Physical properties of deep subducted lithosphere

Basic data for this project

Description

Subduction zones form dominant tectonic features on the Earth and it is clear from observations of plates at the surface, that there should be significant three-dimensional structure and complexity within the decending lithosphere. Tomographic inversions for P- and S-wave seismic velocities as well as geodynamic modelling show that descent of so called slabs into the bottom regions of the mantle is in some cases possible. Yet direct observations of deep slabs are scarce but necessary to make statements concerning structural differences within the slab and its behaviour with depth. In this project we will test the descent of slabs, the physical attributes and internal structure of the sinking lithosphere through the use of seismic data and seismic array methods. Using earthquake-station combinations for which the direct wave travel well away from the subduction region we search for reflections off structures in the deep Earth, that travel outside the source-receiver plane and which can be found using seismic arrays. These out-of plane reflections are then analysed in terms of (amongst others) waveform, polarities, angle of incidence, frequency and travel time. We will test several different regions, including the region beneath Central America, where it is relatively certain that subduction reaches the lower part of the mantle, and the Aleutian subduction zone, where it is possible to investigate the slab from two sides when using appropriate source-receiver combinations. The main aims of this proposal are: the observation of deep subducted lithosphere and possibly crust with the use of array observations of seismic data. We also want to quantify physical properties of the seismic structures using waveform analysis and waveform modelling of the seismic data. By comparing and contrasting the observations in different regions, we aim to understand the processes that may influence the behaviour of slabs with depth. Geodynamic modelling results of different subduction scenarios as well as information on mineral physics will help to further understand the dynamics and structure of subducted lithosphere.