ON THE GEOTHERMAL REGIME OF THE FORELAND OF THE EASTERN CARPATHIANS BEND C. Demetrescu 1, H. Wilhelm 2, M. Andreescu 1, V. Dobrica 1, D.Z. Serban 1, A. Damian 1, M. Ene 1, Ch. Baumann 2, G. Polonic 1 1) The Institute of Geodynamics of the Romanian Academy, Bucharest, Romania 2) The Geophysical Institute, Karlsruhe University, Germany Acknowledgements: This study has been funded from: - Subproject A4 of the Sonderforschungsbereich 461 of Karlsruhe University, Project WI687/15-1 of the Deutsche Forschungsgemeinshaft, Projects IG-2,3/ of the Institute of Geodynamics, Bucharest,

The Focsani Depression Age: Neogene (13-0 Ma) Formations: Sarmatian, Meotian,Pontian, Dacian, Quaternary Main rock types: marls, shales, sands Maximum thickness: 9 km (6 km along the profile) A rich geothermal information has been obtained as a result of new temperature measurements in 41 boreholes (triangles) made by continuous logging within the frame of a cooperation between the Geophysical Institute of Karlsruhe University and the Institute of Geodynamics of the Romanian Academy. To this, a detailed information on sediment lithology and composition has been added, based on available logging data. - boreholes in steady-state thermal regime (months/ years- long rest after shut down); - continuous logging; - sampling rate 1 sec; - descent velocity 3-6m/min; - resolution in the mK range; - precision: ± K

the correlation of geothermal gradient with structure at all scales; - results of extensive modeling of local and basin-scale fluid flow; - no independent evidence on basin- scale fluid flow, point to a conductive regime of the heat transfer in the FD. Several processes may contribute to the observed curvature of the temperature logs or to the increasing thermal gradient with depth: - palaeoclimate changes of the surface temperature; - sedimentation; - low thermal conductivity at certain depths - local and basin-scale fluid flow

yields unreasonable large (~ 40 o C) temperature step at the end of the Weichselian glaciation ( ybp). Other processes must also contribute the observed temperatures. INVERSION OF MEASURED TEMPERATURES FORWARD MODELING OF PALAEOCLIMATE EFFECTS - GSTH of Beck (1977) -

From measured data Corrected for palaeoclimate

The thermal conductivity of sediments has been derived by a geometric mean formula, knowing the properties of components (literature) and their volume fraction (logging data). The conductivity structure of sediments, the crustal heat production and the mantle heat flux are established by trial and error, comparing the model output with the measured temperature, corrected for paleoclimate changes. - Undercompacted shales, of lower thermal conductivity than normally compacted rocks, result in higher geothermal gradients than normal; - The lateral variation of the surface heat flux along profile is a result of combined sedimentation effects and heat production of the upper crust.

Normally compacted shales in Dacian formations Sedimentation only Sedim. + Palaeoclim.

The heat flux budget along a 120 km long profile has been investigated and the thermal effect of sedimentation has been evaluated. The thermal modeling is based on a two- dimensional finite element model which includes the effects of changing rates of sedimentation, vertical and lateral variations in sediment thermal parameters, sediment compaction, as well as lateral variations of mantle heat flux and crustal heat production.

Effect on the temperature field Evolution of geotherms a b c

P - T - t PATHS The deposition of sediments of generally low thermal conductivity changes temperature and pressure in the underlying lithosphere, which in turn can induce changes in the metamorphic state of the crust. The amplitude of the temperature increase depends on the integrated thermal resistivity of the sedimentary layer, its horizontal extent, and the heat flow. The pressure change of crustal rocks depends on their change in depth and the contribution from tectonic stresses, which are reflected in the mode of basin formation (extensional, flexural or compressional). a

CONCLUSIONS - sedimentation, undercompaction, and palaeoclimate effects combine to explain the pronounced curvature of the vertical temperature profiles; - the heat generated in the upper crust and sedimentation effects are responsible for the lateral variation of the surface heat flux from the centre of FD (40 mWm-2) to the margin and foreland platform (70 mW m -2) ; - ~20 mWm -2 in the deepest part of the FD and ~10 mWm -2 at the south-eastern end of the studied profile (Moesian Platform) is the surface heat flux deficit in case of FD; -as a result of the sedimentation process, temperature variations as large as ºC occurred in the crystalline crust immediately under the sedimentary pile and progressively smaller to km depth; - pressure-temperature-time paths for various crustal volumes during sedimentation show that Sarmatian and Jurassic sediments, as well as upper crustal rocks underwent significant phase changes; - significant underestimation of lithospheric temperatures, with the consequence of significant (of km) overestimation of the thermal lithospheric thickness, is produced in the direct steady-state thermal modeling, with no account for palaeoclimate and sedimentation effects.

INSTEAD OF CONCLUSIONS II - A SYNTHESIS OF THE THERMAL STRUCTURE ACROSS THE CARPATHIAN BEND -

M2 M1 M4 M3 M1: > 23 Ma Pre-Miocene oceanic subduction Ma Thermal relaxation M2: > 23 Ma Pre-Miocene oceanic subduction Ma Continental convergenc (70 o ) Ma Thermal relaxation M3: Ma Continental convergence Ma Thermal relaxation M4:> 23 Ma Pre-Miocene oceanicsubduction Ma Continental convergence (30 o ) Ma Thermal relaxation