The long-term evolution of subduction zones: a modelling study

The motion of oceanic and continental lithosphere, volcanic activity, earthquakes, and other tectonic activities are expressions of processes in the Earth's mantle. Nowadays, it is widely accepted that these phenomena are related to convective flow in the mantle, which forms the mechanism to turn heat from the Earth's interior into mechanical work. One example of that mechanism is the creation of oceanic lithosphere at the mid-ocean ridges, which are zones of upwelling hot mantle material. At boundaries where two plates converge one of the plates, preferebly of oceanic type, is consumed by subduction into the mantle. Most of the information about the Earth's structure has been obtained by seismology. Seismological observations have demonstrated that there is a great variety in subduction zone geometries. Subducted slabs with dip angles varying from very small (e.g. Peru) to nearly 90° (e.g. Mariana) have been observed. Slabs can either stagnate at the upper-lower mantle boundary (e.g. Izu-Bonin) or can penetrate into the lower mantle (e.g. Sunda arc). Initiation or cessation of subduction are manifestations of the time-dependence of the dynamical processes in the mantle. Images of the seismic velocity distribution of the mantle interior provide an instantaneous view of time-dependent structures which must be interpreted in a dynamical context. Understanding the long-term evolution of the subduction process is the primary motivation for this study. More specifically, the aim is to investigate the sensitivity of subduction zone geometry to various parameters, as, for example, plate velocities and viscosity structure of the mantle. For that purpose we have chosen to use a computational method. Numerical studies have the advantage that parameters can be varied over a great range. Plate motions and mantle flow can be included as boundary conditions easily, particularly compared to laboratory experiments. It must be emphasized that the model used here includes several shortcomings. The major simplification is probably the neglect of the third dimension. Over the last two decades numerous studies concerning subduction zones have been performed. In Chapter 2 a summary of some influential work and recent studies from different fields of geophysical disciplines is presented. Constraints on the structure and evolution of subducting slabs obtained from seismology, geoid data, plate tectonic analysis, and numerical and experimental studies are discussed. This information forms the basis for the research presented in this thesis.