The influence of temperature, bulk composition, and melting on the seismic signature of the low-velocity layer above the transition zone

Abstract

We report a new technique to describe seismic velocity and impedance anomalies atop a seismic low‐velocity layer (LVL) at 350 km depth. We model shear wave speed reductions detected with Ps conversions beneath the Hawaiian Islands and negative impedance contrasts detected with ScS reverberations beneath the Coral Sea in the South Pacific, by varying the bulk solid composition, reference potential temperature, dihedral angle of melt, and melt composition. For a given bulk solid composition, the effects of elevated temperature and melt volume fraction on the seismic properties trade off with one another. At a given temperature, the calculated melt volume fraction is nearly insensitive to variations in the bulk solid composition. A low volume fraction of low dihedral angle melts mimics the seismic signature of a higher volume fraction of high dihedral angle melts. Despite stronger lateral variations in the LVL structure beneath Hawaii compared to the Coral Sea, both regional averages are similar. For a basalt volume fraction of 0.2 and a dihedral angle of 10°, we estimate regional averages of 1.1 ± 0.8 vol % melt at a depth of 350 km beneath the Hawaiian Islands for a reference potential temperature of 1800 K and 1.2 ± 0.005 vol % melt at a depth of 350 km beneath the Coral Sea region for a reference potential temperature of 1500 K. Our model of the seismic signal is unable to distinguish between melt compositions of mid‐ocean ridge basalt and carbonated peridotite melts at such small melt volume fractions.

Pervasive upper mantle melting beneath the western US

“…Recent studies suggest that the LVL can be present on a global scale (Tauzin et al, 2010;Vinnik and Farra, 2007), with the distance above the 410 discontinuity changing laterally from 20 km to as much as 90 km over a few hundred kilometers. Correlations of these variations with hot (Hier-Majumder et al, 2014;Vinnik and Farra, 2007) or cold (Courtier and Revenaugh, 2007;Hier-Majumder and Courtier, 2011;Song et al, 2004) tectonic environments have remained elusive (Tauzin et al, 2010), suggesting that the variations in position cannot be explained by temperature alone.…”

“…Due to the lack of coverage and absence of extensive analysis involving rock physics and melt microstructure, however, these studies were unable to quantify the spatial expanse and local variations in the melt content in the LVL. Using limited coverage underneath the Coral Sea and Hawaii, the LVLs were estimated to contain approximately 1 vol% melt (Hier-Majumder and Courtier, 2011;Hier-Majumder et al, 2014). The seismic data in these two studies, however, were too sparse to create a detailed regional map of melting.…”

Development of anisotropic contiguity in deforming partially molten aggregates: 2. Implications for the lithosphere‐asthenosphere boundary

“…In this work, we use our numerical data to calculate the shear wave anisotropy. To estimate the effect of deformation in shear wave speed reduction and anisotropy generation, we also present predicted profiles of shear wave speed using the numerical code MuMaP [ Hier‐Majumder et al , ]. In this section, we present a brief description of the numerical methods from DHM that were used to calculate the contiguity, followed by a derivation for calculating shear wave anisotropy from numerical values of contiguity, and a short summary of the technique used to build profiles of shear wave speed using MuMaP.…”

“…We use the numerical model MuMaP [ Hier‐Majumder et al , ] to create vertical profiles of shear wave speed. The elastic properties of the reference mantle were obtained from the work of Xu et al [], for mantle potential temperatures of 1300 K and 1500 K, for a basalt fraction of 0.2 in the bulk composition.…”

“…We then calculated the reduction in shear wave speed when the melt is distributed in tubules in an undeformed state for melt volume fractions of 0.001 and 0.02, for dihedral angles of 5°and 25°, respectively. The detailed method of these calculations have been outlined in Hier‐Majumder et al []. To test the role of deformation in the shear wave reduction, we also calculated the relative reduction in shear wave speed caused by melt films.…”