A review and reinterpretation of previous experimental data on the deformation of partially melted crustal rocks reveals that the relationship of aggregate strength to melt fraction is non-linear, even if plotted on a linear ordinate and abscissa. At melt fractions, U < 0.07, the dependence of aggregate strength on U is significantly greater than at U > 0.07. This melt fraction (U ¼ 0.07) marks the transition from a significant increase in the proportion of melt-bearing grain boundaries up to this point to a minor increase thereafter. Therefore, we suggest that it is the increase of melt-interconnectivity that causes the dramatic strength drop between the solidus and a melt fraction of 0.07. We term this drop the Ômelt connectivity transitionÕ (MCT). A second, less-pronounced strength drop occurs at higher melt fractions and corresponds to the breakdown of the solid (crystal) framework. This is the Ôsolid-to-liquid transitionÕ (SLT), corresponding to the well known Ôrheologically critical melt percentageÕ. Although the strength drop at the SLT is about four orders of magnitude, the absolute value of this drop is small compared with the absolute strength of the unmelted aggregate, rendering the SLT invisible in a linear aggregate strength v. melt-fraction diagram. On the other hand, the more important MCT has been overlooked in previous work because experimental data usually are plotted in logarithmic strength v. melt-fraction diagrams, obscuring large strength drops at high absolute strength values. We propose that crustal-scale localization of deformation effectively coincides with the onset of melting, pre-empting attainment of the SLT in most geological settings. The SLT may be restricted to controlling flow localization within magmatic bodies, especially where melt accumulates.

We present a tectonic map of the Tauern Window and surrounding units (Eastern Alps, Austria), combined with a series of crustal-scale cross-sections parallel and perpendicular to the Alpine orogen. This compilation, largely based on literature data and completed by own investigations, reveals that the present-day structure of the Tauern Window is primarily characterized by a crustal-scale duplex, the Venediger Duplex (Venediger Nappe system), formed during the Oligocene, and overprinted by doming and lateral extrusion during the Miocene. This severe Miocene overprint was most probably triggered by the indentation of the Southalpine Units east of the Giudicarie Belt, initiating at 23-21 Ma and linked to a lithosphere-scale reorganization of the geometry of mantle slabs. A kinematic reconstruction shows that accretion of European lithosphere and oceanic domains to the Adriatic (Austroalpine) upper plate, accompanied by highpressure overprint of some of the units of the Tauern Window, has a long history, starting in Turonian time (around 90 Ma) and culminating in Lutetian to Bartonian time (45-37 Ma).

A large data set of surface wave phase velocity measurements is compiled to study the structures of the crust and upper mantle underneath the Alpine continental collision zone. Records from both ambient‐noise and earthquake‐based methods are combined to obtain a high‐resolution 3‐D model of seismic shear velocity. The applied techniques allow us to image the shallow crust and sedimentary basins with a lateral resolution of about 25 km. We find that complex lateral variations in Moho depth as mapped in our model are highly compatible with those obtained from receiver function studies; this agreement with entirely independent data is a strong indication of the reliability of our results, and we infer that our model has the potential to serve as reference crustal map of shear velocity in the Alpine region. Mantle structures show nearly vertical subducting lithospheric slabs of the European and Adriatic plates. Pronounced differences between the western, central, and eastern Alps provide indications of the respective geodynamic evolution: we propose that in the southwestern and northeastern Alps, the European slab has broken off. The complex anomaly pattern in the upper mantle may be explained by combination of remnant European slab and Adriatic subduction. Along‐strike changes in the upper mantle structure are observed beneath the Apennines with an attached Adriatic slab in the northern Apennines and a slab window in the central Apennines. There is also evidence for subduction of Adriatic lithosphere to the east beneath the Pannonian Basin and the Dinarides down to a maximum depth of about 150 km.

