Mesozoic regional tectonics and South Viking Graben formation: evidence for localized thin-skinned detachments during rift development and inversion

Abstract

A regional thin-skinned model of South Viking Graben development is presented in which Zechstein marginal evaporites and interpreted salt have acted as a key de´collement horizon, along which mechanical decoupling of post-salt Triassic and younger strata, from pre-salt ‘basement’ has occurred. The western margin of the South Viking Graben defines an asymmetrical Late Jurassic half-graben, controlled by episodic extension along a NNE-SSW trending, eastward dipping, fault system. An initial, minor phase of rifting in the Triassic is suggested by the slight westward thickening and associated divergence of interpreted Triassic sequences at depth. The scarcity/absence of Upper Triassic and Lower Jurassic sequences is believed to result from a subsequent phase of uplift which affected the region in the mid-Cimmerian, attributed to the development of a thermal dome at the future site of a Jurassic rift triple-junction to the southwest. The onset of large-scale extension, from the Late Callovian onwards, marked the major phase of half-graben formation and created a regional hanging wall tilt, which locally caused gravity gliding of post-salt section to the WNW, with resultant low-angle extensional faulting upslope and salt-cored buckling downslope. The Late Jurassic half-graben was subsequently tightened by a phase of thin-skinned inversion in the Latest Jurassic-Early Cretaceous, associated with back-steepening of the western margin fault system, with resulting reactivation of the extensional detachment system. The effects of this inversion have been noted in many parts of the northern North Sea. Within the South Viking Graben, shortening estimates vary between 14 and 18% and are expressed on seismic data by localized hanging wall folds and the back-steepening of faults along the western margin fault complex of the graben, together with out of graben thrusting up the hanging wall dip-slope. Variation in both the extension and inversion mechanisms appears to be controlled by interpreted NW-SE transfer systems which compartmentalize the graben into three subgraben, within which differential Zechstein basin development occurred.