Geophysik

History

1. Intersecting remanence small circles:

McClelland-Brown (1983) was the first to shift synfolding remanences along their remanence small circles until they came to better grouping. Surmont et al. (1990) noted that remanences tilted in different directions converge toward a common direction when applying partial tilt correction. Shipunov (1997) stated explicitely that a synfolding remanence should lie on a particular small circle on which the vector moves during tilting, and that the small circles of different remanence vectors tilted in different directions should intersect in a point (statistically a narrow region), which indicates the Earth´s magnetic field during their acquisition.

Probably all three authors cited above found their observations independently. The fundamentally new - yet mostly unrecognized - aspect is, that the approach works with synfolding and prefolding remanences, thus independently from the remanence character (as long as the remanences belong to the same paleofield). Small circle intersections can be used to cross-check tilt correction / fold tests by a method which is geometrically alternative.

Shipunov (1997) was the first to publish a practical method with equations to determine the center of a number of intersecting small circles. Following applications or variations of this method have been called "optimal differential untilting" (Enkin et. al 2000, Enkin et al. 2002).

2. Small circle distribution of folded remanences:

Treatments of small circle distributions have been developed theoretically (Mardia and Gadsden 1977, Gray et al. 1980) and possible small circle distributions of remanences have been reported by various authors (Mardia and Gadsden 1977, MacDonald 1980, Ménard and Rochette 1992). Fisher et al. (1987) in their textbook still concluded that, with a few exceptions, small circle distributions will probably not be realised in nature. Crouzet et. al. (1996) first reported a small circle distribution of remanences and related it to an overall folding in the region. Eventually, Shipunov (1997) was the first to theoretically state that synfolding remanence vectors from differently tilted sites must lie on small circles with poles parallel to the fold axis. Waldhör (1999)* and Waldhör et al. (2001) discribed the directional and geometric properties of remanences when tilted to different directions. Small-circle distributions are in fact occurring in all directionally folded rocks. However, they also may occur in single sites and even in single specimens that carry various components. Further small-circle distributions have been discribed by Schill et al. (2001), Schill et al. (2002), Schill et al. (2003).

3. Small-circle reconstruction:

Once paleofield direction and remanence age are known (from tilt correction, E-W tilted remanences, small circle intersections or an APWP), a remanence can be tilted back to reach its expected inclination. In this way, the vertical-axis rotation is determined also for synfolding remanences (Waldhör 1999 and Waldhör et al. 2001) and fold geometries can be reconstructed for the time of remanence acquisition.

(*) Until 2000 (time of submission of the published paper Waldhör et al. 2001), the authors did not know the papers of McClelland-Brown (1983), Surmont (1990) and Shipunov (1997) and have not thoroughly read the paper of McFadden (1998).

References:

  • Cairanne, C., Aubourg, C. & Pozzi, J.P., 2002. Syn-folding remagnetization and the significance of the small circle test: Examples from the Vocontian trough (SE France), Physics and Chemistry of the Earth, 27, 1151-1159.

  • Delaunay, S., Smith, B. & Aubourg, C., 2002. Asymmetrical fold test in the case of overfolding: two examples from the Makran accretionary prism (Southern Iran), Physics and Chemistry of the Earth, 27, 1195-1203.

  • Crouzet, C., Menard, G. & Rochette, P., 1996. Post-Middle Miocene rotations recorded in the Bourg d'Oisans area (Western Alps, France) by paleomagnetism, Tectonophysics, 263, 137-148.

  • Enkin, R.J., Osadetz, K.G., Baker, J. & Kisilevsky, D., 2000. Orogenic remagnetizations in the front ranges and inner foothills of the Southern Canadian Cordillera: chemical harbinger and thermal handmaiden of Cordilleran deformation, GSA Bull., 112, 929-942.

