Fold and fabric patterns developed within a major Caledonian thrust nappe in NW Scotland reflect a progressive increase in regional D2 strain towards the basal ductile detachment. Within the upper greenschist to lower amphibolite facies thrust sheet, the main gently east-dipping foliations and SE-plunging transport-parallel lineations maintain a broadly similar orientation over c. 600 km2. Associated main phase, thrust-related folds (F2) are widely developed, and towards the base of the thrust sheet display progressive tightening and increasing curvilinearity of fold hinges ultimately resulting in sheath folds. Secondary folds (F3) are largely restricted to high-strain zones and are interpreted as flow perturbation folds formed during non-coaxial, top-to-the-NW ductile thrusting. These features are consistent with a structural model that incorporates plane strain pure-shear flattening with a superimposed and highly variable simple shear component focused into high-strain zones. The increase in strain over a distance of 30 km across strike is similar to the increasing deformation observed when structures are traced along strike to the north, and which are apparently related to proximity to basement-cover contacts. A U–Pb zircon age of 415±6 Ma obtained from a syn-D2 meta-granite confirms that regional deformation occurred during the Scandian phase of the Caledonian orogeny.
Tectonic processes associated with supercontinent cycles result in a variety of basin types, and the isotopic dating of detrital minerals within sedimentary sequences assists palaeogeographical reconstructions. Basins located along the Laurentia–Baltica margin prior to assembly of Rodinia at 1.2–1.0 Ga are dominated by zircon detritus derived from contemporaneous magmatic arcs. Basins formed during assembly are also dominated by zircon detritus with ages similar to that of sediment accumulation, reflecting syn-collisional magmatism and rapid exhumation of the developing Grenville–Sveconorwegian orogen. Post-collision intracratonic basins lack input from syn-depositional magmatism, and are dominated by significantly older detritus derived from the mountain range as well as its foreland. Basins formed during late Neoproterozoic to Cambrian breakup of Rodinia are divisible into two types. Those within the Caledonides lie on the Grenville–Sveconorwegian foreland and incorporate Archaean and Palaeoproterozoic detritus derived from the cratonic interior and Mesoproterozoic detritus derived from the eroded remnants of the orogen. In the Appalachian orogen, such basins are dominated by Mesoproterozoic detritus with older detritus forming only a minor component, suggesting restricted input from the cratonic interior as a result of either the Grenville orogen still forming a drainage divide or the formation of rift shoulders.
The ophiolitic Voltri Group in the eastern part of the Ligurian Alps (NW Italy) is made up of a number of thrust sheets emplaced during Alpine collision. These thrust sheets include (1) the Voltri-Rossiglione calcschist unit of Mesozoic high-pressure calcareous micaschists, metavolcanics and slices of serpentinite, overlain by (2) the Beigua serpentinite unit, of mainly antigorite serpentinite and eclogitic metagabbro, in turn overlain by (3) the Erro-Tobbio peridotites. In the northern part of the Voltri Massif, a conspicuous melange-type lithology occurs along the contact of the Beigua serpentinite unit and the Voltri-Rossiglione calcschist unit. In the vicinity of the contact, the structure in the hanging-wall serpentinites is dominated by kink-type crenulations. Towards the base of the serpentinite nappe these crenulations become intense, and veins and patches of talc + chlorite + tremolite + carbonate replace the original antigorite-dominated assemblage. The thrust itself is marked by a layer, at least several tens of metres thick, of intensely deformed and foliated antigorite-bearing talc-chlorite-tremolite schist, enclosing rounded and lense-shaped (phacoid) blocks, up to 25 metres across, of retrogressed eclogitic metagabbro, antigorite serpentinite, metabasic rock, calcschist and schistose micaceous marble. The main features of this chaotic lithology meet the descriptive criteria of a tectonic melange. The structures and assemblages in the wall rock units and those in the melange indicate that the melange lithology developed at a relatively late stage of greenschist facies ductile thrusting, emplacing the Beigua unit onto the rocks of the Voltri-Rossiglione unit. The structures indicate that the blocks and lenses were formed during localized deformation in a relatively narrow zone along the thrust plane via intense stretching and boudinage of the various lithologies in the foot- and hanging wall. The development of talc-chlorite-tremolite-carbonate assemblages at the expense of the overriding antigorite serpentinites require significant calciummetasomatism, hence extensive fluid activity, whilst the microstructures in the melange matrix suggest that the talc-chlorite-tremolite-carbonate assemblage was mechanically weak. It is suggested that both fluid activity and the associated metamorphic reactions strongly facilitated ductile to semi-brittle deformation along the thrust, leading to progressive fragmentation and mixing of the different lithologies and development of a tectonic melange.
