Thomas B. Phillips1*, Christopher A-L Jackson1, Rebecca E. Bell1, and Oliver B. Duffy2

Using upper crustal fault populations to decipher the geometry and kinematics of lower crustal lineaments: an example from offshore Southern Norway

Thomas B. Phillips1*, Christopher A-L Jackson1, Rebecca E. Bell1, and Oliver B. Duffy2

Contact:

1Basins Research Group (BRG), Department of Earth Science and Engineering, Imperial College, South Kensington Campus, Prince Consort Road, London, SW7 2BP, UK.

2Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, University Station, Box X, Austin, TX 78713-8924, USA

Structures within the lower crust have the potential to locally modify regional stress fields within the upper crust and can thusexert significant influence over the geometry and evolution of shallow tectonic systems. However, due to difficulties in imaging these lower crustal structures, how theymay reactivate during later tectonic events remains poorly understood.Lower crustal structures are typically too deep, or their boundaries too diffuse, to be accurately imaged with geophysical techniques, leading to uncertaintiesregarding their location, geological origin and subsequent behaviour during regional tectonism. Therefore,to better understand the geometry and activityof these structures, we rely upon information obtained from shallower, better-imagedstructural levels, namely upper crustal fault populations.

Rift systems developed above lower crustal structures are likely to be structurally complex and prone to contain non-colinear fault populations, a function of the potential for lower crustal structures to modify regional stresses. Interactions between non-colinear fault populations such as abutting and cross-cutting relationships can reveala large amount of information about rift evolution, including the influence, if any, of the reactivation of lower crustal structures.Therefore, by undertaking a detailed examination of the geometric and kinematic evolution of the overlying fault populations, not only are we able to understand the evolution of the rift itself, but we are also able to determine the potential effect of the reactivation of any lower crustal structures.

In this study, we use borehole constrained 2-D and 3-D seismic reflection data to analyse the evolution of an upper crustal non-colinear fault population located on the southern margin of the Farsund Basin, offshore south Norway. The E-trending Farsund Basin is situated at the westernmost mapped extent of the lower crustal Sorgenfrei-Tornquist Zone (STZ), a megascale NW- to W-trending lower crustal lineament that stretches over 1000 km across Central Europe and roughly delineates the southwestern margin of the stable cratonic lithosphere of Baltica. Previous studies suggest that repeated reactivation of the STZ throughout multiple tectonic events has exerted a strong influence locally over the upper crustal rift system. However, a direct link between the upper crustal rifts and the STZ is yet to be determined. Using seismic stratigraphy and quantitative fault analysis,we examine the geometric and kinematic evolution of the upper crustal non-colinear fault population along the southern margin of the Farsund Basin and use this to infer how the underlying STZ reactivates during regional tectonic events.

The non-colinear upper crustal fault population within the Farsund Basin is characterised by aseries of N-S and E-W striking faults that display cross-cutting and abutting interactions with one another. Throughout the evolution of the Farsund Basin, we observe evidence of multiple phases of oblique reactivationof both the N-S and E-W striking faults. During the Permian-Carboniferous, fault activity associated with regional wrench faultingoccurred along E-W striking faults. Subsequently, in response to regional E-W Triassic rifting, N-S striking faults were active along with the reactivation of an E-W fault segment. The reactivated E-W-striking segment represents a trailing segment betweentwo abutting N-S faults; this segment joins the two N-S striking faults, creating a coherent, linked fault system. As the N-S and E-W faults are oriented perpendicular to one another, some component of oblique slip is required across all faults, implying a local NE-SW extension direction as opposed to the regional E-W extension.Following a period of quiescence during the Middle Jurassic, further evidence of the oblique reactivation of N-S and E-W striking faults was observed during the Early Cretaceous, in response to regional E-W to NW-SE oriented Late Jurassic-Early Cretaceous extension. During this rift phase we observe the upwards propagation and bifurcation of some faults into a series of left-stepping en-echelon segmented fault strands, separated by relay ramps. Other faults propagated vertically and laterally during this interval and wereassociated with oblique reactivation within an overall dextral transtensional regime.

We find that local transtensional stresses within the Farsund Basin cause the oblique reactivation of pre-existing faults during regional tectonic events. The underlying STZ wasreactivated in an oblique manner during regional E-W oriented extension, with this local stress field manifest within the upper crust rift system. In addition, the Early Cretaceous lateral propagation and vertical bifurcation of faults into en-echelon segments is consistent with the formation of a pull-apart basin. Therefore, we also suggest that the Farsund Basin is situated above a releasing bend in the STZ, which underwent dextral transtensional reactivation during the Early Cretaceous.

By deciphering the geometric and kinematic evolution of upper crustal fault populations, we offer insights into the reactivation of, often poorly imaged and defined, lower crustal structures during regional tectonic events, as well as determine the role that this reactivation played in the evolution of the overall rift system