3D instantaneous dynamics modelling of the surface motion associated with East European subduction zones

Abstract

Forecasting earthquakes is a prime future target of Solid Earth research to which geodynamics research can contribute by providing a deep understanding of the physical drivers generating the lithosphere stress driving earthquakes. Processes confined to the lithosphere are known to cause stress build-up. However, the contribution of the Earth’s mantle is less substantiated. Assessing the contribution of deep dynamic drivers to lithosphere deformation can be done by numerical modelling by creating predictions of deformation that are then tested against observations, e.g., the GNSS surface motion field. Here we present the findings of 3D geodynamic modelling of the Vrancea (east Carpathians) and Aegean subduction zones in south-eastern Europe. Two differing tectonic settings - in Vrancea seismically active continent-continent collision and in the Aegean roll-back subduction- of differing scale are present, providing a suitable case to test interplay of mantle drivers and their effect on surface/crustal flow. To this end a 3D initial temperature-density model is designed from published lithospheric thickness and tomography models down to a depth of 800 km. We solve the equations describing the conservation of mass and momentum pertinent to a viscoplastic continuum, using the finite-element code ASPECT (Kronbichler, Heister and Bangerth 2012). Surface motion predictions from these models are then validated against the observed GNSS field (Global Navigation Satellite System, such as the Global Positioning System (GPS)). First a reference model is constructed that shows a good first-order fit of the crustal flow field, namely the characteristic westward movement of Anatolia and a rotation to the SW of the Aegean region towards the Hellenic trench. Predictions of the flow in the upper mantle show that the surface flow is correlated to the predicted mantle flow pattern and models without the Aegean slab fail to explain the rotation of the Aegean region. Experiments on the seismically active Vrancea slab cannot show differences in predicted surface observables between a slab that is continuous or a detached slab, but the models predict that a continuous slab experiences more resistance from the mantle against slab dragging by the Eurasian plate which could increase seismic activity. Second, experiments are performed to determine the sensitivity of the surface flow field for the make-up and rheology of the lithosphere and slabs. Potential model improvements such as STEP faults and a rheologically heterogeneous Eurasian plate are explored. Furthermore, the 3D models provide novel insight into the correlation between mantle flow and the pattern of seismic mantle anisotropy in subduction zones, as well as the 3D interaction between mantle flow, basal tractions on the lithosphere and tectonic plate motions. This research provides a steppingstone to more detailed studies of subduction plate boundary regions, which may lead to a better understanding of the physical drivers of crustal deformation and flow and may provide constraints for future seismic-hazard modelling.