A limited effect of continents on subduction initiation and lithospheric stresses

Earth is the only planet where plate tectonics takes place. Despite significant study over the past few decades, when and how plate tectonics initiated on Earth still remains contentious. Different lines of evidence from the geologic record suggest that the rapid growth of continental crust in the Archean is linked to plate tectonics initiation. In addition, the rocks from Acasta Gneiss Complex even support the idea that incipient continental blocks act as nuclei for subduction. Icelandites, the rocks formed in modern Iceland-like geological settings, formed in shallower depth than the classic Archean TTGs. The Icelandites represent primitive continents and might initiate the subduction at the continent margins and start to produce the Archean TTGs in deeper depths. Prompted by this evidence, we used numerical models of mantle convection with an imposed continent block to test whether continents facilitate subduction initiation due to the stress developed at their margin. Our models assume a grain-damage rheology, so that weak shear zones form by grain size reduction driven by the deformation work done by mantle convection. Our results show that simply including a continent does not contribute to the enhancement in time averaged damage or plate speed. Subduction also does not preferentially initiate at the continent margin. We provide a new plate tectonics diagram showing no significant shift in the boundary between mobile lid and stagnant lid regimes when continents are added. However, even though continental blocks do not necessarily lead to subduction initiation, especially at their margins, we found that stresses are modestly enhanced at the continent margin. These enhanced stresses also lead to additional grain damage and rheological weakening, just not enough to cause subduction initiation. We developed a scaling law for the stress enhancement due to a continental block as a function of the model parameters (e.g., continent thickness, viscosity activation energy, Ra, and viscosity jump between a continent and surrounding mantle). From this scaling analysis we can estimate the amount of lithospheric damage at continent margins that is possible from the continents buoyancy forces. This scaling analysis helps explain why damage enhancement at continent margins is not sufficient for subduction to initiate.