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The Eastern Alps were affected by a profound post‐collisional tectonic reorganisation in Neogene time, featuring indentation by the Adriatic upper plate, rapid uplift and filling of the eastern Molasse Basin, exhumation and eastward orogen‐parallel transport of Paleogene metamorphic units in the orogenic core, and a shift from northward thrust propagation in the European plate to southward propagation in the Adriatic plate. We test the idea that these events were triggered by slab detachment by reconstructing the indentation process. This involves sequentially restoring N‐S and E‐W cross‐sections of the orogenic wedge and correcting for out‐of‐section orogen‐parallel transport with a map‐view reconstruction. We propose two phases of indentation: Initially (23 and 14 Ma), the whole Adriatic crust acted as an indenter. Its northward motion was accommodated by upright folding and orogen‐parallel extensional exhumation in the Tauern Window. This phase was followed (14 Ma to Present) by continued orogen‐parallel transport of the orogenic wedge into the Pannonian Basin and deformation of the leading edge of the Adriatic indenter, forming the Southern Alps fold‐thrust belt. The lower crust of the Southern Alps indented the base of the Venediger Nappes in the Tauern Window, forming a high‐velocity (6.8–7.25 km/s) ridge in map view at 30–45 km depth. By correlating the post‐23 Ma orogenic evolution with presently imaged European slab segments in P‐wave teleseismic tomography, we discern two possible Neogene slab removal events: One from 23 to 19 Ma triggering tectonic reorganisation of the Eastern Alps and its foreland basin, and potentially a second event after 14 Ma. The Eastern Alps were first indented by Adriatic lithosphere (23 and 14 Ma), then by Adriatic lower crust as the indenter deformed (14 and 0 Ma)Shortening of the orogenic wedge since 23 Ma requires 135 km of subduction and 90 km of eastward extrusion of orogenic lithosphereSlab detachment at 23 and 20 Ma and possibly after 14 Ma is constrained by areal balancing of crust and mantle The Eastern Alps were first indented by Adriatic lithosphere (23 and 14 Ma), then by Adriatic lower crust as the indenter deformed (14 and 0 Ma) Shortening of the orogenic wedge since 23 Ma requires 135 km of subduction and 90 km of eastward extrusion of orogenic lithosphere Slab detachment at 23 and 20 Ma and possibly after 14 Ma is constrained by areal balancing of crust and mantle

Graphical Abstract:

We integrate structural, geophysical, and geodetic studies showing that the Dinarides‐Hellenides orogen along the Adria‐Europe plate boundary in the Western Balkan peninsula has experienced clockwise oroclinal bending since Eocene‐Oligocene time. Rotation of the Hellenic segment of this orogen has accelerated since the middle Miocene and is associated with a north‐to‐south increase in shortening along the orogenic front. Within the Paleogene nappe pile, bending was accommodated by orogen‐parallel extension, clockwise block rotation, and thrusting in the hanging wall of the Skhoder‐Peja Normal Fault (SPNF). The SPNF and related faults cut the older Skhoder‐Peja Transfer Zone with its pre‐Neogene dextral offset of the West Vardar ophiolite nappe. Rotation of the SPNF hanging wall involved Miocene‐to‐recent, out‐of‐sequence thrusting that was transferred to the Hellenic orogenic front via lateral ramps on dextral transfer zones. Along strike of the Dinarides‐Hellenides and coincident with the southward increase in Neogene shortening, the depth of the Adriatic slab increases from ~160 km north of the SPNF to ~200 km just to the south thereof, to several hundreds of kilometers to the south of the Kefalonia Transfer Zone. The geodynamic driver of tectonics since the early Miocene has been enhanced rollback of the Hellenic segment of the Adriatic slab in the aftermath of Eocene‐Oligocene slab tearing and breakoff beneath the Dinarides, which focused slab pull in the south. The SW‐retreating Hellenic slab segment induced clockwise bending of the southern Dinarides and northern Hellenides, including their Adriatic foreland, about a rotation pole in the vicinity of the Mid‐Adriatic Ridge. The junction of the Dinarides and Hellenides is the site of a N‐to‐S increase in Neogene shortening and upper‐plate extensionThis shortening reflects oroclinal bending about a pole near the Mid‐Adriatic Ridge that has been driven by Hellenic rollback subductionThe Shkoder‐Peja Normal Fault accommodated Neogene orogen‐parallel extension and clockwise rotation of shallower levels of the orogen

