This is a combined active and passive seismic experiment along the Eastern Lau Spreading Center to test the following hypotheses. 1. Circulation in the mantle wedge is dominated by slab driven flow. 2. Interaction of the arc and backarc magma production controls the character of the ridge by influencing melt flux, petrology, and geochemistry. 3. Variations in the mantle melt supply control ridge crest features such as morphology, thermal structure, and hydrothermal venting. The passive experiment consists of 55 broadband ocean bottom seismographs and five land seismographs deployed for 10 months to image the larger-scale structure of the melt production region and the mantle flow pattern. The active source experiment consists of 100 ocean bottom seismographs deployed along a 250 km section of the spreading center extending from the inflated Vala Fa region to the magma-starved northern Eastern Lau Spreading Center where the axial melt lens is absent.
In broader terms this experiment addresses a first order problem in ridge dynamics in a unique subduction zone setting and is essential for the stated goals of R2K program. Graduate students will be supported by all four PIs.
Through plate tectonics, the Earth is an evolving and chemically differentiating planet that exchanges heat and chemical elements between the deeper mantle and the oceans and atmosphere. A significant part of this differentiation process is the melting of the mantle beneath diverging plates, and above subduction zones, to create new crust. Our project seismically imaged the interior of the crust and mantle along the diverging Eastern Lau Spreading Center in the Lau back-arc basin, behind the Tonga subduction zone - a location in which plate diverging influences on mantle melting interact with subduction-related flux-melting influences on mantle melting. Our project made a major observation fundamental to the creation of new oceanic crust: we showed that crust is compositionally zoned over a large area of the basin and that this zoning probably arises from the changing influence of subduction-derived mantle water on melt generation and crustal petrogenesis as a function of distance from the Lau arc. Near-arc crust is also highly differentiated (vertically stratified) leading to new models of crustal petrogenesis in this environment. As part of this project we mapped out the subsurface magmatic system along a ~120 km section of the spreading center. Detailed images of seismic anisotropy provide the first evidence of upper-crustal variations in hydrologic activity (via the elastic anisotropy of regions experiencing hydrofracturing and tectonic stretching) along the crests of the oceanic spreading centers. Mantle imaging found fundamental differences in the way melt is both generated and extracted at spreading centers versus volcanic arcs zones. And found a variable connection between sub-ridge and sub-arc mantle melting zones that was predicted by chemistry and the crustal studies. Overall this collection of studies, carried out as part of this project, have greatly advanced our understanding of the Earth's dynamic and varied evolution.
