Passive margins and aggressive grains: Mixing, damage and incipient subduction

Abstract:

How subduction -- the sinking of cold lithospheric plates into the mantle -- is initiated is one of the key mysteries remaining in understanding why Earth has plate tectonics. The favored location of subduction triggering at passive margins, where oceanic floor abuts a continental margin, is problematic because the strength of the old, cold ocean lithosphere should prohibit it from bending under its own weight and sinking into the mantle. Some means of mechanical weakening of the passive margin is therefore necessary. Spontaneous and accumulated grain-damage can allow for considerable lithospheric weakening and facilitate passive margin collapse. Grain damage is enhanced where mixing between mineral phases in lithospheric rocks is greatest. Such mixing is driven both by compositional gradients associated with petrological heterogeneity as well as the state of stress in the lithosphere. With lateral compressive stress imposed by ridge push in an opening ocean basin, bands of mixing and weakening can become vertically oriented and occupy a large portion of lithosphere after about 100 million years. These bands lead to anisotropic lithospheric strength that is extremely weak to bending and sinking, thereby greatly facilitating passive margin collapse.