Subtle traps in extensional terranes; a model with reference to the North Sea

The three stages of graben formation (initial, active stretching and thermal subsidence/sedimentary loading) offer different subsidence patterns, and, hence, different patterns of topography and deposition. The pre-rift stage (in rifts without pre-rift doming) is characterized by development of a broad basin with well developed axial and secondary transverse sediment transport. Stratigraphic traps are uniform and display less variation than in the subsequent stages. During the active stretching stage the basin geometry becomes more varied. Rotational fault blocks parallel to the graben margin become activated, and active fault scarps contribute to the instability of the system and to sedimentary transport systems parallel to the graben axis. The active stretching stage generates a topographically complex depositional environment, and potentially offers a great variety of subtle traps. The thermal subsidence/sedimentary loading stage brings the basin back to a less complicated geometry, and several types of unconformity traps will characterize the transition from the active stretching to the cooling stage. The dominant sediment transport system is transverse to the rift axis. The variety in types of subtle traps again is lowered.