Pre-existing intra-basement shear zones influence growth and geometry of non-colinear normal faults, western Utsira High–Heimdal Terrace, North Sea

Abstract

25Pre-existing intra-basement shear zones can induce mechanical and rheological heterogeneities 26 that may influence rifting and the overall geometry of rift-related normal faults. However, the 27 extent to which physical and kinematic interaction between pre-existing shear zones and 28 younger rift faults control the growth of normal faults is less-well understood. Using 3D 29 reflection seismic data from the northern North Sea and quantitative fault analysis, we constrain 30 the 3D relationship between pre-existing basement shear zones, and the geometry, evolution, 31 and synrift depositional architecture of subsequent rift-related normal faults. We identify NE-32 SW-and N-S-striking rift faults that define a coeval Middle Jurassic -Early Cretaceous, non-33 colinear fault network. NE-SW-striking faults are parallel to underlying intra-basement shear 34 zone. The faults either tip-out above or physically merge with the underlying shear zone. For 35 faults that merges with the basement shear zone, a change from tabular to wedge-shaped 36 geometry of the hangingwall synrift strata records a transition from non-rotational to rotational 37 extension faulting, which we attribute to the time of rift fault's linkage with the shear zone, 38 following downward propagation of its lower tip. N-S-striking faults are oblique to, and offset 39 (rather than link with) intra-basement shear zones. These observations highlight the selective 40 influence pre-existing intra-basement shear zones have on evolving rift-related normal faults. 41 42 1. 45 Phillips et al. 2016). Examples of such rift basins include the North Sea rift basin (e.g., Ziegler, 46 1975; Fossen, 2010), the East Greenland rift system (e.g., Rotevatn et al., 2018), the Malawi 47 rift system (e.g., Dawson et al., 2018), the Taranaki Basin, New Zealand (e.g., Collanega et al., 48in press), the Phitsanulok Basin, Thailand (e.g., Morley et al., 2007), and the Potiguar Basin, 3 49 NE Brazil (e.g., Kirkpatrick et al., 2013). These pre-existing intra-basement shear zones not 50 only induce lithological heterogeneity, but also thermal, mechanical and/or rheological 51 heterogeneities at crustal and lithospheric scales that impact the style and duration of rifting, 52 and the final rift geometry. In the North Sea for example, these shear zones are exposed onshore 53 and are imaged in seismic reflection data offshore (e.g., Norton, 1987; Fossen, 1992; Reeve et 54 al., 2013; Phillips et al., 2016; Fazlikhani et al., 2017; Lenhart et al., 2019). Although the 55 recognition and description of intra-basement shear zones in the field may be relatively 56 straightforward, seismic imaging of intra-basement shear zones in subsurface datasets (e.g., 2D 57 and 3D seismic) can be limited by a combination of factors (Phillips et al., 2016). For example, 58 seismic data may not image to the relatively deep depths at which crystalline basement occurs; 59 even when the seismic record length is sufficient, decreasing seismic resolution with depth due 60 to frequency attenuation may n...

From Caledonian Collapse to North Sea Rift: The Extended History of a Metamorphic Core Complex

“…The above interpretation is supported by numerous other studies documenting brittle reactivation of previously ductile shear zones, both onshore Norway (Andersen et al, 1999; Eide et al, 1997; Ksienzyk et al, 2016; Torgersen et al, 2015; Torsvik et al, 1992), in the North Sea rift (Fazlikhani et al, 2017; Fossen et al, 2016; Lenhart et al, 2019; Osagiede et al, 2020; Phillips et al, 2019), and in other areas (e.g., Daly et al, 1989; Piqué & Laville, 1996; Salomon et al, 2015; Smith & Mosley, 1993). Our study gives new insights into the spatial variability of shear zone reactivation by brittle faults that previously could not be recognized in 2‐D seismic data.…”

Structural Inheritance Controls Strain Distribution During Early Continental Rifting, Rukwa Rift

“…Since continental rifts typically rupture previously deformed lithosphere (Wilson, 1966;Dewey and Spall,1975;Buiter and Torsvik, 2014), the distribution of early-phase strain in magma-poor rifts can be complex due to the interaction between faults that exploit or reactivate pre-existing structures, and those that form independently of any pre-existing structure (e.g., Manatschal et al, 2015;Kolawole et al, 2018;Ragon et al, 2019;Schiffer et al, 2019;Phillips et al, 2019a,b;Heilman et al, 2019;Osagiede et al, 2020). Overall, very little is known about the earliest phase of continental extension, and even less of how strain is partitioned along inherited structures; this reflects the fact that the associated structures and related stratigraphic record are typically deeply buried beneath younger (i.e., post-rift or later rift phase) sequences and are thus difficult to image with geophysical data, or are overprinted by later tectonic events (e.g., post-rift plate collision).…”

Strain migration during multiphase extension, Stord Basin, northern North Sea rift

“…Rift fault F1 strikes overall N‐S (Figure 1), although tips of individual fault segments have a more NE‐SW strike. F1 developed perpendicular to the E‐W regional extension direction; however, rotated segment tips are subparallel to the USZ, most likely associated with mylonitic foliation or layering within the shear zone, both of which may be prone to being preferentially reactivated (Gontijo‐Pascutti et al., 2010; Heilman et al., 2019; Kirkpatrick et al., 2013; Morley, 2017; Osagiede et al., 2019; Paton & Underhill, 2004; Salomon et al., 2015). In this case, the orientation of the regional stress field is the primary factor controlling rift fault development, whereas the presence of pre‐existing structures locally influences segment tip reorientation.…”

“…compare faults on time‐structure map in Figure 2a), similar to the trend of pre‐existing structures related to the Caledonian and/or post‐orogenic Devonian tectonic events. These structures may have locally perturbed the regional stress field and influenced rift fault strike and kinematics in the early stages of fault development (Collanega et al., 2019; Osagiede et al., 2019). Later, as extension continues, and fault segments grew and linked laterally, rift fault activity focuses on fault segments that strike at a high‐angle (e.g.…”

“…These pre‐rift structures created a structurally and rheologically heterogeneous crust that was subsequently stretched. Caledonian and post‐Caledonian (Devonian) structures were reactivated several times onshore (Fossen et al., 2016; Ksiensyk et al., 2016) and offshore (Fazlikhani et al., 2017; Lenhart et al., 2019; Osagiede et al., 2019; Phillips et al., 2016; Reeve et al., 2014) West Norway. The Carboniferous and Permian strata in the North Sea region is marked by the deposition of post‐Variscan clastics of the Rotliegend Group and evaporites of the Zechstein Supergroup, the latter being thickest in the southern North Sea (Heeremans & Faleide, 2004; Ziegler, 1992).…”