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The stratigraphic development of foreland basins has mainly been related to surface loading in the adjacent orogens, whereas the control of slab loads on these basins has received much less attention. This has also been the case for interpreting the relationships between the Oligocene to Micoene evolution of the European Alps and the North Alpine foreland basin or Molasse basin. In this trough, periods of rapid subsidence have generally been considered as a response to the growth of the Alpine topography, and thus to the construction of larger surface loads. However, such views conflict with observations where the surface growth in the Alps has been partly decoupled from the subsidence history in the basin. In addition, surface loads alone are not capable of explaining the contrasts in the stratigraphic development particularly between its central and eastern portions. Here, we present an alternative view on the evolution of the Molasse basin. We focus on the time interval between c. 30 and 15 Ma and relate the basin-scale development of this trough to the subduction processes, and thus to the development of slab loads beneath the European Alps. At 30 Ma, the western and central portions of this basin experienced a change from deep marine underfilled (Flysch stage) to overfilled terrestrial conditions (Molasse stage). During this time, however, a deep marine Flysch-type environment prevailed in the eastern part of the basin. This was also the final sedimentary sink as sediment was routed along the topographic axis from the western/central to the eastern part of this trough. We interpret the change from basin underfill to overfill in the western and central basin as a response to oceanic lithosphere slab-breakoff beneath the Central and Western Alps. This is considered to have resulted in a growth of the Alpine topography in these portions of the Alps, an increase in surface erosion and an augmentation in sediment supply to the basin, and thus in the observed change from basin underfill to overfill. In the eastern part of the basin, however, underfilled Flysch-type conditions prevailed until 20 Ma, and subsidence rates were higher than in the western and central parts. We interpret that high subsidence rates in the eastern Molasse occurred in response to slab loads beneath the Eastern Alps, where the subducted oceanic slab remained attached to the European plate and downwarped the plate in the East. Accordingly, in the central and western parts, the growth of the Alpine topography, the increase in sediment flux and the change from basin underfill to overfill most likely reflect the response to slab delamination beneath the Central Alps. In contrast, in the eastern part, the possibly subdued topography in the Eastern Alps, the low sediment flux and the maintenance of a deep marine Flysch-type basin records a situation where the oceanic slab was still attached to the European plate. The situation changed at 20 Ma, when the eastern part of the basin chronicled a change from deep marine (underfilled) to shallow marine and then terrestrial (overfilled conditions). During the same time, subsidence rates in the eastern basin decreased, deformation at the Alpine front came to a halt and sediment supply to the basin increased possibly in response to a growth of the topography in the Eastern Alps. This was also the time when the sediment routing in the basin axis changed from an east-directed sediment dispersal prior to 20 Ma, to a west-oriented sediment transport thereafter and thus to the opposite direction. We relate these changes to the occurrence of oceanic slab breakoff beneath the Eastern Alps, which most likely resulted in a rebound of the plate, a growth of the topography in the Eastern Alps and a larger sediment flux to the eastern portion of the basin. Beneath the Central and Western Alps, however, the continental lithosphere slab remained attached to the European plate, thereby resulting in a continued downwarping of the plate in its central and western portions. This plate downwarping beneath the central and western Molasse together with the rebound of the foreland plate in the East possibly explains the inversion of the drainage direction. We thus propose that slab loads beneath the Alps were presumably the most important drivers for the development of the Molasse basin at the basin scale. [ABSTRACT FROM AUTHOR]

Many convergent orogens, such as the eastern European Alps, display an asymmetric doubly vergent wedge geometry. In doubly vergent orogens, deepest exhumation occurs above the retro‐wedge. Deep‐seismic interpretations depict the European plate dipping beneath the Adriatic, suggesting the pro‐wedge location on the north side of the orogen. Our new thermochronometer data across the Eastern Alps confirm distinct shifts in the locus of exhumation associated with orogen‐scale structural reorganizations. Most importantly, we find a general Mid‐Miocene shift in exhumation (in the Tauern Window and the Southern Alps) and focus of modern seismicity across the Southern Alps. Taken together, these observations suggest a subduction polarity reversal at least since the Mid‐Miocene such that the present‐day pro‐wedge is located on the south side of the Alps. We propose a transient tectonic state of a slow‐and‐ongoing slab reversal coeval with motion along the Tauern Ramp, consistent with a present‐day northward migration of drainage divides. Plain Language Summary: When tectonic plates collide, they bend downwards and form two lithospheric wedges dipping in opposite directions, such as in the Eastern Alps. We present new crustal cooling data along a transect in the Eastern Alps confirming that surface rocks across the central Tauern Window originated from the deepest structural levels along the transect. South of the Tauern Window rocks were exhumed from higher depths compared to those north of it and were exhumed more recently, while seismic