Heat flow models across the Trans-European Suture Zone in the area of the POLONAISE’97 seismic experiment

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Abstract

Heat flow data from the Polish basin show a sharp change in the transition from the East European Craton (EEC) and Teisseyre–Tornquist Zone (TTZ) in the north-east to the accreted terranes in the south west (Paleozoic Platform). The analysis of this data and numerical modelling of the crustal temperatures show evidence of extensive crustal–mantle warming in the area between the Sudetes to the south and the Trans-European Suture Zone to the north. The change in heat flow is 100% when compared with values for the EEC. Heat flow in the anomalous zone is also higher than in the Sudetes. The axis of the anomaly is aligned with the Dolsk Fault and Variscan deformation front. Low crustal/mantle temperatures derived from the relationship between temperature and Pn velocities (more than 8.2 and as high as 8.4 km/s) are at odds with high crustal temperatures calculated from surface heat flow, seismic velocity based heat generation models and thermal conductivity. High heat flow (Variscan platform) and related high temperatures of the crust coincide with small crustal thickness (30–35 km). The opposite is the case for the low heat flow EEC (45–50 km). High heat flow above thin crust and low heat flow above thick crust with no major variation in elevation is supported by a simple isostatic balance model. Crustal heat generation explains part of the high heat flow within the zone with thick meta-sediments reaching down to 20 km depth, however, it is far from explaining high heat flow in Variscan crust and in the transition zone into a cold EEC. 2D numerical models of heat flow based on new seismic data require a contrast of 15 mW/m2 in mantle heat flow. High mantle heat flow (35–40 mW/m2) is likely to occur in the high heat flow zone while cold crust and cold and high-density mantle (mantle heat flow of 20–30 mW/m2) is typical of the EEC. Thermal lithosphere thickness for the craton is 200 km while it is only 100 km in the accreted terranes to the southwest of the TTZ. The TTZ in Poland appears as a relatively cold area.

Introduction

The crustal structure and thermal regime of the transition zone between the Precambrian and Paleozoic Platforms in Poland was one of the main aims of geophysical studies during the last 30 years. The tectonic position of Poland in Europe is unique and complex. Several large geological units of different age meet there: the East European Craton (EEC), Paleozoic mountain belts (the Caledonides and Variscides), the Paleozoic Platform of Central and Western Europe, and the South European Alpine orogene (Carpathian Mts). The SW margin of the EEC, between the North Sea and the Carpathians-Black Sea area, termed the Teisseyre–Tornquist Zone (TZZ), consists of two segments. The first one, the Sorgenfrei–Tornquist Zone, extends NW of Bornholm Island and the other, the TTZ, runs SE from the Baltic coast in Poland to the Carpathians-Black Sea area in the Ukraine. In general, the Tornquist Zone is the longest tectonic feature in Europe separating the EEC with its Paleozoic cover from the Phanerozoic mobile belts of central and western Europe.

The deep crustal structure of that transition zone has been addressed in detail by a number of seismic refraction and wide-angle reflection experiments (Guterch et al., 1999; Jensen et al., 1999; Guterch and Grad, 2000). A large seismic experiment, the POLONAISE’97, was conducted in Poland during May of 1997 and targeted the deep structure of the Trans-European Suture Zone (TESZ). In addition to five POLONAISE’97 profiles (P1–P5) two deep seismic sounding (DSS) profiles (LT-7 and TTZ) pass through the Polish Basin. The quality of the data surpasses any of the previous DSSs for which heat flow modelling was completed (Majorowicz, 1978; Milanowsky, 1984 and Čermak et al., 1989). About 3000 km of high quality deep seismic profiles has shown that the Earth’s crust in the TTZ has highly anomalous properties. The boundary velocities, mean velocities and stratification of the crust vary distinctly along the TTZ. This new seismic database allows a new look at the thermal state of the crust and comparisons with previous models from previous seismic experiments.

