Fault Evolution and Role of Salt in Extensional Tectonics in the Smørbukk Area, Halten Terrace, Offshore Mid-Norway

Abstract

Thick layers of mechanically weak evaporite between basement and cover units are prevalent in many extensional settings. The rheological heterogeneity due to the presence of such layers causes decoupling of the deformation between the shallower and deeper units. The degree of decoupling is dependent on the mechanical stratigraphy of the weak layer and its overburden as well as the amount and rate of fault displacement. Even if the mechanical stratigraphy is favorable for decoupling of deformation, hard linkage of basement and cover fault can occur by an increase in the amount and rate of fault displacement. A structural analysis of the Smørbukk area of the Halten Terrace, offshore Mid-Norway shows the controls of various parameters on the structural style. The area has a thick sequence of Middle-Late Triassic evaporites interbedded with dolomite and anhydrite-rich shale (collectively referred to as ‘salt’) between the Permo-Triassic sedimentary basement and the Jurassic cover. Seismic interpretation of a 3-D time migrated seismic dataset and structural restorations of depth converted sections provided means to investigate the fault evolution and the role of the salt in decoupling the shallower from the deeper units. The results show that extensional deformation was partitioned above and below the salt layer by the development of major faults that detached at the level of base of salt. The positions of such major basement-detached faults are controlled by underlying basement faults. Most of the basement faults have a two-phase activation history: first, during the earliest phase of rift initiation, and second during the rift climax stage. Hard linkage of basement and cover faults occurs in some only during the rift climax stage. The development and activity of newer basement-detached faults and the reactivation of some older basement faults during this stage are probably related to an increase in the strain rate.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015