Thomas Phillips

The influence of pre-existing structures on the geometry and evolution of rift systems – Case studies from offshore southern Norway

Structural heterogeneities are ubiquitous throughout the earth’s crust and upper mantle, and as such have been proposed to exert fundamental controls on the geometry and evolution of rift systems. However, the geometry of these heterogeneities is often poorly defined and their geometric and kinematic links to superposed rift systems simply based on 2D plan-view correlations.

In this talk, I use 2D and 3D seismic reflection data to investigate a series of pre-existing structures located offshore south Norway, and examine the interactions, or lack thereof, with the superposed rift. In the west, we identify a series of intrabasement structures which, based on their seismic character and 3D geometry, are correlated to onshore Devonian shear zones.  These shear zones show a range of different styles of structural inheritance with later rift-related faults. To the east, we interpret a dyke swarm on seismic reflection data, and show that this can act as a mechanical template for the formation of later faults. Finally, we examine the upper crustal expression of a sub-crustal lineament, the Sorgenfrei-Tornquist Zone, which corresponds to a change in lithospheric thickness. The upper-crustal expression of this lineament is repeatedly reactivated during later tectonic events, with the style of this reactivation reflective of the prevailing regional stress field.

Pre-existing structures, situated at various levels throughout the lithosphere, show a range of different interactions with rift-related faults. Those within the upper crust may dictate the geometry and evolution of individual faults in some cases, but be offset in others; whereas pre-existing structures at deeper levels are associated with complex, periodically reactivated, upper-crustal rift systems.