HR: 15:00h
AN: GP73A-05 INVITED     [Abstracts]
TI: Tracking the Deformation Along a Shear Zone Using Anisotropy of Magnetic Susceptibility
AU: * Almqvist, B S
EM: bjarne.almqvist@mag.ig.erdw.ethz.ch
AF: ETH Zuerich, Institut fuer Geophysik NO H27 Sonneggstrasse 5, Zuerich, 8092, Switzerland
AU: Hirt, A M
EM: hirt@mag.ig.erdw.ethz.ch
AF: ETH Zuerich, Institut fuer Geophysik NO H27 Sonneggstrasse 5, Zuerich, 8092, Switzerland
AU: Herwegh, M
EM: herwegh@geo.unibe.ch
AF: Universitaet Bern, Institut für Geologie Baltzerstrasse 1-3, Bern, 3012, Switzerland
AU: Leiss, B
EM: bleiss1@gwdg.de
AF: Universitaet Goettingen, Geoscience Centre of the University of Goettingen Goldschmidtstrasse 3, Goettingen, 37077, Germany
AB: Magnetic sub-fabrics (i.e., diamagnetic, paramagnetic) are investigated for a set of calcite mylonites, collected along a large-scale shear zone (Morcles nappe) in southwest Switzerland, adopting the method described by Schmidt et al. (2007: GJI, 168, 40 - 47). The mylonites consist mainly of matrix calcite, where second phases (i.e. minerals other than calcite) make up less than 5 percent of the total volume. The most important second phases from an AMS perspective are the paramagnetic minerals white mica, chlorite and pyrite. Substitution of paramagnetic iron and manganese in the calcite lattice also occurs. Rock magnetic tests infer that the ferromagnetic content is negligible. The crystallographic preferred orientation (CPO) for the samples was measured with x-ray diffraction goniometry (XRD) and electron backscatter diffraction (EBSD). Generally CPOs show c-axes maxima perpendicular to the shear plane and a-axes concentration along a great circle sub- parallel to the shear plane. With decreasing metamorphic grade the CPO strength decreases. The AMS measured in low- and high-fields, at room temperature, display both normal and inverse magnetic fabrics, with susceptibility shape ellipsoids in the oblate and prolate fields. An inverse magnetic fabric is predominant when the AMS for the mylonites is measured at 77 K, with shape ellipsoids in the prolate field. A relationship between CPO and AMS is not obvious from these results. However, from the separated magnetic sub-fabrics it is possible to establish a quantitative relationship between AMS and CPO. The paramagnetic sub-fabric is related to the specimen bulk susceptibility (i.e., due to sample chemistry), whereas the diamagnetic sub-fabric agrees with the calcite CPO. The strongest CPO, observed in the root zone of the nappe where metamorphic grade is highest (greenschist facies, ~395C) and displacement is large, give a separated diamagnetic anisotropy with site mean k1 - k3 = 4.38E-7 +/- 1.00E-7 (2sigma) SI. The weakest CPO, near the front of the shear zone where metamorphic conditions were sub-greenschist facies (~280C), similarly correspond to a lower separated diamagnetic anisotropy, with site mean k1 - k3 = 2.35E-7 +/- 6.62E-8 (2sigma) SI. Intermediate magnetic fabrics, resulting from the combination of paramagnetic and diamagnetic sub-fabrics, are difficult to interpret but can be resolved through successful separation. In our case the separated diamagnetic sub-fabric is a quantitative representation of the calcite CPO. Separation of magnetic sub-fabrics can in general be performed when the details of matrix mineral and second phases are known.
DE: 1518 Magnetic fabrics and anisotropy
DE: 8012 High strain deformation zones
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2009 Joint Assembly