Tectono-stratigraphic evolution of multi-phase rifts

Continental rifts typically develop from an early rift initiation stage, characterised by the growth of multiple, small, geometrically and kinematically isolated fault segments, through a fault interaction and linkage stage, when co-linear fault segments link to form longer faults with larger throw and other fault segments begin to die, to a rift climax stage, when extension is focused on a small number of large, half-graben bounding normal faults. This three-stage evolution of structural style also explains many of the first-order stratigraphic and sedimentological characteristics of syn-rift sequences. However, many continental rifts have formed in response to multiple phases of extension, thus parameters such as pre-existing basement and cover structures, rheological heterogeneity, and lithosphere thermal structure may modify the three-stage evolutionary model outlined above, and the overall geometry and tectono-sedimentary evolution of rifts. We use results from regional studies of the Northern North Sea and other parts of the Norwegian Continental Shelf to investigate the structural style o f multiphase rifts and their tectono-sedimentary evolution.

Where basement/upper crustal structure is relatively well imaged on seismic data, for example on the Horda Platform and in the Stord Basin, narrow (single- or double-wavelet) reflections and several kilometre-wide zones of reflectivity, some of which extend down into a strongly reflective lower-crust, characterise the intra-crystalline basement structure. Some of these zones form the offshore continuation of major shear zones exposed on the Norwegian mainland. These pre-existing basement structures seem to have played a significant role in controlling the first-order geometry and segmentation of the North Sea rift system by controlling rift segmentation, and the location and strike of major rift-related normal faults. However, in detail, the relationships between basement structures and cover normal faults are more complex. For example, some of the basement structures have been truncated and offset by Permo-Triassic and Late Jurassic-Early Cretaceous normal faults whereas others are reactivated. The degree of physical linkage between basement faults and shear zones and cover faults can vary locally along strike. Similar complexity exists between Permo-Triassic and Late Jurassic-Early Cretaceous normal faults. For example, in some areas, such as the East Shetland Basin, pre-existing normal faults appear not to control the Late Jurassic-Early Cretaceous fault geometry and basin development, with limited fault reactivation and most pre-Triassic faults being cross cut by new faults. In other areas, such as the Horda Platform, some Permo-Triassic faults are reactivated, but others are not, and new, Late Jurassic normal faults also develop that are unrelated to Permo-Triassic faults. The regional scale of this study also highlights the diachronous development of the fault network, thus bringing into question the validity of using rift-related megasequence terms (i.e. pre- syn- and post-rift) at the basin-scale, even in this low-strain setting that became extinct long before breakup.

The results of our study indicate that bespoke tectono-stratigraphic models are required for multi- phase rifts; existing models, based on 'pristine' crust and analogue models for multi-phase rifting, fail to capture the full complexity present in natural multi-phase rifts. To be relevant, physical and numerical models, which provide critical insights into the controls on structural style and evolution in rifts, need to account for this complexity.