Geophysical data, Moine and Outer Isles thrust zones, Northern Highlands of Scotland

From: Johnstone, G S and Mykura, W. 1989. British regional geology: the Northern Highlands of Scotland (4th edition). (Nottingham: British Geological Survey.)

Geophysical data

Recent deep seismic reflection profiling has been carried out in the Pentland Firth extending west from the Orkney Isles to north of the Isle of Lewis (P915470b). The traverse was designed to determine the subsurface profile of the Moine and Outer Isles Thrusts (hence its name MOIST), the depth to the Mohorovicic Discontinuity (or Moho: the base of the continental crust) and the nature of any other major reflectors in the crustal section. In the light of spectacular results from similar work in the Appalachians (COCORP), summarised by Ando and others (1983) and Cook and others (1979), Brewer and Smythe (1984) have interpreted the major seismic reflectors from the MOIST traverse and presented crustal models. These models, which trace major tectonic structures through the crust, also take into account seismic reflection data from oil companies working in the area. They are also constrained by the existing seismic refraction data from the LISPB profile (Bamford and others, 1978) and the results of an electrical conductivity traverse by Hutton and others (1980).

The interpreted MOIST profile is shown on P915470a; the preferred geological interpretation of Brewer and Smythe is in P915470c. The seismic reflection data show clearly that in its western part the Outer Isles Thrust forms a planar discontinuity dipping about 25° to the east. It has apparently been reactivated as a listric (curved) normal fault in connection with the formation of Mesozoic and possibly Torridonian fault-controlled basins. The overall offset of reflections across the thrust is only 2 to 3 km. About 6 km below the Outer Isles Thrust and subparallel to it, a further strong reflector termed the Flannan Thrust has been recognised. This feature displaces the Moho which, on the MOIST section, is normally constant at a depth of around 26 km. If these reflectors represent tectonic structures of Caledonian age, and there is strong evidence of extensive Caledonian mylonitisation in the Outer Hebrides (Sibson, 1977; Fettes and others, in press), then it appears that the foreland to the orogenic belt did not remain a rigid unfractured block. Thus the ‘rules’ of thrust development noted earlier in this section may well apply to the complete crustal section, with the basal detachment lying in a low-velocity zone in the upper mantle, and ramps corresponding to the interpreted major thrust features such as the Outer Isles and Flannan thrusts. Displacement along such ramps cannot be large, however, as unrealistic crustal thickening would result.

The eastern part of the MOIST traverse (P915470c) contains a sequence of shallow, westerly-dipping reflectors which are interpreted as Devonian and Permo-Triassic red beds by Brewer and Smythe. These beds are affected by a series of easterly-dipping listric normal faults which appear to be sited above a series of shallow, easterly-dipping reflectors (discontinuities) in the underlying Moines. Hence the late normal faulting has apparently utilised the earlier thrust planes. The Moine Thrust itself is identified tentatively as a reflector dipping up to 25° E below which a layered sequence is present. As shown on P915470c, Brewer and Smythe interpret these units to be part of an off-shelf sedimentary sequence (?Dalradian) overlain by the Cambro-Ordovician units and overthrust by the Moines.

However, as discussed by Coward (1983), there are problems in interpreting the position of the Moine Thrust within the crust, in particular the distribution of ramps and flats. There is a lack of correlation in dip and outcrop position of the various units between the MOIST profile and the onshore geology of North Sutherland. This has been explained by invoking either a lateral ramp and/or tear fault running approximately east-west immediately offshore.