The geologic and tectonic evolution of the Pan-African/Mozambique Belt in East Africa

Berhe, Seife Michael (1988). The geologic and tectonic evolution of the Pan-African/Mozambique Belt in East Africa. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000dece

Abstract

The presence of ophiolite complexes in NE and E Africa has been documented using Landsat, field and geochemical studies. This has led to the recognition of five ophiolite belts. These ophiolite belts represent sutures marking the position of island arcs and could be traced to Saudi Arabia on a pre-Red Sea drift reconstruction. Most of the ophiolites are dismembered, their mode of occurrence varies widely resulting in different structural relationships. The Yubdo complex in Western Ethiopia is formed of harzburgite which grades into dunites and pyroxenitic units, a cumulate sequence of ultramafic and gabbroic rocks and sheeted dykes. The Baragoi complex in Kenya is formed of tectonised ultramafics with dunite and chromite pods, a cumulate sequence of ultramatic and gabbroic units and a dyke unit. Trace element data of the Baragoi complex shows a transitional MORB to IAT affinity, and the presence of boninites suggest a supra- subduction setting, while data from the Adola- Moyale belt (S Ethiopia- NE Kenya) indicate an island-arc and MORB geochemistry, which developed in a back-arc setting. The chromites of Baragoi and Moyale have high Cr2O3 which follow an ophiolitic trend.

Major and trace element data for granitoids from W Ethiopia, S Ethiopia- NE Kenya and central Kenya indicate three geochemically distinct granitoid groups: volcanic are granitoids, crustal melt granitoids and within-plate granitoids. Calc-alkaline rocks predominate in W Ethiopia, whereas the proportion of crustal melts appear to increase going further south in S Ethiopia/ NE Kenya and central Kenya. Diorites form about 10 percent of Precambrian outcrop in NE Sudan, while further south diorites are almost insignificant. Only in NE Sudan, W Ethiopia and Saudi Arabia do diorites feature prominently. However the lack of extensive cats-alkaline volcanic rocks, could simply reflect relatively narrow oceans and insufficient subduction of oceanic crust to produce large quantities of calc-alkaline melts, while the increase in the proportion of crustal melt granitoids in the southern part of the Mozambique belt indicates crustal thickening due to continent-continent collision.

This study shows that the major lineaments identified in the Horn of Africa trend 010 ± 100, 055-065° and 145-165°. The 010 ± 10° and 145-165° trending lineaments form conjugate sets, while a later deformation episode reactivated 145-165° (NW-SE) trending lineaments and caused 055-0650 (NE-SW) lineaments. Two deformation mechanisms most likely controlled the growth of the major fault zones. Structural and metamorphic evidence suggests that crustal shortening was severe in S Sudan, Kenya and SE Ethiopia as compared to Saudi Arabia, NE Sudan and N and W Ethiopia due to oblique collision from the southeast causing stacking of crustal blocks along NW trending faults.

Regional geologic, tectonic and geochemical studies suggest rifting c. 1200 Ma which subsequently led to the development of intraoceanic arcs and associated marginal basins in the north and narrow basins within the sialic basement gneisses further south in Kenya and Tanzania. This was followed by continent- continent collision which led to accretion of island arcs by mild collision from the northeast in Saudi Arabia and NE Sudan and severe crustal shortening In S Sudan, Kenya and SE Ethiopia as compared to Saudi Arabia, NE Sudan and N and W Ethiopia due to oblique collision from the southeast.

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