Abstract
Integrated geophysical methods (mainly Gravity, Magnetic, and Seismic methods) will be used in this study. It will involve the analysis of the proper data of each of these methods with the borehole data to establish the tectonic subsidence of the basin. And delineate structures and faults. Locate and define any major structures and study the tectonic evaluation of the Gabes-Tripoli basin NW-offshore Libya. The G-T Basin is located within a large intra-cratonic basin situated on the passive continental margin of the African plate. Diapiric phenomena related to Triassic salt movement have affected the study area. Sedimentary units in the Basin were deformed due to halotectonic and halokinetic movements during Upper Cretaceous time. Gravity and magnetic investigations and their interpretations involved analysis of the gravity and magnetic anomaly maps, using the methods of trend surface analysis, second derivative, upward continuation, power spectrum, and 3D Euler Deconvolution. Quantitative interpretation of selected gravity and magnetic profiles was carried out, using two-dimensional models. The broad aim was to produce a geological interpretation of the gravity and magnetic data, delineating the deep, intermediate, and shallow structures, and improve the geological interpretation of the data. Combining several geophysical methods has proved particularly useful in identifying the cause of various anomalies and delineating regional and local structures in the G-T Basin. Uncertainties in interpretation from any method are reduced by this combined approach. The interpretation of those methods shows that three broad highs dominated the southern part of the area believed to be a volcanic intrusion, and a synclinal depocenter, extending from the middle of the map at 12˚ 20ʹ to 12˚ 45ʹ east and form the center of the map at 33˚ 20ʹ to 33 45ʹ North gives anomaly lower than -18 mGal, the anomaly has two average axes 31.5 km and 4 km, show its considerable extent and area which covers approximately 1250 km2. The depth to the top of the largest anomaly structure ranges from ~4300 m–7000 m while The Moho depth as determined by the gravity models is ~25.3 km–31.6 km. The models show quite a deep basement depth (~7 km) under the troughs, while it is relatively shallow (~4 km) beneath the positive anomalies. The analysis of the tectonic subsidence curves produced based on well data of six real boreholes, reveals that the tectonic subsidence of the G-T Basin was caused by two rift phases. Initial syn-rift subsidence began in the Paleocene (~50 Ma) and ended at (~39.9 Ma) mid-Oligocene and was followed by rapid subsidence decreasing with time associated with rift events. This second phase of rifting and rapid subsidence continued through the Mid-Eocene–present, during which ~ 362m_387m of subsidence occurred. Analysis of the tectonic subsidence curves shows the maximum crustal stretching factor in the area is 1.193 (19.3% extension).