Behrmann, Jan

Keppler, Ruth

Stipp, Michael

Ullemeyer, Klaus

English

Behrmann, Jan H.

OceanRep

Crystallographic preferred orientation and elastic anisotropy of high-pressure rocks from the Eclogite Zone of the Tauern Window, Austria

Geophysical Research AbstractsVol. 15, EGU2013-5322, 2013EGU General Assembly 2013© Author(s) 2013. CC Attribution 3.0 License.Crystallographic preferred orientation and elastic anisotropy ofhigh-pressure rocks from the Eclogite Zone of the Tauern Window,AustriaRuth Keppler (1), Klaus Ullemeyer (1), Jan Behrmann (2), and Michael Stipp (2)(1) Institute of Geosciences, University of Kiel, Germany (rkeppler@geomar.de), (2) GEOMAR, Helmholz Centre for OceanResearch, Kiel, GermanyHigh-resolution seismic imaging of the internal structures in subduction channels has so far not been possible.In part, this is due to a lack of knowledge regarding elastic wave velocities and their anisotropy, and textures(crystallographic preferred orientation, or CPO) of the rocks making up subduction channel fills. These arelikely metabasalts of oceanic origin, and metamorphosed deep-sea sediments. All these rock types containpolyphase mineral assemblages, and CPO is difficult to obtain when dealing with polyphase rocks, which are thecommon case in subduction channels. In this study, the mineral CPO of high pressure rocks was determined fromtime-of-flight neutron spectra applying full pattern fit method (’Rietveld texture analysis’). With this method, it ispossible to investigate polyphase samples, since the CPOs of all mineral phases can be determined simultaneouslydespite overlapping Bragg reflections in the spectra. From the CPO, 3D models for P-wave velocity anisotropieswere calculated. This was done using the Christoffel equation, which considers the measured CPO and the singlecrystal elastic constants and density of the constituent phases.Since subduction channels are not directly accessible, samples were collected in the Eclogite Zone of the TauernWindow, Austria. This is a paleo-subduction channel of the Alpine orogen, which originally formed in the Tertiaryduring subduction of the Penninic ocean beneath the Adriatic continent. It comprises eclogites, blueschists andgreenschists, as well as micaschists, marbles and quarzites. The rocks have been exposed to P-T-conditions of20-25 kbar and 600 +/- 30◦C, and were exhumed in a very short time span of 1-2 Ma. A set of metasediments andmetabasites with well-developed foliation were chosen for the study.Here, we present the results of CPO analyses and bulk rock elastic anisotropies predicted from the CPO.In the eclogites, the CPO of omphacite, which is the main constituent phase (∼40%), is well pronounced.Omphacite likewise determines the maximum P-wave velocity of the eclogites, leading to a maximum anisotropyof about 3%. The CPOs of micaschists display a clear foliation and lineation. Accordingly, slowest velocitiesare normal to the foliation plane. Maximum velocity anisotropy of the micaschists is about 7%. These resultsallow conclusions on seismic properties of rocks in subduction zones, as well as deeper structural levels of the Alps.

Crystallographic preferred orientation and elastic anisotropy of high-pressure rocks from the Eclogite Zone of the TauernWindow, Austria

Crystallographic preferred orientation and elastic anisotropy of high pressure rocks from the Eclogite Zone of the Tauern Window, Austria

Crystallographic preferred orientation and elastic anisotropy of high-pressure rocks from the Eclogite Zone of the TauernWindow, Austria

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