Subduction Initiation

The Initiation of Subduction at an Old Fracture Zone.

Post Doctoral Scholar Chad Hall, working with Michael Gurnis and Luc Lavier, has recently discovered a mechanism for initiating an entirely new subduction zone, perhaps the most important unsolved problem in plate tectonics. Using a visco-elastoplastic model, they show that a fracture zone could be converted into a self-sustaining subduction zone after approximately 100 km of convergence. The entire system has a realistic rheology (non-linear, temperature-dependent, and visco-elastic). Modeled initiation is accompanied by rapid extension of the over-riding plate and explains the inferred catastrophic boninitic volcanism associated with Eocene initiation of the Izu-Bonin-Mariana (IBM) subduction zone. They estimate that the forces resisting IBM subduction initiation were substantially smaller than available driving forces at the time.

Hall and company then teamed up with Maria Sdolias and Dietmar Müller at Sydney University to show that the initiation of the IBM arc may have been preceeded by a change in the relative motion of the Pacific Plate with respect to Eurasia between 55 and 45 Ma. They were able to make their inference using global plate reconstructions.

We show an animation of the evolution of an old fracture zone in cross section in which we push on the sides at 2 cm/yr. In this animation, red regions are hot (and have a low effective viscosity) while the blue to green region is cold and has a high viscosity.

We also have another animation of the same model in which deviatoric stress is plotted. In this animation, red regions are under high stress while regions in blue have a low stress. To view this animation, press here.

Reference

Additional references of related material can be found here.