Seismic imaging of the Macquarie Ridge Complex: A search for incipient subduction

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Subduction describes the process in which one tectonic plate descends into the mantle beneath an adjoining tectonic plate as a result of convergence. Currently active subduction zones can be found in many parts of the world, including western South America, the NW Pacific and southeast Asia. Subduction may have been operating on Earth for over three billion years, and plays a fundamental role in the creation and evolution of the continents, the generation of new oceanic crust, the exchange of water between the Earth, oceans and atmosphere and the global distribution of earthquakes and volcanoes. Subduction zones, therefore, are one of the primary keys to understanding how our Earth works, and as a result, they have been the subject of intensive study by Earth scientists from around the world. However, one aspect of the subduction cycle that is still poorly understood is subduction initiation, in which a new subduction zone starts to form. This requires not only localised weakening throughout the thickness of the lithosphere in order to accommodate the formation of a new plate margin, but also a sufficiently large density instability to overcome frictional forces at the boundary and the flexural strength of the plate in order to produce a downgoing slab. These challenges have lead many studies to conclude that it is very difficult to initiate a new subduction zone, and substantial disagreement remains about the relative importance of different processes which appear to influence subduction initiation. One possible way of achieving a weak zone that cross-cuts the entire lithosphere is from inheritance of pre-existing or fossil boundaries, such as old fracture zones, transform faults and extinct spreading centres. Given an appropriate regional stress field, it may then be possible to initiate subduction.The Macquarie Ridge Complex (MRC), which sits at the boundary between the Australian and Pacific plates in the south-west Pacific, is one location on Earth where subduction initiation is though to be taking place. The MRC initially appears to have formed as a spreading centre, but in the last 40 Ma has experienced a 60-90 degree change in spreading direction, which has resulted in a gradual transition from a divergent to a dominantly transform (strike-slip) margin. However, the azimuth of the MRC varies significantly along strike, which results in a transpressional setting along several of its segments. It is thought that transpression can result in subduction initiation, and several recent studies have suggested that the northern and southern segments of the MRC may be experiencing incipient subduction. However, in the central section of the MRC, which includes Macquarie Island, there is insufficient data available to determine whether subduction has begun.This project will undertake a large seismic experiment in collaboration with Australian partners in order to image the structure of the MRC in the vicinity of Macquarie Island. By applying advanced seismic imaging methods to the data which are recorded, we will be able to address fundamental questions regarding the formation of the MRC and the process of incipient subduction. This includes (i) why is Macqurie Island the only piece of ophiolite (oceanic crust) in the world exposed above sea-level, and how is its extreme topography (~5 km above the surrounding ocean basin) supported; (ii) why is the MRC a factory for the world's largest strike-slip earthquakes and is this accommodated by thickened crust; (iii) is there a penetrating fault surface or decollement beneath the MRC; (iv) is there evidence for a density instability in the crust or mantle lithosphere?An exciting aspect of the proposal is that it will take advantage of a successful ~£3.7M bid (by the PI and Australian collaborators) for the Australian Marine National Facility vessel the RV Investigator to undertake a cruise to the MRC to enable the deployment of both ocean bottom and land seismometers.