Graben inversion in nature and experiments

TLDR

The second, brittle‑ductile configuration involving a weak décollement layer can produce a wide range of geological structures. This study investigates basement graben inversion using laboratory experiments and North Sea seismic data, and proposes a mechanically based classification of inversion structures. Laboratory experiments on small‑scale brittle‑ductile models built with sand and silicone putty simulate basement, cover, and décollement layers, testing two configurations and key parameters such as fault obliquity, strength profiles, and salt diapirs. The experiments show that reactivation of normal faults requires compression angles below 45°, that superficial décollements generate low‑dipping thrust faults at graben borders, that salt diapirs or walls at borders preferentially host thrust faults, and that when the cover is decoupled, deformation splits into border thrusts and oblique strike‑slip faults within the graben, findings that align with southern North Sea field examples.

Abstract

The inversion of crustal‐scale basement grabens is studied here through laboratory experiments on small‐scale models and available oil industry seismic lines from the southern North Sea. Two basic configurations are considered. First, both the basement and the sedimentary cover are brittle, and inversion does not involve any potential décollement between them. Second, the basement and sedimentary cover are separated by a weak ductile layer (e.g., salt), which can allow décollement of the cover during both extension and later compression and inversion. The second configuration is more complicated and can lead to a large variety of geological structures. Laboratory experiments were carried out on brittle‐ductile models built with sand to represent brittle layers (basement and sedimentary cover) and silicone putty to simulate the décollement layer between basement and cover. A mechanically based classification of inversion structures is proposed. The effects of some crucial parameters are investigated, including obliquity between the direction of shortening and normal faults, as well as strength profiles, and the presence or absence of salt diapirs. The experimental investigation leads to the following conclusions: (1) the inversion of the graben by reactivation of normal faults implies that the angle between the direction of compression and the graben is less than 45°, (2) if there is a superficial décollement (e.g., basement‐cover interface), inversion initiates low dipping thrust faults in the cover, localized at graben borders, (3) salt diapirs or salt walls localized along the graben borders in the cover are preferential sites for the development of thrust faults, and (4) when the cover is decoupled from the basement by a décollement layer, inversion induced deformation in the cover which is partitioned between thrust faults along the graben borders and strike‐slip faults within the graben trending oblique to the graben borders. Experimental results are compared with field examples, in particular from the southern North Sea.