Seismic observations of mid-mantle discontinuities on a global scale: implications for convection and composition

Seismic observations of mid-mantle discontinuities on a global scale Dr Lauren Waszek & Dr Nicholas Schmerr University of Maryland, Department of Geology Contact: lwaszek@umd.edu Recent tomographical studies have found that some slabs stagnate at the 660 km discontinuity, whereas others stagnate at 1000 km depth. Very few slabs continue subducting into the mid mantle (Fukao & Obayashi, 2013). Conversely, upwelling material also shows deflection at various depths. These depths show some relationship to observed mantle discontinuities. The apparent transition at 1000 km depth is particularly enigmatic, as both subducting slabs and upwelling material are observed to be displaced here (French & Romanowicz, 2013). Although some recent publications suggest that the transition is a viscosity jump (Rudolph et al., 2015) or a compositional difference (Ballmer et al., 2015), the relationship to observed seismic discontinuities is unclear. Here, we present global-scale interrogations of mid-mantle discontinuities.We have compiled a large high quality global dataset of over 45,000 hand-picked SS phases. We use SS precursors to search for the presence (of lack thereof) of discontinuities in the mid-mantle, from 700 km to 1200 km depth. The data are partitioned into spherical caps to generate regional maps, using different cap sizes to investigate the lateral extent of the discontinuities. Differential precursor-SS travel time measurements with respect to AK135 are used to estimate the depth of the discontinuities. Amplitude ratios of precursors/SS help to constrain velocity and density contrasts across the boundaries.Our results display evidence for multiple discontinuities at various depths in the mid-mantle. We analyse the locations of mid-mantle discontinuities for any relationship to SS tomographical models and other mantle structures. We find some correlation to subduction zones, oceanic ridges and LIPS, however the discontinuities cannot be linked to any one mantle feature. We therefore suggest that the discontinuities have varied origins, with different influencing factors. Constraining the link between seismic observations and geodynamical features is essential to provide insight into the continued evolution of Earth's mantle and its convection processes.