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

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.

Who are the non-academic beneficiaries of this research and what can they get out of the science undertaken as part of this project?

(1) Ocean Bottom Seismometer Instrument manufacturers
A major hole in the NERC-GEF national geophysical instrument facility is a lack of access to broadband ocean bottom seismometers which can be used for experiments such as the one proposed here. This has made it difficult to undertake passive marine seismic experiments or combined active and passive marine experiments. As such, the UK is trailing many other nations in its ability to undertake world class marine research. The cost of establishing such a facility is sizeable, and the running costs are also significant, which likely have been the main barriers that have prevented establishment of such a pool of instruments. An alternative approach is to have a "virtual facility", which in effect leases the OBSs to users via pre-existing arrangements with instrument manufacturers. With on-going discussions now taking place to plan the future of NERC-GEF, it is extremely timely that this project is able to test this lease approach. In this case, the PI has arranged to lease the instruments from the Chinese Academy of Sciences, with whom he has pre-existing collaborations. However, if this model proves successful, then it may help pave the way for the development of a more established arrangement via NERC-GEF. Already, OBS manufacturers such as Guralp (based in the UK) and KUM (based in Germany) have indicated their interest in being part of such an arrangement.

(2) The general public
The astronomy community has arguably been far more successful at capturing the imagination and interest of the general public than the Earth Sciences community. Yet, we have a better idea of what is happening 100 light-years above our head than 100 km beneath our feet. It is therefore incumbent on us to get the message out that the Earth we live on is worth understanding, and what happens at depths beyond where we can dig or drill does matter. This project is ideally situated to help transmit this message to the general public because it has the necessary ingredients to generate the "wow" factor needed to turn the head of the average Joe in the street who would otherwise walk on by. First, it involves a cruise by Australia's premier research vessel into a remote part of the southwest Pacific to study a region around one of the most fascinating islands on the planet (Macquarie Island, which has UNESCO world heritage status on account of its unique geology, and hosts very large penguin colonies). Then there is the deployment of ocean bottom seismometers, which can operate at depths of up to 6 km for 12 months, in a region which hosts the largest strike-slip earthquakes ever recorded. Last but not least, we will be probing the structure beneath a transpressional plate boundary which may hold valuable clues into the genesis of subduction. We will take advantage of all of these ingredients to create a substantial and innovative outreach program, which includes a stall at the prestigious Edinburgh International Science Festival, which is the largest science festival in Europe, and an exhibit at the Sedgwick Museum of Earth Sciences in Cambridge, which received nearly 150,000 visitors in 2017.

(3) Schools
Most schools have limited or no educational programs in the Earth Sciences. It is therefore important for them to receive outside help to make their students aware of this major strand of the physical sciences. The PI has extensive experience in outreach to schools, having been part of the seismometer in schools program in Australia, and through his current seismic deployment in North Borneo, where a significant number of stations have been placed in schools. In this project, he will engage with schools during visits to the Sedgwick Museum, and through presentations at Cambridge Science Week, which attracts nearly 50,000 visitors per year.