THE THERMAL FIELD OF PASSIVE MARGINS AS DERIVED FROM 3D DATABASED, REGIONAL BASIN MODELS

The geodynamics of passive margins results from the interaction of a variety of processes acting on different spatial and temporal scales. To understand the present day structure and the mechanisms of subsidence we assess first-order heterogeneities in the sediments, crust and upper mantle. Thus we explore how far a good knowledge of the sedimentary and upper crustal configuration can provide constraints for the deeper parts of the system and how far the preserved record of deposits holds the key to unravel margin history. The present-day geometry and distribution of physical properties within the sediment fill as well as the upper and middle crust is integrated into data-based, 3D structural models, which, in turn, provide the base for the analysis of the deeper crust and the lithospheric mantle. Once constructed these models provide the base for different process-oriented studies. For example, different configurations of the deeper lithosphere can be tested against two independent observables: gravity and temperature, using isostatic, 3D gravity and 3D thermal modelling. Examples from the North and South Atlantic Margins show, that the latter behave differently with respect to their structure, isostatic state and thermal field. Results from the Norwegian passive volcanic margin indicate that there the oceanic lithospheric mantle is less dense than the continental lithospheric mantle, that this is mainly due to thermal effects and that the transition between continental and oceanic lithosphere thickness is sharp. Furthermore, the thickness of the young oceanic lithosphere in the North Atlantic is smaller than predicted by plate cooling models but consistent with seismologically derived estimates. We also find that the oceanic Lithosphere Asthenosphere Boundary strongly influences the shallow thermal field of the margin. In contrast, this is not found for older oceanic lithosphere as first results from the South Atlantic passive margins suggest. There, the shallow thermal field is decisively influenced by the configuration of the radiogenic crystalline crust.