Preprints
https://doi.org/10.5194/se-2021-27
https://doi.org/10.5194/se-2021-27

  31 Mar 2021

31 Mar 2021

Review status: this preprint is currently under review for the journal SE.

Elastic anisotropies of deformed upper crustal rocks in the Alps

Ruth Keppler1, Roman Vasin2, Michael Stipp3, Tomás Lokajícek4, Matej Petruzálek4, and Nikolaus Froitzheim1 Ruth Keppler et al.
  • 1Institute for Geosciences, University of Bonn, Poppelsdorfer Schloss, D-53115 Bonn, Germany
  • 2Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Joliot-Curie 6, 141980 Dubna, Russia
  • 3Institute for Geosciences and Geography, Von-Seckendorff-Platz 3, D-06120 Halle (Saale), Germany
  • 4Institute of Geology of the Czech Academy of Sciences, Rozvojova 269, 16000 Prague 6, Czech Republic

Abstract. The upper crust within collisional orogens is very heterogeneous both in composition and grade of deformation, leading to very variable physical properties at small scales. This yields difficulties for seismic investigations of tectonic structures at depth since local changes in elastic anisotropy cannot be detected. In this study, we show elastic anisotropies of the range of typical lithologies within deformed upper crustal rocks in the Alps. Furthermore, we aim to model average elastic anisotropies for these rocks and their changes with increasing depth due to the closure of microcracks. We therefore sampled rocks in the Adula Nappe of the central Alps, which is typical for upper crust in collisional orogens. The two major rock types found are orthogneisses and paragneisses, however, small lenses of metabasites and marbles also occur. Crystallographic preferred orientations (CPOs) and volume fractions of minerals in the samples were measured using time-of-flight neutron diffraction. Combined with single crystal elastic anisotropies these were used to model seismic properties of the rocks. The sample set shows a wide range of different seismic velocity patterns even within the same lithology, due to the heterogeneity of deformed upper crust. To approximate an average for these upper crustal units, we picked common CPO types of rock forming minerals within the gneiss samples, which represent the most common lithology. These data were used to determine an average elastic anisotropy of a typical upper crustal rock within the Alps. Average mineral volume percentages within the gneiss samples were used for the calculation. In addition, ultrasonic measurements of elastic anisotropies of the samples at increasing pressures were performed. These measurements, as well as the microcrack pattern determined in thin sections of the samples were used to model the closure of microcracks in the average sample at increasing depth. At ≈740 MPa microcracks are assumed to be closed yielding average elastic anisotropies of 4 % for the average gneiss. This value is an approximation, which can be helpful for seismic models at a lithospheric scale. At a crustal or smaller scale, however, it is an oversimplification and local lithological as well as deformational changes shown by the range of elastic anisotropies within the sample set have to be considered.

Ruth Keppler et al.

Status: open (until 13 Jul 2021)

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Ruth Keppler et al.

Ruth Keppler et al.

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Short summary
Rocks in mountain belts have been deformed during continental collision causing a certain alignment of the minerals referred to as crystallographic preferred orientation (CPO). Minerals have anisotropic properties: the velocity of seismic waves, travelling through them is direction dependent. This leads to anisotropy of the rocks. We measured the CPO of common rocks within the Alps. With this data and know anisotropic properties of the minerals we calculated the seismic anisotropy of the rocks.