Exploring the structure and dynamics of slab holes in subduction zones

Numerical subduction models attempt to explain certain subduction phenomena and accelerate our understanding of subduction dynamics. Seismic tomography has shown that certain subduction zones feature slab holes, tears, and windows. These slab hole features and their influence will be explored through several mechanical and thermomechanical models after the initiation of subduction.

We use the geodynamic code Underworld to model subduction zones with various hole geometries and examine the surface expression of such features. We run models with and without an over-riding continental lithosphere, which produce various subsurface (and surface) topographical and geological features of interest. We examine many impacts of slab hole geometry, such as the velocity of the over-riding and subducting plate, perturbations in the ambient mantle viscosity and velocity, trench migration, surface topography, and others. These features are examined by varying the hole size and shape, which can allow different values of heat flux into the mantle wedge and upper subducting lithosphere, as well as intrusion of additional melt at slab hole edges. Thermomechanical models are used to visualize the melt field around the slab hole, giving us an idea of how fast a hole could propagate and influence the surface topography and volcanism. With a larger slab hole present, we see larger surficial expressions and perturbations in surface topography, and many observations indicate there are also compositional changes to surface geology above subduction zones. We examine changes in position of the subduction zone through time, as well as changes in plate velocities when we vary the hole geometry. Observations show there is a significant influence on subduction zone morphology when a hole is introduced, primarily the folding behavior of the slab at significant depths, which alters the direction and magnitude of trench migration. Slab holes influence the behavior of a subduction zone from initiation to cessation by inducing further interactions between the mantle and overlying lithosphere, modifying the locations and magnitudes of buoyant forces present in these regions. We compare our numerical results to other laboratory subduction experiments to better understand the influence of slab holes across all scales.