east african lithosphere

7/30/2019 East African Lithosphere

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Jorge Botala Boloso

02/13/06Geol 345 Term Paper Write-Up


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Abstract:

Most of the information available about the various stages that account in the

evolution of the East African lithosphere comes from active sources; gravity and thermal

anomalies; and, mostly, from the study of mantle xenoliths. My aim in this paper is to

provide up to date information about the geomorphology and the geochemistry

underneath the East African Rift system, which is based on the application of these

models. In addition, these models allow the construction of individual maps of each of

the individual regions that comprise the East African Rift System, which are necessary to

construct the overall picture of the area.


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1. Introduction:

The East African Rift System (Fig. 1a, 1b), which can be considered as the

worlds current example where to study the early stages of continental breakup, gives us

the opportunity to image the dynamic that is going on in the interior of the Earth. Similar

processes like the one that are presently remodeling the East African Lithosphere were

among the factors that initiated the breaking-up of Pangaea ~200 Ma years ago,

producing the Mid Atlantic Ridge. Therefore, understanding the dynamic of the East

African Rift System can provide us with lots of clues about the internal structure of the

Earth. Consequently, this will improve our understanding of plate tectonics and,

therefore, can help us to predict events that are related to the internal earths processes,

such as earthquakes, volcanic eruptions, and tsunamis triggered by earthquakes.

Furthermore, most of the available data from East Africa comes from passive sources;

gravimagnetism and thermal anomalies. Also, modern techniques developed from the

study of the mantle have allowed the study of the mantle xenoliths, which provide

accurate information about the age, the morphology and the geochemistry of the internal

layers of the earth. Moreover, this paper addresses the results and the analyses that have

been concluded regarding the geomorphology and the geochemistry of the East African

Rift System as well as the factors that initiated it.

2. Geographical overview:

Geographically speaking, the East African Rift System extends from the Afar

Triangle in Ethiopia through Kenya into northern Tanzania (Fig. 2), where the Eastern

branch dies out in a diffuse network of graben with little extensional strain (Ring et al,


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2005). Winter (2001) describes the Afar Triangle as a triple junction where three

divergent (spreading) systems come together. He added that the Red Sea and the Golf of

Aden resulted from the two arms that extend to the North between Africa and the Arabian

Peninsula; whereas, the third arm is defined by a failed rift that expands into the

continent.

3. Origin of the East African Rift:

There are lots of controversies about the origin of the East African Rift System.

On this remark, Nyblade and Brazier (2002), combining a new seismic model for the

uppermost-mantle, established that because the two branches of the rift system are clearly

bifurcated around the Tanzania craton (Fig. 3), it has been suggested that the Tanzania

craton, of low-grade granite greenstone belts and high-grade gneiss terrains (A.L. Tesha

et al. 1997), may have influenced the development of the rift system, possibly by

behaving as a rigid tectonic block. Also, adding to their observations the work of

previous geophysics they concluded that the cratons thick, cold lithosphere has

effectively resisted modification by the Cenozoic extensional tectonism and that the

craton therefore has behaved as a rigid tectonic block.

In addition, two major models, the active and the passive-rift models, have been

preferentially used to explain the origin of the rifting in the East African Rift System.

Nablade and Brazier (2002) explain that in the passive-rift model, rifting in East Africa is

caused by far-field stresses associated with the development of the Afar Triple junction.

However, they added that active-rift models for East Africa, with one or more mantle

plume, are favored by many investigators given that the passive-rift models does not


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account for the volcanism in East Africa by pressure-release melting. Furthermore,

seismic studies of crustal structures in Kenya and the observation of gravity lows over

Precambrian sutures throughout the Tanzania Craton suggest that the origin of the rifting

in East Africa may be explained by the evidence of a suture-thickened crust. On this

remark, A.L. Tesha et al. (1997) stated that the development of Cenozoic rift structures

within proximity to suture-thickened crust in northeastern Tanzania and Kenya suggest

that rifting in the Eastern arm of the Eastern African rift system may have been localized

by the presence of the suture-thickened crust.

