Sources of Melt beneath the Lau Back Spreading Center and Tonga Arc as Indicated by Local and Teleseismic Bodywave Tomography

Abstract:

The Lau Backarc Spreading Center (LBSC) and the Tonga Arc are an exemplary location for studying melt production and migration in a complex subduction system. The LBSC is an archetype of backarc spreading centers, but displays significant variation along strike and is composed of three primary segments, the Central Lau Spreading Center, the Eastern Lau Spreading Center (ELSC), and the Valu Fa Spreading Center (VFSC). Magma geochemistry varies with decreasing distance to the arc from north to south, with increasing concentrations of water and fluid-mobile elements in the south.

This study uses body wave tomography to investigate the mantle structure and processes associated with these surficial along-strike variations. We invert more than 50,000 arrival times from local events recorded by 51 ocean bottom seismometers and 16 broadband land stations in 2009-2010, arrivals from a previous experiment in 1994, and teleseismic arrivals for P- and S-wave velocity models.

These body wave velocity models show distinct low velocity regions within the upper 100 km beneath the volcanic arc and ELSC, but these separate anomalies merge beneath the VFSC, indicating a common source for arc and backarc magmas in the south. At depths greater than 50 km, we find slower velocities beneath the ELSC than beneath the VFSC, consistent with previous studies suggesting greater in situ-melt concentration beneath the ELSC. This has been interpreted as resulting from faster and more efficient melt extraction in the south due to the higher water contents (Wei et al., 2015). A dominant first-order feature at depths greater than 100km is a north-south low velocity zone beneath the ELSC that extends to at least 300km depth, tilting westward with increasing depth. This suggests that the melt source region for the ELSC occurs along the upper part of a rising mantle flow pattern sourced at depths of at least 300 km depth in the far backarc region.