In the area of the typical 3D Békés basin ("graben") high anisotropy (difference) appears in the extreme values of the magnetotelluric impedance phases in the period interval corresponding to the indication of the conductive asthenosphere. As proved by the forward 2D/3D modelling, this anisotropy can only be explained by a strong 2D effect. This fact may conform the validity of the results of the 2D inversion of PGT1 magnetotelluric data indicating the upwelling of the asthenosphere below the Békés basin.
In the present study the crustal structures of Nile Delta, including Greater Cairo province, and its surroundings, were evaluated using 3D forward gravity and magnetic modeling. The interpretation is also based on the seismic reflection results, well logs data and previous density models. The present results allow to improve the understanding of both the crustal thicknesses and density distributions between the sedimentary cover and the upper mantle in the study area.The high Bouguer anomalies near the Mediterranean coast are mainly caused by deep-seated structures. The crust beneath the Mediterranean coastal region is typically continental, with a thickness of 24 km beneath Rositta and Damitta branches, which increases toward the south. The Greater Cairo province has been modeled with maximum crustal thickness of ca. 34 km. The negative gravity anomalies with minimum values are due to the effect of sedimentary cover and/or basement relief geometry.The main results of this work suggest that the study area could be divided into three different distinctive tectonic zones according to their Moho depth and crustal structures. The southern zone (unstable shelf zone) which covers the Greater Cairo province is characterized by maximum crustal modeled thickness. It also represents the most seismically active tectonic zone in the study area. On the contrary the middle and northern zones show a thinned crustal layer and a correspondingly thicker sedimentary cover.Furthermore, the magnetic anomalies along the Greater Cairo, as deduced from the 3D magnetic modeling, are mainly caused by the upper crustal structures. In particular, the high magnetic anomalies of the southern part of the studied area are interpreted as related to variation in the basement relief near the surface, resulting from block faulting and/or compressional folds.In the northern region near the Mediterranean coastal area the highest magnetic susceptibility anomaly values is affected by the shallow smooth relief of the lower crustal layer. Finally, limited, shallow-seated basaltic intrusions were modeled beneath the western side of Nile Delta. The existence of these basaltic intrusions suggests that the study area was influenced by the opening of the Red Sea and/or Gulf of Suez during the Oligocene time.
Thermal convection has been modelled in a 3D model box, in order to estimate the areal density of upwellings and compare it to the density of hotspots, assumed as surface imprints of the cylindrical upwellings of the mantle. The number of the hotspots of the Earth is 40 to 100. If this is translated to a nondimensional areal plume density, using the depth of the convecting layer as length unit, a value of 2-6 is obtained for whole-mantle convection, while this value is 0.04-0.10 for a separately convecting upper mantle. The nondimensional theoretical areal plume density has been found about 0.2-1.0 for reasonable numerical models of the mantle. The fact, that the theoretical value lies between the densities estimated for one- and two-layer mantle systems, supports the possibility of a mixed regime, where some of the plumes come from the base of the mantle, some others from the 660 km boundary.