4. Geophysical overview:

4.1. Seismic and thermal anomalies beneath the East African Rift System:

Seismic data along the East African Rift System are based on the observation of P

and S-wave velocity models recorded from teleseismic earthquakes. Thermal anomalies

along the East African Rift System are explained by the presence of one or more mantle

plumes. In addition, Nyblade et al. (2000) observed that these models combined with

topography on the 410 km discontinuity provide evidence for a deep thermal anomaly in

the upper mantle beneath the eastern rift. They attributed this thermal anomaly to a

mantle plume, adding to their argument the fact that the structure of a thermal anomaly

can be readily explained by a plume head under the eastern margin of the Tanzania

craton. These authors also make it clear that the thermal structure beneath the eastern rift

is caused by buoyant (warm) plume head material that has migrated around and laterally

along the eastern side of the cratonic keel, modifying the mantle lithosphere beneath the

eastern rift (Fig. 4). Finally, their observation lead them to the conclusion that a plume


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head situated beneath the eastern side of the craton could also explain the lack of

volcanism in the western rift compared to the eastern rift.

On the other hand, by using the 3-D stacking method to determine the transition

zone thickness (TZT) in the eastern rift, Owens et al (2000) established that 30 to 40 km

of transition zone thinning correspond to a 200-300ok increase in temperature; which,

according to their model, shows a 2-3% reduction in S wave velocities beneath the

eastern rift coincident with the location of the thinned transition zone (Fig. 4). This

thermal anomaly at >400 km depth beneath the eastern rift is consistent with a plume

origin for the Cenozoic rifting, volcanism and plateau uplift in East Africa, based on

these observations. In addition, according to Macdonald et al. (2001), heat flow and

seismic evidence are consistent with the idea of a southward propagating rift system, such

that at least the upper parts of the craton have not been thermally modified by the action

of any inferred plume.

4.2. Gravity Anomalies:

Bouguer gravity gradients vary locally along the different regions that comprised

the eastern rift. Low Bouguer gravity gradients decrease eastward along the area of the

west Lake Tanganyika and Lake Edward in Zaire; whereas, high Bouguer gravity

gradients are common along the western and eastern valleys. In addition, citing Bullard

(1996), MacConnell (1980) states that East Africa generally and the rift valleys in

particular show regional negative Bouguer anomalies (Fig. 5). Accordingly, observations

from Simiyu and Keller (1997) show that broad gravity low in the region of the western

rift results from contributions by both crustal thickening and an asthenosphere at a depth

of 50 km beneath the rift; whereas, a prominent gravity high forms a divide between the


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Kenyan dome of the East African Plateau and the Ethiopia dome. In the same line, they

stated that it is in these regions where most of the magnitude ~ 4.5 earthquakes are

produced.

6. Igneous petrology and Geochemistry

Lots of studies conducted along the East African Rift System aim to determine the

composition of the substratum underneath it through the data obtained from mantle

xenoliths, the chemical and isotopic composition of lavas, and the behavior of the trace

element in the rocks that contain them. Bellow is a list of remarkable observations

established by Winter (2001).

1. Magneto-tellurics reveals high conductivity in the shallow rift mantle that

suggests that the mantle beneath the East African Rift is partially molten.

2. What really distinguishes the magmatism of the East African Rift is the

diversity of chemical composition expressed by the tholeiitic toto ultra-alkaline spectrum

in such a limited area.

3. Silicic lavas and pyroclastics constitude a significant proportion of the

total volume of East African Rift volcanics. In Ethiopia they compose one-six of the

volcanic pile and in Kenya one half (Williams, 1982). In all subprovinces of the East

African Rift system, intermediate lavas(~52 to 57 wt. % SiO2) are subordinate to the

mafic and silicic lavas.

4. Citing the work of Kampunsu and Mohr (1991), Winter established that

prerift volcanism began to the north in Ethiopia in the Eocene (~43 Ma) and in Kenya

(Eastern Branch) in the early Oligocene (33 to 30 Ma) with the extrusion of extrusive


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flood basalts covering the thinning lithosphere. Rhyolites and rhyolitic igneous

ignimbrites accompanied the later flood basalt activity.