A new kinematic reconstruction that incorporates estimates of post‐20 Ma shortening and extension in the Apennines, Alps, Dinarides, and Sicily Channel Rift Zone (SCRZ) reveals that the Adriatic microplate (Adria) rotated counterclockwise as it subducted beneath the European Plate to the west and to the east, while indenting the Alps to the north. Minimum and maximum amounts of rotation are derived by using, respectively, estimates of crustal extension along the SCRZ (minimum of 30 km) combined with crustal shortening in the Eastern Alps (minimum of 115 km) and a maximum amount (140 km) of convergence between Adria and Moesia across the southern Dinarides and Carpatho‐Balkan orogens. When combined with Neogene convergence in the Western Alps, the best fit of available structural data constrains Adria to have moved 113 km to the NW (azimuth 325°) while rotating 5 ± 3° counterclockwise relative to Europe since 20 Ma. Amounts of plate convergence predicted by our new model exceed Neogene shortening estimates of several tens of kilometers in both the Apennines and Dinarides. We attribute this difference to crust‐mantle decoupling (delamination) during rollback in the Apennines and to distributed deformation related to the northward motion of the Dacia Unit between the southern Dinarides and Europe (Moesia). Neogene motion of Adria resulted from a combination of Africa pushing from the south, the Adriatic‐Hellenides slab pulling to the northeast, and crustal wedging in the Western Alps, which acted as a pivot and stopped farther northwestward motion of Adria relative to Europe. Adria has rotated 5 ± 3° counterclockwise and translated 113 km to the NW (azimuth 325°) relative to Europe since 20 MaAdria motion was associated with 110 km convergence relative to Moesia, 125 km in Eastern Alps, and 60 km of extension in Sicily ChannelDifferences between amounts of shortening and plate convergence suggest crust‐mantle decoupling at active Adria‐Europe boundaries

In this study, 3‐D models of P‐wave velocity (Vp) and P‐wave and S‐wave ratio (Vp/Vs) of the crust and upper mantle in the Eastern and eastern Southern Alps (northern Italy and southern Austria) were calculated using local earthquake tomography (LET). The data set includes high‐quality arrival times from well‐constrained hypocenters observed by the dense, temporary seismic networks of the AlpArray AASN and SWATH‐D. The resolution of the LET was checked by synthetic tests and analysis of the model resolution matrix. The small inter‐station spacing (average of ∼15 km within the SWATH‐D network) allowed us to image crustal structure at unprecedented resolution across a key part of the Alps. The derived P velocity model revealed a highly heterogeneous crustal structure in the target area. One of the main findings is that the lower crust is thickened, forming a bulge at 30–50 km depth just south of and beneath the Periadriatic Fault and the Tauern Window. This indicates that the lower crust decoupled both from its mantle substratum as well as from its upper crust. The Moho, taken to be the iso‐velocity contour of Vp= 7.25 km/s, agrees with the Moho depth from previous studies in the European and Adriatic forelands. It is shallower on the Adriatic side than on the European side. This is interpreted to indicate that the European Plate is subducted beneath the Adriatic Plate in the Eastern and eastern Southern Alps. Energy released by earthquakes is converted to seismic waves that radiate through the Earth. Here, we study two types of waves (P and S) in the Eastern and eastern Southern Alps (northern Italy and southern Austria). The aim is to investigate the subsurface structure beneath the mountains. The P‐wave and S‐wave propagate at different velocities between the earthquakes and seismometers. Given the origin time and locations of earthquakes, seismometer coordinates, and wave arrival times, we can calculate the P‐wave velocity (Vp) and the P‐wave and S‐wave velocity ratio (Vp/Vs) of the subsurface volume. Therefore, our velocity model allows for imaging the crust at unprecedented resolution. Moreover, an important structure within the Earth, the boundary between the crust and mantle or the so‐called Moho, can be inferred by the study of the waves. Our results show that during collision and formation of the Eastern and eastern Southern Alps, the lower crust of the Adriatic Plate thickened due to decoupling from both the mantle and upper crust within the orogen. The Moho is shallower on the Adriatic side than on the European side. A possible interpretation is that the European Plate subducted under the Adriatic Plate in the Eastern and eastern Southern Alps. Vpand Vp/Vsmodels of the crust and upper mantle were calculated in the Eastern and eastern Southern Alps using a high‐quality data setThe simultaneous inversion of 3‐D velocity models and earthquake parameters were performed using the well‐established SIMUL2000 algorithmThe structure of Alpine orogenic crust including the tip of the Adriatic Indenter is revealed with the Vpmodel Vpand Vp/Vsmodels of the crust and upper mantle were calculated in the Eastern and eastern Southern Alps using a high‐quality data set The simultaneous inversion of 3‐D velocity models and earthquake parameters were performed using the well‐established SIMUL2000 algorithm The structure of Alpine orogenic crust including the tip of the Adriatic Indenter is revealed with the Vpmodel

In-situ deformation experiments were performed on partially molten analog materials (norcamphor in the presence of a benzamide–norcamphor melt) undergoing pure shearing at a constant melt fraction of 0.13. Melt in the samples induces a strain-dependent transition from purely dislocation creep to dislocation creep associated with minor intergranular fracturing and grain boundary sliding (GBS). Intergranular fractures drain the melt from initially isotropic melt pockets to grain boundaries. Along such boundaries, grain-boundary migration recrystallization is inhibited, while GBS occurs. Intergranular melt pockets occur along grain boundaries oriented subparallel to the shortening direction, but melt must have migrated parallel to the elongation direction of the samples, as indicated by melt accumulations at both extruding ends of the sample. Intergranular melt pockets parallel to the elongation direction were only rarely observed, because melt was rapidly expelled from these sites. Nevertheless, these grain boundaries are the pathways of melt segregation in the samples.

