The role of evaporite mobility in modifying subsidence patterns during normal fault growth and linkage, Halten Terrace, Mid-Norway

Well-calibrated seismic interpretation in the Halten Terrace of Mid-Norway demonstrates the important role that structural feedback between normal fault growth and evaporite mobility has for depocentre development during syn-rift deposition of the Jurassic-Early Cretaceous Viking and Fangst Groups. While the main rift phase reactivated pre-existing structural trends, and initiated new extensional structures, a Triassic evaporite interval decouples the supra-salt cover strata from the underlying basement, causing the development of two separate fault populations, one in the cover and the other confined to the pre-salt basement. Detailed displacement-length analyses of both cover and basement fault arrays, combined with mapping of the component parts of the syn-rift interval, have been used to reveal the spatial and temporal evolution of normal fault segments and sediment depocentres within the Halten Terrace area. Significantly, the results highlight important differences with traditional models of normal fault-controlled subsidence, including those from parts of the North Sea where salt is absent. It can now be shown that evaporite mobility is intimately linked to the along-strike displacement variations of these cover and basement faults. The evaporites passively move beneath the cover in response to the extension, such that the evaporite thickness becomes greatest adjacent to regions of high fault displacement. The consequent evaporite swells can become large enough to have pronounced palaeobathymetric relief in hangingwall locations, associated with fault displacement maxima- the exact opposite situation to that predicted by traditional models of normal fault growth. Evaporite movement from previous extension also affects the displacement-length relationships of subsequently nucleated or reactivated faults. Evaporite withdrawal, on the other hand, tends to be a later-stage feature associated with the high stress regions around the propagating tips of normal faults or their coeval hangingwall release faults. The results indicate the important effect of, and structural feedback caused by, syn-rift evaporite mobility in heavily modifying subsidence patterns produced by normal fault array evolution. Despite their departure from published models, the results provide a new, generic framework within which to interpret extensional fault and depocentre development and evolution in areas in which mobile evaporites exist.