Progressive faulting evolution in a transtensional basin; the Grunnevatnet fault, Western Norway

The Devonian basins in western Norway developed in a contintal environment above a crustal‐scale detachment in a strain field dominated by sinistral transtension. The observatons from the Kvamshesten basin are used to conceptualize scenarios for the basin evoluon. In the southwest, the unconformity below the basin is offset by synsedimentary faults, which show normal/ oblique separtiaon in the present configuraon. The largest is the SW‐NW striking Grunnevatnet Fault (GF), which is characterized by an apparent sinistral offset of >2 km offset along the SW basin‐margin, and just a few 10`s of meters in the north, where alluvial midfan lithofacies can be traced across the fault. This is interpreted as the most important evidence for syndeposional slip (Svendby 2011). The large apparent displacement gradient indicates a development different from many normal faults. The fault terminates against the top of the extensional Nordord Sogn Detachment Zone (NSDZ), providing addional evidence that the main acvity of the fault pre‐dated the last movements on the brile detachment. How can such a large change in fault displacement over such a short lateral distance be explained? We discuss the evoluon of the Grunnevatnet Fault in terms of scenarios where the GF is interpreted as active during different stages of the basin evoluon. Observations in the NW part of the basin indicates a pattern where the oldest part of the basin fill was rotated onto the flank of a syncline that grew laterally as units progressively onlapped basement towards the northeast. The Grunnevatnet fault may represent early deformation related to growth of the main detachment and associated core complex.