The Influence of the Selective Reactivation of Ancient Intrabasement Thrusts on the Geometry and Evolution of Rift Systems

Abstract

Rift basins often form within lithosphere containing structures imparted from previous tectonic events, yet the extent to which the reactivation of these structures affects the evolution of rifts 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 intra-crystalline basement (ICB) structures. However, determining the impact of ICB structure on rift evolution is difficult as crystalline basement is often buried beneath thick sedimentary successions and contains small acoustic impedance contrasts. Therefore, crystalline basement typically appears poorly reflective on seismic reflection data, hampering efforts to interpret internal structure. In this study we use borehole-constrained 2D and 3D seismic reflection data from offshore SW Norway to map a series of prominent ICB reflections, and document how they affect rift evolution. Crystalline basement is exceptionally well-imaged throughout the data due to large impedance contrasts within a highly heterogeneous, shallow basement, allowing for the 3D mapping of ICB 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 (30°) 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 observe multiple phases of extensional reactivation of some ICB structures during Devonian, Permo-Triassic and Late Jurassic-Early Cretaceous rifting, along with reverse reactivation during Late Cretaceous inversion. However, rifting is also associated with the initiation of faults that cross-cut ICB structure; implying selective reactivation of ICB structures, which may be controlled by their relative strength and geometry. From detailed 3D mapping of an ancient thrust belt and overlying rift, we show that the presence of ICB structures can control the structure and evolution of rift basins. Selective reactivation of ICB structures will control the location and orientation of major rift-related faults. Repeated reactivation of these structures during later rifting may modify pre-existing structural traps or migration pathways.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015