A hidden continent birthed a new subduction zone near New Zealand
The new data allowed the researchers to put together a history of the young subduction zone, which Shuck presented at the virtual meeting of the Seismological Society of America on April 22, the same day the study was published in the journal Tectonics. It all started about 45 million years ago when a new plate boundary between the Australian and Pacific plates began to form because of a force called extension — basically, tectonic forces pulled the two plates apart like putty.
The oceanic crust at the plate boundary responded to this extension predictably: As the crust thin, magma from the mantle pushed up through fractures, hardening into new rock. This process is called seafloor spreading, and it’s how new oceanic crust forms.
But there was a catch: The secret continent of Zealandia. Zealandia is a submerged section of continental crust the size of Australia around New Zealand. Zealandia was perched over the north end of this extensional zone. As continental crust is thicker and more buoyant, the extensional forces working at the plate boundary couldn’t crack Zealandia. Instead, the continental crust merely stretched as it spread, creating a thinned-out zone now known as the Solander basin.
Now there were two plates. The Australian Plate, to the west, consisted of continental crust from Zealandia in the north and new oceanic crust in the south. The Pacific Plate, to the east, also consisted of oceanic crust in the south. To the north, the Pacific Plate hosted the thinned-out continental crust of the Solander basin. At the plate boundary, oceanic crust bumped up against oceanic crust and continental crust against continental crust.
Likely little of interest would have happened, if not for another tectonic shift 25 million years ago.
At that time, the Australian-Pacific plate boundary stopped pulling apart. Instead, the plates started to move past each other, creating what’s known as a strike-slip fault. Now, the Pacific plate was moving south, and the Australian plate was moving north. This opposing movement brought the oceanic crust of the Australian plate right next door to the thin Solander basin continental crust on the Pacific plate.
The researchers will return to pick up the instruments and their data in November 2021, as long as the weather allows. If the Puysegur margin is in the “Roaring Forties,” Macquarie Ridge is in the “Furious 50s.” The research ship encountered 68 mph (109 km/h) winds while trying to deploy the instruments and spent 38 percent of the mission in such bad weather that scientists aboard couldn’t do anything but shelter in place and wait.
Nevertheless, they are hopeful that the new ocean-bottom seismometers (OBS) will open their eyes to what’s going on beneath the ridge. Right now, the researchers know there are large quakes originating in the region, but they don’t know much about how deep in the crust they are, what kind of faults they occur on or what kind of tsunami risks they pose to coastal areas in Australia.
“The OBS data will also allow us to image the subsurface beneath-the-plate boundary for the first time using different seismic imaging techniques,” Eakin told Live Science. “Currently, most of our observations tell us about what is happening at the surface or near-surface, but we have no idea what is happening beneath the surface of the plate boundary in the Macquarie Ridge region.”
One question they hope to answer: Will the fault at Macquarie Ridge begin to turn into a subduction zone, too? The Puysegur margin and Macquarie Ridge are related and are experiencing similar changes in plate motion over time, Eakin said, though Puysegur is further along in the process. Macquarie Ridge, being two slabs of oceanic crust coming together, might be more resistant to subduction than the continental crust and oceanic crust boundary at Puysegur, Shuck said; but subduction zones can also spread along a fault from a single point.