5. Most of the evolved magmas in the East African Rift appear to be the

result of fractional crystallization from mantle-derived partial melts without significant

contamination by ancient continental crust.

6. The isotopic data for the more evolved rocks show signatures similar to

the mafic ones. This implies that magmas evolved mostly by magmatic differentiation

with little contamination by assimilated continental crust.

7. Citing Barbery et al. (1975), Winter argues that only in a few localities,

such as the active Quaternary Boina center near Afar are trachytes and pantellerites more

enriched that the associated basalts, suggesting either a different source or crustal

contamination.

8. Ultra-alkaline basalt occurs in highly silica undersaturated volcanic and

hypobyssal rocks at the southern end of the Kenya Rift, becomiing k-rich in the southern

arm; whereas, transitional basalt are known to comprise the bulk of the flood basalt in

Afar, Ethiopia, and Kenya, which were erupted immediately before (and during) the

graben faulting and flank uplift.

9. Rocks in the East African Rift are fairly representative of the alkaline and

peralkaline rocks from around the world. Volcanics vastly predominate over plutonics in

the East African Rift

10. The trace element data for the East African Rift are generally

incompatible. Such enrichment results in high Rb/Sr and Nd/Sm ratios, which should

produce elevated 87Sr/86Sr and 143Nd/144Nd over time. The isotopic ratios, however, are


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fairly low (depleted with respect to bulk earth) suggesting low Rb/Sr and Nd/Sm ration in

the ancient source.

11. The occurrence of the ultra-alkaline (and alkaline) rocks in the east

African Rift, together with the trace element and isotopic data discussed above, suggest

that the mantle beneath Africa may be incompatible-element enriched.

12. Xenoliths from the Alkaline basalts in the Danakil Blocks of NE Afar

include spinel-harzburgite, olivine pyroxene, and lherzolite, suggestive of equilibration in

the spinel peridotite domain; whereas, xenoliths from other areas in the Eastern Branch in

Kenya and Ethiopia are more typical peridotites.

13. The mantle beneath East Africa is clearly heterogeneous, including zones

depleted during the Precambrian and zones enriched by several events involving various

alkaline and carbonatitic melts and fluids.

14. Gravity anomalies suggest that the same plutonic infrastructure exist

beneath East Africa, and that mafic and silicic intrusions may exceed remnant continental

crust at depth beneath the rift. If true, this may explain the paucity of evidence for crustal

contamination in most areas.

On the other hand, Ring et al. (2005) disclose that olivine nephelinites, basalts and

trachyobasalts occur at the border fault of the Manyara Rift between Lake Manyara and

Engaruka. The age of these rocks were determine to be from 0.8 to 3.8 Ma. In addition,

citing the work of earlier authors (Wendland & Morgan, 1982; Boyd & Gurney, 1986),

Foster et al. (1997) stated that melting of carbonates xenoliths within early Terciary

kimbelites and carbonates suggest that the lithosphere beneath the part of the East

African Plateau was at least 170 km thick at the time of their eruption. Furthermore,


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Chesley et al. (1999) argue that Rb-Sr systematics of the Labait xenoliths show that

ancient refractory lithosphereis present to depths of ~140 km. TRD ages between 2.5 to

2.9 Ga,

7. Conclusion

Overall, the study of the evolution of the East African Rift System using

modern techniques (seismic, gravity, thermal, magnetic, and the mantle xenoliths) have

enable us to obtain information about the processes that are shaping the crust and the

lithosphere along this particular region of the world. Accordingly, the study of this region

has broadened our understanding of plate tectonics and other processes that are related to

the interior of the earth. However continuous monitoring of this region through the

techniques mentioned above will provide over the years a lot of useful information that

can help the governments from the countries in this regions to sketch plans for emergency

responses to save their citizens from earthquakes, tsunamis, and other natural disasters,

as this regions is becoming densely populated.


