Reactivation of ancient thrust belts: Implications for the geometry and evolution ofsedimentarybasins
Thomas B. Phillips; Christopher A-L Jackson; Rebecca E. Bell; Oliver B. Duffy; Haakon Fossen
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Rift basins often form within lithosphere containing structures inherited from previous tectonic events, yet the extent to which the reactivation of these structures can control the evolution and structure of rift basins is poorly understood. Several key aspects of petroleum system development, such as the timing of trap formation and distribution of syn-rift sediments, may be influenced by reactivated intrabasement structures.
However, determining the impact of intrabasement structures on rift evolution is difficult as crystalline basement is often buried beneath thick sedimentary successions and contains small acoustic impedance contrasts. Therefore, it typically appears poorly reflective on seismic reflection data, hampering efforts to interpret internal structure.
In this study we use borehole calibrated 2D and 3D seismic reflection data from offshore SW Norway to map a series of intrabasement reflections and document how they interact with the overlying rift. Crystalline basement is exceptionally well-imaged throughout the data as prominent reflections are generated by large impedance contrasts within a highly heterogeneous, shallow basement, allowing for detailed 3D mapping of intrabasement structure. From our analyses, we observe two types of structure: i) Type 1 - thin (100 m) reflection packages, showing a ramp-flat geometry; and ii) Type 2 - thick (1 km), steeply dipping (30o) packages comprising multiple reflections. These structures correlate northwards along-strike to Caledonian orogeny-related structures mapped onshore southern Norway. Based on this spatial relationship we interpret the structures as Caledonian thrusts (Type 1) and large-scale Devonian shear zones (Type 2).
We are able to document multiple, stages of extensional reactivation of some of these structures during Devonian, Permo-Triassic and Late Jurassic-Early Cretaceous extension, along with reverse reactivation during Late Cretaceous inversion. However, during extension we also observe the formation of a number of rift related faults that cross-cut intrabasement structure; implying selective reactivation of intrabasement structures, which may be controlled by properties such as their strength compared to coherent rock and their orientation.
From the detailed 3D mapping of an ancient thrust belt and overlying rift, we show that the reactivation of intrabasement structures exerts some control over the structure and evolution of rift basins. The selectivereactivation of large-scale, weaker intrabasement structures (Type 2), i.e. Devonian Shear zones, dictates the location and orientation of many majorrift-related faults. Repeated reactivation of these structures during later rifting may modify pre-existing structural traps or migration pathways.