A tensile, flexural model for the initiation of subduction

Abstract

We argue that subduction may be initiated at passive continental margins without shortening the lithosphere. Overcoming the lithosphere's high compressive strength requires special circumstances, and these make it difficult to explain the nearly complete recycling of old sea-floor. Instead, we present a model that predicts tensile decoupling of the continental and oceanic lithosphere, passive rifting, and foundering of the seafloor beneath material welling up in the rift. This occurs because the lithosphere in a new ocean basin establishes mechanical continuity with the continent at a depth comparable to mid-ocean ridges. Later subsidence at the margin is therefore inhibited by flexure, which implies shear stresses that promote fault slip and tensile stresses necessary to balance the component of the plate's weight directed down the margin slope. We show that this tension can more than offset ridge push. In our model, an important additional tension arises from basal shear tractions resisting the plate's motion away from the mid-ocean ridge, although these tractions cannot be evaluated with confidence. Slip on a high-angle fault decouples the oceanic and continental lithosphere when shear stresses arising from flexure and the applied tension exceed the lithosphere's shear strength under these loads. A passive rift then forms, allowing a mantle column to rise to the height of mid-ocean ridges, over 3 km above the old seafloor, and flow onto the surface in a gravity current. This load flexes the plate downwards, which enhances the flow and lets the old oceanic lithosphere founder. This model is consistent with the presence of oceanic material in continental forearcs and the youth of ophiolites when they are obducted, as both might be explained by rifting a margin and underthrusting the juvenile crust formed there. Boninites in ophiolite complexes and tectonites at their bases show depleted and hydrated geochemistries consistent with the melting of rifted mantle lithosphere that receives volatiles from foundering oceanic crust.