Shape and origin of the East-Alpine slab constrained by the ALPASS teleseismic model

During the last two decades teleseismic studies yielded valuable information on the structure of the upper mantle below the Alpine-Mediterranean area. Subducted oceanic lithosphere forms a broad anomaly resting on but not penetrating the 670 km discontinuity. More shallow slabs imaged below the Alpine arc are interpreted as subducted continental lower lithosphere. Substantial advances in our understanding of past and active tectonic processes have been achieved due to these results. However, important issues like the polarity of subduction under the Eastern Alps and the slab geometry at the transition to the Pannonian realm are still under debate. The ALPASS teleseismic experiment was designed to address these open questions. Teleseismic waveforms from 80 earthquakes recorded at 75 temporary and 79 permanent stations were collected during 2005 and 2006. From these data, a tomographic image of the upper mantle was generated between 60 km and 500 km depth. Crustal corrections, additional station terms, and ray bending caused by the velocity perturbations were considered. A steeply to vertically dipping shallow slab below the Eastern Alps is clearly resolved down to a depth of similar to 250 km. It is interpreted as European lower lithosphere detached from the crust and subducted during post-collision convergence between Adria and Europe. Below the Pannonian realm low velocities or high mantle temperatures prevail down to similar to 300 km depth, consistent with the concept of a Pannonian lithospheric fragment, which underwent strike-slip deformation relative to the European plate and extension during the post-collision phase of the Alpine orogeny. Between 350 km and 400 km depth, a deep slab extends from below the central Eastern Alps to under the Pannonian realm. It is 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. (C) 2011 Elsevier B.V. All rights reserved.