Slab detachment under the Eastern Alps seen by seismic anisotropy

Abstract

We analyze seismic anisotropy for the Eastern Alpine region by inspecting shear-wave splitting from SKS and SKKS phases. The Eastern Alpine region is characterized by a breakdown of the clear mountain-chain-parallel fast orientation pattern that has been previously documented for the Western Alps and for the western part of the Eastern Alps. The main interest of this paper is a more detailed analysis of the anisotropic character of the Eastern Alps, and the transition to the Carpathian–Pannonian region. SK(K)S splitting measurements reveal a rather remarkable lateral change in the anisotropy pattern from the west to the east of the Eastern Alps with a transition area at about 12°E. We also model the backazimuthal variation of the measurements by a vertical change of anisotropy. We find that the eastern part of the study area is characterized by the presence of two layers of anisotropy, where the deeper layer has characteristics similar to those of the Central Alps, in particular SW–NE fast orientations of anisotropic axes. We attribute the deeper layer to a detached slab from the European plate. Comparison with tomographic studies of the area indicates that the detached slab might possibly connect with the lithosphere that is still in place to the west of our study area, and may also connect with the slab graveyard to the East, at the depth of the upper mantle transition zone. On the other hand, the upper layer has NW–SE fast orientations coinciding with a low-velocity layer which is found above a more-or-less eastward dipping high-velocity body. The anisotropy of the upper layer shows large-scale NW–SE fast orientation, which is consistent with the presence of asthenospheric flow above the detached slab foundering into the deeper mantle.

P wave anisotropic tomography of the Alps

“…The isotropic tomography is simply a snapshot of the Earth's structure, whereas seismic anisotropy provides information on the time‐integrated history of mantle flow. Mountain chain‐parallel FVDs beneath the Alps have been revealed by the previous studies [ Barruol et al , , ; Kummerow and Kind , ; Lucente et al , ; Bokelmann et al , ; Qorbani et al , ], which are interpreted as the pervasive asthenospheric fabric controlled by the shape of the subducting European lithosphere. Similar to the FVDs beneath the Alps, mountain chain‐parallel FVDs have been found in other mountainous regions [ Bormann et al , ; Lavé et al , ; Barruol et al , ; Ivan et al , ].…”

“…Qorbani et al [] show that two layers of azimuthal anisotropy exist in the Eastern Alps. The upper layer is characterized by NW‐SE FVDs, whereas the lower layer exhibits NE‐SW FVDs, similar to those in the central Alps.…”

“…The Alps, a product of continental‐continental convergence and collision between the European and Adriatic plates since the Mesozoic [ Qorbani et al , ; Handy et al , , and references therein], is an arc‐shaped mountain chain which can be divided into four parts, the Western Alps, the central Alps, the Eastern Alps, and the Southern Alps. The subduction of the European plate beneath the African plate initiated at ~80 Ma ago, and the collision between the European and Adriatic plates started at ~35 Ma ago, followed by uplift of the Alpine orogenic belt after 23 Ma [ Castellarin and Cantelli , ; Schmid et al , ; Handy et al , ; Bianchi et al , , and references therein] .…”

“…Much information on the tectonic processes of the Alps has been provided by seismic tomography studies made so far [ Lippitsch et al , ; Piromallo and Morelli , ; Piromallo and Faccenna , ; Kissling et al , ; Koulakov et al , ; Mitterbauer et al , ; Zhu et al , ; Zhao et al , ; Zhu et al , ; L. Zhao et al , ]. These previous studies have revealed two lithospheric roots exhibiting high seismic velocity (high‐V) anomalies, which are located beneath the central western Alps and the Eastern Alps [ Qorbani et al , ]. The two high‐V roots are separated at about 12°E longitude.…”

“…The results of Bokelmann et al [] show that the FVDs beneath the whole Alps are all mountain chain parallel. However, Qorbani et al [] revealed two layers of seismic anisotropy beneath the Eastern Alps, where the anisotropy pattern of the deeper layer is similar to that of the Western Alps which exhibits SW‐NE FVDs, whereas the shallow layer has NW‐SE FVDs. By using the ambient noise method, mountain chain‐parallel FVDs in the crust and orogen‐normal FVDs below the Moho are revealed [ Fry et al ., ].…”