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

Abstract

Abstract. 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.

Orogenic lithosphere and slabs in the greater Alpine area – interpretations based on teleseismic P-wave tomography

“…The images of velocity anomalies in this paper are derived from a 3D model of P-wave velocity in the crust and mantle below the greater Alpine region (Paffrath et al, 2021b). This is obtained by tomographic inversion of teleseismic P-wave travel-time residuals measured on records of the AlpArray Seismic Network (Paffrath et al, 2021a).…”

“…The travel-time database comprises 162 366 onsets of 331 teleseismic earthquakes of magnitude 5.5 or higher at epicentral distances between 35 and 135 • occurring between January 2015 and July 2019. Paffrath et al (2021b) subtracted the array average from these residuals on an event-to-event basis. This procedure avoids errors in earthquake origin time and reduces influences of heterogeneities in earth structure outside the inversion domain (see Paffrath et al, 2021a, on obtaining highly accurate travel-time residuals suitable for teleseismic inversion).…”

“…The purpose of the a priori model is to account for strongly heterogeneous velocity structure, particularly in the crust. In constructing their a priori crustal model, Paffrath et al (2021b) discretize the EuCRUST-07 model of Tesauro et al (2008) and the fully three-dimensional, high-resolution P-wave velocity model of Diehl et al (2009) for the central Alpine region. In addition, information on Moho depth in the Alpine region is refined using the study of Spada et al (2013).…”

“…We refer to this as the standard lithosphere model of continental subduction to distinguish it from a new model here involving the subduction and partial detachment of much thicker, compositionally heterogeneous European mantle. We base this model on recent P-wave images of the AlpArray seismological campaign (Paffrath et al, 2021b) presented below.…”

“…In this paper, we interpret vertical and horizontal tomographic slices of the Alps generated by integrating crustal and mantle tomographic P-wave models gleaned from the AlpArray seismological network (Hetényi et al, 2018). This new method, described in the next section and in detail in Paffrath et al (2021b), employs teleseismic tomography and integrates the crustal and uppermost mantle models of Diehl et al (2009) and Tesauro et al (2008) as a priori information into the teleseismic inversion, weighted according to its reliability. This allows us to image the Alpine slabs and their potential connections with the orogenic edifice as well as the foreland and hinterland lithospheres at an unprecedented resolution.…”