Zur geotektonischen Problematik im Afar-Gebiet
[On the geotectonical problems in the Afar area]
published: Jan 1, 1973
Die erste Phase der intensiveren geologischen Erforschung des Afar-Dreiecks (Äthiopien) ist abgeschlossen. In einem kurzen Überblick über die derzeitig bekannten Ergebnisse wird die Frage diskutiert, ob im Afar-Gebiet Erscheinungen vergleichbar mit dem Seafloor Spreading der Ozeanböden auftreten. Diese Frage muß verneint werden. Die Gründe dafür werden im einzelnen diskutiert. Als Schlußfolgerung wird eine ehemalige Position der Arabischen Halbinsel im Bereich des heutigen Golfs von Aden und eine Driftbewegung nach Norden angenommen.
The Afar Triangle, located on the Ethiopian side of the southern Red Sea attracted geologists and geophysicists in the past ten years due to its position around the intersection of three major structures, partly built up by oceanic magma in a region accessable for detailed geological investigations. The International Upper Mantle Project (IUMP) supported investigations with sophisticated methods of almost all applied geosciences. The first phase of research terminates now. Geological maps of almost the whole region have been made available through the Sponsoring agencies as the result of numerous field campaigns. Photogeological investigations of larger regions filled the gaps left during the field campaigns and geophysical and geochemical investigations added decisive knowledge on the deeper crust underlying the Afar Triangle. The second phase of research in the Afar is now to be started. It will be characterized by detailed investigations of prime targets or key regions, which will enlighten the mechanics of plate tectonics and will modify the partly contradictionary hypothesises dealing with the geological development of this region. The following paper gives a summary of the present knowledge of the geology of the Afar Triangle. The results are in brief the following: The Afar Triangle in its present shape and size is primarily not the result of recent seafloor-spreading on land! The facts leading to this Statement are listed below: The occurrence of oceanic magma is restricted to the northern part of the Afar Triangle, the Danakil region. No major Substitution of open space between drifting blocks can be proven so far. The occurrence of huge volcanoes does not request an equal distribution of oceanic magma below. Gravity surveys and regional geology give many arguments for a modest rotation of the Danakil Block (= the northern part of the Danakil Horst) with a maximum amount of rotation about 8°. To the south the amount of spreading should increase, if we consider the rotation of the Danakil Horst in an anti-clockwise mode in a way, that the western part of the Danakil Horst once had a Position immediately at the present eastern scarp of the Ethiopian Plateau, as done by several authors in the past (e. g. MOHR, 1970; BUREK, 1973; LAUGHTON, 1966). This model of spreading would require an increasing amount of horizontal dilatation to the south, which consequently leads to an increasing amount of new crust to substitute the open space. Huge masses of oceanic basalts would flood the central Afar and high gravity anomalies over the whole region would have been found. However, we observe frequently rhyolitic volcanoes with a magma type equal to those magmas known from the Pierre Pruvost Complex which consists of contaminated magma (TAZIEFF et al., 1969; BARBERI et al., 1970). The deep Danakil Graben fades out south of 13° northern latitude and large regions of Upper Tertiary flood basalts occur. Gravity maps do not show any stronger positiv anomaly south of 13° north (MAKRIS, pers. communication). In the southern and eastern Afar the recent observations show, that the tectonical structures cannot be explained by the eastward drift of the Aisha Horst. However, the Aisha Horst has to be located in the past far to the west, near the mouth of the continental East Ethiopian Graben, if Arabia was situated in the region of to-days Afar as requested by all drift and rotational hypothesises of the Arabian Peninsula. As a result of these evaluations it is assumed, that Arabia performed since Miocene a northward motion instead of a northeastward rotation. Magnetic observations in the Red Sea and the Gulf of Aden central zones have to be reviewed critically under this scope.