Anwendbarkeit magnetischer Gefügemessungen für die Analyse des Deformationszustands von Karbonatgesteinen – Am Beispiel der östlichen Nördlichen Kalkalpen, Österreich

Translated title of the contribution: Applicability of petrophysical anisotropy patterns for the analysis of the deformation stage of carbonates - Using the example of the Eastern Alps, Austria

Karin Gruber

Research output: ThesisMaster's Thesis

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Abstract

Recent geodynamic investigations in the Eastern Alps and surrounding areas gave rise for large neotectonic movements, which are dated younger than 5 Ma. The main objective of the study involves petrophysical analyses of textures in Alpine rocks in order to observe changes of the regional deformation pattern. Measurements of anisotropy of magnetic susceptibility (AMS) were carried out on samples from 34 sites collected in the Eastern Alps. Samples spanning an age range from Upper Permian to Mesozoic were taken along a North-South transect from Scheibbs in the North to Kapfenberg in the South, comprising Helvetic and Penninic Flysh units, most of the Northern Calcareous Alps (NCA) nappes, as well as the Greywacke zone. AMS analysis was carried out on two to six sites per thrust sheet or nappe for a structural investigation of the relationship between magnetic fabrics and tectonic strain in the investigated area. Recent strain data of the Eastern Alps were used as the reference frame. This study focuses on the directional correlation of the AMS axes patterns. Isothermal remanent magnetization measurements and stepwise thermal demagnetization of 3-component isothermal remanent magnetization identify magnetite and haematite as well as goethite as the main magnetic minerals. Stepwise thermal demagnetization defines two magnetic directions carried by phases with low coercivity and different unblocking temperatures: C2, 250°C and C1, 580°C. Blockrotations (Middle – Late Miocene) within the NCA with values ranging from 2° to 57° were recorded by the remanence vector C2. The remanence vector C1 also documents an older blockrotation with higher rotation values than C2 but the remagnetization- and rotation-ages cannot be given due to unsufficient data quality. Particularly within the NCA, AMS is inapplicable on most rocks for strain analysis as they are diamagnetic and/or isotropic. Even so, some sites are anisotropic and yield oblate and triaxial magnetic fabrics. In some cases even tendency to prolate magnetic fabrics can be observed. In the Flysh units and Greywacke zone, the AMS principal axes document a weaker tectonic deformation than in the NCA. All sampled lithologies of the Flysh units and Greywacke zone yield well defined AMS patterns whereas in the NCA only carbonatic sediments, which were accumulated in sub-oxic and reducing environments with terrigenous influence, are suitable. The orientations of the susceptibility tensors can be correlated to strain directions of three different tectonic phases from Early Eocene to Middle Miocene. The magnetic fabrics of the Flysh units indicate a relationship to an Early Eocene NW-SE compression but direct comparison is not possible due to lack of strain data in the Flysh units. Within the NCA, after restoration of the pre-Miocene situation kmax displays a more suitable correlation with σ3 of a Middle Miocene NE-directed compression. Within the NCA nappes the magnetic fabrics were mainly influenced from reverse faults and at the border to the Greywacke zone from sinistral strike-slip faults. The magnetic fabrics of the Greywacke zone can be related with an Early to Middle Miocene N-directed compression. Another group represents a tectonic block with similar magnetic fabric orientation as the NCA. The bulk susceptibility reaches its highest values in these samples observed although paramagnetic minerals are the main carriers of the susceptibility.
Translated title of the contributionApplicability of petrophysical anisotropy patterns for the analysis of the deformation stage of carbonates - Using the example of the Eastern Alps, Austria
Original languageGerman
QualificationDipl.-Ing.
Supervisors/Advisors
  • Scholger, Robert, Supervisor (internal)
Award date17 Dec 2010
Publication statusPublished - 2010

Bibliographical note

embargoed until null

Keywords

  • Eastern Alps
  • magnetic fabrics
  • anisotropy of magnetic susceptibility
  • geodynamics
  • strain
  • remagnetization
  • IRM
  • neotectonics

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