The Alpine Oligocene plutons are spatially and temporally associated with the activity of the Periadriatic Fault System (PFS), an orogen‐parallel, crustal‐scale transpressive mylonitic belt. Excellent three‐dimensional exposure, combined with a wealth of structural, seismic, petrological, geochronological, geochemical, and paleomagnetic data collected over the last decades help to constrain the relationships between deformation, ascent, and emplacement of the plutons. Magmas were channeled from the base of the thickened continental crust into the narrow mylonitic belt of the Periadriatic Fault System, which was used as ascent pathway to cover vertical lengths of 20 to 40 km. Therefore the linear alignment of the plutons at the surface is not the expression of a linear source region at depth. Ascent of the melts is controlled by the mylonitic foliation of the PFS, which forms the only steep anisotropy, continuously traversing the entire Alpine crust. In contrast, the flow direction is not influenced by the specific kinematics of the faults. Final emplacement of the plutons occurred by extrusion from the Periadriatic Fault System into the adjacent country rocks. The transition from ascent to final emplacement is favored by partitioning of transpressive deformation.

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International audienceBased on a review of the surface and deep structure of the Eastern Alps, we link the timing and the inferred displacement fields to exhumation of upper and lower crustal units of the orogenic nappe stack during collision. The discussion focuses mainly on the Tauern Window and its country rocks, the only area of the Eastern Alps where the orogenic wedge, from its uppermost Austroalpine nappes down to its deepest European basement nappes is continuously exposed. We summarize and discuss the long-standing controversy on the mechanisms of exhumation of this nappe stack on the base of a synthesis of structural and geochronological data, and restorations of collisional displacements, both in cross-sections and map views. We conclude that the large amounts of exhumation assessed for the western Eastern Alps resulted from large amounts of thickening and erosion, not observed in the eastern part of the Eastern Alps. Extensional faults, laterally bounding the area of major thickening and exhumation are inferred to nucleate in order to accommodate displacement around the indenter corner in the west, and in order to reduce a large gradient of crustal thickness and surface elevation in the East.Restorations to the pre-indentation stage, document an amount of northward increasing orogen-parallel extension, varying 45 km to 85 km, corresponding to 15% of extension, that is partly accommodated along normal faults. N-S shortening between the Northern Calcareous Alps and the Dolomites Indenter attained 75 km in the west and decreased to 30 km in the east. 55 km out of these 75 were accommodated in the area of the Tauern Window. Our kinematic model shows that lateral extrusion accommodated along conjugate strike-slip faults requires large amounts of north-south shortening in the western part of the Eastern Alps. Such shortening is consistent with the reconstructed upright folding and erosion of the Tauern Window, thus explaining the largest amount of its exhumation. In contrast, the eastern termination of the Eastern Alps represents an area where collisional convergence was barely accommodated by crustal thickening. This transition from a highly shortened, thickened and exhumed wedge in the west, mainly affected by orogen-perpendicular displacements, to a barely shortened and exhumed wedge in the east, mainly characterised by orogen-parallel displacements, spatially coincides with a change in the deep structure of the European slab. Indeed, the inferred continental, European Slab, imaged in the west disappears into a low velocity anomaly, where no slab is detected in the east. An inherited step in the geometry in map view of the European passive margin, causing its crust to enter the subduction zone earlier than the area east of the Tauern Window, may explain the rapid decrease of shortening, of thickening, the different syn-collisional P-T gradients, and the disappearance of the continental slab east of the Katschberg Fault

After the onset of plate collision in the Alps, at 32-34 Ma, the deep structure of the orogen is inferred to have changed dramatically: European plate break-offs in various places of the Alpine arc, as well as a possible reversal of subduction polarity in the eastern Alps have been proposed. We review different high-resolution tomographic studies of the upper mantle and combine shear-and body-wave models to assess the most reliable geometries of the slabs. Several hypotheses for the tectonic evolution are presented and tested against the tomographic model interpretations and constraints from geologic and geodetic observations. We favor the interpretation of a recent European slab break-off under the western Alps. In the eastern Alps, we review three published scenarios for the subduction structure and propose a fourth one to reconcile the results from tomography and geology. We suggest that the fast slab anomalies are mainly due to European subduction; Adriatic subduction plays no or only a minor role along the Tauern window sections, possibly increasing towards the Dinarides. The apparent northward dip of the slab under the eastern Alps may be caused by imaging a combination of Adriatic slab, from the Dinaric subduction system, and a deeper lying European one, as well as by an overturned, retreating European slab.

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