Insight into the continental lithosphere using 3D geophysical and petrological modelling: An example from the Novohrad-Gemer region (Pannonian Basin, Slovakia-Hungary)

Abstract

Percolation of fluids and melts in the crust and the lithospheric mantle produces alteration zones with significantly disturbed physical properties, such as electrical resistivity and seismic velocity. The geophysical signatures of a metasomatized mantle include gravity responses as well because the modified modal and chemical compositions result in density changes. Here, we show how the local anomalous gravity field can be mapped in intraplate tectonic settings and interpreted using three-dimensional integrated modelling, involving intra-crustal structures, deep faults, and various mantle lithologies. The 3D interpretation performed using IGMAS+ software enables the integration of independent geo-datasets. Densities of near-surface (<5 km) bodies have been defined using laboratory measurements of surface and borehole rock samples and from well-logging. To calculate rock densities at greater depths, p-wave velocities have been transformed to in situ densities (ρ), while the densities of the lower lithosphere have been determined using thermodynamic modelling constrained by the chemical composition of xenoliths brought to the surface by alkali basalts. Thermobarometric data on megacrysts helped constrain the vertical extent of mantle metasomatism. To elucidate the inherent ambiguity of the gravity method, several geologically reasonable models conforming to the observed gravity field are tested. Based on the proposed 3D lithospheric models, the following conclusions can be drawn regarding the northern margin of the Pannonian Basin: a) The thickness of Neogene volcanics and sediments is variable, ranging from 0 to 4 km; b) The Hurbanovo-Diósjenő-Darnó fault zone is a steep and deeply penetrating first-order tectonic zone filled with low-density rocks in the upper crust and characterized by low-amplitude gravity anomalies and a maximum of modified horizontal gradient amplitude; c) Garnet-bearing mafic rocks with a density of 3.1 g⋅cm⁻³ are identified at the Conrad discontinuity; d) The subjacent lithospheric mantle is characterized by a sandwich structure consisting of a 9.0–9.5 km thick upper layer of mafic cumulates, a 12.0 km thick middle layer of the metasomatized mantle, and the wehrlitized mantle in a depth interval from 50 km to 77 km. Alternative models admit either a crustal Cadomian(?) segment with a density of 2.84 g⋅cm⁻³, with its upper boundary ranging from 10 to 18 km located beneath the Veporic unit, or a purely hypothetical eclogite layer with a density of 3.51 g⋅cm⁻³ within the mantle. Such an interdisciplinary approach combining geophysical and petrological data is also applicable in other continental tectonic settings. Detection of crustal and mantle sources related to deep-seated deformation zones through specific geophysical patterns could play an important role in global lithosphere research.