Time-dependent thermal state of the lithosphere in the foreland of the Eastern Carpathians bend. Insights from new geothermal measurements and modelling results

Abstract

The time-dependent heat budget, thermal evolution and rheology of the lithosphere in the last 13 Myr, along a 120 km long profile in the foreland of the Eastern Carpathians bend is presented. The study area, a complex tectonic environment in the vicinity of the well-known intermediate-depth seismogenic Vrancea zone, includes the deepest sedimentary basin in Romania (∼18 km), with a recent rapid evolution in the last 13 Myr (8 km of sediments). Detailed high-resolution temperature logs in the depth ranges from 0-700 m to 0-2400 m in eight thermally stabilized boreholes, together with logging and geological information on structure, lithology and time evolution of the sedimentary pile have been used to model the thermal evolution by means of a 2-D finite element model which includes sedimentation history, sediment compaction, lateral and vertical variation of thermal properties of sediments and consolidated crust. Modelling results have been compared to measured temperatures, corrected for palaeoclimate effects, in a trial and error iterative approach. Sedimentation, palaeoclimate, undercompaction and heat refraction effects on one hand, and the heat generated in the upper crust, on the other, combine to explain the observed subsurface temperature field and in particular the pronounced curvature of the vertical temperature profiles. The Neogene-Quaternary sedimentation resulted in a rather uniform deficit of the surface heat flux of about 12 mW m-2 in the study area, in spite of the marked lateral variation of the sedimentary pile thickness. The palaeoclimate effect is responsible for a surface heat flux depression of 7-8 mW m-2, uniform along the study profile. Unlike older sedimentary basins, in the study area transient sedimentation thermal effects add to palaeoclimate effects in establishing the temperature field in the depth range of geothermal measurements. The lateral variation of the palaeoclimatically corrected surface heat flux from the centre of the Focsani Depression (40 mW m-2) to its margin and the foreland platform (70 mW m-2) is mainly the result of the lateral variation of the heat produced in the upper crust where thickness varies from 7 to 21 km, respectively. A heat production of 2.3 μW m-3 for the southeastern end of the profile and of 2.15 μW m-3 for the rest of it has been modelled. As a result of the sedimentation process, temperature variations as large as 70-100 °C occurred in the pre-Neogene sediments and the crystalline crust immediately under the sedimentary pile, with consequences on the metamorphic state and rheological behaviour of the foreland lithosphere. Progressively smaller variations were present to 40-50 km depth. Unlike flexural modelling, which indicate effective elastic thickness values of 10-15 km, our rheological analysis based on the derived temperature field indicates time-dependent lateral variations between 26-39 km, possibly favouring the recent evolution of the foredeep, characterized by depo-centre migration and basin expansion. © 2007 The Authors Journal compilation © 2007 RAS.