2-STAGE RIFTING IN THE NORTH-VIKING-GRABEN AREA (NORTH-SEA) - INFERENCES FROM A NEW 3-DIMENSIONAL SUBSIDENCE ANALYSIS

Backstripping software from the IFP (SUBTEC) was used to realize a thr ee-dimensional subsidence analysis of the Norwegian part of the North Viking Graben. For each considered epoch the software produces restore d depth maps of the selected stratigraphic horizons, as well as tecton ic subsidence maps. That allows, in addition to classical subsidence c urve analysis, an overview of the spatial evolution of the studied bas in. The three-dimensional data set consists of maps representing the p resent day geometry of the graben. For this study, these maps have bee n established on the basis of a new interpretation of available seismi c lines, taking into account new information on the time-depth convers ion and on the age of the different seismic markers. Data from wells g ive detailed information on lateral lithological variations. Sedimento logical information and ana lysis of the geometry of the seismic marke rs have allowed palaeobathymetry and eroded thickness maps to be estab lished and discussed with a reasonable confidence. Computed tectonic s ubsidence maps define three major events and allow discussion of their origin: the lower Jurassic thermal subsidence in relation to the prev ious Triassic rifting event, the Jurassic rifting phase and the subseq uent Cretaceous post-rift thermal subsidence, and, finally, a rapid su bsidence event at the beginning of the Palaeocene. Thermal modelling p rovides theoretical subsidence curves. The comparison with the observe d curves allows testing of and discrimination between several hypothes es about two parameters: the age of the first rifting event (Permo-Tri assic) and its relative amplitude with regard to the Jurassic event. T he corresponding stretching factors were compared with the crustal str etching values observed along the NSDP1 seismic profile. It appears th at the first extensional phase is clearly Triassic (beginning about 24 0 Ma) and that it is characterized by higher stretching factors than t he following Jurassic phase. The ability of different geological proce sses to produce both the Palaeocene subsidence observed in the graben and the contemporaneous uplift of the basin margins is discussed. The preferred interpretation involves buckling of the lithosphere in respo nse to the onset of a compressional stress regime at the end of the Cr etaceous. A simple method (assuming uniform stretching) is proposed fo r computing maps of stretching factors for both the Jurassic and Trias sic events.