  • Enkin, R.J.; Mahoney, J. B.; Baker, J.; Kiessling, M.; Haugerud, R.A.Syntectonic remagnetization in the southern Methow block: Resolving large displacements in the southern Canadian Cordillera, Tectonics, 21(4), 10.1029/2001TC001294, 1-17, 2002

  • Fisher, N.I., Lewis, T. & Embleton B.J.J., 1987. Statistical analysis of spherical data. Cambridge University Press, New York, 329pp.

  • Gray, N.H., Geiser, P.A. and Geiser, J.R., 1980. On the least-squares fit of small and great circles to spherically projected orientation data, J. Math. Geol., 12, 173-184.

  • Henry, B., Rouvier, H., Le Goff, M., Leach, D., Macquar, J.-C., Thibieroz, J. & Lewchuk, M.T., 2001. Palaeomagnetic dating of widespread remagnetization on the southeastern border of the French Massif Central and implications for fluid flow and Mississippi Valley-type mineralization. Geophys. J. Int., 145, 368-380.

  • Kirschvink, J.L., 1980. The least-squares line and plane and the analysis of palaeomagnetic data, Geophys. J. R. Astron. Soc., 62, 699-718.

  • MacDonald, W.D., 1980. Net tectonic rotation, apparent tectonic rotation, and the structural tilt correction in paleomagnetic studies, J. Geophys. Res., Vol. 85, No. B7, 3659-3669.

  • Mardia, K.V. and Gadsden, R.J., 1977. A small circle of best fit for spherical data and areas of vulcanism, Appl. Statist., 26, 238-245.

  • McClelland-Brown,E., 1983. Palaeomagnetic studies of fold development and propagation in the Pembrokeshire old red sandstone. Tectonophys., 98, 131-149.

  • McFadden, P.L., 1998. The fold test as an analytical tool, Geophys. J. Int., 135, 329-338.

  • Menard, G. and Rochette, P., 1992. Utilisation de reaimantations postmetamorphiques pour une etude de l'evolution tectonique et thermique tardive dans les Alpes occidentales (France). Bulletin de la Société Géologique de France, 163. Jg., Nr. 4, S. 381-392.

  • Schill, E., Appel, E., Zeh, O., Singh, V.K. & Gautam, P., 2001, Coupling of late-orogenic tectonics and secondary pyrrhotite remanences: towards a separation of different rotation processes and quantification of rotational underthrusting in the western Himalaya (northern India), Tectonophysics, 337, 1-21.

  • Schill, E., Appel, E., Gautam, P. & Dietrich, P., 2002, Thermo-tectonic history of the Tethyan Himalayas deduced from the palaeomagnetic record of metacarbonates from Shiar Khola (Central Nepal), J. Asian Earth Sci., 20, 203-210.

  • Schill, E., Appel, E., Godin, L., Crouzet, C., Gautam, P. & Regmi, K.R., 2003, Record of deformation by secondary magnetic remanences and magnetic anisotropy in the Nar/Phu valley (central Himalaya), Tectonophysics, 377, 197-209.

  • Shipunov, S.V., 1997. Synfolding magnetization: detection, testing and geological applications. Geophys. J. Int., 130, 405-410.

  • Surmont, J., Sandulescu, M., & Bordea, S. (1990). Mise en évidence d'une réanimation fini crétacée des séries mésozoïques de l'unité de Bihor (Monts Apuseni, Roumanie) et de sa rotation horaire ultérieure. Comptes rendus de l'Académie des sciences. Série 2, Mécanique, Physique, Chimie, Sciences de l'univers, Sciences de la Terre, 310(3), 213-219.

  • Waldhör, M., 1999. The small-circle reconstruction in palaeomagnetism and its application to palaeomagnetic data from the Pamirs. In: Frisch, W., Kuhlemann, J. (Eds.). Tübinger Geowissenschaftliche Arbeiten, PhD-Thesis 45. Universität Tübingen, Germany, 99 p.

  • Waldhör, M., Appel, E., Frisch, W. & Patzelt, A., 2001. Palaeomagnetic investigation in the Pamirs and its tectonic implications. J. Asian Earth Sci., 19, 429-451.