Structural Precursors to Continental Break-Up; the Faroe Islands, NE Atlantic Margin

During the Palaeogene the NE Atlantic margin was subjected to a series of extension events immediately prior to and during continental break-up (at ca. 54 Ma). In the Faroes region of the margin, palaeostress analyses on faults exposed on the Faroe Islands indicate that the extension vector rotated in an anticlockwise sense from NE-SW to NW-SE. Remnants of the Faroe Islands Basalt Group (FIBG) exposed on the islands were emplaced at or around sea-level, to a total stratigraphic thickness in excess of 6.6 km, requiring a comparable magnitude of subsidence; to date, the structures preserved on the Faroe Islands have been inferred as being concurrent with subsidence. However, no onshore studies have accounted for the uplift events that must have occurred to bring the Faroe Islands to their current elevation (the highest peak at 882 m a.s.l.). The purpose of this study is to constrain the relative timings and kinematics of structures exposed on the Faroe Islands in order to investigate the regional tectonic deformation regime during continental break-up and sea-floor spreading; processes that have not hitherto been resolved using geophysical techniques. For the first time, we provide structural evidence that suggests uplift was accommodated by reactivation of pre-existing structures in the period immediately following emplacement of the FIBG. Structures on the islands provide clear evidence for a 3-phase tectonic evolution: (1a) anticlockwise rotation from E-W to NE-SW extension, facilitated first on N-S (dip-slip) faults, followed by NW-SE (dip-slip) faults. NE- SW extension (1b) continued with emplacement of a NW-SE- and NNE-SSW-oriented dyke swarm. Event-1 began prior to the deposition of the coal-bearing Prestfjall Formation, and was sustained through to emplacement of the Enni Formation, resulting in notable thickness variations across the Judd, Brynhild and Westray fault-zones. Further anticlockwise rotation of the extension vector led to (2a) the emplacement of ENE-WSW and ESE-WNW conjugate dykes. This magmatic input resulted in a N-S dilation of the crust that was followed by (2b) N-S extrusion and E-W shortening facilitated by recurrent slip on ENE-WSW (dextral) and ESE-WNW (sinistral) conjugate strike-slip faults. A component of the E-W shortening was accommodated by the development of numerous minor thrusts and low-angle normal faults. The final stages of this event show a rotation of the extension vector into a NNW-SSE orientation, which is taken up predominantly on NW- SE oriented dextral-oblique-slip faults. Event-2 began in immediate succession to Event-1, and continued through to the end of magmatism associated with the FIBG. Both events 1 and 2 display multiple generations of (predominantly calcite and zeolite) mineralisation as tensile (mode-I) and shear hydraulic veins. Finally, (3) uplift occurred with reactivation of some existing faults, characterised by the entrainment of clastic material, and an absence of mineralisation. Event-3 orientations are controlled by the pre-existing anisotropy; the predominantly N-S and NW-SE orientated faults of event-1 were inverted - in the north, the E-W conjugate- fault set and associated thrust faults of event-2 were reactivated. In all observed cases, motion-sense puts the Islands in the (relatively up-thrown) footwall block. These observations emphasise the necessity of carrying out detailed field studies, in addition to the more usual margin-scale modelling studies, in order to fully constrain the kinematics of continental break-up.