The role of pre-existing structure in determining the geometry and evolution of rift systems
Creators: name: Phillips, Thomas Brian
Publisher: name: Imperial College London
Year: 2017

description: Pre-existing structures are ubiquitous throughout the lithosphere, having been imparted during multiple, repeated phases of rifting and orogenesis. These pre-existing structures exist across all scales, from the microstructural to transcontinental, and are present at various structural levels throughout the lithosphere. These pre-existing structures may strongly influence the geometry and evolution of rift systems. Understanding the influence of these pre-existing structures on rifting requires a detailed understanding of their 3D geometry and lithological properties. Seismic reflection data offers detailed imaging of the subsurface over large areas, allowing both pre-existing structures and the overlying rift system to be mapped and analysed. However, in seismic reflection data, it is often difficult to image and constrain the geometry of pre-existing structures which are situated within crystalline basement, at great depths within the lithosphere, or typically display a vertical attitude. In this thesis, I use borehole-constrained 2D and 3D seismic reflection data, covering a 450 x 150 km area offshore southern Norway, to analyse the influence of pre-existing structure on the geometry and evolution of the overlying rift system. In the west of the area, I constrain the offshore continuations and 3D geometry of the Caledonian thrust belt and associated orogenic collapse-related Devonian shear zones within crystalline basement. I document multiple phases of reactivation along these shear zones as well as a range of interactions with rift-related faults. To the east, I identify the upper crustal component of the Sorgenfrei-Tornquist Zone (STZ), a sharp change in lithospheric thickness at sub-crustal depths, represented by a crustal-scale fault system defining the Farsund Basin. Multiple phases of reactivation, including sinistral strike-slip and dextral transtension, are documented along these faults, representing the reactivation of the STZ to accommodate variably oriented regional stress fields. Further east, I identify a Carboniferous-Permian aged dyke swarm, linked to a regional Large Igneous Province (LIP), which represents the first dyke swarm to be identified on seismic reflection data, following passive post-emplacement rotation to shallower dips. Dykes within the swarm form a mechanical anisotropy and thus dictate the geometry of kinematically-linked overlying faults. Geomorphological analyses within the Farsund Basin document a markedly different facies distribution to that in the west, implying a difference in tectono-stratigraphic evolution. I note a major change in the tectonic regime and style of pre-existing structure control between the east of the area, dominated by Proterozoic basement, the dyke swarm and the Sorgenfrei-Tornquist Zone, and the west of the area, dominated by Caledonian orogeny and Devonian orogenic collapse-related upper crustal basement structures. This thesis documents how the presence of different pre-existing structures, and their potential reactivation, can exert a fundamental influence over the evolution of rift systems, displaying a range of relationships with the rift-related faults.