The Structural Evolution of the Faroe Islands, NE Atlantic Margin

Abstract

The NE Atlantic margin plays host to numerous basins, developed in phases from the Devono-Carboniferous through to the Cenozoic, which record the build up to plate separation and formation of the North Atlantic Ocean. Existing models for this invoke broadly NW-SE extension within the basins, which are segmented by regional-scale NW-SE trending strike-slip lineaments, which are commonly termed ‘transfer zones’. However, there is a general paucity of information concerning the true kinematics of the so-called transfer zones. In this study, the Palaeogene and later structural evolution of the NE Atlantic margin is investigated using abundant field data collected on the Faroe Islands, and systematic observations that characterise the related deformation structures developed in the Faroe Islands Basalt Group (FIBG).

Structures in the Faroe Islands provide evidence for a 6-stage tectonic evolution, here split into 3 broad phases: (1a) E-W to NE-SW extension, accommodated by dip-slip N-S and NW-SE trending faults. Continued NE-SW extension (1b) was then accommodated by the emplacement of a regionally significant NW-SE- and NNE-SSW-oriented dyke swarm. Event 1 affects the majority of the FIBG stratigraphy, resulting in thickness variations, most notably across the Judd, Brynhild and Westray (‘transfer’) fault-zones. Continued magmatism and anticlockwise rotation of the extension vector led to (2a) the emplacement of ENE-WSW and ESE-WNW conjugate dykes, followed by intrusion of the large, saucer-shaped sills on the islands. Their intrusion heralded the onset of N-S crustal extension and was followed by (2b) crustal extrusion involving both E-W shortening and further N-S extension facilitated primarily by slip on ENE-WSW (dextral) and ESE-WNW (sinistral) conjugate strike-slip faults, interlinked with minor NE and SW dipping thrust faults. During the final stages of this event (2c), the regional extension vector rotated into a NW-SE orientation that was accommodated predominantly by slip along NE-SW oriented dextral-oblique-slip faults. Event 2 began towards the end of magmatism associated with the FIBG, and most likely continued through to the onset of oceanic-spreading on the Aegir ridge (ca. 55 Ma). Finally, (3) Event 1 and 2 structures were reactivated as extension and extensional-hybrid features, characterised best by the entrainment of clastic material along fault planes. Relative timings of Event 3 structures suggest they formed during a period of compression and uplift following the formation of a through-going mid-ocean ridge system (i.e. on the Reykjanes, Kolbeinsey and Mohns ridges).

The progressive anticlockwise rotation of the extension vector identified here is broadly consistent with the most recent NE Atlantic continental break-up reconstructions. Importantly, this model does not require basin-scale transfer zones during the Palaeogene, suggesting instead that these NW-SE faults formed as normal faults during a pre-cursor margin-parallel extension episode (Event 1) prior to the onset of oceanic spreading in the Faroe-Iceland sector. This study emphasises the importance of carrying out detailed field studies in addition to the more usual regional-scale modelling studies, in order to validate and add further detail to basin kinematic histories.

Mineralised syn- to post-magmatic fault sets display a recurring zeolite-calcite-zeolite trend in mineralisation products, which precipitate during successive phases of fault development during each individual event. Fault style and damage zone width appear to be related to the stage of fault development, with early fault/vein meshes linking to form through-going structures with associated damage zones. Dykes and sills are found to form their own fractures, rather than exploiting pre-existing sets. Dyke propagation appears to be buoyancy-driven, with magmatic pressure overcoming the minimum compressive stress. Sills, however, most likely seeded at an interface in the stratigraphy between a weak, more ductile material (i.e. a sedimentary horizon), and a rigid material (i.e. basalt lavas) above. Following this initial development, sill growth and propagation would likely be controlled by viscous dissipation, leading to the complex ramp and flat architecture, with rapid intrusion resulting in upward ramping of the sill. The alternation from fault events, to dyke events and back again corresponds to a switch from faulting with mineralisation along extensional hybrid veins, to magmatic intrusions into extension fractures followed by extensional hybrids (conjugates), and back to extensional and shear hybrid faults (again as conjugates). This alternation reflects variations in the differential regional stress, as well as the magmatic evolution of the margin, and most likely relates to the migration of lithospheric thinning northwestwards across the area, towards the eventual axis of break-up.

We find that, in particular, faults in basalts are in many ways comparable to faults formed at shallow crustal depths in carbonate rocks and crystalline basement, most likely reflecting the similarities in their mechanical properties under near-surface pressures and temperatures. The nature and style of the post-magmatic fault infills provides compelling evidence to suggest that subterranean cavities associated with faults were persistent open features within the FIBG. Structures equivalent to these late, clastic-filled faults of the Faroes may occur in other parts of the NE Atlantic margin, particularly along the axes of gentle regional-scale folds that are widely developed in the region. The late fault displacements observed are all well below seismic resolution, and such structures may be more widespread across the region than previously anticipated. Importantly, the probable unsealed nature of the clastic infills makes them potential fluid-migration pathways, both up- and across-faults within the Cenozoic volcanic sequences of the NE Atlantic region.