Ebbing, Joerg

We modeled crustal and lithospheric thickness variation as well as the variations in temperature, composition, S wave seismic velocity, and density of the lithosphere beneath the Saharan Metacraton (SMC) applying an interdisciplinary 3‐D modeling. Regardless of the limited data set, we aimed at consistent imaging of the SMC lithospheric structure by combining independent data sets to better understand the evolution of the metacraton. We considered that the SMC was once an intact Archean‐Paleoproterozoic craton but was metacratonized during the Neoproterozoic due to partial loss of its subcontinental lithospheric mantle (SCLM) during collisional processes along its margin. This has permitted the preservation of three cratonic remnants (Murzuq, Al‐Kufrah, and Chad) within the metacraton. These cratonic remnants are overlain by Paleozoic‐Mesozoic sedimentary basins (Murzuq, Al‐ Kufrah, and Chad), which are separated by topographic swells associated with the Hoggar Swell, Tibesti Massif, and Darfur Dome Cenozoic volcanism. The three cratonic remnants are underlain by a relatively thicker lithosphere compared to the surrounding SMC, with the thickest located beneath Al‐Kufrah reaching 200 km. Also, the SCLM beneath Al‐Kufrah cratonic remnant is significantly colder and denser. Modeling of the lithosphere beneath the Chad and Murzuq Basins yielded a complex density and temperature distribution pattern, with lower values than beneath the Tibesti Massif. Further, our modeling indicated a uniform and moderately depleted mantle composition beneath the SMC. The presence of a relatively thinner lithosphere beneath the noncratonic regions of the SMC is attributed with several tectonic events, including partial SCLM delamination during the Neoproterozoic, Mesozoic‐Cenozoic rifting, and Cenozoic volcanism.

This study presents a recent combined regional gravity field model over Egypt, developed by integrating satellite and terrestrial data via applying the remove-compute-restore (RCR) principle and the least-squares collocation (LSC) procedure. A high-resolution digital terrain model was exploited for the computation of the terrain and residual terrain corrections. Hereby, all the signals that can be modelled or deterministically computed are considered known and then removed in order to reduce the order of magnitude of the input gravity data prior to applying the LSC. Several GOCE-only and combined global geopotential models (GGMs) have been thoroughly investigated with respect to the EGM2008, in which the space-wise (SPW) solution, namely the SPW-R5 model, demonstrated the best performance. For the development of the combined model, the SPW-R5 GGM has been integrated with both the EGM2008 GGM and the terrestrial data retrieved from 56,250 gravity stations of the Getech data, acquired in the framework of the African Gravity Project. The combined regional gravity model was compared to the state-of-art XGM2016 global gravity model. The standard deviation of the differences is 18.0 mGal in terms of Bouguer anomalies. The combined regional model fits well with the terrestrial gravity data along the chosen North–South oriented profile through the Nile Delta region. The improvements of the developed combined regional model over the XGM2016 are due to the use of a more extensive terrestrial dataset. In conclusion, our model is more suitable than solely using the ground data or GGMs for regional density modelling over Egypt. As an example, the comparison of using a global or regionally defined gravity model with the forward gravity modelling based on Saleh (Acta Geodaetica et Geophysica Hungarica 47(4):402–429, 2012) density model is performed.

The recently released gravity potential field development derived from the Gravity Recovery and Climate Experiment satellite allows an unprecedented opportunity to use the gravity field to make global comparisons of structures of geological interest. The spatial resolution of the gravity field is sufficiently good to map large-scale or intracratonic and cratonic basins, as the areal extent of these basins is 0.5 × 106 km2 and greater. We present the gravity anomaly, Bouguer, geoid and terrain corrected geoid fields for a selection of nine large-scale basins and show that the satellite-derived field can be used to successfully identify distinctive structures of these basins, e.g., extinct rifts underlying the basins and generally the isostatic state. The studied basins are the Eastern Barents Sea, West Siberian, Tarim, Congo, Michigan, Amazon, Solim ˜ oes, Parnaiba and Paran`a basins. We complete the mapping of the gravity field with a description of the basins in terms of areal extension and depth, sedimentary age and presence and age of volcanism. Interpretation of the satellite gravity anomalies and considerations regarding the crustal thickness as known from seismic investigations, allows us to conclude that for the greater part of the basins there is evidence for high-density material in the lower crust and/or upper mantle. This density anomaly is, at least partly, compensating for the low-density sedimentary infill instead of the crustal thinning mechanism. For our selection of basins, crustal thickness variations and Moho topography cannot be considered as mechanisms of compensation of the sedimentary loading, which is a clear difference to well-defined rift basins.