A new series of two- and three-dimensional deformation regime maps facilitating the simultaneous display of petrological, microstructural and rock-mechanical information illustrates the pressure, temperature and grainsize dependencies of brittle, crystal-plastic and granular creep regimes in the lithosphere. The predictions made using the extrapolated laboratory data are in qualitative agreement with inferences drawn from microstructural studies in naturally deformed monomineralic rocks. The deformation regime maps are used to show that the rheological evolution of a fault zone is related both to its location within a compositionally heterogeneous lithospheric section and to the pressure-temperature-time history of that section. Strain localization into ductile shear zones is accelerated if grainsize reduction leads to a change from grainsize-insensitive to grainsize-sensitive creep. In rocks whose rheology is approximated by the flow of one weak mineral, this process is favored at high stress-low temperature and/or retrograde thermal conditions. For polymineralic rocks in either prograde or retrograde thermal settings, reaction-induced or -enhanced switches to grainsize-sensitive creep can occur at depths corresponding to the intersection of the ambient geothermal gradient or pressure-temperature-time path with the limits of mineral stabilities for a specified rock composition. The consideration of the effects of pressure, temperature, grainsize and syntectonic reactions on fault rheology results in an evolutionary rheological model of an extending lithosphere which is more complex than existing models.

Norcamphor (C7H10O) was subjected to plane strain simple shear in a see-through deformation rig at four different strain rate and temperature conditions. Two transient stages in the microfabric evolution to steady state are distinguished. The grain scale mechanisms associated with the microstructural and textural evolution vary with the applied temperature, strain rate and strain. In high-temperature-low-strain-rate experiments, computer integrated polarization microscopy reveals that the texture evolution is closely related to the crystallographic rotation paths and rotation rates of individual grains. High c-axis rotation rates at low to intermediate shear strains are related to the development of a symmetrical c-axis cross girdle by the end of the first transient stage (γ = 1.5 to 2). During the second transient stage (γ = 1.5 to 6), the cross girdle yields to an oblique c-axis single girdle as c-axis rotation rates decrease and the relative activity of grain boundary migration recrystallization increases. Steady state (γ 8) is characterized by a stable end orientation of the sample texture and the cyclic growth, rotation and consumption of individual grains within the aggregate.

Plane strain simple shearing of norcamphor (C 7 H 10 O) in a see-through deformation rig to a shear strain of γ = 10.5 at a homologous temperature of T h = 0.81 yields a microfabric similar to that of quartz in amphibolite facies mylonite. Synkinematic analysis of the norcamphor microfabric reveals that the development of a steady-state texture is linked to changes in the relative activities of several grain-scale mechanisms. Three stages of textural and microstructural evolution are distinguished: (1) rotation and shearing of the intracrystalline glide planes are accommodated by localized deformation along three sets of anastomozing microshears. A symmetrical c -axis girdle reflects localized pure shear extension along the main microshear set ( S a ) oblique to the bulk shear zone boundary (abbreviated as SZB); (2) progressive rotation of the microshears into parallelism with the SZB increases the component of simple shear on the S a microshears. Grain-boundary migration recrystallization favours the survival of grains with slip systems oriented for easy glide. This is associated with a textural transition towards two stable c -axis point maxima whose skeletal outline is oblique with respect to the S a microshears and the SZB; and (3) at high shear strains (γ > 8), the microstructure, texture and mechanism assemblage are strain invariant, but strain continues to partition into rotating sets of microshears. Steady state is therefore a dynamic, heterogeneous condition involving the cyclic nucleation, growth and consumption of grains.

We investigate a well‐preserved paleo subduction channel that preserves a coherent part of the European continental margin exposed in the central Tauern Window (Eastern Alps), with the aim of testing models of sheath fold nappe formation and exhumation. The subduction zone was active during Paleogene convergence of the European and Adriatic plates, after closure of the Alpine Tethyan ocean. New cross sections and structural data together with new petrological data document a recumbent, tens of kilometers‐scale sheath fold in the center of the Tauern Window that formed during pervasive top‐foreland shear while subducted at high‐pressure (HP) conditions (~2.0 GPa, 500 °C) close to maximum burial depth. The fold comprises an isoclinally folded thrust that transported relicts of the former Alpine Tethys onto a distal part of the former European continental margin. The passive margin stratigraphy is still well preserved in the fold and highlights the special character of this segment of the European continental margin. We argue that this segment formed a promontory to the margin, which was inherited from Mesozoic rifting. In accordance with classical sheath fold theory, this promontory may have acted as an initial structural perturbation to nucleate a fold that was passively amplified to a sheath fold during top‐foreland shear in the subduction zone. The fold was at least partly exhumed and juxtaposed with the surrounding lower pressure units by opposing top‐hinterland and top‐foreland shear zones above and below, respectively, that is, in the sense of a nappe fold formed during channel‐extrusion exhumation. The central Tauern Window preserves a crustal‐scale sheath fold that formed in the Alpine subduction zone in top‐foreland shearThe sheath fold was formed by passive amplification of a rift‐inherited heterogeneity of the European marginExhumation of sheath fold nappe from HP conditions was achieved at least partly by channel extrusion