Searching for Order in the Jungle: Probabilistic Assessment of the Causes of Active Deformation in the East Central Mediterranean Using Spherical Finite Element Models

Abstract

Plate boundary deformation zones represent a challenge in terms of
understanding their underlying geodynamic drivers. Active deformation is
well constrained by GNSS observations in the SW Balkans, Greece and W
Turkey, and is characterized by variable extension and strike slip in an
overall context of slow convergence of the Nubia plate relative to
stable Eurasia. Diverse, and all potentially viable, forces and models
have been proposed as the cause of the observed surface deformation,
e.g., asthenospheric flow, horizontal gravitational stresses (HGSs) from
lateral variations in gravitational potential energy, and rollback of
the Hellenic slab. We use Bayesian inference to constrain the relative
contribution of the proposed driving and resistive regional forces.

Our models are spherical 2D finite element models representing
vertical lithospheric averages. In addition to regional plate
boundaries, the models include well-constrained fault zones like north
and south branches of the North Anatolian Fault, Gulf of Corinth and
faults bounding the Menderes Massif. Boundary conditions represent
geodynamic processes: (1) far-field relative plate motions; (2)
resistive fault tractions; (3) HGSs from lateral density variations; (4)
slab pull and trench suction at subduction zones; and (5) active
asthenospheric convection. The magnitude of each of these is a parameter
in a Bayesian analysis of ~100,000 models and horizontal GNSS
velocities. The search yields a probability distribution of all
parameter values including model error, allowing us to determine
mean/median parameter values, robustly estimate parameter uncertainties,
and identify tradeoffs (i.e., parameter covariances).

The average viscosity of the overriding plate is well resolved 3-4
10^22 Pa.s, which is higher than from published models without faults.
Significant trench suction forces from the Hellenic slab act on the
overriding Aegean Sea, including along the Pliny-Strabo STEP Fault. Slab
pull and convective tractions have a small imprint on the observed
deformation of the overriding plate. HGSs are necessary to explain local
features in the velocity field, particularly in the Aegean Sea, but are
less important for fitting the regional pattern of velocities.
Resistive tractions on most plate boundaries and faults are low.