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

Abstract

Available online xxxx Editor: P. Shearer Keywords: ocean bottom seismograph ambient noise Lau Basin back-arc spreading center mid-ocean ridge seismic surface waveThe Lau Basin displays large along-strike variations in ridge characters with the changing proximity of the adjacent subduction zone. The mechanism governing these changes is not well understood but one hypotheses relates them to interaction between the arc and back-arc magmatic systems. We present a 3D seismic velocity model of the shallow mantle beneath the Eastern Lau back-arc Spreading Center (ELSC) and the adjacent Tofua volcanic arc obtained from ambient noise tomography of ocean bottom seismograph data. Our seismic images reveal an asymmetric upper mantle low velocity zone (LVZ) beneath the ELSC. Two major trends are present as the ridge-to-arc distance increases: (1) the LVZ becomes increasingly offset from the ridge to the north, where crust is thinner and the ridge less magmatically active; (2) the LVZ becomes increasingly connected to a sub-arc low velocity zone to the south. The separation of the ridge and arc low velocity zones is spatially coincident with the abrupt transition in crustal composition and ridge morphology. Our results present the first mantle imaging confirmation of a direct connection between crustal properties and uppermost mantle processes at ELSC, and support the prediction that as ELSC migrates away from the arc, a changing mantle wedge flow pattern leads to the separation of the arc and ridge melting regions. Slab-derived water is cutoff from the ridge, resulting in abrupt changes in crustal lava composition and crustal porosity. The larger offset between mantle melt supply and the ridge along the northern ELSC may reduce melt extraction efficiency along the ridge, further decreasing the melt budget and leading to the observed flat and faulted ridge morphology, thinner crust and the lack of an axial melt lens.

Petrogenesis and structure of oceanic crust in the Lau back-arc basin

“…1). Regional mantle imaging indicates this is approximately the latitude at which the low velocity anomalies in the mantle under the volcanic arc and the spreading centers transition from being connected (at ∼20-35 km depth) to distinct (within the resolution of the imaging) (Zha et al, 2014).…”

Segmentation of mid-ocean ridges attributed to oblique mantle divergence

“…The maximum depth extent of the inferred region of melt storage must extend for several kilometres given the seismic wavelength of Pn phases. Deeper mantle imaging (10-30 km sub-Moho) of multiple non-transform discontinuities along the Lau Spreading Centre reveals that the melt production region is uninterrupted across multiple ridge offsets 44 . Assuming these results are characteristic of other spreading centres, we conclude that the retention of mantle melt beneath second-order offsets occurs within the top few kilometres of the mantle.…”

Waveform Fitting of Cross Spectra to Determine Phase Velocity Using Aki’s Formula

“…Considerable effort has been put into methods for rapid and accurate computation of the correlograms (Bensen et al, 2007). Noise correlation methods have been applied in a wide variety of disciplines, including broadband seismology, for determining the structure of the lithosphere (e.g., Yao et al, 2006;Yang et al, 2007;Lin et al, 2008;Calkins et al, 2011;Zha et al, 2014), and in engineering seismology, for determining site response, where it is referred to as the spatial autocorrelation method (e.g., Apostolidis et al, 2004;Asten, 2006;Xu et al, 2013). Special attention has been given to the extraction of Love-and Rayleigh-wave phase velocity dispersion curves from correlograms, because these surface waves have the highest signal-to-noise ratio (SNR) and can be easily detected in even short-deployment experiments.…”