In recent years conflicting models have been proposed for the late Caledonian closure of the Iapetus ocean between Laurentia, Baltica and the Avalonian terranes. Recently published structural and stratigraphic evidence from Britain, Scandinavia, East Greenland and Newfoundland is reviewed and shows that Western Avalonia, Eastern Avalonia and Baltica all docked sinistrally against Laurentia in the Silurian. Western Avalonia collided sinistrally against the previously accreted Gander and Dunnage arc terranes on the Appalachian margin of Laurentia in mid-Silurian time and then shifted dextrally during the Acadian orogeny in the Devonian. Oblique collision of Baltica with the Greenland margin induced southeasterly crustal stacking in the Scandian orogen and sinistral transpression in the NE Greenland Caledonides, and was followed by more nearly orthogonal convergence. Eastern Avalonia underwent anticlockwise with the Scottish ‘corner’ of Laurentia, rotating into a re-entrant between Laurentia and Baltica. Some implications of this Silurian closure model are that convergence in the Tornquist zone was modest at that time, and that the Acadian (Devonian) deformation in the northern Appalahians and Britain had a subsequent external cause, most likely the impingement of Armorica and Iberia due to the northward drift of Gondwana.
Structural and metamorphic investigations of the northeastern margin of the Bohemian Massif indicate three main sequential Devonian–Carboniferous tectonic events: (1) Devonian rifting; (2) Early Carboniferous oblique underthrusting and formation of a continental accretionary wedge; (3) eduction of the wedge and Late Carboniferous transpression. Devonian rifting of the Brunia microcontinent resulted in the formation of two crustal‐scale boudins associated with the development of two syn‐rift Devonian basins. This extensional template strongly influenced the nature of the ensuing Variscan contractional deformation. Early Carboniferous (350–330 Ma), progressive, highly oblique underthrusting of the two crustal boudins beneath the Lugian terrane to the west, generated syn‐deformational Barrovian metamorphism and the formation of a continental accretionary wedge. The wedge was further compressed by continued underthrusting of Brunia which resulted in the successive vertical extrusion (eduction) of an upper and lower allochthon, derived from the more deeply underthrust crustal boudin. The eduction was terminated by a Late Carboniferous (330–310 Ma) transpressional event resulting from continued plate convergence. Release of mantle‐derived magma during late‐stage eduction thermally softened the transpressional zones in the more external parts of the wedge. The resultant differential displacements gave rise to extensional unroofing of the internal part of the wedge.
SummaryConsideration of the arctic configuration of the Caledonides leads to a distinction between eastern and western geosynclinal belts. The western belt, comprising the East Greenland, East Svalbard and southern Barents Sea Caledonides is postulated to continue northwards into the Lomonosov Ridge, whilst the western Spitsbergen Caledonides are thought to have originated as part of the North Greenland geosyncline which is also thought to continue northwards to form the western part of the Lomonosov Ridge. The eastern Caledonian geosynclinal belt comprising the Scandinavian Caledonides appears to swing eastwards to link with the Timan Chain and possibly the Urals.The already postulated (‘Proto-Atlantic’) ocean concept is reviewed in the light of the Arctic Caledonides and named Iapetus. Faunal provincialism suggests that the ocean was in existence up to early Ordovician but had substantially closed by mid Ordovician times. Possible relics of the suture marking the closure of this ocean suggest that it lay to the west of the Arctic Scandinavian Caledonides trending NE to latitude 70° N and thence veered eastwards separating the southern Barents Sea Caledonides from those of Arctic Scandinavia, possibly connecting with the northern Uralian ocean. A previous branch of the ocean may have separated East Svalbard and East Greenland as an ocean-like trough. A further (pre-Arctic) ocean may have existed to the north of the North Greenland–Lomonosov Ridge geosynclines. This is named Pelagus.The closure of these oceanic areas and the deformation of the bordering geosynclines delineates three principal continental plates, namely, Baltic, Greenland and Barents Plates. Their relative dominantly E–W motion up to Silurian times produced compression between the Greenland and both the Baltic and Barents plates but dextral transpression and transcurrence between the latter plates. In Late Silurian to Devonian times an increasing northward component controlled late Caledonian transpression and sinistral transcurrence between the Greenland plate and the combined Baltic and Barents plates.
