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In this study, we present an estimate of the gravity signal of the slabs beneath the Alpine mountain belt. Estimates of the gravity effect of the subducting slabs are often omitted or simplified in crustal-scale models. The related signal is calculated here for alternative slab configurations at near-surface height and at a satellite altitude of 225 km. We apply three different modelling approaches in order to estimate the gravity signal from the subducting slab segments: (i) direct conversion of upper mantle seismic velocities to density distribution, which are then forward calculated to obtain the gravity signal; (ii) definition of slab geometries based on seismic crustal thickness and high-resolution upper mantle tomography for two competing slab configurations – the geometries are then forward calculated by assigning a constant density contrast and slab thickness; (iii) accounting for compositional and thermal variations with depth within the predefined slab geometry. Forward calculations predict a gravity signal of up to 40 mGal for the Alpine slab configuration. Significant differences in the gravity anomaly patterns are visible for different slab geometries in the near-surface gravity field. However, different contributing slab segments are not easily separated, especially at satellite altitude. Our results demonstrate that future studies addressing the lithospheric structure of the Alps should have to account for the subducting slabs in order to provide a meaningful representation of the geodynamic complex Alpine area. [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]

Some (ultra)high-pressure metamorphic rocks that formed during continental collision preserve relict minerals, indicating a two-stage evolution: first, subduction to mantle depths and exhumation to the lower-crustal level (with simultaneous cooling), followed by intensive heating that can be characterized by a β-shaped pressure–temperature–time (P–T–t) path. Based on a two-dimensional (2D) coupled petrological–thermomechanical tectono-magmatic numerical model, we propose a possible sequence of tectonic stages that could lead to these overprinting metamorphic events along an orogenic β-shaped P–T–t path: the subduction and exhumation of continental crust, followed by slab retreat that leads to extension and subsequent asthenospheric upwelling. During the last stage, the exhumed crustal material at the crust–mantle boundary undergoes heating from the underlying hot asthenospheric mantle. This slab rollback scenario is further compared numerically with the classical continental collision scenario associated with slab breakoff, which is often used to explain the late heating impulse in the collisional orogens. The mantle upwelling occurring in the experiments with slab breakoff, which is responsible for the heating of the exhumed crustal material, is not related to the slab breakoff but can be caused either by slab bending before slab breakoff or by post-breakoff exhumation of the subducted crust. Our numerical modeling predictions align well with a variety of orogenic P–T–t paths that have been reported from many Phanerozoic collisional orogens, such as the Variscan Bohemian Massif, the Triassic Dabie Shan, the Cenozoic Northwest Himalaya, and some metamorphic complexes in the Alps. [ABSTRACT FROM AUTHOR]

