The Structure and Evolution of the Sabratah Basin, West Offshore Libya.

Abstract:

The Sabratah Basin is situated on the continental margin of Africa. In this presentation we use a series of grids of 3D seismic data, supported by potential field data, to document the structure and evolution of the basin. Although the basin is thought to be underlain by Paleozoic rocks, the oldest encountered in wells are Triassic in age. These are continental passing upwards into evaporites, a sequence thought to represent the progressive subsidence of the margin during major Mesozoic extension related to the opening of the Tethyan Ocean. Continued subsidence through the Jurassic and Early Cretaceous led to the deposition of marine sequences. The basin deepens towards the northeast, as the crust thins in the same direction. Crustal thinning in this direction is supported by an increase from southwest to northeast in the gravity. However, since the Late Cretaceous, there has been compression between Africa and Europe as a result of the Alpine collision, causing the inversion of the former extensional features, and creating en echelon folds trending ENE-WSW and segmented by en echelon northwest trending right-lateral faults. The geometries we observe are consistent with a right lateral shear roughly parallel to the continental margin. The Bouguer gravity anomaly map of the Sabratah Basin has relatively a smooth gradient with gravity increasing seaward to the northeast. The strong southwest-to-northeast increase in gravity values is probably related to the seaward swallowing of Moho as the crust thins in the that direction. This gradient is interrupted by two local gravity anomalies associated with magnetic anomalies and bounded the Sabratah basin, the WNW trending Jarrafa anomaly to the NE and to the SW the NS trending Jeffara anomaly with varying amplitudes. The high density and magnetisation of the source bodies indicates that they probably comprise mafic igneous rock. The depth to magnetic source estimated by source parameter imaging (SPI) and Euler Deconvolution techniques indicates that, the base of the magnetic bodies may be controlled by the depth of the Curie isotherm.