Open Session - SWISS GEOSCIENCE MEETINGs

Figure 1. Transtensional to extensional brittle evolution of the Lepontine dome with relative chronology between the different states of
stress recorded, showing a rotation of around 90° of the extensional axes. Mean direction of σ3 axes calculated from previous studies on
brittle deformation in the extended Central Alps are reported as follow: Bi: Bistacchi and Massironi (2000); Ch: Champagnac et al (2006); Ci:
Ciancaleoni and Marquer (2008); Gr: Grosjean et al (2004).
REFERENCES
Allanic, C., et al. (2006), A paleo-seismogenic Lepontine dome? New insights from pseudotachylytes-generating faults,
Geochimica et Cosmochimica Acta, Vol. 70 (18), Suppl. 1, p9 Goldschmidt conf. Melbourne.
Bistacchi, A., and Massironi M. (2000), Post-nappe brittle tectonics and kinematic evolution of the north-western Alps: an
integrated approach, Tectonophysics, 327, 267-292.
Champagnac, J. D., Sue, C., Delacou, B., Burkhard, M. (2006), Miocene orogen-parallel extension in the inner Western Alps
revealed by dynamical fault analyses, Tectonics, 25, 1-26.
Ciancaleoni, L., and Marquer D. (2008), Late Oligocene to early Miocene lateral extrusion at the eastern borders of the
Lepontine dome of the Central Alps (Bergell and Insubric areas, Eastern Central Alps), Tectonics, 27, TC4008, doi:10.1029/
2007TC002196, 2008.
Grosjean, G., Sue, C., Burkhard, M. (2004), Late Neogene brittle extension in the vicinity of the Simplon fault zone, central
Alps, Switzerland., Eclogae Geologicae Helvetiae, 97, 33-46.
1.2
Origin of the magnetic anisotropy in a two marble lithologies from the
Morcles Nappe shear zone
Almqvist Bjarne*, Hirt Ann*, Herwegh Marco**
*Institut für Geophysik, ETH Zurich, Schaffmattstrasse 30, CH-8093 Zurich (bjarne.almqvist@mag.ig.erdw.ethz.ch)
**Institut für Geologie, Universität Bern, Baltzerstrasse 1-3, CH-3012 Bern
Anisotropy of magnetic susceptibility (AMS) arises from the sum of ferromagnetic (sensu lato), paramagnetic and diamagnetic
minerals in a rock. The AMS is generally controlled by the ferromagnetic and paramagnetic phases, since these have the
strongest magnetic susceptibility. However, when the ferromagnetic contribution to the AMS is negligible, it may be possible
to separate the paramagnetic and diamagnetic fabrics using high-field magnetic-torque measurements at room temperature
and 77K. This is illustrated in a study of two marble lithologies from the Morcles Nappe shear zone. The AMS of the marbles
are compared with that of a set of quartzites from the Navia-Alto Sil slate belt in northwestern Spain, which have been studied
using the same technique.
From the room temperature AMS studied in a weak magnetic field it is possible to distinguish each marble based on its bulk
susceptibility and degree of anisotropy. Texture mapping using electron backscatter diffraction (EBSD) shows that the AMS
is controlled by the microtexture of the marbles. High-field AMS measurements, performed at 77K, produce an anisotropy
that is in some cases more than 100 times stronger than that measured at room temperature, indicating that paramagnetic
phases contribute to the total AMS. Separation of the paramagnetic and diamagnetic fabrics shows an inversion of the AMS
ellipsoid, from oblate and triaxial shapes at room temperature to strongly prolate shape at 77K. This is commonly seen in
marbles from the shear and root zones of the Morcles Nappe, and can be attributed to the presence paramagnetic cations
(e.g. Fe 2+ and Mn 2+ ) in the calcite crystal lattice (Figure 1). Further texture mapping together with determination of the chemical
composition of the calcites and secondary phases in the marbles should help identify the origin of their magnetic
anisotropy.
In contrast, the diamagnetic quartzites have similar bulk susceptibility as the marbles, but are only a few times more anisotropic
at low temperature as compared to the room temperature measurements. This suggests that the quartzites have an
AMS with a significantly lower paramagnetic contribution, and that the inherent AMS of the quartzite is less than that of
the marble.
13
Symposium 1: Structural Geology, Tectonics and Geodynamics