How important are intrabasement structures in controlling the geometry of sedimentary basins? Insights from the Taranaki Basin, offshore New Zealand

Intrabasement structures often are envisaged to have acted as a structural template for normal fault growth in the overlying sedimentary cover during rifting (e.g. Barents Sea; Egersund Basin, offshore southern Norway). However, in other settings, the geometry of rift-related faults was apparently unaffected by the pre-existing basement fabric (e.g. Maløy Slope, offshore western Norway). Understanding the nucleation and propagation of normal faults in the presence of basement structures may elucidate how and under what conditions basement fabric can exert an influence on rifting. This study is based on borehole constrained 3D seismic data from an area of the Taranaki Basin, offshore New Zealand, situated at the boundary between two basement terranes generated during the Mesozoic convergence along the margin of Gondwana. The relatively shallow basement (<3.5 km) is overlain by a late Paleocene to Pleistocene sedimentary cover scarcely affected by the late Miocene inversion and Pliocene rifting, resulting in excellent imaging of basement structures. We mapped the 3D geometry and distribution of throw on the fault planes for clarifying the relationships between basement and cover structures and the kinematic history of the faults. Our analysis has highlighted two types of intrabasement structures. In the northern part of the survey, a N–S-striking, west-dipping lineament marks the transition between two basement units, characterized by different seismic facies. This lineament was reactivated during the late Miocene inversion. In addition, a network of arcuate, N–S-elongated, west-dipping high-amplitude reflectors cut through a largely homogenous low-amplitude basement throughout the whole study area and is only partly reactivated during the inversion phase. Two classes of normal fault segments affected different intervals of the sedimentary cover. The lower fault segments are hard-linked with the intrabasement structures and nucleated within few hundreds of metres from the basement-cover interface. They are blind and swing from NW-SE to NNE-SSW trends. We document different styles of interaction between them and the overlying faults. The segments diverging from the regional NNE-SSW trend are confined in the lower 500 m of the sedimentary succession, whilst the aligned ones are connected with the upper segments. The upper fault segments mostly strike according to the regional NNE-SSW trend; they nucleated within the late Miocene strata and were active during the Pliocene. Above the N–S-striking basement lineament, the upper fault segments strike parallel to it and are systematically hard-linked with the lower ones, generating a single fault zone affecting the whole sedimentary cover. Conversely, away from this lineament, deep and shallow fault planes are only occasionally linked, with some shallow faults totally lacking any connection to basement features. Our study suggests that basement fabric can effectively constrain the geometry of later normal faults in the proximity of the top basement and at the transition between basement units, whilst elsewhere the deformation seems to respond to the regional stress field. The interplay between intrabasement structures and regional stress generates complex geometric relationships between structures at different levels of the sedimentary cover.