Study of the asthenospheric flow around cratonic keels from numerical modelling.

In this work, the thermochemical convection in the terrestrial mantle and the relative displacement between the asthenosphere and lithosphere were simulated to study the mantle flow around cratonic keels. These processes are probably related to the preferential orientation of seismic anisotropy in the upper mantle. Numerical models were performed using the program package developed by Sacek (2017), where the Stokes Flow for an incompressible Newtonian fluid is numerically solved. The keel geometry was based on seismic P-wave time-tomography results in the region of the Paraná Basin region in Brazil (Rocha et al., 2019), where high-velocity anomalies are related with a cratonic block and high amplitude can be related with thicker regions. The results allowed us to observe that the velocity vector directions resulting from the asthenospheric flow are oriented around the thickest portion of the cratonic lithosphere. We observed that the viscosity structure in the upper mantle is a predominant factor that governs the flow pattern in the asthenosphere. For low viscosity values (< 5x1019 Pa s), the small-scale convection cells (SSCC) in the asthenosphere around the cratonic keel can suppress the flow induced by the relative movement of the lithosphere and the lower mantle, resulting in a chaotic asthenospheric flow pattern without a clear relation with the horizontal movement of the lithosphere. On the other hand, higher mean viscosity values for the asthenosphere decreases the vigor of the SSCC, resulting in a flow pattern correlated with the horizontal movement of the cratonic lithosphere relative to the surrounding mantle.