Viability of Archean Subduction Initiation from Gravitational Spreading

The development of plate tectonics and Earth's early tectonic environment are currently not well understood. Modern plate tectonics are characterized by the sinking of dense lithosphere at subduction zones; however this process may not have been feasible if Earth's interior was hotter in the Archean, resulting in thicker and more buoyant oceanic lithosphere than observed at present (van Hunen and van den Berg, 2008). Previous studies have proposed gravitational spreading of early continents at passive margins as a mechanism to trigger early episodes of plate subduction (Rey et al., 2014). This study utilizes 2D thermo-mechanical numerical experiments using the finite element code MVEP2 (Kaus, 2010; Thielmann et al., 2014) to investigate the viability of this mechanism for subduction initiation with Archean mantle conditions. The model is comprised of a 55-km-thick continent above 170 km of strongly depleted lithospheric mantle and surrounded by a 15-km-thick oceanic lid. A range of possible densities and viscosities were investigated for the different layers, and results show that lithospheric stresses may vary between 250-750 MPa. Because lithospheric stresses are crucial to subduction initiation, the model includes elasticity in order to better accommodate this large stress range. This model also includes free-surface boundary conditions to allow the development of isostatic topography, which is a factor not considered previously. Preliminary results indicate that the magnitude and location of lithospheric stresses varies for cases with and without isostatic uplift being taken into account. Subduction initiation was previously shown to occur due to large intra-lithospheric gravitational stresses from a spreading continent, but these intra-lithospheric stresses may be insufficient in cases with lithospheric elasticity and continental uplift. Critical factors are expected to be the magnitude of continental buoyancy and the degree of decompression melting from the depleted mantle layer.