Toward a comprehensive understanding of transition zone discontinuities: A new constraint near the stagnant slab region beneath China.

Date: 
Tuesday 24 January 2017 - 15:00 to 16:00
Location: 
NOC Southampton - Node Room (074/02) (Waterfront Campus).
Speaker: 
Dr Teh-Ru Alex Song

Toward a comprehensive understanding of transition zone discontinuities: A new constraint near the stagnant slab region beneath China.

Teh-Ru Alex Song1,Xuzhang Shen2, Lars Stixrude1, Carolina Lithgow-Bertelloni1
1. Department of Earth Sciences, University College London, London, UK
2. Lanzhou Institute of Seismology, China Earthquake Administration, Lanzhou, China

Plate tectonics and subduction operating over much of the Earth’s history can induce long- term mantle mixing, chemical heterogeneities and recycle volatiles into the mantle. While some slabs are penetrating into the deep lower mantle, there are instances where slabs are stagnated near the transition zone. Presumably, the thermochemical state of the transition zone is a consequence of delicate balance and feedback between the short-term and long- term mixing. In the region near the stagnant slab, what’s the thermochemical state of the transition zone? what’s the degree of hydration in the transition zone?

Transition zone seismic discontinuities, among all, hold the key to resolve the mystery of mass and heat transport in the Earth’s mantle as well as the composition of the Earth’s interior. However, deciphering discontinuity properties are not trivial. In previous efforts, the data are typically limited to either upper mantle triplications, converted waves (P-to-S or S-to-P) or mantle reflections (e.g. SS precursors, ScS reverberations). These observations place constraints on the velocity gradient near the discontinuity as well as discontinuity reflectivity, but they hardly offer independent information on the density jump or/and density gradient. In few cases where multiple datasets are jointly analysed to resolve the density jump, the region of sensitivity (or the fresnel zone) of different dataset does not necessarily coincide. Finally, the use of short period (~1 Hz) data (e.g., P’P’ precursors) or long period (~> 0.1 Hz) data (e.g., SS precursors) does not allow us to simultaneously address the transition width and the gradient near the discontinuity.

To complement previous efforts and circumvent aforementioned difficulties, we advocate a simple and effective strategy. Specifically, we involve broadband direct converted waves (e.g., P410s, P660s) and the topside reflections (the multiples, e.g., PpP410s, PpP660s) in the context of P wave receiver function technique. Such a tactic not only minimizes tradeoffs between velocity and density jumps, but also allows self-consistent estimates of the shear velocity jump, the density jump, the transition width and the velocity/density gradient near the boundary. In this talk, we will detail our first attempt near the region of stagnant slab beneath Chinese continent. These new observations, along with the thermodynamic framework, HeFESTo, allow us to test and validate hypotheses including the state of mantle mixing and equilibrium, compositional heterogeneities and the degree of hydration in the transition zone. 

Seminar category: 
G3 Seminars