Caledonian-Appalachian Orogen Research Articles

Miogeoclines and suspect terranes of the Caledonian–Appalachian Orogen: tectonic patterns in the North Atlantic region

The Caledonian–Appalachian Orogen was formed by the closing of a Paleozoic Iapetus Ocean. The continental margins of Iapetus are identified in the deformed early Paleozoic miogeoclines of the Caledonian–Appalachian Orogen. Ophiolitic vestiges of Iapetus, its oceanic plateaus, microcontinents, and volcanic arcs are Caledonian–Appalachian suspect terranes. These were assembled in interior parts of the orogen and locally they were emplaced structurally upon the adjacent miogeoclines.The modern North Atlantic Ocean opened along an axis that traversed the Paleozoic orogen longitudinally. Its opening dispersed the elements of the Paleozoic orogen and led to the present arrangement of disjunct Paleozoic miogeoclines and suspect terranes throughout the North Atlantic borderlands.The western or North American margin of Iapetus is represented by the miogeoclines along the west flank of the North American Appalachians and Caledonides of east Greenland. A small North American miogeoclinal segment occurs in the British Isles, and suspect terranes with North American faunal affinities occur in Scandinavia. The eastern margin of Iapetus is represented by the miogeoclines of the Scandinavian Caledonides and the Mauritanides of northwest Africa. Ophiolitic vestiges of Iapetus and suspect terranes occur in the Appalachians, the Caledonides of Scandinavia and the British Isles, and the Variscan foldbelt of Morocco, Iberian Peninsula, and western France.In the scenario of a closing Iapetus and opening North Atlantic, the Paleozoic margin of eastern North America expanded by the acquisition of Appalachian suspect terranes, the Paleozoic margins of Greenland and Scandinavia remained essentially unchanged, and Africa lost parts of its Paleozoic margin.Modern continental margins and the geometry of the North Atlantic mimic Paleozoic miogeoclines and the geometry of Iapetus. The Paleozoic miogeoclines, in turn, follow Grenvillian deformed zones of the Precambrian North Atlantic craton. Thus, patterns for the opening of the North Atlantic may have been set by the geometry of the Grenvillian deformed zones.

Evidence against large-scale Carboniferous strike-slip faulting in the Appalachian–Caledonian orogen

Kent and Opdyke1 have argued that during the late Devonian and early Carboniferous, the eastern part of the northern Appalachians (Acadia) was situated ∼1,500 km further south relative to cratonic North America than it is at present, and that it moved to its present position sometime later in the Carboniferous (Fig. 1). Van der Voo and Scotese2 proposed an even greater displacement of 2,000 km which they attribute to motion between cratonic North America and Europe, Acadia being attached to Europe (Fig. 1). The argument is that palaeolatitudes (λp) derived palaeomagnetically from Upper Devonian and Lower Carboniferous rocks of cratonic North America are 15–20° more northerly than those of Acadia and Europe. The proposed shear zone passes through central Newfoundland. Using palaeomagnetic results from eastern and western Newfoundland we show here that no such motion occurred. We also show that Kiaman (late Carboniferous and Permian) overprinting is widespread in Newfoundland, and that these secondary magnetizations agree (with two exceptions attributed to dextral rotation of the Colorado Pleateau) with observations from late Devonian and early Carboniferous rocks of the North American craton, confirming the proposal3 that magnetizations of the cratonic early Carboniferous rocks are Kiaman, not early Carboniferous in age. Our results also enable us to extend this proposal3 to late Devonian rocks of the craton. Hence the palaeolatitudinal offset of Fig. 1 is almost certainly not tectonic, but is an artefact of the wrong assumption of the equivalence of rock and magnetization ages.

MOIST and the continuity of crustal reflector geometry along the Caledonian-Appalachian orogen

