This post is part of the GeoConnect³d blog.

A bit about the regional structure

The geologic structure and faulting in NE Slovenia reflects a rich and varied tectonic history since the middle part of the Mesozoic. Buried under thick Neogene marine and terrestrial sediments of Paratethyan origin, the underlying basement is as complex as anything encountered at the Alpine-Dinaric junction to the west. The complex structure of both the Neogene sediments and the pre-Neogene basement was controlled the evolution of the area through Neogene and Quaternary. The Pannonian domain, which includes NE Slovenia in its northwest part, consists of several structural blocks, mainly the ALCAPA and TISZA crustal blocks/mega-units, that underwent thinning due to crustal extension and thermal collapse of the Pannonian basin (e.g. Handy at al., 2014; Horváth et al., 2015).

The northeast part of Slovenia is part of the ALCAPA mega-unit, divided from the Southern Alps by the Periadriatic fault system. The ALCAPA mega-unit comprises the Austroalpine unit of the Alps (Schmid et al., 2020 and references therein). The structure and activity in the eastern part of Slovenia are mostly controlled by the interaction between the ALCAPA and TISZA mega-units. TISZA is an accreted tectonic block/mega-unit composed of composite terranes of Eurasian origin formed in the Mesozoic and final emplacement present-day configuration in Paleogene-Miocene (Csontos et al., 1998; Handy et al., 2014; Schmid et al., 2020). The boundary between the ALCAPA and TISZA mega-units is the Miocene-formed Mid-Hungarian Zone (MHZ). The northern edge of the MHZ, also referred to as the Balaton fault/line is the eastward continuation of the Periadriatic fault system. The MHZ is zone of repeated tectonic inversions (Csontos et al., 1998), currently a sinistral strike-slip zone resulting from the TISZA eastward motion outpacing the eastward motion of ALCAPA (Serpelloni et al., 2016).

Fault systems in NE Slovenia

There are two fault systems in the GeoConnect³d project area in northeast Slovenia: the Raba fault system and the Balaton fault system, which itself is part of the Mid-Hungarian zone. The lower Neogene rifting phase produced a multiple systems of normal faults, with general WSW-ESE strikes and northerly and southerly dips. Two deep basins formed in NE Slovenia: the Radgona-Vas sub-basin in the north, with the Burgenland swell its north and the Haloze-Ljutomer-Budafa sub-basin in the south, separated by the Murska Sobota extensional block. The Radgona-Vas sub-basin formed along the Raba fault system normal faults, it is approximately 50 km long and 10 km wide and up to approximately 3000 m deep.

The Haloze-Ljutomer-Budafa basin is an extremely deep basin, with maximum depths >6000 m, was formed along another system of normal faults, which was inverted in late Neogene as reverse faults (Fodor et al., 2002). These faults now form the Ljutomer fault and associated smaller faults. The Ljutomer fault zone is part of the Balaton line (c.f. Schmid et al., 2020), the northernmost part of the Mid-Hungarian Zone, the contact zone between the TISZA and ALCAPA mega units. The segments that make up the Ljutomer fault zone initially formed as normal faults, undergoing later inversion and reverse, likely transpressive movement.

The Ljutomer fault is a spectacular blind reverse fault, offsetting the upper Neogene strata by up to several hundred meters and forming a textbook fault propagation fold and growth triangle above the fault tip. It is considered a probably active reverse to sinistral transpressive fault. No definite activity has been associated with it, no systematic GNSS or other geodetic data, paleoseismological or age dating data has been acquired so far. Probable activity is indicated by indirect evidence, such as dog leg deviations in the Drava and Mura rivers as they cross the fault. The Ljutomer fault and other reverse faults in the area are structurally connected with large fault propagation anticlines in the Boč-Ormož-Budafa-Lovaszi antiform (such as the Petišovci anticline) that are structural traps for hydrocarbons and have been exploited for decades.