13The Heshan coals, with very high organic sulphur content, are found in the Upper Permian marine carbonate successions 14 (Heshan Formation) in the Heshan Coalfield, central Guangxi, southern China. The petrography, mineralogy, and geochemistry 15 of coals and non-coal partings from the Suhe and Lilan coal mines of the Heshan Coalfield have been investigated using 16 proximate, petrographic, inductively coupled plasma mass spectrometry (ICP-MS), X-ray fluorescence (XRF), X-ray 17 diffraction (XRD), and scanning electron microscopy with an energy-dispersive X-ray (SEM-EDX) techniques. The sulphur 18 content in the coals (with ash less than 50%) ranges from 5.3% to 11.6%, of which more than 90% is organic sulphur, 19 reflecting a strong marine water influence on the palaeomire. The high vitrinite reflectance (1.89 -2.18%Ro max ) indicates that 20 the coals in the Heshan Coalfield are mainly low-volatile bituminous coal. Microscopic observation has revealed that the coal 21 is mainly composed of vitrinite and inertinite macerals with relatively low TPI and high GI values, suggesting an unusual, 22 strongly alkaline palaeomire, with high pH. XRD analysis plus optical and scanning electron microscopy show that the 23 minerals in these coals are mainly quartz, calcite, dolomite, kaolinite, illite, and pyrite, although marcasite, strengite, and 24 feldspar, as well as some oxidised weathering products such as gypsum, are also present. Most trace elements in the Heshan 25 coals are enriched with respect to their world mean, with Mo, U, and W highly enriched, more than 10 times their world 26 means. The trace elements are believed to be associated either with organic compounds (Mo and U) or minerals such as 27 aluminium -iron-silicates (Sc, Ge, and Bi), aluminium-silicates (Cs, Be, Th, Pb, Ga, and REE), iron-phosphates (Zn, Rb, and 28 Zr), iron-sulphides (As, Cd, Cr, Cu, Ni, Tl, and V), and carbonates (Sr, Mn, and W). Abnormally high organic sulphur content, 29 high ash yields, relatively high GI values, very low TPI values, very high U contents, and very low Th/U ratios suggest that the 30 Heshan coals accumulated in low-lying, marine-influenced palaeomires, developed on carbonate platforms. Many of these 31 characteristics have also been recorded in the Tertiary coals of the circum-Mediterranean coal basins, where no marine . Previous studies have focused on 53 facies and microfacies relationships and coal-forming 54 models (Zhang et al., 1983;Zhang and Shao, 1987; 55 Chen, 1987; Jin and Li, 1987;Shao and Zhang, 1992; 56 Huang et al., 1994;Shao et al., 1995Shao et al., , 1998 Hou et al., 57 1995) and have demonstrated that the Heshan coals 58 were deposited in marine carbonate platform settings. 59 Geochemical data of this type of coal have seldom 60 been provided. Because of their high sulphur content 61 and therefore potential impact on the atmosphere 62 when burnt, the coal mines producing the Heshan 63 coals are being closed down. However, the Heshan 64 coal will continue to be the major feed coal i...
Consideration of six balanced cross-sections through parts of the Finnmark Caledonides, N Norway indicates that shortening varies between 25% and 75%. A restored long cross-section across the width of the orogen, constructed with the aid of a branch line map, demonstrates a foreland propagating thrust system, with earlier formed more internal metamorphic nappes thrust SE 330 km under ductile conditions and then carried piggyback ESE a further 296 km on later brittle thrust sheets. Total shortening is 78·7% with a translation of the most internal thrust sheet of 626 km.The restored section suggests that: (1) the rate of propagation of deformation from hinterland to foreland is c. 2·27 cm y−1; (2) incorporation of basement into the nappes resulted from inversion of extensional faults formed during Iapetus rifting; (3) during rifting a Finnmark basement ridge separated a 220 km wide southeasterly Gaissa basin from the passive Iapetus continental margin which was at least 423 km wide; (4) the Finnmark Caledonides resulted from a continent-microcontinent collision which obducted continental crust at least 600 km across the Baltic margin; and (5) the Caledonian Baltoscandian margin prior to Iapetus suturing extended at least 400 km W of the Norwegian coast. On a Bullard reconstruction this overlaps with Laurentian rocks in Greenland. The excess continental crust is accounted for by shortening of the Baltoscandian margin during collision.
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