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We perform a teleseismic P-wave travel-time tomography to examine the geometry and structure of subducted lithosphere in the upper mantle beneath the Alpine orogen. The tomography is based on waveforms recorded at over 600 temporary and permanent broadband stations of the dense AlpArray Seismic Network deployed by 24 different European institutions in the greater Alpine region, reaching from the Massif Central to the Pannonian Basin and from the Po Plain to the river Main. Teleseismic travel times and travel-time residuals of direct teleseismic P waves from 331 teleseismic events of magnitude 5.5 and higher recorded between 2015 and 2019 by the AlpArray Seismic Network are extracted from the recorded waveforms using a combination of automatic picking, beamforming and cross-correlation. The resulting database contains over 162 000 highly accurate absolute P-wave travel times and travel-time residuals. For tomographic inversion, we define a model domain encompassing the entire Alpine region down to a depth of 600 km. Predictions of travel times are computed in a hybrid way applying a fast TauP method outside the model domain and continuing the wave fronts into the model domain using a fast marching method. We iteratively invert demeaned travel-time residuals for P-wave velocities in the model domain using a regular discretization with an average lateral spacing of about 25 km and a vertical spacing of 15 km. The inversion is regularized towards an initial model constructed from a 3D a priori model of the crust and uppermost mantle and a 1D standard earth model beneath. The resulting model provides a detailed image of slab configuration beneath the Alpine and Apenninic orogens. Major features are a partly overturned Adriatic slab beneath the Apennines reaching down to 400 km depth still attached in its northern part to the crust but exhibiting detachment towards the southeast. A fast anomaly beneath the western Alps indicates a short western Alpine slab whose easternmost end is located at about 100 km depth beneath the Penninic front. Further to the east and following the arcuate shape of the western Periadriatic Fault System, a deep-reaching coherent fast anomaly with complex internal structure generally dipping to the SE down to about 400 km suggests a slab of European origin limited to the east by the Giudicarie fault in the upper 200 km but extending beyond this fault at greater depths. In its eastern part it is detached from overlying lithosphere. Further to the east, well-separated in the upper 200 km from the slab beneath the central Alps but merging with it below, another deep-reaching, nearly vertically dipping high-velocity anomaly suggests the existence of a slab beneath the eastern Alps of presumably the same origin which is completely detached from the orogenic root. Our image of this slab does not require a polarity switch because of its nearly vertical dip and full detachment from the overlying lithosphere. Fast anomalies beneath the Dinarides are weak and concentrated to the northernmost part and shallow depths. Low-velocity regions surrounding the fast anomalies beneath the Alps to the west and northwest follow the same dipping trend as the overlying fast ones, indicating a kinematically coherent thick subducting lithosphere in this region. Alternatively, these regions may signify the presence of seismic anisotropy with a horizontal fast axis parallel to the Alpine belt due to asthenospheric flow around the Alpine slabs. In contrast, low-velocity anomalies to the east suggest asthenospheric upwelling presumably driven by retreat of the Carpathian slab and extrusion of eastern Alpine lithosphere towards the east while low velocities to the south are presumably evidence of asthenospheric upwelling and mantle hydration due to their position above the European slab. [ABSTRACT FROM AUTHOR]

We perform a teleseismic P-wave traveltime tomography to examine the geometry and structure of subducted lithosphere in the upper mantle beneath the Alpine orogen. The tomography is based on waveforms recorded at over 600 temporary and permanent broadband stations of the dense AlpArray Seismic Network deployed by 24 different European institutions in the greater Alpine region, reaching from the Massif Central to the Pannonian Basin and from the Po plain to the river Main. Teleseismic traveltimes and traveltime residuals of direct teleseismic P-waves from 331 teleseismic events of magnitude 5.5 and higher recorded between 2015 and 2019 by the AlpArray Seismic Network are extracted from the recorded waveforms using a combination of automatic picking, beamforming and cross-correlation. The resulting database contains over 162.000 highly accurate absolute P-wave traveltimes and traveltime residuals. For tomographic inversion, we define a model domain encompassing the entire Alpine region down to a depth of 600 km. Outside this domain, a laterally homogeneous standard earth model is assumed. Predictions of traveltimes are computed in a hybrid way applying a fast Tau-P method outside the model domain and continuing the wavefronts into the model domain using a fast marching method. For teleseismic inversion, we iteratively invert demeaned traveltime residuals for P-wave velocities in the model domain using a regular discretization with an average lateral spacing of about 25 km and a vertical spacing of 15 km. The inversion is regularized towards an initial model constructed from an a priori model of the crust and uppermost mantle and a standard earth model beneath. The resulting model provides a detailed image of slab configuration beneath the Alpine and Apenninic orogens. Major features are an overturned Adriatic slab beneath the Apennines reaching down to 400 km depth still attached in its northern part to the crust but exhibiting detachment towards the southeast. A fast anomaly beneath the western Alps indicates a short western Alpine slab that ends at about 100 km depth close to the Penninic front. Further to the east and following the arcuate shape of the western Periadriatic Fault System, a deep-reaching coherent fast anomaly with complex interior stucture generally dipping to the SE down to about 400 km suggests a slab of European origin extending eastward to the Giudicarie fault. This slab is detached from overlying lithosphere at its eastern end below a depth of about 100 km. Further to the east, well-separated from the slab beneath the western and central Alps, another deep-reaching, nearly vertically dipping high-velocity anomaly suggests the existence of a slab beneath the Eastern Alps of presumably European origin which is completely detached from the orogenic root. Our image of this slab does not require a polarity switch because of its nearly vertical dip and full detachment from the overlying lithosphere. Fast anomalies beneath the Dinarides are weak and concentrated to the northernmost part and shallow depths. Low-velocity regions surrounding the fast anomalies beneath the Alps to the west and northwest follow the same dipping trend as the overlying fast ones, indicating a kinematically coherent subducting tectosphere in this region. In contrast, low-velocity anomalies to the east suggest asthenospheric upwelling presumably driven by retreat of the Carpathian slab and extrusion of eastern Alpine lithosphere towards the east while low velocities to the south are presumably evidence of asthenospheric upwelling and mantle hydration due to the backarc position behind the European slab. [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]