Understanding the Structure and Evolution of Oceanic mantle lithosphere using 2D geodynamic models

Understanding the structure and dynamics of oceanic lithosphere is essential for unraveling the processes of plate formation and mantle evolution. Heat-flow and bathymetry observations over aging oceanic lithosphere, suggest that oceanic plates conductively cool and thicken up to a given age (Hasterok, 2013, Lucazeau 2019).  More direct observations of the lithosphere-asthenosphere boundary (LAB) come from seismological observations of the LAB across different oceanic basins. Surface wave tomography studies of shear wave velocity and azimuthal anisotropy interpretations reveal that the oceanic lithosphere thickness is strongly age-dependent, primarily controlled by its thermal structure (e.g., Burgos et al., 2014, Beghein et al., 2014). In contrast, radial anisotropy observations representative of the lattice preferred orientation of olivine indicate that, for ages > 50Ma, the interpreted anisotropy gradient is at nearly constant depth of ~70-80 km (Burgos et al., 2014). This apparent age-independence of radial anisotropy diverges from the age-dependent patterns observed in azimuthal anisotropy and isotropic velocities and can be an artifact of tomography inversion techniques (Beghein et al., 2019, Kendall et al., 2022) or representative of distinct processes shaping the oceanic lithosphere during its evolution (Hansen et al., 2016). This discrepancy along with observations of scattered wave imaging of LAB-related discontinuities (e.g., Tharimena et al., 2017) and active source seismic observations of oceanic lithosphere (e.g., Adhukasi and Singh, 2022) raises important questions about the thermo-mechanical definition of the lithosphere, how it differs from the weaker asthenosphere below, and what constitutes the LAB. To address these questions, we use 2D geodynamic models to investigate the thermal and viscosity evolution of the oceanic lithosphere, from continental breakup to oceanic plate formation. Our goal is to reconcile these contrasting seismological observations with geodynamic model results to enhance our understanding of the processes that influence the structure of the oceanic lithosphere.