Gird-search Modeling the Lowermost Mantle Structure Beneath Central America and the Central Pacific

Abstract:

The D’’ layer, the lowermost few hundred kilometres of the mantle, has been regarded as a thermal boundary layer and a chemically distinct region above the core-mantle boundary (CMB). It plays a critical role in the mantle and core convection. A sharp change in seismic velocities at the top of D’’, designated as the D’’ discontinuity, has been observed in several regions of the lowermost mantle with substantial variation in depth, sharpness and magnitude. However, the origin of D’’ and its role in the dynamic process of mantle and core is still under debate. Previous waveform modeling studies have characterized the detailed shear velocity structure of the D’’ discontinuity beneath the central Pacific and Central America, two regions with significantly different velocity features in the lowermost mantle, a large low shear-wave velocity province (LLSVP) and a high velocity zone in the circum-Pacific ring. However, previous studies have not been able to unambiguously quantify D’’ layer properties due to subjective model-parameterization choices, and inherent non-uniqueness of solutions. In particular, these studies did not specifically constrain the velocity gradient at the top of the D’’ layer, which is crucial for deciphering the composition and thermal conditions of the lowermost mantle. In this study, we use a grid-search scheme to resolve the detailed velocity structure at the top of D’’ and provide quantitative assessment of the uncertainty of 1D models in these two regions. We use events that occurred in the Fiji-Tonga region recorded at broadband seismic stations in Alaska, western Canada and North America to sample D’’ beneath the central Pacific and events that occurred along the South America subduction zone recorded at broadband stations in North America to sample D’’ beneath Central America. We choose events with Mw magnitudes greater than 5.5 and depths larger than 150 km between 2014 to 2018 and selected event-station pairs with a distance range between 70° to 85°. We identify lateral variations in shear wave velocity in D'' region beneath Central America and the central Pacific with distinct velocity contrasts due to their different temperature and compositional conditions. The inferred velocity profiles from this study help improve our understanding of the existence of the D'' discontinuity and its possible relationship with slab debris at the core-mantle boundary both along the circum-Pacific and beneath the Pacific LLSVP.