The heat flow at Moho and the temperature distribution in the crust and upper mantle were studied for the northern part of the TESZ (Balling, 1995). Mantle heat flow of 20–30 mW/m2 was estimated for the northern and central Baltic shield transects, increasing to 30–40 mW/m2 beneath the shield margin, deep basins (Danish and North German), and Tornquist zone (Balling, 1995). Föerster and Föerster (2000) estimated mantle heat flow of 20–30 mW/m2 for the Variscan Erzgebrige in Germany.

In this paper we construct two-dimensional (2D) numerical models of the crustal and upper mantle temperatures in the transitional zone between the craton (Baltica) and the areas south west of the TTZ.

Section snippets

Geological background

The area of investigation in the Polish lowland is located at the border between Precambrian crustal terranes of the EEC and the younger Phanerozoic terranes in the southwest (Berthelsen, 1992a, Berthelsen, 1992b, Berthelsen, 1998). Much of Poland and northern Germany is covered by the deep (>10 km) sedimentary basin, filled with Permian and Mesozoic sedimentary rocks. The Polish Basin forms the easternmost part of this Central European Basin. It is situated on the contact between and on the

Geophysical investigations of the study area

The TESZ is clearly visible as a tectonic unit of distinct crustal structure (Guterch et al., 1986), outlined by the morphology of magnetic and gravity fields (Tornquist, 1908; Jankowski, 1967; Grabowska and Raczyńska, 1991; Grabowska et al., 1991; Królikowski and Petecki, 1995; Królikowski and Wybraniec, 1996) as well as the heat-flow data (Majorowicz and Plewa, 1979; Čermak et al., 1989), contrasting with neighbouring lithosphere blocks. In general, the EEC in NE Poland is characterised by a

Geothermal data and heat flow

A recent critical review of Polish well temperature logs (Plewa, 1994; Karwasiecka and Bruszewska, 1997; Majorowicz et al., 2002; Szewczyk, 2001) addressed their limitations due to well conditions (equilibrium state) and quality of industrial continuous logging. Continuous temperature logs obtained in 231 wells (Fig. 1(a)) with commercial logging equipment is of low accuracy (0.1–0.5 K). Ten precise (better than 0.03 K) temperature-depth measurements with thermistor probe were recorded in wells

Crustal temperature modelling

Assumption of constant mantle heat flow of 20–30 mW/m2 (25 mW/m2) derived from previous models of heat generation in the crust based on the relationships between heat generation and seismic velocities for the older DSS profiles and the surface heat flow (Čermak et al., 1989) results in large lateral variations of crustal heat flow from as high as 65 mW/m2 in south-western Poland to 5 mW/m2 in north-eastern Poland (Fig. 7). The analysis of derived crustal heat flow variations shows little

Discussion

The new results of modelling show that mantle heat flow increases after crossing the TTZ zone southward by 15–20 mW/m2. The small-scale variations of mantle heat flow, e.g. along profile P1 (Fig. 8), are one possible cause of variations of the observed surface heat flow. Reliability of the observed surface heat flow is not high so small variations in mantle heat flow cannot be resolved. Surface heat flow variations can also be caused by lateral changes of heat production or thermal

Conclusions

The analysis of heat flow data in the Polish basin and 2D numerical modelling of the crustal temperatures shows evidence of extensive crustal–mantle warming with the axis of the anomaly aligned with the Dolsk Fault and Variscan deformation front. High heat flow above thin crust and low heat flow above thick crust with no major variation in elevation is supported by a simple isostatic balance model.

Crustal heat generation explains part of the high flow in the zone with thick meta-sediments

Acknowledgements

Integrated seismic–thermal studies of the NW Poland have been completed within the NFOSiGW research project no. 2.94.0004.00.0 (to PK). The Czech participation on the project was partly financed from the research grant no. A3012005 provided by the Grant Agency of the Czech Academy of Sciences. The authors want to express their sincere thanks to Ilmo Kukkonen and to the anonymous reviewer for their valuable comments, which helped to prepare the final text.

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