Abstract
Pre-existing intra-basement shear zones can induce mechanical and rheological heterogeneities that may influence rifting and the overall geometry of rift-related normal faults. However, the extent to which physical and kinematic interaction between pre-existing shear zones and younger rift faults control the growth of normal faults is less-well understood. Using 3D reflection seismic data from the northern North Sea and quantitative fault analysis, we constrain the 3D relationship between pre-existing basement shear zones, and the geometry, evolution, and synrift depositional architecture of subsequent rift-related normal faults. We identify NE-SW- and N-S-striking rift faults that define a coeval Middle Jurassic – Early Cretaceous, non-colinear fault network. NE-SW-striking faults are parallel to underlying intra-basement shear zone. The faults either tip-out above or physically merge with the underlying shear zone. For faults that merges with the basement shear zone, a change from tabular to wedge-shaped geometry of the hangingwall synrift strata records a transition from non-rotational to rotational extension faulting, which we attribute to the time of rift fault's linkage with the shear zone, following downward propagation of its lower tip. N-S-striking faults are oblique to, and offset (rather than link with) intra-basement shear zones. These observations highlight the selective influence pre-existing intra-basement shear zones may (or may not) have on evolving rift-related normal faults.
Highlights
Rift basins often evolve on a template of crystalline basement that, due to complex pre-rift tectono-magmatic histories, are associated with strong lithological, mechanical and rheological heterogeneities, such as those imposed by mylonitic shear zones (e.g., Phillips et al, 2016)
Detailed fault mapping allows us to determine the overall geometry of the rift fault network at the Base Sleipner Formation (Middle Jurassic) stratigraphic level; this represents the base of the Middle Jurassic – Early Cretaceous rift phase (RP1) in this part of the North Sea (Figs. 1b and 5)
A key observation we make here is that the normal faults within both domains are noncolinear, they initiated and/or slipped contemporaneously, and are related to the same Middle Jurassic – Early Cretaceous rift event. This observation underpins the fact that normal faults do not just slip perpendicular and strike normal to the maximum prin cipal stress, and is key to assessing the role pre-existing intrabasement shear zones have on the development of non-colinear fault networks (e.g. Reeve et al, 2015)
Summary
Rift basins often evolve on a template of crystalline basement that, due to complex pre-rift tectono-magmatic histories, are associated with strong lithological, mechanical and rheological heterogeneities, such as those imposed by mylonitic shear zones (e.g., Phillips et al, 2016) Examples of such rift basins include the North Sea rift basin (e.g., Zie gler, 1975; Fossen, 2010), the East Greenland rift system (e.g., Rotevatn et al, 2018), the Malawi rift system (e.g., Dawson et al, 2018), the Taranaki Basin, New Zealand (e.g., Collanega et al, 2019), the Phitsa nulok Basin, Thailand (e.g., Morley et al, 2007), and the Potiguar Basin, NE Brazil (e.g., Kirkpatrick et al, 2013). Our results have implication for understanding the palaeo-stress orientation during the Middle Jurassic – Early Cretaceous rift phase, and emphasizes the un certainty in using the strike of normal faults alone to infer extension direction
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