activity is also focused across the Southern Alps. These observations suggest a subduction polarity reversal because they are inconsistent with the original southern and northern locations of overriding and subducting plates, respectively, >15 million years ago. This interpretation is contrary to lithosphere‐scale tomography that shows no change in subduction polarity. Therefore, we propose a transient tectonic state, that is, a slow‐and‐ongoing subduction polarity reversal that initiated when Tauern Window rocks began their steep ascent to the surface along a deep‐seated fault known as the Tauern Ramp. This study bridges observations in the mantle, crust and on the surface over geologic time. Key Points: Thermochronologic data in the Eastern Alps is consistent with a transient tectonic state toward complete slab reversalThe pro‐wedge has switched from north to south of the Periadriatic Fault along TRANSALPMid‐Miocene motion along the Tauern Ramp is the consequence of slab‐reversal [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Carbonate-silicate interaction within subduction channels influences the chemical and physical properties of subducting carbon and is crucial to understanding the fate of carbonates in subduction zones. However, geochemical evidence for such interaction is still limited. Here we investigate zinc isotopes (expressed as δ 66 Zn JMC-Lyon) to decipher potential carbonate-silicate interaction in subducting slabs given the remarkable Zn isotopic difference between silicate and carbonate components. A suite of high- to ultrahigh-pressure metasediments containing variable amounts of carbonates from the Schistes Lustrés nappe (western Alps) record subduction processes to various depths (∼15–90 km) in a cool geothermal condition (∼8 °C/km). Despite a broad range of metamorphic grades (0.8–2.9 GPa and 300–630 °C) and carbonate contents (0–49 wt%), the bulk metasediments have nearly constant δ 66 Zn values (av. 0.21 ± 0.05 ‰ , 2sd, n = 12), which are indistinguishable from their putative protoliths (0.25 ± 0.07 ‰ , 2sd, n = 5). The lack of systematic variation of bulk sediment δ 66 Zn with metamorphic grades implies a limited transfer of Zn during prograde metamorphism. Therefore, sediments will preserve their original Zn isotopic compositions after subduction into the mantle. By contrast, the carbonate components in sediments obtained by leaching exhibit a progressive decline in δ 66 Zn from 0.82 ± 0.01 ‰ to 0.20 ± 0.04 ‰ and a modest increase in Zn concentration with pressure. These variations are best interpreted as a result of prolonged closed-system Zn isotopic exchange between isotopically heavy carbonates and light silicate components in the subducting slab. Our results demonstrate that a considerable amount of carbonates in subducting sediments can be retained at least down to the depths of the sub-arc mantle (>90 km). The strong pressure-dependent Zn isotopic fractionation (R 2 = 0.87) during carbonate-silicate interaction makes Zn isotopes a novel proxy for the storage depth of subducting carbonates. • A systematic zinc isotope study on western Alpine metasediments. • Prograde metamorphism has limited impact on Zn isotopes of bulk subducting sediments. • Variation of δ 66 Zn in carbonates reflects interaction with silicates in a closed system. • Zn isotopes are an indicator for the storage depth of subducting carbonates. [ABSTRACT FROM AUTHOR]

Despite being one of the most-studied orogens on Earth, recent deep-seismic data found profound heterogeneities in the deep mantle along-strike the Alpine orogen: in the Eastern Alps the subducting slab is steeply inclined, whereas in the Western Alps it is shallower and consistent with southward directed subduction after slab break-off possibly during Plio- to Pleistocene times. A goal of this study is to explore the long-term effects on the upper crustal structural and exhumation history by contrasting the western NFP-20E, central TRANSALP and eastern EASI seismic profiles utilising thermochronological and structural analyses. We achieve this in three steps: (1) by obtaining new low-temperature thermochronology data along the three profiles (apatite and zircon [U-Th/He]), (2) by modelling structural kinematics based on newly created balanced cross-sections for each profile, and (3) using these kinematics as input for an integrated thermo-kinematic forward model to predict cooling ages that can be compared to those measured at the surface. This approach will enable us to constrain rock uplift potentially associated with slab break-off in the Western Alps, and whether this mechanism is able to explain the overall higher topography in surface elevation compared to the Eastern Alps. Here we present new thermochronological ages along the NFP-20E and TRANSALP profiles together with preliminary thermo-kinematic forward models. These data are in accordance with recent exhumation focussed in the central part of the sections. The cooling ages show a slight north-to-south increase from ~5 Ma to ~10-15 Ma in apatite fission track ages and from ~10-15 Ma to ~20-25 Ma in zircon (U-Th/He) ages within the Tauern Window, and a sharp increase in cooling ages north of the Periadriatic Line. Apatite (U-Th/He) ages remain within ~5-10 Ma. North and South of the Tauern Window cooling ages tend to successively increase. We propose that the cooling age distribution measured at the surface is consistent with an in-sequence activation of mid-crustal ramps coeval with fault activity in the immediate hanging wall of the Periadriatic Line since the Oligocene. The unique setting, comparing the structural and exhumational history along sections above contrasting deep-seismic profiles, enables us to better understand, and possibly link, mantle processes and their influence on the crust and the surface during Palaeogene to Neogene Alpine orogenesis, while providing new insights into the formation of collisional mountain belts in general. [ABSTRACT FROM AUTHOR]