeable that the magnetic and gravity anomalies are roughly coincident. The relative ease and convenience of gravity 3D modelling relative to magnetic 3D modelling, in the context of this study, dictated the placing of a greater emphasis on the former in the quantitative interpretation.
The gravity anomaly is interpreted in terms of an inverted funnel like body of density 3.1 gcm-3, with its top surface at about l km depth. Estimates from the magnetic map yield a similar value for the depth to the top of the body.

Most of the magnetization in these rocks is the induced type and is in the direction of the ambient field. The large grain size has an appreciable effect on the magnetization in these rocks.
It appears from the study, that both the magnetic and gravity anomalies are caused by a single body. The gravity anomaly depicts the effect of the entire body while the magnetic anomaly that due to a relatively small central and shallow portion of it. The exact geometry of this portion was not determined. However a sphere fitted to the anomaly gave satisfactory results.

By comparison with the geology of the area and with the characteristics of other known alkaline complexes in Eastern Africa, the gravity model is considered geologically reasonable.

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