Oblique reactivation of lithosphere-scale lineaments controls rift physiography – the upper-crustal expression of the Sorgenfrei–Tornquist Zone, offshore southern Norway

Abstract

Pre-existing structures within sub-crustal lithosphere may localise stresses during subsequent tectonic events, resulting in complex fault systems at upper-crustal levels. As these sub-crustal structures are difficult to resolve at great depths, the evolution of kinematically and perhaps geometrically linked upper-crustal fault populations can offer insights into their deformation history, including when and how they reactivate and accommodate stresses during later tectonic events. In this study, we use borehole-constrained 2-D and 3-D seismic reflection data to investigate the structural development of the Farsund Basin, offshore southern Norway. We use throw–length (T-x) analysis and fault displacement backstripping techniques to determine the geometric and kinematic evolution of N–S- and E–W-striking upper-crustal fault populations during the multiphase evolution of the Farsund Basin. N–S-striking faults were active during the Triassic, prior to a period of sinistral strike-slip activity along E–W-striking faults during the Early Jurassic, which represented a hitherto undocumented phase of activity in this area. These E–W-striking upper-crustal faults are later obliquely reactivated under a dextral stress regime during the Early Cretaceous, with new faults also propagating away from pre-existing ones, representing a switch to a predominantly dextral sense of motion. The E–W faults within the Farsund Basin are interpreted to extend through the crust to the Moho and link with the Sorgenfrei–Tornquist Zone, a lithosphere-scale lineament, identified within the sub-crustal lithosphere, that extends >1000km across central Europe. Based on this geometric linkage, we infer that the E–W-striking faults represent the upper-crustal component of the Sorgenfrei–Tornquist Zone and that the Sorgenfrei–Tornquist Zone represents a long-lived lithosphere-scale lineament that is periodically reactivated throughout its protracted geological history. The upper-crustal component of the lineament is reactivated in a range of tectonic styles, including both sinistral and dextral strike-slip motions, with the geometry and kinematics of these faults often inconsistent with what may otherwise be inferred from regional tectonics alone. Understanding these different styles of reactivation not only allows us to better understand the influence of sub-crustal lithospheric structure on rifting but also offers insights into the prevailing stress field during regional tectonic events.