At most mid-ocean ridges, a wide region of decompression melting must be reconciled with a narrow neovolcanic zone and the establishment of full oceanic crustal thickness close to the rift axis. Two competing paradigms have been proposed to explain melt focusing: narrow mantle upwelling due to dynamic effects related to in situ melt or wide mantle upwelling with lateral melt transport in inclined channels. Measurements of seismic attenuation provide a tool for identifying and characterizing the presence of melt and thermal heterogeneity in the upper mantle. Geoscientists from Lamount-Doherty Earth Observatory, Columbia University use a unique data set of teleseismic body waves recorded on the Cascadia Initiative’s Amphibious Array to simultaneously measure seismic attenuation and velocity across an entire oceanic microplate. They observe maximal differential attenuation and the largest delays (Embedded Image s and δTS ~ 2 s) in a narrow zone <50 km from the Juan de Fuca and Gorda ridge axes, with values that are not consistent with laboratory estimates of temperature or water effects. The implied seismic quality factor (Qs ≤ 25) is among the lowest observed worldwide. Models harnessing experimentally derived anelastic scaling relationships require a 150-km-deep subridge region containing up to 2% in situ melt. The low viscosity and low density associated with this deep, narrow melt column provide the conditions for dynamic mantle upwelling, explaining a suite of geophysical observations at ridges, including electrical conductivity and shear velocity anomalies.
Fig. 1 Example of S waves recorded at JdF OBS stations from an event on 3 April 2014 at a distance of ~84°, from a back azimuth of ~129°. T-component displacement seismograms are aligned by cross-correlation, colored by age, arranged by distance to ridge, and plotted with a 0.05- to 2-Hz fourth-order Butterworth filter. Dashed lines, data window for calculation of spectra. DF, deformation front.
Fig. 2 S-wave t* (left) and δT (right) recorded at OBS stations.Radial spokes show individual arrivals at their incoming azimuth, whereas central circles show least-squares station average terms. Open circles show land stations used to link JdF and Gorda arrays. Boxes show three areas: north JdF (blue), south JdF (red), and Gorda (yellow).
Fig. 3 Station-averaged S-wave t* and δT as a function of crustal age, relative to the mean value for 4- to 8-My seafloor. A representative west-northwest–east-southeast bathymetric profile at 46.8°N (top) includes deformation front and isochrons reflecting an irregular age-distance relationship. Colors relate to geographic area (Fig. 2) and point size scales with the number of individual observations contributing to the average. Superimposed lines show predictions of differential attenuation due to the effect of temperature alone, assuming half-space cooling and using laboratory-derived anelastic scaling relationships (see the Supplementary Materials). 2σ uncertainties for Δt* are shown where they exceed the symbol size. δT uncertainties are ~0.3 s.
Fig. 4 Schematic of MOR showing several contributions to seismic structure, including temperature (green lines are 200°C isotherms except where stated), strain rate [solid flow streamlines in brown, modified after the work of Braun et al. (41)], and melt regimes. Profiles of QS and VS along rays incident at stations on 0-My (solid line) and 10-My (dashed line) crust are shown on the right. Melt fraction in the carbonated melt region is 0.01%, melt fraction in the hydrous melt region is 0.01 to 0.2%, and melt fraction in the dry melt region is 0.2 to 2%. See Materials and Methods for details.
Reference:
Zachary C. Eilon and Geoffrey A. Abers. High seismic attenuation at a mid-ocean ridge reveals the distribution of deep melt. Science Advances, May 2017 DOI: 10.1126/sciadv.1602829