Geophysical Institute of the ASCR

Fig. 16. Initial results of the shear-wave splitting measurements evaluated from data of the RETREAT array
(green diamonds), permanent observatories (yellow diamonds) and the NAP (North Apennine Profile – blue
diamonds). Thick symbols are the very high quality measurements, thin symbols are fair measurements. The big
arrows on the top mark the SW-NW (blue) and N-E (red) azimuths of arriving shear waves. The green dashed
lines mark boundaries of domains of the mantle lithosphere with similar orientation of anisotropic structures
resulting from the initial stage of our study.
c) We have applied the three-dimensional analysis of anisotropic parameters of body waves to the
Northern Apennines, the tectonically active region affected by a progressive collision between the
Eurasian and African plates, associated with a steady growth of the mountain range. Simultaneous
extension in the convergent margin of the Adriatic and Tyrrhenian domains results in a retreat of
the subducting Adriatic plate from the orogenic front caused by sub-lithosphere mantle processes.
The study will contribute to developing a 3D self-consistent dynamic model of the syn-convergent
extension in the Northern Apennines within the multidisciplinary international project RETREAT.
Shear-wave splitting evaluated from recordings of the RETREAT array, formed by the network of
permanent and 35 portable stations, evidences seismic anisotropy within the upper mantle. Ten of
the portables are equipped by STS2 seismometers and GAIA data acquisition systems belonging to
the MOBNET of the GI. Initial analysis of the splitting reveals both distinct lateral variations of
anisotropic parameters and their dependence on the direction of propagation within the upper
mantle (Fig. 16). Variations of the splitting time delays and orientation of the fast shear waves
exclude a simple sub-lithosphere mantle flow, associated with the Apennines subduction, as the
only source of the observed effective anisotropy. A part of the anisotropic signal, observed both in
the shear-wave splitting and the P-residual spheres, can be generated by a frozen-in fabric of the
Adriatic and Tyrrhenian lithosphere domains (Plomerová et al., 2005c).
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