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| Titel |
Transition in magnetic fabric types in progressively deformed, fine-grained sedimentary rocks of Central Armorica (Brittany, France) |
| VerfasserIn |
Tom Haerinck, Ann M. Hirt, Timothy N. Debacker, Manuel Sintubin |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250089229
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| Publikation (Nr.) |
 EGU/EGU2014-3425.pdf |
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| Zusammenfassung |
| The anisotropy of magnetic susceptibility (AMS) of progressively deformed, fine-grained
sedimentary rocks is determined for different tectonometamorphic settings in Central
Armorica (Brittany, France). Low-temperature AMS and high-field torque magnetometry on
a representative selection of samples indicate that the magnetic fabric is dominantly
paramagnetic and the ferromagnetic (s.l.) contribution can be neglected. The AMS
documents a progressive transition of intermediate fabrics to tectonic fabrics and increasingly
stronger developed tectonic fabrics. An integrated magnetic-mineralogical approach is
performed in order to assess whether we can use this evolution as a quantitative indicator for
the intensity of cleavage development in Central Armorica.
During the magnetic fabric transition, the maximum susceptibility axis (K1) remains
stationary oriented parallel to the bedding–cleavage intersection, whereas the minimum
susceptibility axis (K3) orientation distribution changes from a moderate girdle distribution in
the intermediate fabric types, to a strongly clustered distribution parallel to the cleavage pole
for the tectonic fabric types. A Woodcock two-axis ratio plot is used to evaluate this change
in K3 distribution. This shows a regional pattern with intermediate fabrics in the
southern part of Central Armorica and tectonic fabrics in the northern part of Central
Armorica.
Quantitative analysis of the observed magnetic fabrics shows that the fabric transition
described above is accompanied by an evolution from prolate susceptibility ellipsoids with a
relatively low degree of anisotropy to oblate ellipsoid with an increasingly higher degree of
anisotropy. In a graph of the shape parameter T against the corrected degree of anisotropy PJ,
this evolution has a hockey-stick shaped pattern with the vertical branch reflecting the
actual transition from intermediate to tectonic fabric type and the horizontal branch
reflecting progressively stronger developed tectonic fabrics. The analysis confirms the
general northward increasing intensity of tectonic fabric development for Central
Armorica.
Thus, based on the K3 orientation distribution and PJ and T values, we could infer a
general magnetic fabric transition and an increasingly stronger tectonic fabric development,
which is in line with the overall geodynamic model of Central Armorica describing a
northward increasing strain gradient. However, exceptions to this general trend do occur with
both prolate intermediate fabrics in the northern part of Central Armorica, and oblate tectonic
fabrics in the southern part of Central Armorica – resulting in a sometimes relatively large
spread in AMS for samples from an area with a homogeneous tectonometamorphic history.
This suggests that the amount of strain is not the only major factor determining magnetic
fabric development, thus jeopardizing a quantitative AMS interpretation in terms of
strain.
Preliminary mineralogical analyses suggest that fabric transition and spread in PJ and T
values between samples of a single sampling area, affected by a homogeneous
amount of strain, reflect variations in mineralogy, i.e. the relative amount of quartz
versus platy clay minerals. This mineralogical influence on the AMS evolution is
currently being further investigated in order to get a better understanding of the
magnetic fabric transition in deformed, fine-grained sedimentary rocks and assess the
applicability of AMS as a (semi)quantitative strain marker in Central Armorica. |
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