Estimating forces and rheology from kinematic observations in the western Mediterranean

Abstract

The Gibraltar Arc (GA; Western Mediterranean) is located in between the slowly converging African and Eurasian continents and runs from the Rif in northern Morocco to the Betics in southern Spain. Geodetic velocities in this wide plate boundary region are different from the motions of both stable Eurasia and Africa. Various lithospheric and asthenospheric drivers have been called upon to explain the observed surface motions. We constrain the magnitude of these driving forces and the effective mechanical properties of the lithosphere and fault zones using regional geodynamic Finite Element models (FEMs). All of our FEMs have lithospheric forces that arise from lateral variations of gravitational potential energy, mainly from lateral variations in topography and Moho topology. They also all include far-field Africa-Eurasia convergence. The well-imaged Gibraltar slab suggests that slab pull and/or slab suction may contribute to the surface motions. Also, tractions that arise from lateral transport of the slab through the upper mantle are potentially relevant. We adopt parameterized versions of these slab forces in the FEMs. In our FEMs, we impose shear tractions on observationally constrained fault zones.
We quantify the model parameter resolution and tradeoffs in light of the available kinematic surface observations with a Markov Chain Monte Carlo (MCMC) approach, implementing the Metropolis-Hastings algorithm. We compare the model results to the geodetic velocity observations, which have the smallest error margins of all the kinematic observations, and to the sense of shear along (potentially) active fault zones. Preliminary results based on a search of 60,000 models indicate that the observed GNSS velocity field and sense of slip on regional faults in the Gibraltar Arc appear to result mainly from Africa-Europe plate convergence and lateral GPE variations. Slab pull from the Gibraltar slab is very likely transmitted poorly into the overriding plate and probability distributions for the trench suction force do not display any favorable value to affect the kinematics in the Gibraltar Arc region. The best models have rms-misfits of 0.23 mm/yr, which is mostly due to a systematic SW motion of up to 4 mm/yr in the SW Rif.