Skip to main content

Magnetic anisotropy and X- ray diffraction study of clay minerals in the decollement horizons of the western Helvetic nappes, SW Switzerland

  • P. Dick (a1) and M. Burkhard (a1)

Anisotropy of magnetic susceptibility (AMS) and X-ray diffraction studies on shale decollement horizons in the Western Helvetic nappes were conducted to delineate the interactions between clay particles and deformation during a non-coaxial thrusting deformation. The reorientation of magnetic lineations in the decollement horizons, from a NW—SW thrusting direction to a NE—SW oriented lateral escape, provides the opportunity to study the evolution of magnetic fabrics and clay mineralogy during a two-step deformation history. The AMS of these shales originates from paramagnetic phyllosilicate minerals: illite, phengite and chlorite. Clay mineral studies indicate that magnetic lineations parallel to the thrust directions correspond to the spatial organization of the basal planes of phengites, whereas those parallel to the lateral escape movement are caused by illites. This work indicates that the change in direction of magnetic lineations from orogenic contraction to lateral escape not only records the strain history but also the dissolution and neoformation of paramagnetic minerals.

Corresponding author
Hide All
Aubourg, C., Rochette, P. & Vialon, P. (1990) Direction of transport revealed by the magnetic fabric of the subalpine Terres Noires, French Alps. C.R. Acad. Sci., Paris, 310, 1341–1346.
Aubourg, C., Rochette, P. & Vialon, P. (1991) Subtle stretching lineation revealed by magnetic fabric of Callovian-Oxfordian black shales (French Alps). Tectonophysics, 185, 211–223.
Bailey, S.W. (1972) Determination of chlorite compositions by X-ray spacings and intensities. Clays Clay Miner. 20, 381–388.
Borradaile, G.J. (1987) Anisotropy of magnetic susceptibility; rock composition versus strain. Tectonophysics, 138, 327–329.
Borradaile, G.J. & Henry, B. (1997) Tectonic applications of magnetic susceptibility and its anisotropy. Earth Sci. Rev. 42, 49–93.
Brindley, G.W. & Gillery, F.H. (1956) X-ray identification of chlorite species. Am. Miner. 41, 169–186.
Brown, G. (1955) The effect of isomorphous substitutions on the intensities of (001) reflections of micaand chlorite- type structures. Mineral. Mag. 30, 657–665.
Brown, G. & Brindley, G.W. (1980) X-ray diffraction procedures for clay mineral identification. Pp. 305–359 in. Crystal Structures of Clay Minerals and their X-ray Identification (Brown, G. & Brindley, G.W., editors). Monograph 5. Mineralogical Society, London.
Burkhard, M. (1988) L’Hélvetique de la bordure occidentale du massif de l’Aar (évolution tectonique et métamorphiqu e). Eclogae Geol. Helv. 81, 63–114.
Chagnon, A. & Desjardins, M. (1991) Determination of the composition of chlorite from X-ray diffraction and microanaly sis data. Canad. Mineral. 29, 245–254.
Dick, P. (2000) Tectonic transport directions in the Helvetic-Subalpine thrust system of the NW-Alpine arc. PhD thesis, Univ. Neuchâtel, Switzerland.
Dick, P. & Burkhard, M. (1999) Comparison between magnetic fabrics and paleostress from major decollement horizons in the Helvetic Nappes (SW Switzerland); implications for the formation of the Helvetic Arc. Eur. Union Geosci. Conf., EUG 10, 4, 626.
Esquevin, J. (1969) Influence de la composition chimique des illites sur leur cristallinité. Centre de Recherches de Pau (Societe Nationale des Petroles d’Aquitaine), Bulletin, 3, 147–153.
Goy-Eggenberger, D. (1998) Faible métamorphisme de la nappe de Morcles: minéralogie et géochimie. PhD thesis, Univ. Neuchâtel, Switzerland.
Hamilton, N. & Rees, A.I. (1970) The use of magnetic fabric in palaeocurrent estimation. Palaeogeophysics, 445–464.
Hirt, A.M., Lowrie, W., Clendenen, W.S. & Kligfield, R. (1988) The correlation of magnetic anisotropy with strain in the Chelmsford Formation of the Sudbury Basin, Ontario. Tectonophysics, 145, 177–189.
Hounslow, M.W. (1985) Magnetic fabric arising from paramagnetic phyllosilicate minerals in mudrocks. J. Geol. Soc. 142, 995–1006.
Housen, B.A. & Van der Pluijm, B.A. (1991) Slaty cleavage development and magnetic anisotropy fabrics. J. Geophys. Res. 96, 9937–9946.
Housen, B.A., Tobin, H.J., Labaume, P., Leitch, E.C., Maltman, A.J., Shipley, T., Ogawa, Y., Ashi, J., Blum, P., Brueckmann, W., Felice, F., Fisher, A., Goldberg, D., Henry, P., Jurado, M.J., Kastner, M., Laier, T., Meyer, A., Moore, J.C., Moore, G., Peacock, S., Rabaute, A., Steiger, T., Underwood, M., Xu, Y., Yin, H., Zheng, Y. & Zwart, G. (1996) Strain decoupling across the decollement of the Barbados accretionary prism. Geology, 24, 127–130.
Kligfield, R., Owens, W.H. & Lowrie, W. (1981) Magnetic susceptibility anisotropy, strain, and progressive deformation in Permian sediments from the Maritime Alps (France). Earth Planet. Sci. Lett. 55, 181–189.
Mosar, J. (1988) Metamorphisme transporte dans les Prealpes; Transported metamorphism in the Prealps. Schweiz. Mineral. Petrogr. Mitt. 68, 77–94.
Oinuma, K., Shimoda, S. & Sudo, T. (1972) Triangular diagrams in use of a survey of crystal chemistry of chlorites. Proc. Int. Clay Conf., Madrid, 161171.
Petruk, W. (1964) Determination of the heavy atom content in chlorite by means of the X-ray diffractometer. Am. Miner. 49, 61–71.
Ramsay, J. (1981) Tectonics of the Helvetic Nappes, Pp. 293–309 in. Thrust and Nappe Tectonics (McClay, K.R. & Price, N.J., editors). Spec. Publ. 9, Geological Society, London.
Rey, J.-P. & Kubler, B. (1983) Identification des micas des séries sédimentaires à partir de la série harmonique (001) des préparations orientées. Schweiz. Mineral. Petrogr. Mitt. 63, 13–36.
Rochette, P., Jackson, M. & Aubourg, C. (1992) Rock magnetism and the interpretation of anisotropy of magnetic susceptibility. Rev. Geophys. 30, 209–226.
Recommend this journal

Email your librarian or administrator to recommend adding this journal to your organisation's collection.

Clay Minerals
  • ISSN: 0009-8558
  • EISSN: 1471-8030
  • URL: /core/journals/clay-minerals
Please enter your name
Please enter a valid email address
Who would you like to send this to? *



Full text views

Total number of HTML views: 0
Total number of PDF views: 0 *
Loading metrics...

Abstract views

Total abstract views: 0 *
Loading metrics...

* Views captured on Cambridge Core between <date>. This data will be updated every 24 hours.

Usage data cannot currently be displayed