Are we exposed to crossfire here in the middle of Europe? Heating from the atmosphere, heating from the crust and the mantle. The life of the Earth is changing and challenges us again and again. But geology gives us many ways to solve the problems as well. While the heat “from above” is a challenge, the heat “from below” can be an opportunity, as long as it provides energy, power for making changes.
The Pannonian Basin lives in our minds, especially in the endless lowlands, as a peaceful and free landscape, a realm of birds, clouds, wind and sunshine. But below the surface we can find more energetic places in the Earth crust: not in terms of movements, earthquakes or volcanic bodies anymore, but very hot rocks and water near the surface.
What causes the concentration of heat in the middle of Europe? Very simply: a Europe-Africa love story, a dance of two fragments of the two continents in the Carpathian embayment. The two dancers are the Alcapa and the Tisza Megaunits and the leading partner is the former one. Let see them in a bit more professional way.
Together with the shifting and rotating dance movements the Alcapa Megaunit moved east-northeastward. The Alcapa Megaunit turned the Tisza Megaunit into the Magura ocean, Carpathian embayment, contemporaneously deforming the pinched, sheared blocks of South Alpine relation and the sediments of the flysch ocean between them. By the Middle Miocene, the Alcapa and Tisza Megaunits were settled into their present position, together with the sheared, consolidated sediments, and with the remnants of the flysch ocean. Between the two megaunits the Mid-Hungarian Shear Zone can be interpreted as the suture of a former ocean.
The several-million-year-long dance ended here, the two dancers remained embraced together but something had changed. During the dance, the dancers’ fervor made the floor, the crust of the Magura ocean, subduct. This led to the formation of the Pannonian basin. Extension was induced by the extensional stress created by the roll-back of the subducted oceanic slabs beneath the Carpathian arc, and by the potential energy of collapsing former Alpine nappe structures. The extension was accompanied by the attenuation of the crust, which resulted in asthenosphere upwelling that was strengthened by the asthenospheric flows created by the roll-back process. Subsidence was practically due to the isostatic movement of the attenuated and low density crust. In summary, the thermal flux of the crust had been increased. So what is the source of the heat “from below”? It is the late remnant of the fever of two microcontinents in love. And as it often happens with lovers, after reaching the isostatic balance, the thermal balance has also started to be achieved, which has resulted in a cooling down effect in the crust leading to a thermal-induced subsidence in the Late Miocene.
And what is the lovers’ heritage for us? Relatively thin Earth’s crust (~25 km), anomalously high terrestrial heat flow (~0.09 W/m2), high geothermal gradient (~0.05°C/m) and hundreds of geothermal wells and baths. It is cool (or warm…), isn’t it?
MBFSZ, Magyar Bányászati és Földtani Szolgálat – Mining and Geological Survey of Hungary
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