Kinematics of Polygonal Fault Systems: Observations from the Northern North Sea

Abstract

Layer-bound, low-displacement normal faults, arranged into a broadly polygonal pattern, are common in many sedimentary basins. Despite having constrained their gross geometry, we have a relatively poor understanding of the processes controlling the nucleation and growth (i.e., the kinematics) of polygonal fault systems. In this study we use high-resolution 3-D seismic reflection and borehole data from the northern North Sea to undertake a detailed kinematic analysis of faults forming part of a seismically well-imaged polygonal fault system hosted within the up to 1,000 m thick, Early Palaeocene-to-Middle Miocene mudstones of the Hordaland Group. Growth strata and displacement-depth profiles indicate faulting commenced during the Eocene to early Oligocene, with reactivation possibly occurring in the late Oligocene to middle Miocene. Mapping the position of displacement maxima on 137 polygonal faults suggests that the majority (64%) nucleated in the lower 500 m of the Hordaland Group. The uniform distribution of polygonal fault strikes in the area indicates that nucleation and growth were not driven by gravity or far-field tectonic extension as has previously been suggested. Instead, fault growth was likely facilitated by low coefficients of residual friction on existing slip surfaces, and probably involved significant layer-parallel contraction (strains of 0.01-0.19) of the host strata. To summarize, our kinematic analysis provides new insights into the spatial and temporal evolution of polygonal fault systems.

Influence of ductile substrates and layer thickness on the spacing and topology of layer bound fault systems

“…The T10 horizon was chosen because 1) the faults have b-type displacement depth-profiles (e.g. Wrona et al, 2017) and therefore exhibit displacement maxima located towards to the lower tip, 2) it is a continuous reflection which can be mapped with high confidence through the faulted interval, and 3) it exhibits the most structural complexity of any single level within the fault system. Fault topology was analysed by using 3 sample areas (Fig 2.).…”

“…Increasingly analysis has shifted from qualitative descriptions to quantified structural analysis, including, but not limited to throw and displacement, orientation and topological analysis (e.g. Wrona et al, 2017;Morley and Binazirnejad, 2020). Here, observations and interpretations from 3D seismic data from the North West Shelf, Australia are used to describe the spacing and topology of layer bound fault systems, and the implications for their genesis and growth examined.…”

A regional CO2 containment assessment of the northern Utsira Formation seal and overburden, northern North Sea

“…The upper section of the Hordaland Group contains the Skade Fm. close to the ESP, with mudstones in the distal part of the basin, which were later polygonally faulted (Rundberg & Eidvin, 2005; Wrona et al., 2017). The Hordaland and Nordland Groups are separated by the Top Hordaland Group Unconformity (THGU), reflecting a phase of plate tectonic reorganisation and associated uplift of the North Sea basin margins and fall in relative sea level (Eidvin et al., 2013; Galloway, 2002; Løseth et al., 2013; Martinsen et al., 1999; Rundberg & Eidvin, 2005).…”

Giant seabed polygons and underlying polygonal faults in the Caribbean Sea as markers of the sedimentary cover extension in the Grenada Basin

“…It also implies that, at great burial depths, the compaction through the process of volumetric contraction ends and the dissolution-precipitation mechanisms become dominant (Gay et al, 2004). Clausen et al (1999) have suggested that PF developed within a broadly polygonal array due to gravitational sliding influenced by far-field tectonic stresses, even if Wrona et al (2017) have recently suggested that such processes played no or an imperceptible role in the growth of this specific North Sea system. However, the hypothesis that layer-parallel displacements of these faults are accommodated by regional extension of the host strata has been confirmed in various basins, such as in the Lower Congo Basin (Gay et al, 2004) or in the Angola basin (Ho et al, 2018), and more recently by field studies showing that a radial extension may account for PFS development (Antonellini & Mollema, 2015;Petracchini et al, 2015).…”

“…Other mechanisms different from lithology variation have been referenced in the literature as responsible for polygonal fault initiation, propagation or reactivation, such as sediment loading (Gay et al, 2007). In any case, this kind of shrinkage is due to diagenetic reactions in the host-rock unit during early burial causing volumetric contraction of fine-grained clay-rich sediments accommodated by small scale normal faults (Gay et al, 2004;Neagu et al, 2010;Ireland et al, 2011;Davies et al, 2009;Wrona et al, 2017).…”