Moine Thrust Belt - notes
Rob Butler

The Moine Thrust Belt forms the outer edge of the Caledonian mountain belt in northern Scotland. It separates the poly-deformed and metamorphosed orogenic interior of Moine and Dalradian rocks, to the ESE, from the undeformed foreland of the Lewisian crust with its sedimentary cover of Torridonian and Cambro-Ordovician successions, to the WNW. The thrust belt runs from the north coast near Whiten Head to Sleat on Skye. Estimating the timing of displacements within the Moine Thrust Belt relies on radiometric ages from various alkaline igneous intrusives. The Loch Borrolan complex of Assynt, which appears to cut the Ben More Thrust (Parsons, 2000) has been dated at 430+/- 4 Ma (e.g. van Breeman et al., 1979). The Loch Ailsh intrusion, which is cut by Caledonian structures has yielded an age of 439+/- 4 Ma (Halliday et al., 1987). However, direct dating of mylonite formation in the hanging-wall to the Moine Thrust implies that ductile movements associated with recrystallisation continued on this until about 410 Ma (Freeman et al., 1999). There are no clear indications of the final end of movement from geological relationships in NW Scotland. No igneous rocks suture the thrust belt and there are no sediments earlier than Triassic which seal the structures. Nevertheless, it is probable that thrust tectonics had ended by earliest Devonian times (c. 400 Ma). 
	A prime reason for the successful mapping of the Moine Thrust Belt, together with its continued popularity as a training ground for students, lies in the well-differentiated lithostratigraphy. The irregular unconformity at the base of the Torridonian is spectacularly exposed at Slioch where there is almost 500m of relief upon it. Regionally the Precambrian rocks are capped by a remarkably planar unconformity which underlies a surprisingly layer-cake succession of Cambro-Ordovician sediments. The Cambro-Ordovician forms three distinctive Formation. The oldest is represented by about 150m of quartzites (the Eriboll Sandstone Formation), the middle one is a highly differentiated collection of sands, silts, muds (“Fucoid Beds”) and clean quartzites (“Salterella Grit”, collectively the An-t Sron Formation) and the upper one is a thick (> 1500m) succession of carbonates (the Durness limestone). These units are readily identified, commonly from a great distance, allowing exceptionally complex tectonic interleavings to be unravelled by field geologists. They also provide outcrops of unrivalled clarity of thrust and fold structures.
The early work culminated in the publication of the famous NW Highlands Memoir (Peach et al., 1907) and the special geological map of the Assynt district (Geological Survey of Great Britain, 1923).	For the following fifty years, geological research within the thrust belt concentrated to relating minor structures, particularly folds and schistosities, to the regional geology, principally by correlating these from place to place. But in the late 1970s renewed interest in the structural evolution of the Moine Thrust Belt was propelled by the work of Dave Elliott (Elliott & Johnson 1980) and by the acquisition by BIRPS of Europe’s first deep seismic profile, the MOIST line (Soper & Barber, 1982; Brewer & Smythe, 1984). 
	Mike Coward began his interest in the Moine Thrust Belt in the mid-late 1970s. His early work developed from undergraduate field teaching in the Eriboll area, from looking at displaced Lewisian structures in northern Assynt and in the structural evolution of the fold-thrust complexes of the southern part of the system (e.g. Coward & Whalley, 1979). His early papers (e.g. Coward et al. 1980; McClay & Coward 1981) formed a springboard for a NERC grant and a bunch of PhD projects in the thrust belt. The studies started looking at the evolution of thrust systems and strain in thrust sheets but developed into some of the earliest descriptions of coherent, apparently extensional structures in an otherwise compression belt (Coward, 1982). The vast extent of Mike’s legacy can be judged from the appended reference list.
This guide is a minimalist compilation of diagrams drawn largely from Mike Coward’s papers. If you’d like to go further into Moine Thrust Belt geology, why not access the web-site:
http://earth.leeds.ac.uk/assyntgeology
The site has lots of images, references, booklists and web links. 

Day 1 Glencoul
An introductory transect through the thrust belt, in part by boat, in part on foot. 

Drive to Kylesku (about 1 hour from Ullapool) for 9.00 am to catch “The Statesman” up Glencoul. Plan to return to Kylesku 5-6 pm. 

The Glencoul area is classic ground in Highland Geology. It was here that Charles Callaway made the first description of thrust structures in 1883. The existing maps were made largely by Clough in the same decade. The Stack of Glencoul – arguably the finest exposure of the Moine thrust – has excellent exposures of mylonites, worked on by John Christie in the 1960s. Mike Coward worked in the area through the late 1970s and early 80s. The Pipe Rock in the Glencoul thrust sheet was the focus for his strain-in-thrust-sheets researches, using the pipes to identify zones of apparent wrench shear in the belt. He also traced out arrays of normal faults, mapping them down to link with thrusts and giving the thrust world the concept of “surge zones”. The ground south of Glencoul and on into Assynt was particularly important. 