The process of continent–continent collision in shaping mountain ranges and adjacent foreland basins has been discussed for many decades. The orogeny driving forces — whether vertical or horizontal — are central for our understanding and it seemed settled in favor of horizontal forces. For the central European Alps, however, this traditional concept fails to explain first-order observations of the mountain belt. Recent stratigraphic, palaeo-altimetry and lithosphere structural evidence suggest that a Slab Rollback Orogeny model is capable of explaining the construction of thick nappe successions and the large-scale evolution of the Swiss Alps (Kissling and Schlunegger, 2018). This new concept questions the traditionally assumed bulldozing effect of Adria indenter as a necessary component of the dynamic system and proposes the buoyancy forces of the postulated post-collisional rollback is sufficient to drive the evolution and to shape the Central Alps as we know them today. Here we explore these challenging questions using a high-resolution, rheologically consistent, two-dimensional visco-elasto-plastic thermo-mechanical numerical model (Dal Zilio et al., 2018), which simulates both tectonic and seismic processes in a subduction and subsequent continental collisional setting. Our numerical experiments reproduce the self-driven stages of oceanic subduction, continent-continent collision, and spontaneous oceanic lithosphere slab breakoff. The post-collisional evolution of the orogen shows how slow — but persistent — flexural bending of the post-breakoff residual slab in combination with the crustal delamination control the most recent stage of the orogen. Bending-related slab suction and coeval development of crust-mantle and intracrustal delamination and nappe stacking through the slab bending zone, leads to the appearance of crustal shortening on the foreland basin, and widening of the orogen. We find good correlations of our numerical results with the previously conflicting tectonic observations in the Central Alps and the foreland (Molasse) basin. In particular, we discuss how the current crustal seismicity pattern implies the occurrence of extensional forces at work beneath the Molasse Basin and within the Alps. Our results thus support the emerging hypothesis that the remaining slab exerts a first-order control on the motions and deformations of the orogen. In light of these results, we propose that the tectonic history of the Central Alps does require an alternative view.Dal Zilio, L., van Dinther, Y., Gerya, T. V., & Pranger, C. C. (2018). Seismic behaviour of mountain belts controlled by plate convergence rate. Earth and Planetary Science Letters, 482, 81-92.Kissling, E., & Schlunegger, F. (2018). Rollback orogeny model for the evolution of the Swiss Alps. Tectonics, 37(4), 1097-1115. [ABSTRACT FROM AUTHOR]

The ongoing formation of the alpine mountain belt is a major indicator of the active geological processes in the Mediterranean that for example causes volcanism in Italy as well as considerable hazard. To understand this complex system the European project AlpArray, with the German contribution 4D-MB, was funded in order to investigate the structure, dynamics and geology of the alpine area in more detail and through all scales. We focus on the large scale geodynamic processes that drive this complex system of multiple subduction zones, ranging from the surface to the mantle. We will present new preliminary results obtained by thermomechanical modelling of the area. The models are based on recent large-scale seismological studies. As such we take into account complex slab gemoetries slab gaps and delaminated slabs. The Moho is created as a combination of Tesauro et al. (2008), Spada et al. (2013) and Molinari et al. (2011). We present several instantaneous simulations with varying temperature structure, rheological parameters, boundary conditions and slab configurations, unraveling the sensitivity of the surface velocity to the existence and shape of slabs in order to answer, e.g. the question which slab is most important for the rotation of Adria? We compare the results to measured surface observables like GPS-velocities or stress directions. In addition we automatically optimize the misfit between the data and the simulation results using adjoint gradient based inversions for the rheology of the lithosphere and the slabs. Tesauro, Magdala, Mikhail K. Kaban, and Sierd APL Cloetingh. "EuCRUST‐07: A new reference model for the European crust." Geophysical Research Letters 35.5 (2008).Spada, M., et al. "Combining controlled-source seismology and receiver function information to derive 3-D Moho topography for Italy." Geophysical Journal International 194.2 (2013): 1050-1068.Molinari, Irene, and Andrea Morelli. "EPcrust: a reference crustal model for the European Plate." Geophysical Journal International 185.1 (2011): 352-364. [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]

