general location of the moine thrust belt
In 1907, we discovered the Moine Thrust Belt situated in northwest Scotland. This was truly important scientifically as it was the first thrust belt recognized as such and it substantiated the tectonic plate theory (Wikipedia). The Moine Thrust Belt, and its associated Moine Supergroup outcrop (Neoproterozoic metamorphic rocks), was formed during the Caledonian orogeny (Kocks et al. 2014) (Johnson 1965). This mountain building event followed the closure of the Iapetus ocean (Kocks et al. 2014). There first was an arc-continent collision named Grampian event during the Ordovician (Kocks et al. 2014). This was followed by an oblique collision of three other proto-continents in the Silurian period (Kocks et al. 2014). Scotland suffered a great compression as the European plate was carried toward the west above the old Lewisian gneisses present on the Laurentian plate (Wikipedia). The Moine Thrust Belt joins Loch Eriboll, on the north coast of Scotland, to the Sleat peninsula on the Isle of Skye further southwest (see the two red dots on Figure 1) (Wikipedia).
Image credits: By Eric Gaba (Sting – fr:Sting) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 4.0-3.0-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/4.0-3.0-2.5-2.0-1.0)], via Wikimedia Commons
Image credits: By Eric Gaba (Sting – fr:Sting) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 4.0-3.0-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/4.0-3.0-2.5-2.0-1.0)], via Wikimedia Commons
The Moine Thrust Belt forms the western margin of the Caledonian orogen (see dashed line in Figure 2) (Kocks et al. 2014). It separates the Hebridean Terrane to the northwest from the Northern Highlands Terrane to the southeast (Wikipedia). Moine rocks of the Morar group form its top. (Kocks et al. 2014). Large slabs of Lewisian gneisses lying on top of a conglomerate base mostly characterize these. Both layers are separated by an unconformity (Wikipedia). They are formed of unmigmatized psammites with subordinate pelitic horizons (Kocks et al. 2014). At its most, the width of the thrust belt is 10 km and it is over 190 km long (Wikipedia). The Highlands of Scotland are full of rolling hills and rugged terraced mountains with steep sides. Unknown to most people, the mountains close to the Moine Thrust are the vestiges of a much higher reaching mountain belt. They are formed of complicated layers often characterized by a hard crystalline rock cap at the summit on top of softer sedimentary layers. We can often find a valley predominantly composed of limestone that goes up and forms sandstone terranes topped by a quartzite cap (Wikipedia).
Many rock types can be found in Northern Scotland. The basement is made of Lewisian granitic gneiss, a Precambrian metamorphic rock dating from the mid-early Proterozoic to the Archean (3.0-1.7 Ga ago) (Butler 2004). The Torridonian and Moine Supergroup sediments were deposited on the previously named rock (Butler 2004). They both date from the late Proterozoic (Butler 2004). Some Cambro-ordovician rocks are also present because they formed a cover on the foreland basin. Because their layers were highly differentiated and they possessed a layer-cake nature before the deformation, they can be used to observe the deformation, which was bestowed upon them by the Moine Thrust Belt (Butler 2004). We know that the Moine Thrust Belt dates back to the Caledonian orogeny, from the end of the Silurian period to the beginning of the Devonian. Additional precisions can be brought from the U-Pb dating synkinematic unusual igneous intrusions in Southern Assynt (Kocks et al. 2014). Indeed, the Borrolan complex dates back to 435-425 MA and there is evidence that places the Moine Thrust Belt active at the time of intrusion (Butler 2002). We can also say that the main displacement occurred from 435-430 MA and that there was only slight movement in early Devonian (Kocks et al. 2014). |
The underlying structures forming the Moine Thrust Belt have not yet been positively identified. Some theories are able to explain most of what can be observed at the surface. One such structure is a duplex (Watkins et al. 2014). A duplex is formed of 2 low dipping thrust faults: the floor thrust deep in the crust and the roof thrust close to the surface. They are linked by an array of steeper thrust faults called imbricate faults (see Figure 4) (Butler 1982). Each imbricate fault travels upward in the stratigraphic layers toward the direction of displacement (Butler 2004). In the Pipe rock formation, the Cambrian quartzites create shortening on the scale of 10's of kilometers by forming duplexes from 1 cm to 10's of meters large (Butler 2000). However, there are no more imbricate faults south of Assynt until the Torridon area where they return, strongly fracturing Torridonian and Cambrian strata (Butler 2000). Another aspect that still confuses geologist is that the roof thrust seems to both predate and postdate the imbricate faults (Butler 2004). Evidence pointing toward a roof thrust coming before that array of faults is when the aforementioned thrust is folded by underlying fault culmination (Butler 2004). However, we can also find proof of the opposite when the roof thrust truncates the imbricate faults (Butler 2004). This creates a real conundrum because we can find both type of deformation in close physical locations, like at the Glencoul Thrust (Butler 2004)
Mylonites are present everywhere in the Moine Thrust Belt and they can reach 100 meters of thickness in some areas. They are formed on either side of the thrust by intense shearing and streaking out. The grains themselves are streaked out and aligned like needles in the direction of sheer (Butler 2002). |
The Moine Thrust Belt's movement is constant throughout its three fold systems and all of its active life (Johnson 1965). The trend of the three fold systems is also the same as the Moine Thrust Belt (Johnson 1965). The fault structure moves west-northwest and it continuously folds and thrusts the slices of foreland units under its power (Butler 2004). It also crystallizes new minerals during transport, which are simultaneously suffering from vertical flattening (Johnson 1965). Knowing the original thickness, we can calculate the original length of an outcrop and determine that it shrunk from more than 50 km long to only a few km (Butler 2002). It was established that structurally higher units were situated to the east. A mylonitic texture and crystal plastic deformation microstructures are typical of such units (Butler 2002). To the west, cataclastic fault rocks are more likely to be found (Butler 2002). This leads to the conclusion that the time sequence at play here is that early, hot, deep and ductile shearing with crystalline plasticity was brought to higher cataclastic faults with brittle faulting and fracture processes (Butler 2002) (Butler 2000).