The Assynt District in the Northwest Highlands is a veritable treasure trove of geological features, the most famous – amongst geologists – being the Moine Thrust. Perhaps less well known is that Assynt also hosts the most extensive cave systems in Scotland, developed in the Cambro-Ordovician Durness limestone. These two features are, of course, linked: Caledonian thrusting and folding resulted in thickening and fracturing of the limestone strata, allowing/enhancing the development of large cave systems.
Restoring primary depositional frameworks from orogenic settings is challenging. To demonstrate a robust determination of original, but now highly deformed, depositional frameworks and their first-order sequence-stratigraphy, we analyse the Dalradian succession of Tyndrum–Glen Lyon (Breadalbane) in the southwestern Grampian Highlands of Scotland. In Breadalbane, several distinctive Appin and Argyll group Dalradian formations are absent. Omission has been attributed to ductile shearing on the Boundary Slide structure, during the Grampian Orogeny ( c. 470 Ma). Alternatively, we restore and describe a primary depositional framework and widely developed intra-Dalradian basin unconformity in Breadalbane, preserved in the relatively low-strain lower limb of the Grampian D 2 Ben Lui Syncline. On this unconformity, locally distinctive strata of the Easdale Subgroup, and more regionally typical strata of the Crinan Subgroup, were deposited directly on strata of the Lochaber Subgroup. Northeastward loss of strata of the Ballachulish, Blair Atholl and Islay subgroups, observed SW of Tyndrum, contrasts with gradual reappearance of correlative units northeastwards from Glen Lyon. Onlap and/or overstep relationships are well preserved; although strain is enhanced locally along pronounced stratal or rheological contrasts, the stratigraphical framework remains essentially intact. Our Scotland-wide analysis of the Dalradian depositional framework recognizes other probable basin-scale unconformities that locally influenced patterns of superimposed orogenic deformation.
Abstract. Basal ice motion is crucial to ice dynamics of ice sheets. The classic Weertman model for basal sliding over bedrock obstacles proposes that sliding velocity is controlled by pressure melting and/or ductile flow, whichever is the fastest; it further assumes that pressure melting is limited by heat flow through the obstacle and ductile flow is controlled by standard power-law creep. These last two assumptions, however, are not applicable if a substantial basal layer of temperate (T ∼ Tmelt) ice is present. In that case, frictional melting can produce excess basal meltwater and efficient water flow, leading to near-thermal equilibrium. High-temperature ice creep experiments have shown a sharp weakening of a factor 5–10 close to Tmelt, suggesting standard power-law creep does not operate due to a switch to melt-assisted creep with a possible component of grain boundary melting. Pressure melting is controlled by meltwater production, heat advection by flowing meltwater to the next obstacle and heat conduction through ice/rock over half the obstacle height. No heat flow through the obstacle is required. Ice streaming over a rough, hard bed, as possibly in the Northeast Greenland Ice Stream, may be explained by enhanced basal motion in a thick temperate ice layer.
Synopsis New British Geological Survey mapping has examined the stratigraphy and structure of Dalradian strata in the Gaick region of the Central Grampian Highlands of Scotland. In the north of that area, turbiditic strata in the Creag Dhubh Psammite Formation (Corrieyairack Subgroup) pass, via a well-defined sedimentary transition, into the stratigraphically younger Gaick Psammite Formation (Glen Spean Subgroup). This latter formation dominates the lithostratigraphy of the Gaick region, records shallow-water marine shelf conditions throughout, and was probably 1 to 2 km thick prior to deformation. Ordovician (Grampian) orogenesis affected the sedimentary rocks now arranged in a stack of Caledonian recumbent kilometre-scale F2 folds with gently ENE-plunging axes. Regional facing on these recumbent folds is typically sideways to the south. Southeast of the Gaick region, the folds, and thus facing, become progressively inclined and dip to the SE beneath the outcrop of the Appin Group Dalradian. No significant Fl folds, and hence no related facing changes, have been detected within this D2 fold stack. The structure of the Gaick–Drumochter area is therefore essentially a flat belt formed in the D2 deformation event and here named the Gaick Fold Complex. The D2 recumbent structures of this flat belt were rotated and steepened by D3 deformation into the Tummel Steep Belt to the SE, and adjacent to the Glen Banchor High to the NW. Tectonic transport in D2 in the Gaick Fold Complex is interpreted to be oriented on a north–south azimuth, similar to that in the Tay Nappe farther south. Such an interpretation implies that the NE–SW ‘Caledonian trend’ is a consequence of D3 deformation and reorientation, rather than a primary feature of the Grampian Orogeny.
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