Crustal and uppermost mantle shear velocity structure adjacent to the Juan de Fuca Ridge from ambient seismic noise

Abstract

[1] Based on 6 months of OBS data from the Cascadia Initiative experiment near the Juan de Fuca Ridge, we obtain Rayleigh wave group and phase speed curves from 6 s to about 20 s period from ambient noise cross correlations among all station pairs. We confirm the hypothesis that the dispersion data can be fit by a simple age-dependent formula, which we invert using a Bayesian Monte Carlo formalism for an agedependent shear wave speed model of the crust and uppermost mantle between crustal ages of 0.5 and 3.5 Ma. Igneous crustal structure is age invariant with a thickness of 7 km, water depth varies in a prescribed way, and sedimentary thickness and mantle shear wave speeds are found to increase systematically with crustal age. The mantle model possesses a shallow low shear velocity zone (LVZ) with a velocity minimum at about 20 km depth at 0.5 Ma with lithosphere thickening monotonically with age. Minimum mantle shear velocities at young ages are lower than predicted from a half-space conductively cooling model (HSCM) and the lithosphere thickens with age faster than the HSCM, providing evidence for nonconductive cooling in the young lithosphere. The shallow LVZ is consistent with expectations for a largely dehydrated depleted (harzburgite) mantle with a small, retained near-ridge partial melt fraction probably less than 1% with melt extending to a lithospheric age of approximately 1 Ma (i.e., $30 km from the ridge).

Seismological imaging of ridge–arc interaction beneath the Eastern Lau Spreading Center from OBS ambient noise tomography

“…The center of the LVZ progressively shifts to the west toward the n-ELSC and lies beneath the ILSC at the northern terminus of ELSC. The width of the LVZ at 10 km depth is about 30 km, similar to that imaged beneath the EPR (Toomey et al, 2007). The LVZ appears continuous across three overlapping spreading centers (OSCs) between four ELSC segments, consistent with observations of a continuous mantle velocity anomaly at the EPR 9 • N (Dunn et al, 2001;Toomey et al, 2007).…”

“…The increasing offset between the center of mantle melt production and the ridge may result in less efficient melt transport to the ridge and lead to a reduced magma supply to the n-ELSC. Similar along-ridge contrast has been observed at EPR 9 • N, where spreading segments with ridge-centered mantle melt delivery show more volcanism, while segments with off-axis mantle melt delivery have reduced volcanic activity and more extensive seafloor faulting (Fornari et al, 1998;Toomey et al, 2007;Wright et al, 1995).…”

“…The width of the LVZ at 10 km depth is about 30 km, similar to that imaged beneath the EPR (Toomey et al, 2007). The LVZ appears continuous across three overlapping spreading centers (OSCs) between four ELSC segments, consistent with observations of a continuous mantle velocity anomaly at the EPR 9 • N (Dunn et al, 2001;Toomey et al, 2007). However, separate narrow LVZs as observed in the crust (Dunn et al, 2013) would probably not be detectable due to the small offset of these OSCs (1.5-8 km) and the limited horizontal resolution.…”

Pn Tomography of the Juan de Fuca and Gorda Plates: Implications for Mantle Deformation and Hydration in the Oceanic Lithosphere

“…The abrupt increase in Pn velocities at ~4 Ma coincides with a similarly abrupt increase in upper mantle Vs and decrease in teleseismic body wave attenuation (Byrnes et al, ; Eilon & Abers, ). Ambient noise (Tian et al, ) and Rayleigh wave (Bell et al, ; Ruan et al, ) tomography reveals a thin JdF lithosphere at young plate ages that thickens rapidly beyond ~2.5 Myr. These anomalous gradients in seismic properties with plate age are difficult to reconcile with conductive cooling models alone and have been attributed to dynamic upwelling beneath the JdF ridge.…”

Long-range correlations of microseism-band pressure fluctuations in the ocean

“…This effort is motivated, in part, by successful applications of wave interferometry to the microseism-band seismic noise recorded by seismometers located on land (Sabra et al 2005;Shapiro et al 2005;Gerstoft et al 2006;Bensen et al 2007;Brooks et al 2009) and on the seafloor (Harmon et al 2007;Yao et al 2011;Takeo et al 2014;Tian et al 2013;Zha et al 2014). Using methodology developed by the seismic community for extracting fundamental and higher mode Rayleigh wave arrivals (Yao et al 2011) from ambient noise, we obtain robust dispersion measurements from long-range correlations in the microseism band for the first time using moored hydrophone data.…”