P wave tomography of the mantle under the Alpine-Mediterranean area

We study the upper mantle P wave velocity structure below the Euro-Mediterranean
area, down to 1000 km depth, by seismic travel time tomography. We invert summary
residuals constructed with both regional and teleseismic first arrival data reported by the
International Seismological Centre (ISC) (1964–1995), introducing some alternative
strategies in the travel time tomographic approach and a new scheme to correct teleseismic
data for global mantle structure. Our high-resolution model PM0.5 is parameterized with
three-dimensional (3-D) linear splines on a grid of nodes with 0.5 spacing in both
horizontal directions and 50 km vertical spacing. We obtain about 26% root-mean-square
(RMS) reduction of residuals by inversion in addition to roughly 31% reduction after
summary rays formation and selection. Sensitivity analyses are performed through several
test inversions to explore the resolution characteristics of the model at different spatial
scales. The distribution of large-scale fast anomalies suggests that two different stages of a
convection process presently coexist in very close regions. The mantle dynamics of
western central Europe is dominated by blockage of subducted slabs at the 660 km
discontinuity and ponding of seismically fast material in the transition zone. Contrarily, in
the eastern Mediterranean, fast velocity material sinks into the lower mantle, suggesting
that the flow of the cold downwelling here is not blocked by the 660 km discontinuity. On a
smaller scale, the existence of tears in the subducted slab (lithospheric detachment) all
along both margins of the Adriatic plate, as proposed by some authors, is not supported by
our tomographic images.

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