Proterozoic folds on the Northwest Caledonian Foreland

Soper, Nathaniel J.; Barber, Asa H.

doi:10.1144/sjg15010001

Cited by 27 publications

(7 citation statements)

References 18 publications

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“…Directly beneath the Moine thrust in the northeastern part of the Assynt culmination are two narrow horses of Torridonian rocks (cf. 1"= 1 mile geological map, but see Soper & Barber 1979). These may have come from yet another basin of Torridonian sediments which originally lay E of the Assynt basin.…”

Section: Distribution Of Torridonian Rocksmentioning

confidence: 99%

Structural evolution in the northern part of the Moine thrust belt, NW Scotland

Elliott

Johnson

1980

Transactions of the Royal Society of Edinburgh: Earth Sciences

347

201

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Four balanced cross sections, supported by longitudinal sections, structure contour maps, stratigraphic separation diagrams and hangingwall sequence diagrams are keys to this interpretation of the Moine thrust, which forms the western margin of the Caledonides in NW Scotland. New basement and cover correlations between foreland and thrust belt give new slip estimates for the Moine thrust (∼ 77 km), the Loch More klippe (≥ 43 km), Glencoul sheet (20–25 km), Ben More sheet (∼28 km), Achall and Dundonnell ‘sheet II’ (∼28 km).Like other major thrusts the Moine thrust moved in a smooth or rough fashion at different places and times, and many structures are a footwall response to its passage. Widely developed duplexes vary in thickness so that the roof thrust is folded and occasionally faulted; many late Caledonian folds in the Moine metasediments are of this origin. The presence of igneous bodies with contact aureoles increased the propensity to rough slip and this, by causing thickening in the footwall to the Moine thrust, is partly responsible for the Assynt culmination.The previously accepted sequence of thrusting from foreland to hinterland, which has been deduced from the concept of ‘overstep’ of the Moine thrust across lower thrusts, is considered to be a misconception of thrust geometry. Instead, a ‘piggy-back’ sequence of thrusts, from higher to lower, is proposed.

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Section: Distribution Of Torridonian Rocksmentioning

confidence: 99%

Structural evolution in the northern part of the Moine thrust belt, NW Scotland

Elliott

Johnson

1980

Transactions of the Royal Society of Edinburgh: Earth Sciences

347

201

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“…8), was taken to indicate the true age of pegmatite formation during a late Proterozoic event which was distinct from the Grenville orogeny (see also Piasecki & van Breemen 1979a, b;Piasecki 1980;Bowes 1980;Bowes & Gaal 1981). Soper and Barber (1979) related this event, across the Moine thrust zone, to an episode of open folding and major erosion of the Torridon Group, under the sub-Cambrian unconformity of the Caledonian foreland. However, there is no unanimity in the interpretation of the c. 750 Ma ages, since they have also been interpreted in terms of post-Grenville cooling, Caledonian thermal disturbance of Grenville-related pegmatites (Brewer et al 1979) and as a tectonothermal "event" lasting from c. 1000 to 720 Ma (Lambert etal.…”

Section: Central Highlands Northern Highlandsmentioning

confidence: 98%

“…In the latter, a lower, Stoer Group deposited at c. 970 Ma is separated by an angular unconformity from an upper, Torridon Group laid down at c. 790 Ma (Moorbath 1969;Stewart 1969Stewart , 1975. The Precambrian rocks have undergone an episode of open fold development and of major peneplanation (Soper & Barber 1979) prior to the deposition of a Cambro-Ordovician quartzite-carbonate shelf sequence (Swett 1969). At a late stage in the Caledonian orogeny this foreland was overthrust in a northwesterly direction by the metamorphosed Moine assemblage along a belt of imbricate thrusts, the Moine thrust zone (Elliott & Johnson 1980;McClay & Coward 1981;Soper & Barber 1982).…”

mentioning

confidence: 99%

Field and isotopic evidence for a c. 750 Ma tectonothermal event in Moine rocks in the Central Highland region of the Scottish Caledonides

