CONTROLS ON SUBDUCTION DIP ANGLES AND THEIR EFFECTS on DEFORMATION IN THE OVERRIDING PLATE

Subduction processes have important impacts on lithospheric and crustal deformation at the surface including orogeny processes. For example, shallow dipping subduction of the Farallon plate is a potential mechanism to explain surface deformation far from the plate boundary during the Laramide orogeny. It is not clear, however, why some subducted slabs have different dip angles, and how the subduction affects topography and deformation in the overriding plate.

One approach for understanding subduction has been to assume that plate geometry is the result of steady state processes. Deformation and stresses within the mantle and in the overriding plate can then be infered as an application of fluid dynamic corner flow. Many authors have used this method to study the dips of subducting plates, stresses in the wedge and at the base of the overriding lithosphere, and seismic anisotropy resulting from flow within the mantle wedge.

Although kinematic models of subduction are very informative of the basic physics, the assumption of a steady state subduction process is likely inappropriate when many studies indicate time dependent slab behavior and more complex mantle flow. Laboratory experiments and numerical models that consider subduction as a dynamic time dependent process account for the internal deformation and buoyancy of slabs, and are hence more complete.

We examine the process of subduction to understand what causes slabs to descend at shallow dips. Our particular focuses are on the role of continental keels adjacent to subduction zones and overriding plate motion. The presence of a nearby continental keel can be expected to alter the pressure field in the wedge. Overriding plate motion can control the rate at which material subducts in relation to the rate at which it sinks into the mantle. However, it is unknown to what degree these factors can affect slab dip. We perform dynamic numerical calculations to quantify the subduction process and study the relation between keels, overriding plate motion, and shallow angle subduction.