Two-stage rifting in the North Viking Graben area (North Sea): inferences from a new three-dimensional subsidence analysis

Abstract

Backstripping software from the IFP (SUBTEC) was used to realize a three-dimensional subsidence analysis of the Norwegian part of the North Viking Graben. For each considered epoch the software produces restored depth maps of the selected stratigraphic horizons, as well as tectonic subsidence maps. That allows, in addition to classical subsidence curve analysis, an overview of the spatial evolution of the studied basin. The three-dimensional data set consists of maps representing the present day geometry of the graben. For this study, these maps have been established on the basis of a new interpretation of available seismic lines, taking into account new information on the time-depth conversion and on the age of the different seismic markers. Data from wells give detailed information on lateral lithological variations. Sedimentological information and analysis of the geometry of the seismic markers have allowed palaeobathymetry and eroded thickness maps to be established and discussed with a reasonable confidence. Computed tectonic subsidence maps define three major events and allow discussion of their origin: the lower Jurassic thermal subsidence in relation to the previous Triassic rifting event, the Jurassic rifting phase and the subsequent Cretaceous post-rift thermal subsidence, and, finally, a rapid subsidence event at the beginning of the Palaeocene. Thermal modelling provides theoretical subsidence curves. The comparison with the observed curves allows testing of and discrimination between several hypotheses about two parameters: the age of the first rifting event (Permo-Triassic) and its relative amplitude with regard to the Jurassic event. The corresponding stretching factors were compared with the crustal stretching values observed along the NSDP1 seismic profile. It appears that the first extensional phase is clearly Triassic (beginning about 240 Ma) and that it is characterized by higher stretching factors than the following Jurassic phase. The ability of different geological processes 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 response to the onset of a compressional stress regime at the end of the Cretaceous. A simple method (assuming uniform stretching) is proposed for computing maps of stretching factors for both the Jurassic and Triassic events.