Fault activity and seismicity

Seismicity in this area is low and much below average for Slovenia, which itself is an area of moderate seismicity. There are only two noteworthy historical events: the 1838 M_w=4.5 Ormož Kog and 1839 M_w=4.8 Ormož Zavrč earthquakes. Even for these two events, the epicentral uncertainty is very large, so no causative faults or fault systems can be attributed reliably (Ribarič, 1982; Živčić, 2009).

The same structural trend of the Mid-Hungarian Zone extends also to the south into the Međimurje region in NE Croatia, where the epicenter of the 1738 M_L=5.1 Međimurje earthquake is located. This is by far the most studied event in this area and is likely typical of significant earthquake events in this fault system. The causative fault is the reverse Čakovec fault, similar to the Ljutomer fault. It was a shallow event, with a hypocentral depth of only 6 km and a relatively small area of maximum intensity (Herak et al., 2020).

While the Ljutomer fault and the entire Balaton fault system is a significant regional structural element, they produce relatively little seismicity. This is attributed to the thinned lithosphere, evidenced by the very shallow MOHO discontinuity and high heat flux, which in turn changes the rheological properties of the lithosphere, decreases the brittle-ductile transition depth and decreases the seismogenic crustal thickness.  Nevertheless, as the 1738 event indicates, seismic hazard is significant and noteworthy even in this area of relatively low seismicity.

Jure Atanackov
GeoZS – Geological Survey of Slovenia

References

Csontos, L., Andras, N., Nagymarosy, A., (1998).  The Mid-Hungarian line: a zone of repeated tectonic inversions.  Tectonophysics 297, 51–71. Schmid et al., 2020

Fodor, L.., Jelen, B., Marton, E., Rifelj, H., Kraljić, M., Kevrić, R., Marton, P., Koroknai, B., Baldi-Beke, M. 2002. Miocene  to  Quaternary  deformation,  stratigraphy  and  paleogeography in  Northeastern  Slovenia  and  Southwestern  Hungary = Deformacije,  stratigrafija  in  paleogeografija  severovzhodne  Slovenije in  jugozahodne  Madžarske  od  miocena  do  kvartarja. Geologija, 45/1, 103-114.

Handy, M.R., Ustaszewski, K., Kissling, E. (2014). Reconstructing the Alps–Carpathians–Dinarides as a Key to Understanding Switches in Subduction Olarity, Slab Gaps and Surface Motion. Int. J. Earth Sci. (Geol Rundsch). doi: 10.1007/s00531-014-1060-3

Herak, D., Živčić, M., Vrkic, I., Herak, M. (2020). The Međimurje (Croatia) Earthquake of 1738. Seismological Research Letters 91 (2A), 1042–1056. DOI: 10.1785/0220190304

Horváth, F., Musitz, B., Balázs, A., Vegh, A., Uhrin A., Nador, A., Koroknai, B., Pap, N., Toth, T. and Worum, G. (2015).  Evolution of the Pannonian basin and its geothermal resources.  Geothermics 53, 328-352.

Ribarič, V. 1982. Seizmičnost Slovenije. Katalog potresov (792 n.e.–1981). = Seismicity of Slovenia. Katalogue of earthquakes (792 A.D.–1981). Ljubljana, Seizmološki zavod SR Slovenije: 649 p.

Serpelloni, E., Vannucci, G., Anderlini, L., Bennett, R. A. (2016).  Kinematics, seismotectonics and seismic potential of the Eastern sector of the European Alps from GNSS and seismic deformation data.  Tectonophysics 688, 157-181.

Schmid, S.M., Fügenschuh, B., Kounov, A., Maţenco, L., Nievergelt, P., Oberhänsli, R., Pleuger, J., Schefer, S., Schuster, R., Tomljenović, B., Ustaszewski, K.van Hinsbergen, D.J.J., (2020). Tectonic units of the Alpine collision zone between Eastern Alps and western Turkey, Gondwana Research, Vol. 78, p. 308-374. DOI: 10.1016/j.gr.2019.07.005. Živcic M., (2009). Earthquake Catalogue of Slovenia.

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