Structural development and trap formation in the Central North Sea HP/HT play

Abstract

This paper describes a new family of structures and class of tectonism; it is not wholly detached nor basement linked but falls between the two. The 3D interplay between basement extension and salt movement is used to describe the formation of a continuum of genetically linked trap styles in the Central North Sea HP/HT province. This style of deformation is progressive and manifests itself from simple low relief tilted fault blocks to large diapiric structures common to the UK eastern Central Graben. The proposed kinematic model attempts to unite basement structure, behaviour of salt, and observed map patterns into a simple continuous process. The model is composed of a succession of 6 time-ordered elements: an initial basement (Permian/Triassic) framework of north–south and east–west normal faults; Zechstein salt infill; deposition of Triassic and Jurassic strata; Late Jurassic basement extension, initiation of salt movement; grounding and rotation of Triassic/Jurassic strata about the edges of basement fault blocks; and upward movement of salt localized at the apex of the Triassic/Jurassic fault pattern. The relative amount of salt involved varies, and produces structures of similar architecture, but of varying scale. The Triassic/Jurassic fault pattern and block orientation reflect this structural process in an apparently random nature (i.e. with no dominant trend in either fault or bedding strike). The orientation of each block and the resultant faulted boundaries are mainly an indirect reflection of the underlying 3D-basement configuration. Rotation occurs about a fulcrum between the Triassic/Jurassic section and the point of first contact with a basement fault block, with subsequent rotation into an adjacent graben. This allows for a range of possible fault and bedding orientations inherited from the initial and evolving basement structure, combined with interaction between the rotating Triassic/Jurassic blocks. Although the overall process is simple, the observed map patterns may be complex. This model inherently decouples faults in the basement from the faults in the Triassic/Jurassic section. The Upper Jurassic and basement faults are different ages, and formed in response to different regional tectonics. Thus, common appeals to wrench tectonics, based on opposing offset along a single fault, seem to be unwarranted. This model is additionally supported by experimental analogues, where the seismic based observations can be reproduced in a qualitative manner.