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Cenozoic Subsidence and Lithospheric Stretching Deformation of the Ajdabiya Trough Area, NE Sirt Basin, Libya

Abstract

The Ajdabiya Trough forms the deepest part of the NE Sirt Basin, Libya and formed due to Mesozoic rifting followed by subsidence and possible flexural loading of underlying Tethyan rifted margin. We have analysed a data set that includes over 10,000 km of onshore 2D seismic reflection profiles, potential field data, and 5 wells to characterize and quantify the Cenozoic thermal subsidence and prior lithospheric stretching within the Ajdabiya Trough. Existing stratigraphic data shows that this margin is characterised by an early syn-rift sequence of mainly Triassic to Early Cretaceous age overlain by a post-rift sequence of Eocene to Miocene age. Structural and stratigraphic interpretations of the 2D seismic profiles highlight three stages of fault formation and basin subsidence. The structural style within the deepest parts of the trough is defined by a system of fault blocks bound by normal faults and bald basement highs devoid of syn-rift sediments, which are buried under a thick succession of post-rift deposits. Mainly NW-trending normal faults have been inferred from the seismic and potential field data. These structures dissect Cretaceous and older rocks and are sealed by Paleocene – Eocene sediments. Other faults trend NE-SW and E-W, and mainly cut Miocene and older strata. These faults are probably reactivated Hercynian or older basement structures. Some of these faults have strike slip components and are possibly linked to seismic activity in Sirt Basin and the Cyrenaica Platform. Vertical displacements on these faults are several hundred meters and are defined by large throws on Cretaceous and underlying horizons. One-dimensional back-stripping assuming Airy isostasy, shows that Cenozoic subsidence in the Ajdabiya Trough can be divided into three episodes of post-rift subsidence 1.(65 – 54.8 Ma), 2. (50 – 33.7 Ma), 3. (33.7 – 0 Ma). The analysis shows that the syn-rift and post-rift sequences in the Ajdabiya Trough were affected significantly by basement structural relief. There is a ca. 1000 m syn-rift sequence correlated with the basement structural relief, and post rift sequences of more than 4000 m in sag or thermal subsidence stage. The lithospheric stretching factor ß = 1.3 was calculated based on gravity, magnetic modelling and preliminary observations from the syn-rift character, show that the post-rift subsidence has the same trend as whole crust with greater lithosphere stretching and equivalent basement relief.