Joint inversion of PP and SS precursor waveforms and Rayleigh wave phase velocities for global mantle transition zone structure

Abstract

SUMMARYWe have compiled a new data set of global PP and SS precursor waveforms that we jointly invert in combination with fundamental-mode and higher-order Rayleigh-wave phase velocities for upper mantle and mantle transition zone (MTZ) structure. We observe clear S410S, S520S, S660S and P410P precursor arrivals, but not P660P, because of interfering phases. Traveltimes and amplitudes of precursor phases reflect a complex interplay of data and modelling factors, implying that MTZ structure is best resolved through direct inversion of waveforms. To model waveforms as accurately as possible, we account for effects arising from data processing, shallow structure, incoherent stacking, attenuation and source effects, among others. As part of the inversion, we consider two independent model parametrizations to obtain quantitative insights into the seismic and thermochemical constitution of the MTZ. These include a ‘classical’ seismic parametrization based on a layered seismic velocity structure and a thermodynamic parametrization, where seismic profiles are self-consistently built from mineral physics data. The results show lateral variations in thermal, compositional and discontinuity structure that partly correlate with tectonic setting. The mantle beneath continents and subduction zones is found to be colder in comparison to oceans and hotspots as reflected in MTZ thickness. In terms of composition, we find that subduction zones are enriched in basalt. Mid-MTZ structure shows a trend from simple sub-ocean single- to complex circum-Pacific subduction-zone-related dual-discontinuity structure—the possible signature of oceanic crustal transport to the MTZ. Statistical analysis indicates that a mechanically mixed mantle matches seismic data better than an equilibrated mantle across ∼2/3 of the globe. Finally, while a large part of the seismic data can be matched by an iso-chemical and adiabatic mantle, complexities within the MTZ are not entirely captured by this assumption.