GRABEN INVERSION IN NATURE AND EXPERIMENTS

The inversion of crustal-scale basement grabens is studied here throug h laboratory experiments on small-scale models and available oil indus try seismic lines from the southern North Sea. Two basic configuration s are considered. First, both the basement and the sedimentary cover a re brittle, and inversion does not involve any potential decollement b etween them. Second, the basement and sedimentary cover are separated by a weak ductile layer (e.g., salt), which can allow decollement of t he cover during both extension and later compression and inversion. Th e second configuration is more complicated and can lead to a large var iety 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 d ecollement layer between basement and cover. A mechanically based clas sification of inversion structures is proposed. The effects of some cr ucial parameters are investigated, including obliquity between the dir ection of shortening and normal faults, as well as strength profiles, and the presence or absence of salt diapirs. The experimental investig ation leads to the following conclusions: (I) the inversion of the gra ben by reactivation of normal faults implies that the angle between th e direction of compression and the graben is less than 45 degrees, (2) if there is a superficial decollement (e.g., basement-cover interface ), inversion initiates low dipping thrust faults in the cover, localiz ed at graben borders, (3) salt diapirs or salt walls localized along t he graben borders in the cover are preferential sites for the developm ent of thrust faults, and (4) when the cover is decoupled from the bas ement by a decollement layer, inversion induced deformation in the cov er which is partitioned between thrust faults along the graben borders and strike-slip faults within the graben trending oblique to the grab en borders. Experimental results are compared with field examples, in particular from the southern North Sea.