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Geological Structures
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
In intensely-folded mountain regions such as the Alps, overthrusts associated with recumbent folds occur on a large scale. A recumbent fold driven forward on a thrust surface is called a nappe, or thrust-sheet; structures of this kind form the basis of an interpretation of the complex geology of the Alps (Fig. 2.17b). The figure illustrates the structural units along a line from Lausanne to Italy. The Swiss plain at Lausanne, betweeen the Jura Mountains and the Pre-Alps, is covered by thick coarse sediments known as molasse, mainly of Oligocene age, which were accumulated by rapid denudation of the rising mountain-mass to the east. West of the Pre-Alps the Jura Mountains show a series of upright broken anticlines and synclines that are believed to have slid westwards on a weak layer of Triassic rock-salt (TL in the figure). East of the Pre-Alps rises a pile of gigantic recumbent folds, now deeply eroded; the lower members are covered by higher and later-formed overfolds which have been moved for large distances as nappes. Relics detached from a nappe by later erosion are called nappe-outliers or klippen (the plural of klippe); the Pre-Alps are probably parts of such folds that have been thrust over the molasse. South-east of the Pre-Alps in the section are the High Calcareous Alps, composed of Mesozoic sediments. They are one of four tectonic units each of which is a group of nappes, and are named in ascending order Helvetid, Pennid, Grisonid, and Dinarid. The Helvetid nappes of the High Calcareous Alps are overridden by the Pennid group of nappes (numbered I to VI in the figure); the Matterhorn pyramid, with a thrust-plane at its base, is an erosion remnant of the Dent Blanche nappe (VI). Beyond the pile of Pennid nappes the rocks turn vertically downwards in a root-zone, beyond which the Dinaric structures show movements towards the south-east. The broad structure of this area is thus asymmetric rather than bilateral as in many orogenic belts (e.g. Fig. 7.6a).
Structure and landforms
Published in Richard J. Chorley, Stanley A. Schumm, David E. Sugden, Geomorphology, 2019
Richard J. Chorley, Stanley A. Schumm, David E. Sugden
Complex folding involves large-scale disharmonic features and the production of large asymmetrical or recumbent folds, often broken along their axial planes by thrusts of kilometres or tens of kilometres displacement, of which the European Alps are the prime example (Figure 7.17). The root zone of such folds (i.e. where the core of the recumbent anticline is seen in relation to the older basement) has usually been subjected to regional metamorphism (see Section 4.4). Autochthonous folds are connected with their appropriate root zone, whereas allochthonous ones have been separated from their roots by large-scale thrusting. These thrust sheets, commonly composed of the back limbs of recumbent folds, are termed nappes (French, ‘cover’; German equivalent: Decke), parts of which isolated by erosion are called klippe (German, ‘reef’) and erosional depressions through which are windows (German: Fenster) (Figure 7.18). Most mountain ranges which developed from the violent collapse of geosynclines exhibit complex folding with thrusts oriented outwards from the geosynclinal axis. However, most ranges show a dominant thrusting towards the marginal craton (i.e. foreland), and this is particularly so in the case of the European Alps which were formed by the collision of two crustal plates (Figure 7.19A). North of the root zone (Figures 7.17 and 7.19B) are, successively, the pile of recumbent folds of the Pennine nappes (i.e. Simplon, Great St Bernard, Monte Rosa and Dent Blanche); the Helvetic nappes, a complex thrusted mass of calcareous Mesozoic and Eocene rocks pierced by the massifs which are exposed upthrusted splinters of the crystalline basement; and the Pre-Alps, which are the leading-edges of the huge thrusted recumbent folds, where a series of smaller closely spaced subparallel imbricate thrusts characteristically occur. The huge size and distance of travel of the allochthonous Alpine nappes has led to their explanation, at any rate in part, by the concept of gravity tectonics (see also Chapter 10). This concerns the sliding of huge rock masses down the flank of a squeezed-up crustal belt under the action of gravity, rather than primarily by lateral thrusting from the rear.
Anatomy of an obducted ultramafic unit (Tiébaghi Massif – Peridotite Nappe – New Caledonia): Polyphase brittle tectonics constrained by fault-slip data and crack seal mineralogy
Published in New Zealand Journal of Geology and Geophysics, 2023
Pierre Maurizot, Bernard Robineau, Julie Jeanpert, Marion Iseppi, Stéphane Lesimple, Farid Juillot, Michael Meyer, Patrick Fullenwarth, Vincent Mardhel
In this paper, we present several new sets data collected in the Tiébaghi Massif, a klippe of the Peridotite Nappe located in the north of New Caledonia. These data were collected by investigating two different types of mining works: the underground works of the Chromical hypogene chromite deposit and the open-cast works of the Société Le Nickel (SLN) supergene nickel deposits. In nickel mine, ground electro-magnetic (EM) survey and drilling data have been used to constrain the geometry of the mineralisation. In both the hypogene chromite deposit and supergene nickel works, structural, micro-structural measurements and oriented samples have been systematically collected to determine the chronology and kinematic parameters of the structural evolution of the ultrabasic unit. Raman spectrometry and TEM microscopy provided additional information on the composition of the minerals associated with the different kinematic indicators.
Timing of deformation, metamorphism and leucogranite intrusion in the lower part of the Seve Nappe Complex in central Jämtland, Swedish Caledonides
Published in GFF, 2021
Yuan Li, David G. Gee, Anna Ladenberger, Håkan Sjöström
The upper 500 m of the Åreskutan mountain (Fig. 2) comprise a well exposed klippe of garnetiferous paragneisses and migmatites of the Åreskutan Nappe (Arnbom 1980) that have recently yielded evidence of UHPM metamorphism, with the presence of microdiamonds (Klonowska et al. 2017). A major shear zone separates these high grade rocks from underlying, lower grade psammitic metasedimentary rocks and amphibolites (Fig. 2) that are less well exposed and have provided the location of the COSC-1 drillhole. These lower parts of the SNC (the Lower Seve Nappes) include at least two thrust sheets, the main unit being referred to as the Fröå-Bjelke Nappe (Helfrich 1967), or the Kall Nappe (Yngström 1969). These authors recognized a basal part dominated by amphibolites and quartzites, passing up into more varied, often calcareous paragneisses, marbles and psammites. Amphibolitized dolerites and gabbros, and minor leucogranites have been recognized in these host rocks; locally, also ultramafic rocks. A sheet of granitic gneisses occurs in the basal part of the SNC in central Jämtland (Strömberg & Karis 1984), possibly of similar age as an orthogneiss at a comparable tectonostratigraphic level in Västerbotten dated to c. 1645 Ma (Zachrisson et al. 1996). Metamorphism appears not to have exceeded amphibolite facies (Arnbom 1980), with grossular, vesuvianite and wollastonite in the calcareous paragneisses. The presence of almandine and staurolite (kyanite reported, but not confirmed) in subordinate more pelitic gneisses suggests the possibility of a somewhat higher pressure history, but nothing comparable with that in the overthrust Åreskutan Nappe. Greenschist facies retrogression is widespread, particularly in shear zones.