A 3D crustal density model for Egypt was compiled. It is constrained by available deep seismic refraction, receiver functions analysis, borehole, and geological data. In Egypt, seismic data are sparsely and irregularly distributed. Consequently, we developed the crustal thickness model by integrating seismic and gravity data. Satellite gravity data was inverted to build an initial model, which was followed by a detailed 3D forward gravity modelling. The initial crustal thickness is determined by applying seismically constrained non-linear inversion, based on the modified Bott's method and Tikhonov regularization assuming spherical Earth approximation. Moreover, the gravity inversion-based Moho depth estimates are in good agreement with results of seismic studies and are exploited for the 3D forward modelling. Crustal thicknesses range from 25 to 30 km along the rifted margins of the Red Sea, which thin toward the Mediterranean Sea. Thicknesses in southern Egypt reach values between 35 and 40 km. A maximum crustal thickness of 45 km is found in the southwestern part of Egypt. Within the Sinai Peninsula, the thickness varies from the shallow southern edge (∼ 31 km) and increases toward the North (∼ 36 km). Our model revealed a thick lower crust beneath the southern part of Egypt, which can be associated with crustal modification that occurred during the collision of East Gondwana and the Saharan Metacraton along the Keraf suture zone during the final assembly of Gondwana in the Neoproterozoic. Finally, the isostatic implications of the differences between the seismic and gravity-derived Mohos are thoroughly discussed. In conclusion, the developed 3D crustal thickness model provides high-resolution Moho depth estimates that closely resembles the major geological and tectonic features. Also, the existing correlation between the topography, Bouguer anomalies, and Moho depths indicates that the investigated area is close to its isostatic equilibrium.

A 3-D joint inversion of seismic and gravimetric data is performed to re-investigate the subsurface structure of Mt. Vesuvius (Italy) utilizing an improved joint inversion method. The aim is to derive models of the 3D distribution of velocity and density perturbations that are consistent with both data sets and with local velocity models. Mt. Vesuvius is a strato volcano located within a graben (Campania Plain) formed in Plio-Pleistocene. Campania Plain is bordered by mostly Mesozoic carbonaceous rocks. Mt. Vesuvius is the southernmost and the youngest of a group of Pleistocene volcanoes, three of which (Ischia, Campi Flegrei and Mt. Vesuvius) have erupted in historical times. The most recent eruption of Mt. Vesuvius occurred in 1944 and since then the volcanic activity has been characterized by moderate low magnitude seismicity and low temperature fumaroles at the summit crater. We modified the coupling mechanism between velocity and density models in the JI-3D optimized joint inversion method (Jordan and Achauer, 1999). This method was designed to provide stable and high resolution results and involves iterative optimized parameterization, 3D ray tracing, and the incorporation of a priori information. The coupling of the velocity and density models, vital to the joint inversion, is based on a cross-gradient approach (e.g. Gallardo and Meju, 2004), which has been proven to work very well in a variety of cases involving seismic, magnetic, CSEM, MT and gravity data sets. We implemented the cross-gradient coupling for our 3-D irregular adaptive grid parameterization. In contrast to conventional joint inversion methods this approach encourages structural similarities in the models and does not rely on predefined relationships between velocity and density parameters. As a consequence, the resulting velocity-density relations are not contaminated by a priori assumptions and can be utilized to derive rock physical parameters. We apply this method to data from the TomoVes project (Gasparini et al. 1998), combining seismics and Bouguer gravity and local high resolution velocity models as a priori information. The starting models for the joint inversion are derived by separate inversions of the individual data sets. We show 3D distributions of velocity perturbations and density variations from the joint inversion of teleseismic relative traveltimes and Bouguer anomaly data with the aim of extracting further information about the physical status of the volcano- tectonic system.

The oil- and gas-rich West Siberian Basin is underlain by a layer of flood basalts of late Permian-Triassic age that are coeval with the Siberian traps. The extent and thickness of the basalts is unknown, but knowing their thickness is important for discussions on the end- Permian mass extinction because basalt volume constrains estimates of emitted volatiles. We have used GRACE satellite and terrestrial gravity data to study the structure of the crust and basalt distribution. Published seismic sections are used to constrain the sediment isopachs and to estimate a depth-density function. We use published models of crustal thickness and basement depth to reduce the observed gravity field to the basement level. The resulting 3D density model gives information on density anomalies in the lower crust and upper mantle and on the basalt thickness. We identify several rift-graben structures which are presumably filled with basalt. The lower crust below the West Siberian Basin shows considerable density variations and these variations allow the region to be divided into four major blocks. The eastern part of the basin, towards the Siberian platform, shows an arch-shaped density increase in the lower crust that is accompanied by a linear high-density anomaly at shallower depths. Our work demonstrates the way in which the GRACE-gravity field can be applied to map geological structures like buried rifts and large basins. The same techniques can be used for other large, remote basins such as those in cratonic South America.