Lithosphere thermal structure and evolution of the Transylvanian Depression — insights from new geothermal measurements and modelling results

Abstract

The surface heat flow density pattern of the Transylvanian Depression (TD) represents a marked high amplitude short wavelength low in a region of elevated heat flow. Detailed temperature–depth profiles obtained by continuous temperature logging, combined with a finite element modelling of topographic and fluid flow effects, support the conclusion that the observed thermal gradient in the TD truly represents the rate of heat loss of the subsurface. The surface heat flux is 30 mW m −2 in the centre of the depression, increasing to about 60 mW m −2 at margins. Climatic correction is of the order of 2–10 mW m −2 depending on the investigated depth interval. The measured temperatures are compatible with the cooling of the Earth surface during the Weichselian glaciation followed by the climate warming which started 10–11,000 years ago. The transient effects of Neogene sedimentation and erosion mean an overall 2–3 mW m −2 reduction in surface heat flux. A low mantle heat flux (30 mW m −2) and a low crustal heat production rate in the TD (0.5 μW m −3 in the centre) are necessary to explain the heat flux anomaly. The surface heat flux pattern of the TD and the extreme low value in the centre of the depression point to an exceptionally cold and strong lithosphere in the Transylvanian region, as shown by strength envelopes characteristic to the entire depression. The evolution of the TD generally can be understood in terms of an exceptionally strong lithosphere block caught in a compressive tectonic environment.