Tectonic history and structural development of the Zallah-Dur al Abd Sub-basin, western Sirt Basin, Libya

Abstract

The Zallah-Dur al Abd Sub-basin area lies in the western part of the Sirt Basin of Libya. 2D seismic data covering an area of about 32,000 km² were studied along with the formation tops from 240 wells. We mapped a complex network of normal and probable strike-slip faults, generally striking NNW-SSE that control the asymmetry of the basin. Subordinate NE–SW structures acted as transverse faults controlling local depocentres that segment the Zallah-Dur al Abd Sub-basin.A number of active faults in the intra-basin area have been identified in seismic sections with generally moderate to high dip angles, and displaying evidence for positive and negative flower structures. The bordering extensional fault (the Gedari fault) passes at depth into a moderately SW-dipping structure crossing most of the Upper Mesozoic to Cenozoic stratigraphic section. Thickness variations adjacent to other major faults suggest also an original extensional system where inherited high-angle faults were reactivated throughout this time.A detailed analysis of the available seismic reflection and drill hole data shows that an obliquely rifted, multi-cyclic, NNW-SSE trending basin developed during the complex Upper Mesozoic Cenozoic rearrangement of Mediterranean tectonics. Multiple phases of rifting can be observed in the study area affecting a number of different horizons from Upper Cretaceous to Eocene.In the study area, the basin was initiated as a result of a Tethyan oblique extensional rift system that began in the Early Cretaceous and peaked in the Late Cretaceous. The basin reached its rift maturation phase during the Upper Cretaceous as a result of the continuing extensional tectonics on the marginal bounding NNW-SSE trending normal growth faults. During the Alpine-related tectonic pulses of Middle–Late Eocene the Sirt Basin underwent compression resulted in northward tilting of the basin, causing abrupt subsidence in the north and uplift on the basin southern shoulders, possibly driving a late stage of minor regional subsidence.