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

Abstract

S U M M A R YThe time-dependent heat budget, thermal evolution and rheology of the lithosphere in the last 13Myr, 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 wellknown intermediate-depth seismogenic Vrancea zone, includes the deepest sedimentary basin in Romania (∼18 km), with a recent rapid evolution in the last 13Myr (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

Surveillance of Vrancea active seismic region in Romania through time series satellite data

“…Thermally defined lithosphere thickness is considered to be of about 160 km. The lithosphere has not yet reached the thermal steady-state, the present temperatures being about 80-90% of the equilibrium temperatures, depending on depth and location along the geologic profiles [25]. • N, and 26.622…”

From nappe stacking to out‐of‐sequence postcollisional deformations: Cretaceous to Quaternary exhumation history of the SE Carpathians assessed by low‐temperature thermochronology

“…The paleogeothermal gradient is based on the present-day geothermal gradients obtained from the present-day surface heat flow, which is ∼40-60 mW/m 2 for the analyzed transect [Veliciu and Visarion, 1984;Demetrescu et al, 2001;Andreescu et al, 2002;Demetrescu et al, 2007]. Including thermal conductivities of 1.7-4.5 W/m/°C for the sampled rocks, a paleo-geothermal gradient of 20 ± 5°C/km was calculated for the entire transect.…”

“…[39] To derive denudation rates from the cooling rates (Table 5 and Figure 7), regional heat flow data [Veliciu and Visarion, 1984;Demetrescu et al, 2007] were used to calculate a constant (paleo)-geothermal gradient. The paleogeothermal gradient is based on the present-day geothermal gradients obtained from the present-day surface heat flow, which is ∼40-60 mW/m 2 for the analyzed transect [Veliciu and Visarion, 1984;Demetrescu et al, 2001;Andreescu et al, 2002;Demetrescu et al, 2007].…”

Satellite thermal infrared anomalies associatedwith strong earthquakes in the Vrancea area ofRomania

“…Thermally defined lithospheric thickness is considered to be about 160 km. The lithosphere has not yet reached the thermal steady-state, the present temperatures being about 80-90% of the equilibrium temperatures, which is a function of the depth and location along geologic profiles [15].…”