Key features
The Glencoul Thrust sheet.
Largely made of Lewisian basement. Coward et al. (1980) estimate 25-33 km displacement on the Glencoul Thrust (and underlying imbricates) on the basis of offset Laxford Front structures correlated from thrust sheet to foreland.  

The imbricates
Repetitions of Fucoid Beds and Salterella Grit (local slice of Durness limestone) are well exposed on the island and on the sides of Glencoul. Key issue – relationship to Glencoul Thrust.

The Moine thrust and related mylonites.
Seen at the Stack of Glencoul, both hanging-wall and footwall are characterised by mylonites. 

Extension in footwall to Moine thrust.
Nature of strain in the mylonites, origin of “exotic” sloice of limestone within quartzites. Possible distant views to S towards “surge zone country” – Beinn Uidhe. 

Day 2. Eriboll
Classic Moine Thrust Belt outcrops, especially for looking at the relationships between major thrusts (Arnabol and Moine) and their respective footwalls. Then the other sites show the relationship between these Caledonian thrusts and later (basin-forming) structures. The final road-cutting is one not to be missed.

Eriboll is a 2 – 2.5 hour drive, via Durness, from Ullapool but it’s worth it!

The Eriboll area has a unique place in the history of Highland Geology. It was here that Charles Lapworth demonstrated stratigraphic repetition of beds both through faulting and folding, in the early 1880s. He went on provide early descriptions of fault rocks, coining the term “mylonite” here. Lapworth inspired Peach and Horne in their studies here, and they in turn convinced Geikie of the role of tectonics in disrupting what had hitherto been regarded as intact stratigraphy. Geikie coined the term “thrust” – on the basis of field relationships on Ben Arnaboll. Peach and Horne, through their mapping, established the relationship between the Moine rocks in Durness with those to the East of Eriboll, recognising the implications for thrust displacements.
	Mike Coward worked in the Eriboll area, revising structural geometry and models of structural evolution of the major thrusts and imbricates through a series of papers in the early 1980s. He then used the Eriboll area as a forcing ground for looking at strain in fold-thrust complexes, developing quantitative descriptions of ramp geometry. Later he turned his attention to the offshore (first working on the seismic in a period of damp weather, laying out the paper lines in the lounge in the Ceilidh Place!). The links between onshore and offshore post-Caledonian faulting – as typified by the conundrum at Durness – remain controversial. 

Key features
1.	Arnaboll
The Arnaboll thrust and its relationship to imbricated Cambrian strata in its footwall. Excellent field examples of breaching. 
 
Walk out the Moine thrust for a couple of km

2.	Moine thrust. 
Relationships between folds, mylonites and the Moine Thrust above Kempie. Trace folds into the mylonites. 

Return to Durness
3. Sango Bay –
Late normal faults (associated with the development of the West Orkney Basin?) that down-throw the Moine Thrust. Transect across Sango Bay. 

4.	Laxford Brae road section.
A brief stop at spectacular road sections through “Laxfordianised” Lewisian. These outcrops form part of the classic Laxford Front deformation zone. 

Continue to Kylesku and hence to Ullapool. 

Day 3. Southern Assynt
A half day looking at the relationship between thrusts and normal faults.

The Assynt culmination has drawn geologists for over a century. Peach and Horne together with the other early survey workers recognised the unrivalled complexity of thrust structures here. They led many field excursions, most based out of the Inchnadamph Hotel, had a 3D model built and published the famous “one inch” geological map (still published in facsimile by BGS). The culmination was the focus of much of the re-interpretation efforts of Dave Elliott and Mike Johnson – published in 1980. It was natural therefore that Mike Coward should focus his attentions during his programme of remapping in the 1980s. Working in the ground above Inchnadamph he mapped out a suite of “surge zones”. The thrusts in the toe of these structures are exposed near the roadside at Inchnadamph. He also found a series of relatively small normal faults that offset thrust structures in southern Assynt. 
	One of the key features of central and southern Assynt are the klippen of various “far-traveled” thrust sheets. Elliott & Johnson (1980) used branch line methods to establish correlations between the klippen and the main outcrop traces of thrusts. These interpretations were greater modified by Mike Coward who suggested that the base of the Moine sheet, hitherto considered to be the Moine Thrust (by definition) was, in southern Assynt, an entirely late structure. He suggested that it was a major, low-angle normal fault that developed late in the evolution of the Caledonian mountain belt. Similar proposals have subsequently been made for many other thrust belts. 

Key features
1.	Sronchrubie cliff (Inchnadamph)
Imbricated Durness limestone seen from the road.

2.	Elphin
Road section through normal faults in Durness limestones, apparently detaching on the Sole Thrust.

3.	Knockan Crag.
Scottish Natural Heritage’s visitor attraction at the Moine thrust in southern Assynt. A nature trail leads visitors to key outcrops – and a short diversion gets us to sites where the Moine thrust truncates imbricates in its footwall.

Return to Ullapool. 
Drive to Inverness in the early afternoon.