The role of continental lithosphere in subduction initiation

Abstract:

Plate tectonics is a unique feature of Earth, not found on any other planets or rocky moons in our solar system. However, how and when plate tectonics initiated on Earth remains enigmatic. Numerous studies have suggested that continents enhance stresses and deformation in the lithosphere near their margins, and therefore might promote modern-style subduction initiation by enhancing the formation of weak shear zones in the lithosphere. We perform numerical mantle convection models with an imposed buoyant continental block to test this hypothesis. While previous studies of subduction initiation were mostly done in plastic yielding rheology that only allows lithospheric shear zones to form during deformation, our models assume grain damage rheology including weak zone memory. However, our preliminary model results show that the subduction initiation does not occur at the continental margin, even though the continent causes additional damage and grain size reduction occurs at its margins. Monitoring where the gradient of surface horizontal velocity field become negative, we track the location of subduction initiation events in the models to demonstrate that these events rarely occur at continental margins. This is true when we insert continents into an already developed mobile lid mode of convection, and also when continents are included in an initially stagnant lid mode then later initiates subduction and mobile lid convection. We are developing scaling analysis for the enhancement of stresses, and therefore damage at continental margins, and for the conditions that lead to subduction initiation. We hypothesize that although continents lead to rheological weakening at their margins, their positive buoyancy resists the formation of new subduction zones.