Reactivation of intrabasement structures during rifting: A case study from offshore southern Norway
Thomas B. Phillips1*, Christopher A-L Jackson1, Rebecca E. Bell1, Oliver B. Duffy2, and Haakon Fossen3
Contact:
1Basins Research Group (BRG), Department of Earth Science and Engineering, Imperial College, South Kensington Campus, Prince Consort Road, London, SW7 2BP, UK.
2Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, University Station, Box X, Austin, TX 78713-8924, USA
3 Department of Earth Science/Natural History Collections, University of Bergen, Norway
Crystalline basement often contains an array of pre-existing structures imparted from previoustectonic events. During subsequentphases of rifting these structures may: i) directly reactivate; ii) act as nucleation sites for rift-related faults; andiii) locally modify regional stress fields; thereby fundamentally altering the geometry and evolution of the overlying rift system. Alternatively, these structuresmay not be reactivated during later tectonic events andaccordingly have no influence on the overlying rift.
Interactions between intrabasement structures and rift-related faults are often solely based on plan-view correlations between the structures, with little consideration given to the three-dimensionalgeometric relationships and kinematic interactions between the two. The range of interactions observed is likely to be much more complex in 3D than previously described. The greatest obstacle to understanding how intrabasement structures may affect later rift systems is obtaining detailed constraints on the geological origin and 3D geometry of intrabasement structures beneath rift systems. Such structures are often buried deeply beneath younger rift systems and are therefore rarely sampled directly by boreholes or imaged in seismic reflection data. Furthermore,low impedance contraststypically occur between different lithologies within crystalline basement,meaningdiscrete structures are often acoustically transparent in seismic reflection data.
The North Sea rift formed within an area of relatively heterogeneous crystalline basement, containing a range of structures imparted from previous tectonic events, including the Caledonian orogeny and its ensuing collapse during the Devonian. Such structures are clearly visible onshore, yet in many cases, the offshore extension and three-dimensional geometry of these structures is poorly constrained. As a result, we have a limited understanding of whether and how these structures are reactivated during subsequent tectonic events, and how this impacts the structural development of later rift systems. However, offshore southern Norway, beneath the Egersund Basin and Stavanger Platform,structures within crystalline basement are exceptionally well-imaged. This is due to a combination of shallow burial depth, with the study area being located close to the rift margin, and large intrabasement impedance contrasts resulting from a highly heterogeneous Caledonian basement. As such, in this study, we use borehole-constrained 2D and 3D seismic reflection data to constrain the 3D geometry of a series of intrabasement structures beneath the North Sea rift system, linking them explicitly to the established onshore geology, before examining, in both plan-view and cross-section, how they interact with and may control the overlying rift structure.
We observe two types of intrabasement structure: (i) thin (100 m) reflections displaying a characteristic trough-peak-trough wavetrain; and (ii) thick (c. 1 km), sub-parallel reflection packages dipping at c. 30°. Through 1D waveform modelling, we recreate the observed reflection patterns and find that they are likely generated from a layered sequence, as opposed to a single interface. We interpret this layered sequence to represent layered mylonitic sequences, as typically observed within shear zones onshore Norway and worldwide. Asthe study area is located close to the Norwegian mainland, we can confidently correlate the individual intrabasement structures to discrete structures mapped onshore. The intrabasement structures represent the offshore continuations of the onshore Caledonian thrust belt and Devonian extensional shear zones, thereby extending these structures up to 100km beneath the North Sea rift system.
We demonstrate how the intrabasement structures experienced multiple phases of reactivation during Mesozoic riftingwithinthe North Sea rift. Along with a strong plan-view correlation between the intrabasement structures and the later, overlying, rift-related faults, we also observe a number of novelinteractions between intrabasement structures and rift-related faults in cross-section: i) faults that exploit internal planes of weakness within the shear zone; ii) faults that initiate within the hangingwall of the shear zone and subsequently merge with the structure at depth; and iii) faults that initiate independently from and cross-cut any intrabasement structure. We find that reactivation preferentially occurs along the thicker, steeper intrabasement structures, i.e. the Devonian shear zones, with faults exploiting the internal intra-shear zone mylonites.
Using a detailed 3D framework of intrabasement structure, correlated to established onshore geology, we find that later rift-related faults typically inherit the location and orientation of underlying basement structure. In particular, kilometre-scale Devonian shear zones define a long-lived structural template that largely controlled the structural style of the Mesozoic rift in this area of the North Sea.