Fault Growth and Strain Localization in a Multilayer Brittle-Ductile System, Halten Terrace, mid-Norway
Tectonic subsidence in rift basins is often characterised by an initial period of slow subsidence (“rift initiation”) followed by a period of more rapid subsidence (“rift climax”). Previous work shows that the transition from rift initiation to rift climax can be explained by interactions between the stress fields of growing faults. Despite the prevalence of evaporites throughout the geological record, and the likelihood that the presence of a regionally-extensive evaporite layer will introduce an important, sub-horizontal rheological heterogeneity into the upper crust, there have been few studies that document the impact of salt on the localisation of extensional strain in rift basins. Here we use well-calibrated 3D seismic reflection data to constrain the distribution and timing of fault activity during Early Jurassic-earliest Cretaceous rifting in the Åsgard area, Halten Terrace, offshore Mid-Norway. Permo-Triassic basement rocks are overlain by a thick sequence of interbedded halite, anhydrite and mudstone. Our results show that rift initiation during the Early Jurassic was characterised by distributed deformation along blind faults within the basement, and by localised deformation along the major Smørbukk and Trestakk faults within the cover. Rift climax and the end of rifting showed continued deformation along the Smørbukk and Trestakk faults, together with initiation of new extensional faults oblique to the main basement trends. We propose that these new faults developed in response to salt withdrawal and/or gravity sliding on the evaporite layer above the tilted basement fault blocks. Rapid strain localisation within the post-salt cover sequence at the onset of rifting is consistent with previous experimental studies showing that strain localisation is favoured by the presence of a weak viscous substrate beneath a brittle overburden.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009