Imaging structure and geometry of slabs in the greater Alpine area – A P-wave traveltime tomography using AlpArray Seismic Network data

Abstract

Abstract. 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.

Moho and uppermost mantle structure in the Alpine area from S-to-P converted waves

“…Figure 19b compares our profile 9 (Fig. 11b) with mantle tomographic results from Paffrath et al (2021), geologically interpreted by Handy et al (2021). The NVG in Fig.…”

“…East of this area the SE dip of the European lithosphere laterally terminates, giving way to a different scenario that indicates eastward shallowing of the Moho (our data) that coincides with the lateral disappearance of the NVG (see yellow area marking velocities < 8.0 km s −1 in Fig. 17b) according to the velocities provided by the model of Paffrath et al (2021). Geissler et al (2010) presented a map of (what was called) the lithosphere-asthenosphere boundary (LAB) of central Europe obtained from an S-receiver function study (their Fig.…”

“…Further studies and high-resolution S-wave velocity models are needed to resolve this discrepancy. In spite of the discrepancies between the two data sets in the 12-14 • E sector, the data of Paffrath et al (2021) confirm that a first-order lateral change occurs in the area of the Tauern window. East of this area the SE dip of the European lithosphere laterally terminates, giving way to a different scenario that indicates eastward shallowing of the Moho (our data) that coincides with the lateral disappearance of the NVG (see yellow area marking velocities < 8.0 km s −1 in Fig.…”

“…In the shallow mantle we also observe, extending from 10 to 14 • E along an E-W section along 47 • N combined with N-S sections, steeply SE-dipping areas of downward velocity reductions referred to as NVGs that extend to at least 140 km depth at 14 • E and disappear east of there. The base of the NVG, which roughly coincides with the top of a lowvelocity zone revealed by mantle tomography west of 12 • E (Paffrath et al, 2021), is related to the European slab. The termination of the NVG at 14 • E suggests an important alongstrike change also in terms of the shallow mantle structure taking place at 14 • E, i.e.…”

“…17b, depicting the E-W profile 10, we compare our data regarding the NVG in the upper mantle below the Moho (previously discussed and illustrated in Fig. 12b) with results from the teleseismic body wave tomography by Paffrath et al (2021) by superimposing the P-wave velocity contours that these authors used for calculating velocity anomalies along the same E-W profile 10. There is fairly good agreement between the two data sets in the western part of the profile between 5 and 12 • E. In this area the eastward-dipping base of the red NVG zone roughly coincides with the top of an area associated with abnormally low P-wave velocities below 8.0 km s −1 marked in yellow in the western part of the profile.…”