The Brenner base tunnel (BBT) natural laboratory – From cross-section construction over fabric and elastic anisotropy analysis to 4D structural modeling

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The railway tunnel in the Brenner pass area (Brenner Basistunnel, BBT), which is currently under construction and is planned to reach a total length of 64 km, provides a unique geological view into the Eastern Alps. The tunnel transect crosses the western side of the antiformal nappe stack of the Tauern window including Austroalpine, Penninic, and Sub-Penninic units as well as basement of the Southern Alps. Hence, this transect covers most of the major tectonic units of the Eastern Alps, approximately 25 km west of the TRANSALP seismic cross section. To better understand the tectonic architecture and development, and to correlate the upper crustal structure with geophysical imaging results, we plan to combine geological surface information, seismic, petrophysical, microstructural and geochronological data, as well as tectonic modeling, using the software 3D MOVE.The BBT offers the unique possibility to get subsurface information of a very long cross section, including the opportunity to investigate carefully the available drill cores that cover most of the BBT transect. Drill core scanning and description will be combined with microfabric studies that apply neutron diffraction texture analysis and Vp/Vs-anisotropy modeling. The structural data provide information about strain localization as well as fault and shear zone formation in this long crustal section, which cannot be done by surface investigation in any close by resolution. Additional geological mapping and tunnel data compilation allows to construct the geological cross-section in the highest detail. Based on current tectonic models of the exhumation of the Western Tauern window, the activity of related fault zones in the area can be tested using 3D MOVE. A detailed analysis of all the major structures, correlation between the surface and the tunnel, and 2D-, 3D-modeling may help to identify crucial structures that may or may not indicate changing tectonic transport directions due to a subduction polarity change. Microfabrics and petrophysical data and especially the elastic anisotropy results from the drill core scanning and logging as well as from the texture modeling and the triaxial experiments, will be included in the cross section construction and the modeling with 3D MOVE. Based on our modeling and our laboratory data and the available time constraints we finally intend to image the patterns of our structural, microstructural and petrophysical parameters of the upper crustal BBT section in both, space and time. Furthermore, our elastic anisotropy results can be implemented into the processing of the seismic data and other deep sounding imaging techniques collected in the framework of the SPP 4D-MB and the entire AlpArray project. In that we also contribute to the interpretation of the deeper crustal investigation that focus on the anatomy of the lithosphere and any indication of a reversal in subduction polarity in the western Tauern area.