Crustal exhumation and indications for Snowball Earth in the East African Orogen: north Ethiopia and east Eritrea

The northeast Ethiopian plateau is composed of a low-grade metamorphic complex which includes the Tsaliet Metavolcanics overlain by the NE-trending upright folded metasediments of the Tambien Group including a thick stromatolitic carbonate sequence. High-grade metasediments and orthogneisses structurally underlie this unit in the Red Sea Lowlands of east Eritrea. In east Eritrea. the low-grade Bizen Domain metasediments and metavolcanics structurally overlie the Ghedem Domain high-grade rocks with a subhorizontal moderately W-dipping transition zone of tectonic origin. Field studies augmented by remote sensing analysis of Modular Optoelectronic Multispectral Stereo Scanner on Priroda (MOMS-2P) images are used to outline the structure, stratigraphy and geomorphology and the continuation of the Negash Synclinorium with adjacent structures up to the Bizen Domain in east Eritrea. The protolith age of the low- and high-grade rocks is around 850 and 800Ma, respectively. Peak metamorphic conditions in the high-grade metasediments reached 700degreesC and up to 12 kbar (depth of similar to40 km) at similar to650 Ma. This indicates that an elevated mountain belt with a very thick crust existed in Eritrea during the evolution of the East African Orogen (EAO). Rapid exhumation of the high-grade metamorphic rocks occurred between 640 and 545 Ma involving extension along a tectonic transition zone. The transition zone is a ductile detachment overprinted by brittle low-angle faulting. Although partly pertaining to a Tertiary deformation related to the opening of the Red Sea, we suggest that this low-angle zone was a detachment surface in the Neoproterozoic that accommodated extension during orogenic collapse to restore normal crustal thickness which was thickened due to collision between East and West Gondwana. The sedimentary rocks of the Tambien Group in north Ethiopia and the Bizen Domain in east Eritrea (stromatolitic limestone, stromatolitic dolostone, polymict conglomerate-diamictites), their stratigraphy and age of deposition suggest association with "Snowball Earth". Light delta(13)C (-6.74 and -6.88parts per thousand) were measured for the stromatolitic dolostone and algal mat of the Bizen Domain which are characteristic for cap-carbonates associated with Snowball Earth. (C) 2003 Elsevier Science B.V. All rights reserved.