Subsalt Seismic Stratigraphy of Southern Offshore Eritrea: New Evidence for Spatially and Temporally Varying Rifting Episodes
PIGOTT, JOHN D., and ESHETU GEBRETSADIK
Refraction velocity constrained seismic stratigraphy of some 2000 line km of 24-fold reflection seismic of Southern Offshore Eritrea provides new images of the sedimentary-structural record of Late Cenozoic rifting. Tectonic subsidence analyses determined from Secca-Fawn 1 and Thio 1 boreholes and incorporating subsalt seismic sequence analyses reveals at least three well defined rifting episodes: at 25 ma, 14 ma, and 5 ma. Temporally, stretching magnitude varies nonsystematically with each rift episode. Spatially, cumulative stretching is minimal at the downwarped rift flank (beta = 1.01), moderate in the symmetrical horsts and grabens (beta = 1.1), major within the en-echelon tilted block zone (beta = 1.25), and infinite at the axial trough. The first rifting episode is predominantly pure-shear, basement-involved, and is capped by a wide-spread unconformity increasing in erosional magnitude toward the axial trough. The second rifting episode reveals localized WNW-ESE trending simple shear faults along a regionally pervasive NNW extensional regional strike. During this episode strain was accommodated by the reactivation of older rift faults and the creation of new ones. The third rifting episode was mainly pure-shear and involved the modification of the existing evaporite blanket into swells in phase with underlying horsts and grabens. Halokinetic modification continued with evaporite-withdrawal, counter-regional faults, and imbricated sediment rafts. This documentation of rifting episodicity in the southern Red Sea destroys the myth of one rifting event and opens up many new hydrocarbon play concepts.