The Nature, Consequences and Controls of Deformation During Superimposed Rifting: the Inner Moray Firth Basin

Abstract

The Inner Moray Firth Basin (IMFB) is a superimposed sedimentary basin that experienced a complex structural history with many deformation episodes of regionally or local extent. Strictly, the IMFB rift forms the western arm of the North Sea trilete rift system that initiated mainly during the Late Jurassic-Early Cretaceous, with the widespread development of major NE-SW-trending faults. However, the basin overlies older Caledonian basement, the pre-existing Devonian-Carboniferous Orcadian Basin, and part of a regionally developed Permo-Triassic basin system in the wider North Sea region. The IMFB also experienced later episodes of uplift and fault reactivation during the Cenozoic. The resulting superimposed structures can be challenging to separate and characterise.
This study uses detailed field observations of key onshore outcrops across the entire IMFB, coupled with U-Pb dating of syn-faulting calcite-mineralised veins to constrain the absolute timing of faulting events. These findings are then integrated with the offshore interpretation of 2D and selected 3D seismic reflection data to constrain the absolute timing of fault populations and decipher the kinematic history of the superimposed basin development.
This holistic approach allows up to six deformation events to be identified and characterised regionally: Devonian rifting associated with the older Orcadian Basin; Variscan inversion and regional uplift/exhumation; Permo-Triassic thermal subsidence with evidence of only minor fracturing (e.g. no clear evidence for rift-related faulting at this time); Upper Jurassic – Lower Cretaceous rifting; possible Upper Cretaceous rifting, and Cenozoic strike-slip-dominated faulting and reactivation of older syn-rift faults.