Discussion on the structural evolution of the northern Viking Graben and its bearing upon extensional modes of basin formation

R. E. Frost writes: Badley et al . (1988) have attributed the structural evolution of the northern Viking Graben to phases of extensional rifting and thermal subsidence. Their ideas are dependent on the recognition of the rift-associated unconformities in the vicinity of block faulting. In particular, the authors recognize a pre-rift sequence dipping down into a listric normal fault, with a wedge shaped syn-rift package thickening towards the fault. Well and seismic data from these locations within the northern Viking Graben, which will be presented, indicates that this is virtually never the case. Indeed, the top of the pre-rift sequence invariably dips in the opposite sense from that predicted by Badley et al . Also Badley et al .’s extensional and isostatic explanation of footwall uplift is challenged in the light of published data, where more than 1500 m of sediments have been removed from the crest of one of the blocks. Not even in areas throughout the world where more advanced stages of rifting have been reached has this amount of uplift been observed. In addition, further data will be presented which contradicts the observation or assumption that the faults associated with the western margin of the Horda Platform are listric normal; these are clearly shown to be reverse in origin. These major discrepancies cast serious doubts on the validity and relevance to the northern Viking Graben of the model proposed by Badley et al . The key to the understanding of the evolution of the northern Viking Graben