ULTRA HIGH VELOCITY ZONES AT THE CORE-MANTLE BOUNDARY

Abstract:

A new kind of seismic structure is found at the base of the mantle beneath Central America and Gulf of Mexico: ultra-high velocity zones (UHVZs). Transversely polarized S wave core reflections (ScS) are stripped from seismograms using an empirical wavelet algorithm that utilizes direct S wave adaptively fit to ScS. The ScS-stripped data are then stacked in regional bins, which enhances precursory energy (if present), from any structures at the core-mantle boundary (CMB). We observed precursor energy for over ½ of our study area that is the same polarity as ScS, indicating anomalous structure with a seismic wave speed increase. Observations are modeled with local 1D reflectivity synthetics processed the exact same way as the data. The imaged UHVZ structures are variable in velocity: from ~ 1% high, up to 18% ultra-high, relative to a standard reference model (PREM), though trade-offs are present. UHVZ thicknesses range from ~5 to 50 km thick. Some geographic bins have best-fit UHVZ solution models that are up off the CMB by 10’s of km. Density is also explored in the modeling, and some bins are best fit with density increases up to 10% over reference models. UHVZs possessing strong shear velocity increases (e.g., > 5-10%) likely involve compositional heterogeneities. We will present mineral physics hypotheses and geodynamics experiments to explore possible origins of UHVZs, which focus on mineralogies that elevate shear velocity and mechanisms to collect materials to be seismically observable. Our preferred models suggest that UHVZs are linked to chemical interactions between the mantle and the core.