SLAB HOLES IN SUBDUCTION ZONES: A NUMERICAL INVESTIGATION

Subduction zones contain many important but not well understood phenomena. One curious feature is holes, windows, or tears in the subducting plate, the origin of which is highly debated. Visualized almost exclusively from seismic tomography, there is evidence for these slab holes in South America, the Cascades, East Java, and Anatolia to name a few. In each of these cases the interpreted slab holes can have vastly different hole geometry because the initiation of a hole is different for each case in time and space. This research is interested in the overall geologic implications of slab holes and the geometric differences.

We use the geodynamic coding package 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 general cases featuring holes of various sizes and apply them to individual subduction zones like those named above. In each model 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, and surface topography. Variations in hole geometry allow variations in heat flux into the mantle wedge and upper subducting lithosphere, as well as the intrusion of melt at slab hole edges. 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 the position of the subduction zone through time, as well as changes in plate velocities with various hole geometries. Observations also show there is a significant influence on subduction zone morphology when a hole is introduced, primarily the folding behavior of the slab at depths nearing the lower mantle, which alters the direction and magnitude of trench migration. Overall, we find that 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.