ALBERT

Description: 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 2D and 3D seismic reflection data to investigate the structural development of the Farsund Basin, offshore southern Norway; this E-trending basin represents the upper crustal expression of the Sorgenfrei-Tornquist Zone, a major lithosphere-scale lineament extending 〉1000 km across Central Europe. The southern margin of the Farsund Basin is characterised by N-S and E-W-striking fault populations, the latter extending down through the Moho and potentially linking with the Sorgenfrei-Tornquist Zone as imaged within sub-crustal lithosphere. Due to this geometric linkage, we can analyse the upper crustal fault populations to infer the kinematics of the Sorgenfrei-Tornquist Zone. We use throw-length (T-x) analysis and fault displacement backstripping techniques to determine the geometric and kinematic evolution of upper-crustal fault populations during the multiphase evolution of the Farsund Basin. We document a period of sinistral strike-slip activity along E-W-striking faults during the Early Jurassic, representing a hitherto undocumented phase of activity along the Sorgenfrei-Tornquist Zone. 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 phase of dextral transtension along the Sorgenfrei-Tornquist Zone. We show 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.