Abstract

The northern end of the Danakil horst block in Eritrea has undergone major asymmetric subsidence but only minor separation along the Ethiopian rift, the block being covered by a veneer of Pleistocene evaporites. The supporting evidence includes stratigraphic relations in pre-rifting Jurassic formations, lithofacies variations in the Pleistocene evaporites, and structural information. Recent studies of basalt age-space relations and petrochemistry in the Afar region are also in accord with this view. Additional studies will be required to determine o t what degree this condition persists southward in form of a subsided sialic crustal block beneath the basalt-floored Afar depression, or conversely to what degree there has here been separation of the Danakil horst from the Ethiopian plateau due to rotation, with asymmetric tilting, of the horst block. Although the data are less compelling than in the case of the Danakil horst, recent offshore geophysical surveys and drilling in the southern Red Sea suggest the possibility of there being a second, older, more deeply subsided sialic block covered by a thick Miocene evaporite and volcanic sequence beneath the shallow margins of the Red Sea itself.

If correct, this interpretation would require only minor separation by drift of the Arabian and Nubian plates across the Ethiopian and Red Sea rifts. It would, instead, involve major subsidence, with extensive evaporite deposition and basaltic volcanism covering the subsiding blocks, as the explanation for the large apparent separation.

Although the presence of subsided sialic blocks beneath the Red Sea and southern Afar is interpretive and requires further substantiation, the existence of such a block beneath the north end of the Danakil horst is probable. It is therefore suggested that subsidence may be an important tectonic development along the margins of newly rifted continental plates. Fault-block tectonics in the style suggested for the Danakil-Afar area, characterized by asymmetrically subsiding sialic blocks, may offer an explanation for many features common to some continental margins. These include shallow continental shelves with incised submarine canyons and deep but local offshore evaporite basins.

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