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.