Piasecki

Breemen

1983

Transactions of the Royal Society of Edinburgh: Earth Sciences

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Evidence is presented for a late Proterozoic, tectonothermal event which affected the rocks of the Moine assemblage in the Central Highland region of the Scottish Caledonides c. 750 Ma ago. This is about 200 Ma before the early Palaeozoic Grampian orogeny, whose effects are superimposed on the Moine rocks as well as dominating the tectonism in the Dalradian Supergroup.Field and isotopic studies are integrated in zones of ductile thrusting (sliding) which are typified by belts of tectonic schists with related swarms of quartz and muscovite-bearing pegmatite veins. Of particular significance is a ductile thrust (the Grampian slide) which separates deeper crustal rocks (the Central Highland division), interpreted as showing the imprint of the Grenville orogeny, from shallower rocks (the Grampian division) representing a supracrustal assemblage formed between the Grenville and the c. 750 Ma events.The Grampian slide is the structurally highest member of a system of related, previously unrecorded slides affecting the Central Highland division. New structural, petrographic and Rb-Sr isotopic data, obtained largely from a recently recognised inlier of the Central Highland division at Laggan, bear out that the quartz and pegmatite veins are segregations formed during ductile shearing under amphibolite facies conditions. Muscovites from these veins yield ages between 780 and 730 Ma, and a regression analysis of tectonic schists and the muscovites gives an age of 740 ± 40 Ma. These data substantiate our previous hypothesis that the deeper-seated Moine rocks were affected by a distinct orogenic event at 750 ± 30 Ma.In the Northern Highlands, similar vein swarms are related to the Sgurr Beag slide and to belts of previously unrecorded tectonic schists in the Glenfinnan division of the Moine assemblage. A new, 755 ± 8 Ma age obtained from such a tectonic schist at Kinloch Hourn, combined with previous, similar age data from lensoid pegmatites, imply that the c. 750 Ma event may have also affected the Morar and Glenfinnan divisions.

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“…These sedimentary beds were gently folded before the deposition of Cambrian rocks; the differences in dip between the Torridonian and the Cambrian rocks are about 15°, and Torridonian strata trace out large-scale, gently southward-plunging folds with a wavelength of about 45 km (Soper and Barber, 1979).…”

Section: Stratigraphymentioning

confidence: 99%

The Moine thrust and the Scottish Caledonides

Coward

1988

Geological Society of America Special Papers

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In the Moine thrust zone, Proterozoic (Moine) metasedimentary rocks of the Caledonian belt are carried over the foreland sequence of Archaean to Proterozoic (Lewisian) gneiss, upper Proterozoic (Torridonian) sandstones, and the unconformable sequence of Cambro-Ordovician shelf quartzites and limestones. The thrust structures occur on all scales, from minor duplex zones a few centimeters across to large thrust sheets on a kilometer scale. The thrust sequence is generally piggy-back; thus, the easternmost (highest level) thrust formed first. The thrust direction was toward N65°W ±10°.The lower thrust sheets have produced folds by the stacking of imbricate slices, although in Eriboll on the north coast, buckle folds are common in the hanging wall and footwalls of minor thrusts. The higher thrust sheets contain buckle folds, which locally are sometimes large-scale structures and generally oblique to thrust transport. This suggests that the buckle folds formed by differential movement, with the northern part of the thrust zone moving farthest to the west-northwest. Textural studies suggest that this movement in the north occurred at a slower rate, under more ductile conditions. Back thrusts and break-back faults developed at tip zones where the resistance to movement and/or fault propagation was high.The amounts of displacement estimated from offsets of Lewisian structures are 35 to 45 km. Balanced cross sections in the region between Foinaven and Assynt show shortening values from 35 km + to 55 km + . The Foinaven imbricates do not affect the lowest Cambrian strata or their basement; that is, the basement rocks must continue back some 55 km beneath the Moines, the distance equivalent to the restored middle to upper Cambrian. This implies that any crustal ramp to the Moine thrust zone must lie more than 55 km east of the present outcrop of the thrust. However, off the north coast of Scotland, deep seismic reflection profiles show moderately dipping reflections much farther to the northwest, and if these represent the crustal-scale ramp, this ramp must be offset by a major tear or transfer fault along the north Scottish coast.In the southern part of Assynt, in the central part of the thrust zone, there are several extensional fault systems as well as a late (extensional?) fault at the local base of the Moines that cuts across earlier thrust and extensional faults. These extensional movements suggest a gravity-spreading mechanism for some Caledonide thrusting.The timing of thrusting can be bracketed by the 430-Ma age for the Borralan igneous intrusion, which predates most major thrust movements, and by the Devonian age of molasse deposits. It is difficult to find the driving mechanism for the Moine thrust at this time.

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Proterozoic folds on the Northwest Caledonian Foreland

Cited by 27 publications

References 18 publications

Structural evolution in the northern part of the Moine thrust belt, NW Scotland

Structural evolution in the northern part of the Moine thrust belt, NW Scotland

Field and isotopic evidence for a c. 750 Ma tectonothermal event in Moine rocks in the Central Highland region of the Scottish Caledonides

The Moine thrust and the Scottish Caledonides

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