Along Strike Variations in Structure and Melt Production in the Lau Back Arc Spreading Center from Body-wave Tomography

Abstract:

The Lau Backarc Spreading Center associated with the Tonga Subduction System is often considered to be an archetype of backarc spreading centers.This system offers an excellent opportunity to study the complex relationships between magma production in subduction arcs and backarcs and the dynamic characteristics of the subduction system. The Tonga subduction system exhibits significant changes along strike in surficial and subsurface characteristics - including rift morphology, backarc spreading and subduction rates, rift-arc separation, erupted magma volume, and crustal thickness. These variations, together with geochemical evidence, suggest that arc and backarc magmas may be sourced from shared or connected production centers at depth beneath the southern Lau Backarc Spreading Center, where the backarc spreading center is closest to the Tonga Arc. This study leverages data from a 2009-2010 amphibious deployment of 51 OBS and 16 land stations, and from a prior 1993-1994 experiment, to build a tomographic image the upper mantle beneath the Tonga Arc and the Lau Backarc Spreading Center using a joint inversion of local and teleseismic body wave travel times. Early results from this study show distinct changes in along strike structure, including an apparent separation of low velocity zones associated with the Lau Backarc Spreading Center and the Tonga Arc in the north, but apparent merging of the low velocity zones at depth in the south, supporting prior geochemical evidence for a common source of arc and backarc magmas in the south.