The long-lasting debate on the origin of Periadriatic magmatism in the European Alps has led to the formulation of one of the most successful theories of modern geology, the slab breakoff model (Davies and von Blanckenburg 1995) that is now extensively adopted worldwide (see review in Garzanti et al. 2018). In the Alpine region, recent tomographic data outline an unbroken European slab beneath the Western and the Central Alps (e.g., Zhao et al. 2016), which is incompatible with the hypothesis of oceanic slab detachment taking place in the early Oligocene, after continental collision between Adria and Europe (e.g., Schmid et al. 1996). In order to understand what triggers Alpine magmatism in the absence of slab breakoff, we present here the first comprehensive dataset of zircon U-Pb ages and Hf isotopic compositions from Periadriatic intrusives in the Western and Central Alps (Ji et al. 2019). Our data provide the first evidence of Eocene magmatism in the Western Alps (42-41 Ma in Traversella), and demonstrate that magmatism started synchronously in different segments of the Alpine belt, when subduction was still active. Zircon U-Pb ages define younging trends perpendicular to the strike of the European slab as imaged by seismic tomography, suggesting a progressive Eocene-Oligocene slab steepening. We propose that slab steepening enhanced the corner flow. This process was more effective near the edge of the European slab, and may have triggered Alpine magmatism in the absence of slab breakoff.CITED REFERENCESDavies JH and von Blanckenburg F (1995) Slab breakoff: a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens. Earth Planet Sci Lett 129, 85–102Garzanti E, Radeff G and Malusà MG (2018) Slab breakoff: A critical appraisal of a geological theory as applied in space and time. Earth-Sci Rev 177, 303-319Ji WQ, Malusà MG, Tiepolo M, Langone A, Zhao L and Wu FY (2019) Synchronous Periadriatic magmatism in the Western and Central Alps in the absence of slab breakoff. Terra Nova (in press)Schmid SM, Pfiffner OA, Froitzheim N, Schönborn G and Kissling E (1996) Geophysical–geological transect and tectonic evolution of the Swiss-Italian Alps. Tectonics 15, 1036–1064Zhao L, Paul A, Malusà MG, Xu X, Zheng T, Solarino S, Guillot S, Schwartz S, Dumont T, Salimbeni S, Aubert C, Pondrelli S, Wang Q and Zhu R (2016) Continuity of the Alpine slab unraveled by high-resolution P wave tomography. J Geophys Res Solid Earth 121, 8720-8737 [ABSTRACT FROM AUTHOR]

Collisional shortening across the entire Alpine belt has often been estimated in the Central Alpine sections, using sedimentary markers in the Southern Alps and more loosely constrained line-length balancing techniques in the Penninic and European domains. In the western Alps shortening has been assessed along numerous sections in the External Zone (European basement and cover), but shortening estimates for the entire Chain were always derived by projections of shortening amounts obtained in the Central Alps, either by assuming the amount of dextral displacement along the Insubric Line (Schmid and Kissling, 2000), or resolving the inferred amount of Central Alpine shortening on an assumed convergence direction in the western Alps (Handy et al., 2010).In the present study we estimate collisional shortening in the entire Alpine Chain, by reassessing displacements along the main structures of the orogen, without extrapolating displacements from one segment of the chain to the other. We note that collisional shortening progressively increases from less than 90 km in the south-western Alps, to nearly 200 km in the western Central Alps, where the Alpine chain starts to strike in an E-W direction. Therefore, we conclude that the convergence direction during the collisional phase must have been oriented NNW, as suggested by paleomagnetic studies, rather than WNW, as often inferred by tectonic investigations. The distribution of shortening described above is consistent with the distribution of collisional metamorphism: where the largest amounts of shortening are accommodated the nappes affected by the highest T of metamorphism are exhumed.The distribution of collisional shortening can also be used to constrain the interpretations of tomographic models on the geometry of slabs at depth. If the estimates of shortening are correct, along-strike variation of shortening are significant both along the upper and the lower plates. Based on these values it can be suggested that the central Alpine slab, well visible in all tomographic models, may largely consist of subducted European continental plate. In contrast, the eastern Alpine slab, in the area underlying the Tauern Window cannot be derived from the South-alpine continental plate, due to shortening amounts that are by far too small in this area, compared to the imaged slab length. Below the western Alps, some tomographic models image a slab break off whereas others image a continuous slab. Only the upper part of this slab, whether broken off or continuous, can be derived from the European plate based on the small amounts of inferred collisional shortening. References: Handy MR, Schmid SM, Bousquet R, Kissling E, Bernoulli D (2010). Earth Sci. Rev. 102:121–158.Schmid, S. M., and E. Kissling (2000). Tectonics, 19, 62–85, doi:10.1029/1999TC900057. [ABSTRACT FROM AUTHOR]