Crustal structure beneath the Nógrád-Gömör Volcanic Field from 3D density modelling

During the last 21 Ma, widespread and geo-chemically variable volcanism took place in the Pannonian Basin and surrounding areas. The Nógrád-Gömör Volcanic Field (NGVF) is the northernmost Neogene monogenetic alkali basalt volcanic field of the Carpathian–Pannonian region, where the magma transported numerous upper mantle xenoliths to the surface. Alkaline basalt volcanism in this area represents a typical intraplate association, which is a result of decompression melting at the interface of the mantle and asthenosphere. The deep structure of this area has long been of interest to the geologists, volcanologists, geophysicists and geochemists.

 

Long period MT data collected along a ~50 km long NNW-SSE profile helped to explain the electric conductivity behaviour of the lithospheric rocks and to indicate the LAB too (Patkó et al. 2021). A massive conductive wehrlitic cumulates were indicated at ~30-60 km depths which arose as a product of the mantle metasomatism. Wehrlite-bearing xenolith suites found in the central part of the NGVF supports this interpretation. We are aiming to understand the crustal architecture and interpret the rather complicated gravity field of the NGVF. Therefore, a robust 3D density model was constructed using the 3D potential field modelling tool IGMAS+.

 

Only the gridded gravity data were utilized in the modelling, as the amplitudes of multiple magnetic anomalies aligned in a belt formation indicates rather shallow sources related to basalt volcanism along the Hurbanovo-Diósjenő fault. To be able image the deeper structures we have constructed bigger starting 3D model containing all important geological interfaces, i.e. pre-Cenozoic basement, UC/LC boundary, Moho and LAB. Then all available geophysical and geological constraints (seismic, MT, faults positions, main tectonic units) were applied to produce a more detailed, structural model in the central part of the studied area.

 

The Hurbanovo-Diósjenő fault is confirmed to be a steep and deeply penetrating tectonic zone beneath the central part of the NGVF, separating the Trans-danubian Range and Bükk units from the Veporic and Gemeric units of the Inner Western Carpathians. Thanks to a higher density of wehrlite (3 350 kg/m³; Aulbach et al. 2020) we could identify the deep-seated geobody (located in a depth range of 30-55 km) through the gravity modelling. We assume that this mantle lithosphere geobody is closely related to alkaline basalt volcanism in the NGVF. It contributes with a smaller gravity effect of +5.7 mGal maximally to the overall positive gravity anomaly over the volcanic field. The observed Bouguer anomalies contain superimposed effects of the following upper crustal units too: Gemeric, South Veporic and crystalline basement probably of the Cadomian age.

 

Acknowledgement:

This work was supported by the projects Nos. APVV-16-0482, APVV-16-0146 and VEGA projects Nos. 2/0002/23 and 2/0047/20.

 

References:

Aulbach S. et al. 2020: Wehrlites from continental mantle monitor the passage and degassing of carbonated melts. Geochemical Perspective Letters 15, 30–34.

Patkó L. et al. 2021: Effect of metasomatism on the electrical resistivity of the lithospheric mante – An integrated research using magnetotelluric sounding and xenoliths beneath the Nógrád-Gömör Volcanic Field. Global and Planetary Change 197, 103389.