THREE-DIMENSIONAL NUMERICAL MODELS OF COMPLEX SUBDUCTION ZONES AND IMPLICATIONS FOR GEODYNAMICAL PROCESSES

In the southern Andes, the downgoing Nazca plate has adjacent zones with slab dip angles of 10° and 30°. To better understand the nature of mantle flow across the transition region between the juxtaposed angles, we calculate a steady-state, three-dimensional finite element numerical model using the software package COMSOL. Models solve the conservation equations of mass, momentum, and energy, neglecting heat production and thermal buoyancy. Isoviscous solutions explore first-order patterns of mantle flow. The overall three-dimensional model domain contains a rigid overlying plate, two subducting slabs (with dips of 10° and 30°), and a mantle wedge with a geometry that changes in the trench-parallel direction. The model space is generated by using two-dimensional solutions as boundary conditions for the trench-perpendicular “endcaps” of the numerical domain. The sensitivity of the models to overriding plate thickness, coupling between the downgoing and overriding plates in the wedge corner region, and convergence velocity is also investigated. The models predict a significant amount of trench-parallel flow as a result of the juxtaposition of the two slab angles. The three-dimensional variations in the calculated temperature and velocity fields have implications for plate deformation processes, seismic anisotropy, and seismicity for settings similar to the southern Andes, as well as other global settings in which subduction geometry varies significantly along the strike of the trench.