High resolution seismic imaging of the Hawaiian ULVZ: A large-scale layer of varying topography

Abstract:

Ultra-low velocity zones (ULVZs) are typically observed as small patches (10s km high by 100s km wide) of extreme seismic wave speed reduction (up -50% Vs and -25% Vp) atop the core-mantle boundary (CMB). While these extreme values require significant heterogeneity in mantle temperature and/or composition, the underlying cause of ULVZ observations is not currently known, with end member models ranging from purely thermal (presence of partial melt) to purely compositional (e.g. Fe enrichment) explanations. To understand the underlying cause of ULVZs and their role within the larger mantle convecting system requires improved seismic constraints of ULVZ characteristics and detailed morphology.

Towards this end we use core reflected ScS waves sensitive to a broad region of the core mantle boundary beneath Hawaii to create the first high-resolution map of the Hawaiian ULVZ. Positive ScS-S differential times identify regions of slow velocity anomalies in the lower-most mantle, and the presence of pre/post-cursors around the main ScS phase confirm the sharp top of a basal ULVZ layer. Pre/post-cursor arrivals are migrated into a 3D volume to produce a detailed image of ULVZ morphology. While there are trade-offs between ULVZ height and velocity contrast, the wide range of velocity variation required to explain observations suggests they are more likely to reflect varying ULVZ topography.

Instead of a small distinct patch of material we show the Hawaiian ULVZ is a large-scale regional feature of variable height (~5-25km). Maximum ULVZ thicknesses are observed to the west of Hawaii, coincident with an offset plume stem imaged in recent tomographic models. The thickest ULVZ regions are bound by the Pacific large low-velocity province (LLVP) to the south, and fast seismic velocity anomalies to the north, suggesting ULVZ material may be piled up against the LLVP by slab remnants. The highly variable topography, and position at the edge of an LLVP are more indicative of the Hawaiian ULVZ being formed of a distinct dense material, rather than partial melt.