Abstract
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 > 1000 km 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.
Highlights
Palaeozoic–Mesozoic deformation in the Farsund Basin was accommodated by both E–W- and N–S-striking faults, fault activity was strongly partitioned in time and space
We have shown that the upper-crustal part of the Sorgenfrei–Tornquist Zone (STZ) was repeatedly reactivated in a range of different tectonic styles, which we have linked to regional tectonic events, prevailing stress fields, and potentially broader geodynamic context (Fig. 16)
We use the geometric and kinematic evolution of a complex upper-crustal fault population to better understand the kinematic behaviour of a linked deeper structure, the lithosphere-scale Sorgenfrei–Tornquist Zone
Summary
Pre-existing structures, such as prior fault populations, shear zones and terrane suture zones, are present throughout the lithosphere, where they may influence the geometry and evolution of upper-crustal rift systems forming during later tectonic events (e.g. Bellahsen et al, 2013; Bird et al, 2014; Bladon et al, 2015; Brune et al, 2017; Daly et al, 1989; Doré et al, 1997; Gontijo-Pascutti et al, 2010; Graversen, 2009; Mogensen, 1995; Morley et al, 2004; Phillips et al, 2016; Salomon et al, 2015; Whipp et al, 2014). Structures within the sub-crustal lithosphere are often associated with complex upper-crustal rift systems, which may locally follow structural trends oblique to those predicted by extension of homogeneous lithosphere (Bergerat et al, 2007; Daly et al, 2014; Graversen, 2009; Holdsworth et al, 2001; Le Breton et al, 2013; Tommasi and Vauchez, 2001) Exactly how these anomalous rift systems link to deeper structures is uncertain, their geometry and kinematic evolution can record the regional tectonic history, often throughout multiple stages of reactivation and under the influence of pre-existing structures at deeper levels We find that the sense of motion and style of reactivation along the STZ, as identified from the upper-crustal fault populations, reflects the prevailing regional stress field during these tectonic events
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