Towards understanding the dynamics of subduction zone diversity

Subduction zones play a key role in the tectonic and chemical evolution of the Earth and are the site of the largest earthquakes and most explosive volcanic eruptions. Subduction zones are diverse, varying in rates, shape of the trench and slab, relative contribution and direction of trench motion versus plate motion, coupling between the two plates, and state of stress of the upper plate.

To understand what controls such diversity, my group and several others have been building a systematic understanding of subduction dynamics, starting from the simplest system of a single subducting plate, free subduction.  Free subduction models illustrate how sensitive the subduction system is to the balance between slab density which drives it, the resistance of the plate to bending at the trench (and base of the transition zone) and drag by the mantle below the unsubducted plate and around the slab. Tectonic reconstructions and seismic tomography show that in response to the extra resistance to sinking encountered at the transition to the lower mantle most slabs retreat and flatten. Such observations constrain the magnitude of slab strength relative to the other forces. Any dynamic models of subduction, even for investigating more complex dynamic settings, need to ensure plate properties yield such an earth-like sinking mode of subduction.

Varying trench shapes can be understood from variations in plate width, which lead to simple C-shaped trenches for small subduction zones or W-shapes for trenches that are long relative to slab bending lengths. Although slab pull is the dominant driver of mantle convection, local (plate-age) dependent slab buoyancy does not have a strong expression in trends of plate and trench velocities, indicating the importance of considering spatially varying plate buoyancy. Models show that buoyant features such as aseismic volcanic ridges can lead to either slab steepening or flattening depending on the background plate buoyancy and strength and position relative to the free-subduction shape of the trench. Together, these factors explain quite a bit of the complexity seen in natural subduction zones. Further influences come from global plate interactions, which limit the motions of upper and lower plates, and mantle flow including upwellings and flow driven by previously subducted slab remnants.

The resulting imbalance between the bending a free slab tries to achieve and the bending it undergoes to adjust to the net forces acting on the system affects how much of the deformation is viscous versus elastic. An initial study showed that a measure of this elasticity (the Deborah number) may correlate with the proportion of larger relative to smaller intraplate earthquakes.  In my talk, I will present a summary of some of these key previous insights into subduction dynamics and natural examples.