Pre-inversion normal fault geometry controls inversion style and magnitude, Farsund Basin, offshore southern Norway

Abstract

Compressional strains may manifest along pre-existing structures within the lithosphere, far from the plate boundaries along which the causal stress is greatest. The style and magnitude of the related contraction is expressed in different ways, depending on the geometric and mechanical properties of the pre-existing structure. A three-dimensional approach is thus required to understand how compression may be partitioned and expressed along structures in space and time. We here examine how post-rift compressional strains are expressed along the northern margin of the Farsund Basin during Late Cretaceous inversion and Palaeogene–Neogene pulses of uplift. At the largest scale, stress localises along the lithosphere-scale Sorgenfrei-Tornquist Zone, where it is expressed in the upper crust as hangingwall folding, reverse reactivation of the basin-bounding normal fault, and bulk regional uplift. The geometry of the northern margin of the basin varies along strike, with a normal fault system passing eastward into an unfaulted ramp. Late Cretaceous compressive stresses, originating from the convergence between Africa, Iberia, and Europe, selectively reactivated geometrically simple, planar sections of the fault, producing hangingwall anticlines and causing long-wavelength folding of the basin fill. The amplitude of these anticlines decreases upwards due to tightening of pre-existing fault propagation folds at greater depths. In contrast, later Palaeogene–Neogene uplift is accommodated by long-wavelength folding and regional uplift of the entire basin. Subcrop mapping below a major, uplift-related unconformity and borehole-based compaction analysis show that uplift increases to the north and east, with the Sorgenfrei-Tornquist Zone representing a hinge line rather than a focal point to uplift, as was the case during earlier Late Cretaceous compression. We show how compressional stresses may be accommodated by different mechanisms within structurally complex settings. Furthermore, the prior history of a structure may also influence the mechanism and structural style of shortening that it experiences.