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Abstract

We used a reprocessed 2D seismic line across the ultra-deep water area offshore Angola to map a thick pre-salt sequence divided by a prominent angular unconformity into two main packages. The seismic line shows a strong reflector at ca 9 seconds two-way time (twt), possibly the crust-mantle boundary (Moho).

We interpret that the pre-salt strata records major crustal extension, followed by rapid sedimentation. Presalt decompacted sedimentation rates were in excess of 200–600 m/My, in contrast to a maximum post-salt deposition rate of 50 m/My. This suggests that the transition from pre-salt to post-salt deposition coincided with a dramatic change in climate and/or drainage pattern. This is also indicated by the change from mainly lacustrine clastics in the pre-salt sequence to mainly carbonates in the immediate post-salt sequence.

The total post-rift subsidence in the study area indicates a crustal extension factor between 2.3 and 3.4, increasing from east to west. An alternative estimate of crustal extension, ranging from 3.6 to 4.9, is obtained by assuming that the 9 seconds twt reflector is the Moho. These differing estimates may be reconciled if it is assumed that the study area is uplifted by ca. 750 m relative to the McKenzie (1978) subsidence model predictions. It is noteworthy that an uplift of this magnitude corresponds to the current uplift of continental southern Africa.

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