Discovery of flat seismic reflections in the mantle beneath the young Juan de Fuca Plate

Yanfang Qin,Milena Marjanović,W Roger Buck,Ingo Grevemeyer,Satish C Singh

doi:10.1038/s41467-020-17946-3

Abstract

Crustal properties of young oceanic lithosphere have been examined extensively, but the nature of the mantle lithosphere underneath remains elusive. Using a novel wide-angle seismic imaging technique, here we show the presence of two sub-horizontal reflections at ∼11 and ∼14.5 km below the seafloor over the 0.51–2.67 Ma old Juan de Fuca Plate. We find that the observed reflectors originate from 300–600-m-thick layers, with an ∼7–8% drop in P-wave velocity. They could be explained either by the presence of partially molten sills or frozen gabbroic sills. If partially molten, the shallower sill would define the base of a thin lithosphere with the constant thickness (11 km), requiring the presence of a mantle thermal anomaly extending up to 2.67 Ma. In contrast, if these reflections were frozen melt sills, they would imply the presence of thick young oceanic lithosphere (20–25 km), and extremely heterogeneous upper mantle.

Highlights

Crustal properties of young oceanic lithosphere have been examined extensively, but the nature of the mantle lithosphere underneath remains elusive
The observed crustal thickness is consistent with that observed along the orthogonal seismic reflection profile[22,30]
We find that a decrease in velocity of −30% in a 300-m-thick layer would produce a strong reflection that is not observed in our data

Summary

Introduction

Crustal properties of young oceanic lithosphere have been examined extensively, but the nature of the mantle lithosphere underneath remains elusive. In order explain the receiver function signal, a 26–30% of anisotropy in 8–15-km-thick layer was required, requiring unacceptably large velocity variations (e.g. a P-wave velocity 8.0 ± 1.2 km s−1) below the Moho Another receiver function study covering 0–8 Ma of the Juan de Fuca (JdF) Plate provided a phase converted S-wave sub-horizontal image at ∼30-km depth beneath the sea surface. This receiver function image spans in a wide depth range (±10 km)[14], indicating that the receiver function imaging methods have a limited resolution to precisely decipher the thickness of an young lithosphere. A wide-angle seismic method revealed mid-lithospheric discontinuities between 37 and 59 km depth over the 128–148 Ma old Pacific Plate, which are interpreted as frozen melt sills[18]

Methods

Results

Conclusion