Strength profiles of subduction megathrusts and implications for the seismogenic zone

Melodie French, Rice University
Cailey Condit, University of Washington
Jonathan Delph, Purdue University

Abstract

Strength profiles of continental and oceanic lithosphere provide first order predictions for the depth of the frictional to viscous deformation and the corresponding transition from localized brittle to distributed ductile deformation within a single lithology. These predictions are generally well-supported by field studies of exhumed rocks. However, despite the utility of this approach, a similar understanding of the frictional to viscous transition for the subduction megathrust and its relationship to the brittle to ductile transition of the different lithologies does not exist. This is because of the lithologic complexity along the subduction megathrust and differences between subduction zones in sediment input and thermal gradient. Here, we summarize what is known about the constitutive relations of the most abundant lithologies along the plate boundary. We create strength profiles for 6 segments of 3 active subduction megathrusts considering differences in thermal gradient and sediment inputs. We find that along the megathrust, the transition from dominantly frictional to viscous deformation is predicted to occur due to a change in the weakest lithology rather than a transition in mechanism within a single lithology. We find the products of metasomatism to be weakest and deform by frictional mechanisms at relatively shallow depths and that a mechanical transition to viscous deformation occurs when the strength of pressure solution creep and dislocation creep occurs at lower stress in metapelites. To a first order, the depth of these transitions correlates with the depth of the seismogenic zone and provides constraints on the conditions of deep slow slip.

Session

Subduction zone geology