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.
Subduction zones such as the Cascadia margin in the Pacific Northwest and the Alaskan margin are the source of the greatest seismic, volcanic and tsunami hazard on Earth. The subducting plate drives flow within the overlying mantle wedge releasing fluids that cause magma production and volcanism. However it is difficult to image mantle flow and the effects of fluids in margins such as Cascadia because of an absence of frequent seismicity and the shallow dip of the subducting plate. The Tonga subduction produces abundant seismicity to depths as great as 600km, making it the best site on Earth for studying these processes. The generation and transport of melt beneath oceanic spreading centers is an important geological process shaping the earth; it produces over two thirds of the global crust and is a primary means of geochemical differentiation in the Earth. Yet the physical mechanisms controlling melt aggregation, transport, and collection within the axial crust are poorly understood. The Eastern Lau Spreading Center (ELSC) was selected for a large scale active and passive seismic experiment to permit careful documentation of both crustal and upper mantle structure along a single ridge segment in a region where both surface morphology and geochemical outputs varied substantially along and between ridge segments. The observations are being combined with modeling of mantle flow and melting to develop our understanding of melt dynamics beneath ridges and above subducting slabs. Detailed 3-D imaging of the upper mantle and crust was designed to (1) Characterize the mantle flow pattern and the magma production and transport regions; (2) Understand the origin and consequences of gradients in lava composition along the ELSC; (3) Understand the spatial- temporal variations of crustal architecture. A combined passive and active seismic experiment along the ELSC was designed to image upper mantle and crustal properties and their along-strike variation over a 250 km long section of the ELSC to evaluate the following, previously proposed hypotheses: 1) Circulation in the mantle wedge is dominated by slab driven flow, 2) Interaction of the arc and backarc magma production regions 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 results of this project provide some of the clearest experimental constraints on patterns of mantle flow in the subduction wedge and on the influence of fluids released from the down going plate and back arc ridge volcanism. The seismic imaging from ambient noise and surface wave tomography provide confirmation of a direct connection between crustal properties and uppermost mantle processes at a back arc ridge, and support the prediction that as the back arc ridge 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 may also 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. The research confirms previous numerical modeling efforts and the observations of anisotropy should help to further constrain our understanding of mantle flow within subduction zones. A better understanding of mantle flow within subduction zone and the influence of fluids will feed back into numerical modeling of flow, melt generation and volcanism in subduction zones. The research should influence the understanding of the geochemistry of arcs and geochemistry of the Earth, as suduction zones represent the primary means the lithosphere of the Earth is returned to the interior of the earth. A better understanding of mantle flow and magmatism beneath subduction zones may aid in understanding volcanic, tsunamigenic and seismic hazard from subduction zones worldwide. The results may aid in understanding Earth history.