New aspects of the deep structure of the Scottish Caledonides are revealed by the Moine and Outer Isles deep seismic reflection traverse (MOIST). The Caledonian foreland is underlain by an easterly-dipping, strongly reflecting surface cutting through the Moho and traceable to more than 45 km depth. Thrusts within the foreland basement and Caledonian orogen have been reactivated as normal faults bounding half-grabens filled with sediments of late Palaeozoic to Mesozoic age. The Moine Thrust, which carries rocks of the orogen over Lewisian foreland, dips at 20–25° to the E on MOIST, and is either: (a) the westernmost of a series of easterly-dipping reflections (thrusts) which flatten or terminate at 17–20 km depth, or (b) a thrust lying further E which structurally overlies these easterly-dipping reflectors. By comparison with easterly-dipping reflections on COCORP lines in the northern and southern Appalachians the latter interpretation is preferred, implying that many of the easterly-dipping reflections on MOIST correspond to off-shelf metasedimentary rocks, with slivers of basement, stacked and imbricated against the Lewisian basement edge. Regional seismic refraction and conductivity data from northern Scotland suggest that regions of the Caledonian orogen have lower mid-crustal velocity gradients and higher conductivities than the foreland, which might be explained by this interpretation of the easterly-dipping reflections. Despite similarities in crustal reflector geometries, there are important differences between the MOIST and COCORP lines, including (1) a remarkably continuous, relatively horizontal Moho seen on MOIST data at about 25 km depth, and (2) the sedimentary basins offshore from northern Scotland which have formed by reactivation under crustal extension of the easterly-dipping thrusts. Furthermore, it is probable that the easterly-dipping reflectors in these areas of the MOIST and COCORP lines were formed at different times in the early Palaeozoic.

Timing and Development of the Orthotectonic Zone in the Appalachian Orogen of Northwest Newfoundland

Examination of part of the Fleur de Lys Supergroup, within the orthotectonic zone of north-west Newfoundland, reveals that regional deformation and metamorphism may not have commenced prior to the Early Ordovician and was not completed until the Devonian. The onset of deformation could have been as young as Llanvirnian.Fleur de Lys metasediments are correlated to the west with moderately deformed and undeformed rocks of the western platform. To the east, they are shown to be in part the lateral age equivalent of ophiolitic rocks. The ophiolites may be fragments of an older foundation to the mafic volcanic and volcanoclastic Baie Verte Group.Polyphase-deformed and regionally metamorphosed terrains (orthotectonic zones) such as the Fleur de Lys, do not form 'instantaneously', but evidently result from a critical sequential and progressive development history, covering a recognizable time span, and probably display characteristics of paratectonic zones at an intermediate stage. Sequential or simultaneous opening and closing of marginal basins is suggested as a mechanism, not only to explain the formation of such zones, but also the regional timing variation that apparently exists within the Appalachian-Caledonian orogen. Such processes are known to have occurred consistently since the Early Tertiary, and can presently be observed along the Western Pacific margin.

On the Origin and Deformation of the Fleur de Lys Metaconglomerate, Appalachian Fold Belt, Northwest Newfoundland

The results of a study of a deformed metaconglomerate from the Fleur de Lys Supergroup, which occupies the northwest metamorphic zone of the Appalachian–Caledonian orogen in northwest Newfoundland, are described.This Eocambrian conglomerate unconformably overlies a Precambrian basement complex and has previously been interpreted as a glacial deposit. Sedimentological evidence does not support such an interpretation, and the present study shows the conglomerate to be of probable fluviatile origin.The deformation of the deposit, reflected in the state of finite strain of the clasts, is clearly inhomogeneous on all scales. The clasts range from nondeformed to intensely deformed and their shapes vary from oblale to prolate ellipsoids. The finite strain is thought to be due to buckling and three-dimensional differential flattening at the basement-conglomerate interface.

Timing of Caledonian Evorogenic Activity in the Scandinavian Caledonides

RECENT discussion of the precise timing, extent and intensity of late Cambrian and early Ordovician movements and metamorphism in the Caledonian orogen and in Scandinavia in particular, has concentrated on graptolite faunal studies1–5 and radiometric age determinations6,7. In Norway, interest has centred on the West Finnmark and Trondheim regions following the revelation of an early Ordovician evorogenic event in Finnmark; in compiling evidence correlatable with this period of folding, intrusion and metamorphism, Sturt et al.6 tended to overemphasize the significance of the Trondhjem (Trondheim) disturbance8 so that later workers3,5,9 have mistakenly promoted this parorogenic episode to the status of an important deformation and metamorphism. Concern has been voiced in Scandinavian circles over this repeated misuse of Holtedahl's term8, which was originally used to define a local unconformity where a conglomerate overlies a greenstone sequence. Similar crustal movements recognized higher up in the stratigraphy, the Ekne and Horg disturbances of Vogt10, have analogues in minor breaks occurring in the Appalachian11 and British Caledonian12 Lower Palaeozoic successions. As the evolution of the Caledonian-Appalachian orogen is now being reassessed in the light of the plate tectonics philosophy13,14, it is timely to review aspects of the Norwegian evidence before further models are proposed.