Deutsche Tagung f ¨ur Forschung mit ... - SNI-Portal

Magnetismus Poster: Do., 13:00–15:30 D-P222
Correlation between magnetic fabric and chlorite preferred orientation in
clay sediments: new insights from neutron diffraction analysis (TEX-2)
Francesca Cifelli 1 , Sabine Lenser 2 , Massimo Mattei 1 , Martin Chadima 3 ,
Heinz-Günter Brokmeier 2
1 Dipartimento Scienze Geologiche, Università Roma TRE – 2 Institute of Materials
Science and Engineering, Technische Universität Clausthal – 3 Institute of Geology,
Academy of Sciences of the Czech Republic
In poorly consolidated fine-grained sediments syn-deformation fabric may not be easily
detected for their scarce potential in transmitting the differential stress accumulated
during tectonic deformation. Hence, in some sedimentary basins where sediments show
no evident pervasive tectonic structures conventional structural methods cannot be applied.
The question is whether it is possible to find any qualitative, or even quantitative,
record of deformation in these sediments, before brittle failure occurs. In this study,
an integrated approach of magnetic and mineral fabric investigations is proposed as a
possible alternative tool for detecting grain scale and regional deformation patterns in
weakly deformed sediments, where macroscopic evidence of deformation is not visible.
We first discriminated between the fabric arising from the ferrimagnetic fraction with
that from the paramagnetic matrix in order to understand which minerals contribute
to the susceptibility and which fraction is responsible for the orientation of the principal
axes of the AMS ellipsoid. Results show the paramagnetic minerals, i.e. chlorite,
are the main contributors to the magnetic susceptibility. The chlorite preferred orientations
were then quantified by neutron pole figure measurements, using the TEX-2
in GKSS-Research Centre Geesthacht, Germany. Results show that neutron texture
analysis represents a unique tool for mineral fabric investigation in clays. In fact, the
high penetration of neutron beam and the possibility to utilize the same cylindrical
specimen used for AMS analysis without any specimen preparation, make this method
particularly suitable in detecting the mineral fabric in sediments, such as clays, whose
original mineral fabric may be easily modified by specimen cutting and polishing. Magnetic
results were compared with mineral results in order to independently determine
the rock fabric and to establish a quantitative relationship between AMS and the preferred
orientation of minerals carrying the magnetic fabric. Chlorite distribution is
well correlated with the orientation of the high-field paramagnetic (HPF) and low-field
(at room temperature RT and low temperature LT) AMS ellipsoids. The magnetic
lineations (square symbols) are always perpendicular to the girdle distributions of the
chlorite c-axes, which suggests that it is an apparent lineation. Therefore results indicate
that the magnetic fabric is carried by chlorite and that its spatial arrangement has
been modified in the strain field acting at regional scale and detected in other contiguous
sediments. In this study, well show results from Neogene sedimentary sequences
outcropping in the Rif chain (Morocco) and in Calabria (Southern Italy).