Controls on the development and evolution of structural styles in the Inner Moray Firth Basin

Abstract

Although it has generally been believed that structural styles in the Inner Moray Firth (IMF) have been largely controlled by strike-slip movements on the Great Glen Fault (GGF), integrated seismic and field studies suggest otherwise. Instead, most structural styles appear to have developed and evolved as a result of dip-slip extension and thermal subsidence consequent upon two phases of rifting during the Permo-Triassic and Late Jurassic and subsequent regional uplift and local inversion during the Tertiary. The integration of demonstrable thickening of Kimmeridgian–Portlandian intervals across the GGF with sedimentological information from onshore outcrops and cored wells suggests that the basin had a half-graben geometry during the Late Jurassic with a depocentre adjacent to the Helmsdale Fault, analogous to half-graben geometries which characterized other Late Jurassic sequences in Greenland and the South Viking Graben. Progressive marine onlap suggests that more gentle regional (thermal) subsidence took place in an underfilled basin, during subsequent Early Cretaceous deposition. However, the seismic data and subcrop information show that the geometries resulting from such classic rift- and thermally-driven phases of extension were modified by Cenozoic regional uplift and inversion in response to intraplate compression resulting from NE Atlantic (Thulean) and Alpine events. These events also appear to have effected minor strike-slip motion on the GGF, with the development of spectacular ‘flower structure’ and ‘helicoidal’ geometries, and caused limited oblique-slip reactivation of some extensional structures. The most notable modification of structural styles occurs in areas adjacent to the major basin-bounding faults. Particularly complex structural inversion geometries occur in the northwest corner of the basin adjacent to the Wick Fault while anomalously-trending folds developed in response to space problems in the Sutherland Terrace, between the Helmsdale Fault and the GGF, as a result of opposing senses of slip on these faults.