Bio

My 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.

Publications


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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