What drives and resists plate sinking through the transition zone?

Mantle circulation is largely driven by the sinking ('subduction') of cold and dense tectonic plates. When these cold slabs reach the transition zone between the upper and lower mantle (from 400 to 800 km depth), their progress is hampered by rapid increases in mantle density and viscosity, as mantle minerals change phase. X-ray type images of the interior of the Earth made using earthquake waves ('seismic tomography') reveal that this zone only forms a barrier for some slabs, while others seem to pass through unhindered. Furthermore, when the seismic images are compared with plate motions through time, it becomes clear that slabs penetrated the lower mantle in the past, where shallower parts of the plate are trapped in the transition zone today. This, as well as evidence of fast and slow subduction phases in plate motions (Goes et al., Nature 2008) indicate that this is a time dependent process, where plate material may pond until a critical mass of material has accumulated, and then flush rapidly into the lower mantle.

It is important to understand this fundamental part of mantle circulation as it controls how efficiently the Earth cools and how well heterogeneities like sediments, crust, fluids and CO2 are mixed into it, or brought back up. Furthermore, sudden slab flushing events into the lower mantle have been linked to periods of continental crust formation, changes to the early atmosphere and reorganisations of plate motions.

We will investigate slab behaviour in the transition zone using 3D dynamic models of subduction, and evaluate which of the modelled mechanisms are consistent with observational data from the Pacific. Previous numerical models have investigated how individual factors like slab strength, slab density, coupling to the upper plate, and mantle phase transitions affect whether a slab goes straight through the transition zone or stalls there. However, none of these individual parameters can explain the observed variations of plate-transition zone interaction. Nor is it clear for how long slabs may be stalled, and hence on which time scales the upper and lower mantle mix, and on which time scales plate motions and accompanying surface deformation vary.

As single properties do not explain the variability of slab-transition-zone interaction, the interplay between mantle, downgoing and upper-plate properties must be crucial. Studying the interaction between these different factors requires 3D dynamic models that let plate motions and slab morphology develop freely, something that is numerically challenging. In recent years, our groups developed such dynamic models and elucidated how combinations of plate density, strength and width control upper-mantle slab morphology.

The newest generation of these dynamic models, developed by the group of the PI at Durham, are now capable of modelling all potentially relevant plate and mantle parameters. With these models, we will explore a wide range of parameters to determine which combinations lead to slab ponding and penetration. Next we will compare modelled conditions for stalling and release with those that can be inferred for major subduction zones throughout the last 100-200 million years of Earth history from seismic tomography, plate motion histories and earthquakes in downgoing slab. This will be done using the expertise in model-data comparison in the group of the Co-I at Imperial, in collaboration with partners Prof. Spakman and Prof. Torsvik, leading experts in seismic imaging and plate motion reconstructions, respectively.

With these new models and this interdisciplinary team, we will be able to answer the fundamental question of how the transition zone traps and releases subducting slabs, a process that plays a pivotal role in the Earth's internal and plate-tectonic evolution.

Who will benefit from this research?

The main beneficiaries of this research will be the geodynamic and other Earth scientific communities. The UK has a long record of excellence in almost all aspects of subduction related science, including geophysical imaging, geochemical characterization, earthquake and volcano research and hazard assessment, climate science, and modelling of the lithosphere and mantle. This has led to recent efforts to set up a national program that would link together the subduction work from different groups within the UK (led by PIs from Universities of Durham, Bristol, Cardiff and the British Antarctic Survey). Our work complements existing modelling efforts in the U.K., with its fully dynamic and regional mantle-scale approach. Results from the proposed work will facilitate future efforts to integrate U.K. subduction work.

The primary non-academic beneficiaries are the students from secondary to undergraduate level and the general public in the UK and elsewhere. The operation of plate tectonics and subduction is of primary importance for the thermal evolution of the Earth, the composition of the oceans and atmosphere, and the occurrence of the largest earthquakes and most hazardous volcanic eruptions on Earth. These topics naturally fascinate school children and students, and therefore form an excellent way to engage them with science. The changing surface of the Earth through time forms part of the secondary-school national science curriculum, and A-levels in geography and geology.

There is substantial interest from the general public in plate tectonics, and in how it shaped the surface of our Earth, including Britain (which was assembled by subduction about 450 million years ago), and in how it generates mountains, earthquakes and volcanic arcs. This was demonstrated by the invitation to explain our Nature results on BBC radio 4's Material World (broadcast 3 March 2008).

How will they benefit from this research?

We will set up a website which features movies and images from our research with broadly accessible explanations. Through outreach programmes to schools, as coordinated by Dr. Paula Martin in Durham (some salary requested), and including Imperial's involvement in the BGS-led Seismology-in-Schools project, students (as well as their teachers) will directly benefit from this research. In addition, we propose to develop a publicly available and intuitive user interface to the codes (see 'Pathways to Impact' for more information), This will allow secondary school children and undergraduate students, at different levels of difficulty, to perform their own experiments to obtain insight in why plates move. In this era where sustainability of our natural environment plays such an important role, it is essential that the next generation of scientists will be raised with a proper concept of how our planet operates, and plate tectonics and subduction are key processes in our changing planet. In addition, through public lectures, the interested attendant will be provided with cutting-edge research and state-of-the-art insights in the plate tectonics