Mountain building is a transient process involving a host of interactive mechanisms operating at depth and on the surface on widely varied time scales. Transience is manifested in the rock record, as well as in the structure and motion history of an orogen. Switches in the directions of subduction are first-order examples of transience and two such switches have been proposed in the Alps: one at the transition of the Western Alps to the Apennines that occurred in Paleogene time (Vignarolli et al. 2008, Molli et al. 2010) and the other, controversially, at the transition of the Western to the Eastern Alps in the Miocene (Lippitsch et al. 2003, Kissling et al. 2006). The latter switch challenges the traditional notion of uniform-sense subduction in the Alps, even as it confirms previous knowledge of opposite Cenozoic thrusting directions in the Alps and northern Dinarides (Ustaszewski et al. 2008).
In the Western Alps, the predominantly northwest-vergent thrusting direction is consistent with southward subduction of the European plate beneath the Adriatic plate as interpreted from seismic images of the Moho and of the slab itself (Fig. 2, sections AA’ and DD’, Zhao et al. 2015). In contrast, in the Eastern Alps (defined as the part of the Alps east of a north-south line defined by the Giudicarie Fault, Fig. 1a), the nappes in the vicinity of the Tauern tectonic window are folded and dissected by Miocene-age normal and strike-slip faults that accommodated orogen-parallel extension (Ratschbacher et al. 1991, Selverstone 2005, Scharf et al. 2013). There, the slab is steeply to moderately inclined to the north (Fig. 2, section CC’) and is separated from the European slab to the west by a conspicuous, if poorly defined, gap in positive P-wave velocity anomalies at ~11°E (Fig. 2b). In this gap, which coincides with the TRANSALP profile (Fig. 2), teleseismic tomography indicates that the slab is short (≤ 150 km) and dips vertically (Fig. 2, section BB’).
However, other seismic studies image a south-dipping European Moho (Lüschen et al. 2006, Bleibinhaus & Gebrande 2006) that extends 20-30 km south of the Periadriatic Fault System and underlies the Adriatic Moho (Kummerow et al. 2004, Brückl et al. 2007). Thus, the southward subduction geometry in the crustal sections appears to be inconsistent with the northward dip of the slab anomaly in this part of the Eastern Alps. Moreover, existing crustal models for the TRANSALP section do not agree regarding the inclination and depth of the Adriatic Moho (Kummerow et al. 2004, Lüschen et al. 2004, Ebbing et al. 2006, Bleibinhaus & Gebrande 2006, Diehl et al. 2009). The newest Moho map of this area based on combined controlled-source and receiver-function information indicates a Moho gap beneath part of the Eastern Alps (white area in Fig. 4c, Spada et al. 2013), one of the targets in this SPP (section V). Given these discrepancies, it is hardly surprising that interpretations of subcrustal structure in the Eastern Alps are contradictory. For example, both south-directed (Castellarin et al. 2006a, Lammerer et al. 2008) and north-directed (Schmid et al. 2004, Kissling et al. 2006) subduction has been proposed along the TRANSALP section. Controversy also persists over whether the slab anomaly beneath the Eastern Alps is part of the Adriatic or European plate (Mitterbauer et al. 2011, Handy et al. 2015). Whatever its origin, this anomaly is oriented obliquely to the crustal structure (Schmid et al. 2013), as well as to the orogen-parallel motion vector of the Eastern Alps towards the Pannonian Basin (Grenerczy et al. 2005). One of our challenges is therefore to determine whether decoupling occurs within the orogenic crust (Oldow et al. 1990) or along the crust-mantle boundary beneath the Eastern Alps.
Geodetic measurements indicate that the Adriatic plate is currently rotating counterclockwise and indenting the Eastern Alps at ~2 mm/yr (Vrabec et al. 2006, D’Agostino et al. 2008), whereas in the Western Alps near the rotational pole of Adria, the convergence rate is at or below GPS resolution (≤ 1mm/yr, Nocquet & Calais 2004, Tesauro et al. 2005). Thus, the Alps afford a unique opportunity to study different styles of mountain building along the Adria-Europe plate boundary, from ongoing indentation and orogen-parallel escape in the Eastern Alps (Scharf et al. 2013) to orogenic arcuation, post-collisional extension and isostatic uplift in the Central and Western Alps (Sue & Tricart 2003). 4D-MB may shed light on why the Alpine slabs are largely aseismic, even in the eastern part of the Southern Alps where Adria-Europe convergence is still active.