Collaborative Research: SISIE: South Island, New Zealand, Subduction Initiation Experiment

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In this project, scientists from the California Institute of Technology and The University of Texas at Austin will conduct an experiment south of New Zealand to investigate key unknowns about how plate tectonics works. Plate tectonics describes how the surface of the earth moves and has been very successful providing an understanding of the occurrence of most earthquakes and volcanic eruptions on the surface of Earth. One of the key aspects of plate tectonics is subduction when the cold plate (which is relatively dense) dips beneath another. A puzzling element of this is that new subduction zones must start where none existed in the past. The purpose of this project is to understand how a new subduction zone starts. The region selected for this study is the only known location on Earth where we can see a new subduction zone. The first target of study is the Australian plate as it first dives into Earth's interior. The second target is the new fault that defines the boundary between the Australian and the Pacific plates. All of the data collected will be integrated into a dynamic model of the formation of a new subduction zone and how the forces operate and change. Students from both the University of Texas and the California Institute of Technology will get valuable experience at sea learning about data collection, computational techniques, and data interpretation. New Zealand scientists will conduct onshore seismic recordings on the nature of the over-riding plate concurrent with this experiment. The project is an integrated, active-source seismic experiment on the Puysegur-Fiordland boundary south of South Island of New Zealand. The study will test the hypothesis that this type-example of incipient subduction is making a transition from a forced to a self-sustaining state. The project was motivated to understand the forces that drive subduction, specifically by models that suggest that during subduction initiation there is a fundamental transition from far-field plate stresses to the negative buoyancy of the slab. The subduction zone selected is a segment of the Macquarie Ridge Complex (the boundary between the Australian and Pacific plates), and is likely the best margin globally to study the early stages of subduction. The team will address several fundamental questions including: What controls the transition from a forced to self-sustaining state during subduction initiation? And, what are the mechanical conditions of the oceanic plate during incipient subduction? The PIs will test the hypothesize that a strengthening of the slab pull force causes the subsidence along the ridge. This plate tectonic scenario will be addressed with refined gravity models of the Puysegur region and seismic constraints on the crustal structure and sedimentary basin history. Moreover, initially steep fracture zones and normal faults transition or evolve to shallow thrust planes during subduction initiation, key aspects of the geodynamics. The project is guided by forward modeling of the seismic source-receiver configuration that shows that the dip and other features of the slab at two positions along strike, each with a different degree of compression, will be resolvable with the seismic methods employed.