Moho Topography Estimation using Interactive Forward Modeling of Gravity Data

The Moho discontinuity is a boundary between the crust and upper mantle that reveals the difference between them with changes in seismic velocity, density, chemical structure, and constituents. Estimating the Moho depth and studying its lateral changes is one of the important goals of geophysical studies. The current study aims to estimate the depth and topography of the Moho discontinuity in the southwestern part of the Baltic Sea, including parts of the central European system, the Trans-European Suture Zone, Caledonian Crustal Suture, and the Ringkobing-Fyn High. This area has been one of the most attractive regions for Geoscientists in the last decades due to its complicated geological structures caused by different tectonic events. For this purpose, a three-dimensional model of the crustal structures based on gravity data forward modeling in the study area has been presented. Previous seismic / non-seismic results have been used to constrain the model and reduce its degree of freedom. This model includes sedimentary sequences, crustal thickness, Moho topography, and high-velocity lower crust expansion in the region and shows the tectonic structures of the study area. This study used a combination of marine, land, and EGM2008 gravity data and modeled them with IGMAS+, Interactive Gravity and Magnetic Application System. The interactive modeling program allows the user to change the geometry as well as the density and susceptibility of the primary model and observe results quickly during the processing. In the software, the model structure could be be more user friendly by eliminating additional details and dividing the whole model into vertical sections. Our primary model consists of three main layers of sediments, crust and upper mantle. The sedimentary layer is divided into two major parts, pre-Permian and post-Carboniferous. Also, the crustal layer is divided into the upper crust and the high-density lower crust. Besides, the upper crust is composed of the upper crust of the Baltica and the upper crust of Avalonia. The last layer of the model is a part of the upper mantle. The model space consists of 16 vertical planes stretching 385 kilometers east-west with an equal distance of 15 kilometers, covering the entire study area. The initial model was developed based on seismic sections and previous models, and it has been improved using interactive forward modeling of gravity data. The result shows a good agreement between the measured and modeled Bouguer anomaly, and the Root Mean Square Error of the model is 1.12 mGal. The model correlates clearly with major tectonic units. It indicates that the Caledonian collision resulted in the amalgamation of Baltica and Avalonia is the most prominent tectonic event in the area, and the Caledonian crustal suture between them is interpreted from changes in physical parameters at crustal levels. There is a relatively thick crystalline crust in the area, and the depth of Moho discontinuity varies from 26 to 42 km. The results also indicate that the transition from the Paleozoic crust of the Central European Basin to the Precambrian crust of the Eastern European Craton occurs within the Tornquist Zone.