GNGTS 2017 - 36° Convegno Nazionale

564 GNGTS 2017 S essione 3.1 the eastern Alps and the slab geometry at the transition to the Pannonian realm are still under debate. Several studies, e.g. Lippitsch et al. (2003), Kissling et al. (2006), Mitterbauer et al. (2011), constructed tomographic images of the upper mantle between 60 km and 500 km depth (Fig.1). They found a steeply to vertically dipping “shallow slab” below the eastern Alps down to a depth of ~250 km, interpreted as European lower lithosphere detached from the crust and subducted during post-collision convergence between Adria and Europe. Between 350 km and 400 km depth, a “deep slab” extends from below the central easternAlps to under the Pannonian realm, interpreted as subducted lithosphere of the Alpine Tethys. At greater depth, there is a continuous transition to the high velocity anomaly above the 670 km discontinuity. Due to the sparse station network at that time typical resolution in the upper-most mantle has been on the order of 70 x 70 x 30 km (lat x lon x depth). Based on these results the SWATH-D experiment will allow testing the hypothesis that re-organizations of Earth’s mantle during the collision of tectonic plates have both immediate and long-lasting effects on earthquake distribution, crustal motion and landscape evolution in mountain belts. This aim can only be achieved by integrating geophysical 3D images of the entire crust- mantle system with geologic observations and modelling in time, the 4th dimension. The primary target of the AlpArray network is imaging the large-scale configuration of the slabs and the variation of crustal and mantle properties throughout the whole Alps. However, the average station spacing of 40+ km in the AlpArray “backbone” network is not sufficient to image the details of the collision in the central zone. As generally, at least one station is required within the epi-central distance, this is also not enough to determine reliably the absolute depths of earthquakes <~15 km deep. Furthermore, due to the obliqueness of Ps conversion phases used in receiver function imaging, rays from neighbouring AlpArray network stations only start to overlap at a depth of ~65 km, i.e. too deep to image reliably the 3D geometry of crust-mantle transition and the shallowest slab geometry. Finally, the relatively small scale of geological variation in the central and eastern Alps requires denser spatial sampling than provided by the AlpArray network to model the 3D crustal structure adequately. An example for this is the Tauern Window, a block of European derived crustal units surrounded by African derived units only ~40 km wide in N-S direction. The SWATH-D experiment focuses on the provision of seismic data from a dense seismic network in the Central and EasternAlps. This dense deployment of 154 stations will complement the larger-scale AlpArray “backbone” network (AlpArray, 2016). SWATH-D will provide high- Fig. 1 - Structure of the upper mantle – tomographic P-velocity depth slice between 135 and 165 km depth (after Lippitsch, 2003). The red box indicates the location of the SWATH-D experiment, covering the two slabs and the suggested gap between them.