Crustal lithology vs. thermal state and Moho heat flow across the NE part of the European Variscan orogen: a case study from SW Poland
Geotherms of four crustal profiles across the Variscan orogen in SW Poland have been constructed to better understand the variation in thermal state of the region. Seismic and drilling data as well as rock sequences in exposed parts of the orogen were used for Wilcza Góra (Sudetic Block), Polkowice–Rudna (Middle Odra Horst), Święciechowa (Wolsztyn–Leszno Horst) and Września (Szczecin–Miechów Synclinorium within TESZ) profiles. The palaeoclimate-corrected surface heat flow map and the heat-production calculations based on literature data enabled the construction of geotherms and estimation of Moho heat flow. Exception is the Polkowice–Rudna profile, where the surface heat flow was calculated using temperature measurements in underground copper mines, and K, U and Th contents and density/heat conductivity measurements in rocks drilled in the Middle Odra Horst were used to calculate geotherm. The basaltic lava migration through the lithospheric mantle and its effect on crustal geotherm in Wilcza Góra (area of Cenozoic alkaline volcanism at ca. 30–18 Ma) was shown to increase slightly both current surface and current Moho heat flow (by ca. 4 mW/m2). The presented approach couples geological and geophysical information and thus differs from standard purely geophysical assessment of lithosphere thermal state. It shows that surface heat flow is enlarged to 84 mW/m2 by granitic plutons in the upper crust in the Lubin–Polkowice site. Heat flow on the Moho varies from 28 to 34 mW/m2 in sites located in the Sudetic- /Fore-Sudetic Blocks, Middle Odra Horst and Wolsztyn–Leszno Horst. Locally it may slightly increase because of extraneous heat input by Cenozoic basaltic volcanism, but the values close to 30 mW/m2 are specific for mantle root of Saxothuringian Zone in SW Poland. Mantle heat flow is 36 mW/m2 in Września site and supposedly is a manifestation of different nature of lithospheric mantle underlying TESZ.