Fault-induced deformation in a poorly consolidated, siliciclastic growth basin: A study from the Devonian in Norway

The extensional Berge fault (Devonian Kvamshesten Basin, West Norway) displays 430 m of syntectonic stratigraphy with fluvial sandstones and red fines exposed in a hanging wall growth section. The fault consists of three linked strands, where the offset diminishes and tips out stratigraphically upwards. Folds in the growth basin include a rollover and drag fold that record cumulative deformation during the main phases of fault slip, and a monocline that records the death and burial of the fault. Deformation styles in both the subbasin fill and the fault core indicate that the sediments were unconsolidated to poorly lithified during deformation.The upward-narrowing fault core consists of indurated breccias derived from footwall conglomerates, and mainly laminated fault gouge of subbasin affinity. Towards the hanging wall there is a mixed layer of sandstone lenses enclosed in fault gouge; this unit is variably sheared. In the damage zone deeper in the subbasin, truncating-style small-scale tabular shear bands show a general increase in frequency towards the fault, with abundant peaks in frequency next to the fault core. Smearing-style shear bands are merely encountered near the master fault. In the upper monocline realm, an overall broad zone of deformation reveals a moderate frequency of shear bands, characterized by clear distinctions between variably deformed layers. Some tabular dilation structures are found locally as layer-confined strain throughout the basin.We reason that the mixed layer is a product of fluid mobilization in/along the fault core. Fluid induced weakening combined with differential compaction would augment aseismic creep, as advocated for the creation of the smearing shear bands. We discuss a conceptual model in which damage zones grow by repeated rejuvenation and expand during propagation events, advocating that a distinctive damage zone becomes better expressed with increasing faulting events and depth (consolidation) in a growth basin.

► Deformation took place in poorly lithified sediments of the Devonian Kvamshesten Basin (Norway)
► The growth basin offers 430 m of sandstones and red fine units with fault offset tipping out upwards
► The basin margin is an upward-narrowing fault core including a mixed layer that is variably sheared
► The damage zone in the growth section shows a general increase in deformation intensity towards the fault
► Both the fault core and damage zone show influence of fluid mobilization during fault growth