Red Sea extension influenced by Pan-African tectonic grain in eastern Eritrea

Middle to lower crustal rocks with dominantly flat-lying Pan-African fabric s at amphibolite metamorphic facies are exposed along the actively extendin g Red Sea lowlands of Eritrea. West of a major escarpment, these rocks are structurally overlain in the plateau by greenschist facies metamorphic rock s with steep fabrics dominant. Three Pan-African phases of deformation in e astern Eritrea (PAD1-3) were superposed during the Cenozoic by three phases of Red Sea lateral extension (RSE1-3), PADI is characterised by steep pene trative foliation S-1, which is axial planar to upright F-1 folds. These fo lds were distorted at depth by F-2 recumbent folds and subhorizontal shear zones during PAD2. PAD3 deformation resulted mainly in steep strike-slip sh ear zones. All phases of NE-SW lateral extension of the Red Sea exploited steep PAD1 a nd PAD3 and flat-lying PAD2 fabrics and structures. RSE1 was semi-brittle a nd resulted in top-to-basin low-angle (= 35 degrees) NW-SE-trending normal faults that sole out to subhorizontal detachments at deep exposure levels. RSE2 involved seaward block tilting on a new system of moderate to steep (g reater than or equal to 40 degrees) domino-style normal faults and dykes wi th NW-SE strikes above a younger detachment inferred beneath exposure level s. RSE2 structures concentrate in zones of maximum crustal flexure across t he escarpment and were preceded and/or accompanied by reverse faults near t he foot of the escarpment. RSE2 structures truncate RSE1 and PAD2 subhorizo ntal shear zones in the lowlands by exploiting steep PAD3 structures along the escarpment and the in the plateau. The away-from-basin dip of the RSE2 faults and dykes is attributed to isostatic rise of the asthenosphere offsh ore. RSE3 involved NNW-SSE- to NNE-SSW-trending strike-slip faults associat ed with counterclockwise opening of the hinge where the Danakil block is st ill joined to Africa. The overall geometric relationships between Cenozoic faults and dykes and the Pan-African tectonic grain suggest that the Red Se a escarpment in eastern Eritrea is closer to a monoclinal flexure rather th an a rift. (C) 2000 Elsevier Science Ltd. All rights reserved.