Keppler, Ruth (2015) Deformation processes and petrophysical properties of subduction channel rocks from the Eclogite Zone of the Tauern Window, Austria. (PhD/ Doctoral thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 136 pp.

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Abstract

The exhumation of high pressure metamorphic rocks at convergent plate margins can be explained by the existence of a so-called subduction channel, a shear zone of a few kilometer thickness above the downgoing lithospheric plate, in which material is transported deep into the mantle and exhumed back to crustal depth. The exact processes taking place within these highly dynamic shear zones, however, are still not completely understood. The Eclogite Zone (EZ) of the Tauern Window is an exhumed subduction channel of the Alpine Orogen and provides the opportunity to study subduction channel rocks at the surface. It forms a typical melange consisting of eclogite lenses in a matrix of high pressure metasediments. Crystallographic preferred orientation (CPO) analysis of rocks from the EZ offers the possibility to calculate their elastic properties. This data is valuable for seismic imaging of subduction channels at convergent plate margins active today. CPO analysis of rocks from the EZ furthermore allows drawing conclusions on deformational processes and the tectonic history of subduction channels. This PhD thesis provides CPO data of high pressure subduction channel rocks from the EZ. It gives an overview on the reliability of time-of-flight (tof) neutron diffraction as a method for CPO analysis and uses the obtained data to calculate rock physical properties and to provide a detailed deformational history of subduction channel rocks. In the first study, Rietveld texture analysis is performed on rock samples of different complexity, using tof neutron diffraction at the SKAT diffractometer in Dubna, Russia. The recently upgraded SKAT provides three interchangeable multidetector-systems, offering the opportunity to use the optimum experimental set-up for different samples. This study illustrates that it is possible to obtain reliable CPO results for mineral phases constituting more than 10 vol.% in complex polymineralic rock samples. A comparison of CPO results with reference pole figures showed coinciding maxima for multiples of a random distribution of crystal lattice planes and similar texture strength, which indicates that the acquired data is suitable for the calculation of rock physical properties, as well as for geological interpretations. This study further showed that the application of full pattern fit methods allows a vast reduction in the number of tof spectra used for CPO calculation. 150 spectra for monomineralic samples and 350 spectra for samples with medium complexity are sufficient. This shows that a vast reduction in measuring positions is possible without loss of information. In the second study, the CPO of fresh and retrogressed eclogites, as well as metasediments, from the EZ was used to calculate elastic properties of a complete set of subduction channel rocks. The elastic anisotropy of fresh eclogites is fairly low (~1.5%), but increases for progressively higher grades of retrogression (up to 3.7%). While in fresh eclogites anisotropy is mainly determined by a distinct omphacite CPO, in retrogressed samples the elastic properties are additionally influenced by the CPO of retrograde amphibole. Elastic anisotropies of metasediments in the subduction channel are much higher due to a pronounced quartz and mica CPO (up to 7.5%). Vp/Vs ratios of metasediments (~1.5), as well as eclogites with different grades of retrogression (1.70-1.78) yield distinctly different values. The combination of P-wave velocity, elastic anisotropy and Vp/Vs ratios is specific to each rock type found in the EZ and in the future, these results could simplify the detection of internal structures in subduction channels of active subduction zones at depth. The third study applies CPO and microstructural analyses to the same rocks investigated in the second study to determine the deformational history of the EZ and subduction channel rocks, in general. Electron backscatter diffraction (EBSD) analysis revealed signs of dynamic recrystallization of omphacite in the eclogites during their final stage of subduction. CPO analysis of retrogressed eclogites showed a complex deformation of the rocks within the subduction channel during exhumation. Although plain strain was dominant, locally prolate as well as oblate strain conditions occurred. These local strain variations were consistent from eclogite facies to blueschist fades conditions, as confirmed by matching omphacite and glaucophane CPO in all eclogite samples. Asymmetric quartz CPO in the metasediments points to simple shear. Deformation of the sedimentary matrix was more pervasive and outlasted deformation of the eclogite lenses, which was confirmed by lower silica contents of mica in the metasediments compared to the eclogites. This study elaborates the deformational history of the EZ, illustrating the complexity of processes taking place in subduction channels.

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