It has not yet been determined if the slip on the roof, imbricate and floor thrusts happens simultaneously or not. It is known that the resulting movement appears to be continuous but a cycle of alternations between roof, imbricate and floor thrusts would also create the effect of a simultaneous movement (Butler 2004).
It has not yet been determined if the slip on the roof, imbricate and floor thrusts happens simultaneously or not. It is known that the resulting movement appears to be continuous but a cycle of alternations between roof, imbricate and floor thrusts would also create the effect of a simultaneous movement (Butler 2004).
One of the most interesting parts of the Moine Thrust Belt is situated at Loch Eriboll. As well as being easily accessible, Loch Eriboll presents a lot of exposed outcrops that reveal key information about the Thrust belt. We can clearly see the individual thrust and the intermittent flats and ramps that compose the Moine Thrust as well as sheets of Lewisian gneiss and small-scale imbrications of Cambrian stratigraphy (Butler 2000). Many of these structures can be seen in the Pipe Rock (mostly quartz), which contains markers, trace fossils, for layer-parallel shortening and shear strain. At Loch Eriboll, many geological layers can be found. The structurally highest is the Moine Thrust Sheet, which is made of mylonites originating from Moine metasediments, the Lewisian basement and greatly deformed Lewisian slices. Next come mylonites of Lewisian gneiss origin combined to Cambrian quartzites (Butler 2000). A part of the Lewisian basement was not deformed by the penetrative Caledonian strain. Thrust sheets comprised of such basement rocks follow the mylonites. Finally, we can found different imbricated Cambrian sediments at the bottom of the structure (Butler 2000).
On the geological map of the Moine Thrust zone (Figures 7 and 8), we can see all the major geological features present in this region. A line punctuated by small triangles represents the thrust belt itself. A dashed line represents other faults. The different colors indicate what type of rocks can be found at the surface. The principal rocks found in this part of Scotland are form the Morar group (dark brown-green). However, in some areas, erosion exposed some older and normally hidden rocks, like the Lewisian basement in dark purple. |
References:
Butler, R.W.H., 2004, The Nature of Roof Thrusts in the Moine Thrust Belt, NW Scotland: Implications for the Structural Evolution of Thrust Belts: Journal of the Geological Society, London, Vol. 5, pp. 849-859.
Butler, R.W.H., 1982, The Terminology of Structures in Thrust Belts: Journal of Structural Geology, Great Britain, Vol. 4, No. 3, pp. 239-245.
Butler, R.W.H., 2000. "The Moine Thrust Belt". Leeds University. Retrieved 2015-03-21
Butler, R.W.H., 2002. "Assynt's Geology". Leeds University. Retrieved 2015-03-21
Christie, J.M., 1965, Moine Thrust: A Reply: The Journal of Geology, Chicago, Vol. 73, No. 4, pp. 677-681.
Johnson, Michael, 1965, The Moine Thrust: A Discussion: The Journal of Geology, Chicago, Vol. 73, No. 4, pp. 672-676.
Kocks, H and 3 others, 2014, Contrasting magma emplacement mechanisms within the Rogart igneous complex, NW Scotland, record the switch from regional contraction to strike-slip during the Caledonian orogeny: Geological Magazine, Cambridge, Vol. 151, No.5, pp. 899–915.
Various authors, 2007, Moine Thrust Belt: Wikipedia encyclopedia.
Various authors, 2003, Moine Supergroup: Wikipedia encyclopedia.
Watkins, Hannah and 2 others, 2014, Identifying multiple detachment horizons and an evolving thrust history through cross-section restoration and appraisal in the Moine Thrust Belt, NW Scotland: Journal of structural geology, Aberdeen, Vol. 66, pp. 1-10.
Butler, R.W.H., 2004, The Nature of Roof Thrusts in the Moine Thrust Belt, NW Scotland: Implications for the Structural Evolution of Thrust Belts: Journal of the Geological Society, London, Vol. 5, pp. 849-859.
Butler, R.W.H., 1982, The Terminology of Structures in Thrust Belts: Journal of Structural Geology, Great Britain, Vol. 4, No. 3, pp. 239-245.
Butler, R.W.H., 2000. "The Moine Thrust Belt". Leeds University. Retrieved 2015-03-21
Butler, R.W.H., 2002. "Assynt's Geology". Leeds University. Retrieved 2015-03-21
Christie, J.M., 1965, Moine Thrust: A Reply: The Journal of Geology, Chicago, Vol. 73, No. 4, pp. 677-681.
Johnson, Michael, 1965, The Moine Thrust: A Discussion: The Journal of Geology, Chicago, Vol. 73, No. 4, pp. 672-676.
Kocks, H and 3 others, 2014, Contrasting magma emplacement mechanisms within the Rogart igneous complex, NW Scotland, record the switch from regional contraction to strike-slip during the Caledonian orogeny: Geological Magazine, Cambridge, Vol. 151, No.5, pp. 899–915.
Various authors, 2007, Moine Thrust Belt: Wikipedia encyclopedia.
Various authors, 2003, Moine Supergroup: Wikipedia encyclopedia.
Watkins, Hannah and 2 others, 2014, Identifying multiple detachment horizons and an evolving thrust history through cross-section restoration and appraisal in the Moine Thrust Belt, NW Scotland: Journal of structural geology, Aberdeen, Vol. 66, pp. 1-10.