Thermomechanical modeling of mantle dynamics and plate motion in the Mediterranean
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Abstract
Over millions of years, the Mediterranean has been shaped by various geological pro
cesses. The region comprises several rollback subduction systems, some still actively
retreating, and is influenced by the convergence of the African plate in the south and
the Eurasian plate in the north. The dynamic behaviour is also reflected in the crust,
as the region is characterized by numerous orogens as well as a complex plate motion
pattern. The Adriatic microplate is located in the center of the Mediterranean and is
bounded by the African plate and the Eurasian plate. This configuration makes the
Mediterranean, and in particular the Adriatic microplate, an ideal setting to investi
gate the interplay between mantle dynamics and plate motion.
In this work, 3D thermomechanical modeling of the Mediterranean region is used
to investigate the complex geological history of this region over the last 35 Myr.
The results show that the subduction zones strongly interact with each other. The
retreat of both the Apennines-Calabrian subduction system to the west of Adria and
the Dinaric-Hellenic subduction system to the east of Adria leads to an increased
dynamic pressure under the Adriatic microplate. This results in mantle flow to the
north and south. In the north, the mantle flow is blocked by the Alpine slab, so that
mantle flow parallel to the Alpine trench emerges starting at 20 Ma. Another aspect
of the study is the plate motion of the Adriatic microplate. The results indicate that
this is primarily controlled by the convergence of the African and Eurasian plates, the
horizontal distance between the Calabrian and Hellenic trenches and the Alpine slab
retreat to the north of Adria.
The simulation results are used to calculate synthetic shear-wave splitting param
eters to quantify seismic anisotropy. The comparison with observations reproduces
f
irst-order features, such as the trench-parallel orientation of the fast polarization di
rection south of the Alpine slab. However, the comparison between the model and
observations is not perfect, as some regions, such as the Liguro-Provençal Basin, ex
hibit differences. These differences may stem from features absent in the model, such
as a slab gap of the Apennines slab or lower mantle structures. Overall, the compar
ison demonstrates the value of shear wave splitting observations as an observational
constraint and shows how the model results can help interpret observations.
Thermomechanical simulations typically involve a large number of material param
eters, and it is often difficult to quantify the individual influence of each parameter on
a specific output of interest, such as the velocity of a subduction plate. With the dis
crete adjoint method, it is possible to calculate sensitivities that show the influence of
material parameters on a specific output of interest. Here, we illustrate the implemen
tation of the discrete adjoint method with automatic differentiation. This approach
has the advantage that it allows functions from the Stokes solver to be reused, en
abling the design of a problem-agnostic adjoint solver. The resulting sensitivities are
scaled to allow a comparison between different parameters. Finally, this method is
applied to a 2D subduction zone with a nonlinear visco-elastic rheology, where the
effects of slab pull and ridge push on the plate motion are compared quantitatively.