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

Abstract

Middle to lower crustal rocks with dominantly flat-lying Pan-African fabrics at amphibolite metamorphic facies are exposed along the actively extending Red Sea lowlands of Eritrea. West of a major escarpment, these rocks are structurally overlain in the plateau by greenschist facies metamorphic rocks with steep fabrics dominant. Three Pan-African phases of deformation in eastern Eritrea (PAD1-3) were superposed during the Cenozoic by three phases of Red Sea lateral extension (RSE1-3). PAD1 is characterised by steep penetrative foliation S 1, which is axial planar to upright F 1 folds. These folds were distorted at depth by F 2 recumbent folds and subhorizontal shear zones during PAD2. PAD3 deformation resulted mainly in steep strike-slip shear zones. All phases of NE–SW lateral extension of the Red Sea exploited steep PAD1 and PAD3 and flat-lying PAD2 fabrics and structures. RSE1 was semi-brittle and resulted in top-to-basin low-angle (=35°) 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 (≥40°) domino-style normal faults and dykes with NW–SE strikes above a younger detachment inferred beneath exposure levels. RSE2 structures concentrate in zones of maximum crustal flexure across the escarpment and were preceded and/or accompanied by reverse faults near the foot of the escarpment. RSE2 structures truncate RSE1 and PAD2 subhorizontal 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 offshore. RSE3 involved NNW–SSE- to NNE–SSW-trending strike-slip faults associated with counterclockwise opening of the hinge where the Danakil block is still joined to Africa. The overall geometric relationships between Cenozoic faults and dykes and the Pan-African tectonic grain suggest that the Red Sea escarpment in eastern Eritrea is closer to a monoclinal flexure rather than a rift.