Models for Moine and Outer Isles Thrust Zones development, Northern Highlands of Scotland
|Johnstone, G S and Mykura, W. 1989. British regional geology: Northern Highlands of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
Models for Moine and Outer Isles Thrust zones development
There has recently been renewed interest about which events fashioned the Northwest margin of the Caledonides in Scotland. The concepts of ‘piggyback’ thrusting and balanced sections, in concert with the new geophysical data, all constrain models of development of the Moine Thrust Zone.
Coward (1983) has noted that the main thrusts appear, overall, to climb stratigraphy westwards with a dip of about 3° over at least 25 km. If estimates of total translation on the thrust zone are near correct, then foreland may underlie almost all of the Northern Highlands. However, the seismic reflection work off the north coast (MOIST) has indicated major thrust features commonly dipping about 25° to the east. The Outer Isles Thrust and the newly recognised Flannan Thrust (which apparently cuts the Moho) both affect the whole crustal section and imply that at least some Caledonian deformation apparently affected a large part of the western Foreland. If such a dip were valid for the Moine Thrust Zone on the mainland then high-grade metamorphic rocks should have been upthrust if translation exceeded 70 km, as seems probable.
Stewart (1982) has suggested that a Torridonian basin about 80 km wide Extended between bounding faults near the present positions of the Outer Isles and Moine Thrusts. He envisaged that the eastern fault formed the locus for thrusting during the Caledonian Orogeny (see Chapter 3 and P915465). There is no evidence from the Torridonian in the Moine Thrust Zone for any eastward termination of the sequence, merely for planation prior to deposition of the Cambrian. There are also insuperable difficulties in reconciling the geometry of a Moine Thrust which steepens to near vertical 25 to 30 km east of its current outcrop if amounts of translation exceed 70 km. Soper and Barber (1982) used the LISPB deep seismic refraction data (Bamford and others, 1978) and conductivity data (Hutton and others, 1980) to suggest a stepped crustal profile. They modelled major changes in geophysical properties which they correlated with the presence of the Moine Thrust Zone extending down to the Moho with a curved profile dipping about 25° east. They envisaged the ‘Zone of Complication’ of the Moine Thrust Zone as largely a product of latestage thrusting with about 30 km of westward translation, earlier movement being taken up on the Moine mylonites and within slide zones in the Moines themselves. Soper and Barber cite other larger-scale examples of deep crustal duplex structures from various orogenic belts and interpret this Caledonian margin similarly. Coward (1983) reviews the evidence from Moine Thrust Zone and related rocks and, like Soper and Barber, favours a two-stage evolution of the Moine Thrust Zone. He proposes that late-stage low-angle extensional movement was responsible for reactivation of the Moine Thrust and for its widespread, locally downcutting, extensional movements. The main compressional thrust profile must extend eastwards at an angle of about 3° for approximately 35 km (cf. Soper and Barber, c.25 km), and possibly up to 70 km, before a steep ramp occurs. The role of the MOIST data in the light of this latter model is discussed in the previous section.
Although during the past decades considerable advances have been made with regard to the geometry and deep-level structure of the Moine Thust Zone, it can be seen that the nature of this orogenic margin still presents considerable problems.