Abstract
The aim of this work on Transylvanian Basin is to bring new data and concepts, improving the knowledge of this intricate area of Central – Eastern Europe. The work benefited from a large database of industrial reflection seismics and wells combined with recent literature, data and models. Our focus was
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to improve the overall Transylvanian Basin architecture, to define key moments of evolution and to review possible mechanisms of basin genesis and evolution during Middle-Late Miocene.
The first chapter presents an image of the whole Carpathian hinterland describing the mechanisms and processes that controlled its evolution, while the Transylvanian Basin formation and classification is explained by comparing our observations and interpretations with appropriate literature examples.
The second chapter describes the general and detailed Middle-Late Miocene geometry of the basin, stratigraphy and deposition settings, main structures and their kinematics, salt kinematics receiving a particular focus. We demonstrate the absence of large extensional structures necessary to accommodate the 3.5km basin subsidence, the basin being dominated by compressional structures, occasionally exaggerated by coeval salt diapirism. This chapter reviews and debates a series of possible genetic mechanisms, the overall upper crustal geometry allowed us to locate this mechanism(s) in the lithospheric mantle, a possible deep effect of Pannonian Basin extension.
The third chapter debates the overall configuration of Transylvanian lithosphere by means of numerical modelling using real thermal parameters. Our top-down approach considers the measured heat flux and geometry of the basin and investigates possible causes of the depressed thermal regime. The deposition induced transient effects concur up to 20% in reducing the basin temperature, thus the average heat flux rises to 50mW/m2. In order to match the measured heat flux our models require thinner ophiolite section, therefore an upper crust significantly enriched in radiogenic elements or a 100km thick lithosphere. The latter is in contrast with thinner lithospheric models and with the low velocity anomaly shown by seismic tomography. We consider that Middle Miocene to Pliocene lithosphere thinning effects are not present yet in the basin’s heat flux, the current values representing the Late Cretaceous-Eocene evolution.
Chapter 4 addresses by means of 3D numerical modelling the coupling between tectonic subsidence and salt migration. The salt migrated from the centre towards margins, while on a detail scale, the migration was complex, one area being affected by successive in and out migration due to the interplay subsidence sediment loading – salt accumulation. Three demonstrated peak moments of salt migration are coeval with moments of Carpathians shortening, linking basin dynamics and far-field stresses. The subsidence analysis shows two different wavelength mechanisms affected the basin, a Late Badenian short-wavelength mechanism with localised effects and a Middle Miocene to Quaternary long-wavelength acting at regional scale, both correlating with Miocene Pannonian Basin extension and Carpathians subduction.
All results are integrated in the Chapter 5, underlining the most plausible processes that controlled the basin formation and evolution during Middle-Late Miocene. A short further outlook subchapter suggests that deep lithospheric or mantle processes should be investigated in detail.
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