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Fig.1. A:(e.g. Precambrian terrains & volcanic provinces).

B: W-E cross section topography across East Africa.

(Owens et al, 2000)

Fig.2: Precambrian terrains, Cenozoic rift fault, and Cenozoic volcanism. KRISP

lines give Pn (upper mantle) velocities in Kenya, and gray Scale shading gives Pn

velocities in Tanzania.

(Nyblade et al, 2002)

Fig.3: S-wave velocity model. Horizontal/vertical uncertainties


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Structures ~ 50 and 100 km. Velocity structures above 100 km

and below 500 km (poorly resolved) are not given.

(Nyblade et al, 2000)

Fig.4: Schematic Cross Section at ~4.5oS showing plume

Head beneath eastern margin of Tanzania craton.


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Fig.5: Bouguer gravity map of northwestern Tanzania. N-s-trending line is the

Tanzania Craton-Mozambique Belt boundary. Lake and indian ocean are shaded


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Work Cited:

Chesley, J.T; Rudsnick, R.L., and Lee, C.-T., 1999, Re-Os systematics of mantle

xenoliths from the East African rift: Age, structure and history of the Tanzanian

craton: Geochemica et Cosmochimica Acta, v. 63, p.1203-1217

Foster, A; Ebinger, C., Mbede, E., and Rex, D;1997, Tectonic development of thenorthern Tanzania sector of the East African rift system: Geological Society

[London] Journal, v. 154, p. 689-700

MacConnell, R.B., 1980, A resurgent taphrogenic lineament of Precambrian origin ineastern Africa: J. geol. Soc. London, Vol. 137, 1980, pp. 483-489, 2figs. Printed

in Northern Ireland.

Macdonald R., Rodgers, N.W.,Fitton, J.G., Black and Smith M.; 2000, Plume-LithosphereInteraction in the Generation of the Basalts of the Kenya Rift, East

Africa: Journal of Petrology; v.42; no 5; p. 877-900

GEOLOGY, July 2000: http:// geoscienworld.org; July 2000; v.28; no. 7; p. 599-

602; 2 figures.

Nyblade, A.A. & Brazier, R.A. 2002. Precambrian Lithospheric Control on thedevelopment of the East African Rift system. Geology 30, 755-8.

Nyblade, A.A., Owens, T.J., Gurrola, H., Ritsema, J., and Lonston, Ch., 2000, Seismicevidence for a deep upper mantle thermal anomaly beneath east Africa: Geology;

July 2000; v. 28; no 7; p.599-602; 2 figures.

Owens, T.J; Nyblade, A. A., Gurrola, H., and Langstone, C.A., 2000, Mantle transition

zone structure beneath Tanzania, east Africa: Geophysical Research Letters, v.27,p. 827-830

Ring, Uwe., Hilde, S.L., Bromage T.G. & Sanaane, Ch., 2005, Kinematic and

sedimentological evolution of the Manyara rift in Tanzania, East Africa: Geol.Mag. 142 (4), 2005, pp. 355-368 2005 Cambridge University Press; doi:

10.1017/s0016805000841 Printed in the United Kingdom

Ritsema, J., Nyblade, A. A., Owens, T.J., and Longston, C.A., 1998, Upper mantle

seismic velocity structure beneath Tanzania, East Africa: Implications for thestability of cratonic lithosphere: Journal of Gyophysical Research, v. 103, p.

21,201-21-213.

Simiyu, S.M., and Keller, G.R., 1997, An integrated analysis of lithospheric structureacross the east African plateau based on gravity anomalies and recent seismic

studies: Tectonophysics, v. 278, p.292-313

Tesha, A.L., Nyblade, A.A; Keller, G.R., and Doser, D.I., 1997, Rift location in suture-thickened crust: evidence from Bouguer gravity anomalies in northeastern

Tanzania, East Africa: Tectonophysics 278 (1997)315-328.

Winter, J.D., 2001, An Intoduction to Igneous and Metamorphic Petrology: Department

of Geology Whitman College, QE461 .W735-2001

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