Recent seafloor drilling has revealed that the “hidden continent” of Zealandia — a region of continental crust twice the size of India submerged beneath the southwest Pacific Ocean — experienced dramatic elevation changes between about 50 million and 35 million years ago. New findings from this expedition, published in Geology, propose this topographic upheaval may have been due to a widespread reactivation of ancient faults linked to formation of the western Pacific’s infamous Ring of Fire.
Since the 1970s the prevailing scientific wisdom has been that Zealandia’s unusually low profile is due to the thinning of its crust as it separated from Gondwana, the ancient supercontinent that included Antarctica and Australia, around 85 million years ago. After these tectonic fireworks, says Rupert Sutherland, a geophysicist at New Zealand’s Victoria University of Wellington and the paper’s lead author, this model has Zealandia “doing nothing but gently cooling and subsiding.”
But fossils in the drillcores collected in 2017 by International Ocean Discovery Program Expedition 371 indicate that during the early Cenozoic, portions of northern Zealandia rose 1-2 kilometers while other sections subsided about the same amount before the entire continent sank another kilometer deep underwater. The timing of these topographic transformations, say Sutherland and his co-authors, coincides with a global reorganization of tectonic plates evidenced by the bend in the Emperor-Hawaii seamount chain, the reorientation of numerous mid-ocean ridges, and the onset of subduction — and the related volcanism and seismicity — in a belt that still encircles much of the western Pacific.
Although subduction drives Earth’s plate tectonic cycle, says Sutherland, scientists don’t yet understand how it starts. The drilling expedition to Zealandia may offer new insights into this fundamental process. “One of the amazing things about our observations,” says Sutherland, “is that they reveal the early signs of the Ring of Fire were almost simultaneous throughout the western Pacific.”
Because this timing predates the global tectonic plate reorganization, he says, scientists need to find an explanation for how subduction began across such a broad area in such a short time.
Sutherland and his co-authors propose a new mechanism: a ‘subduction rupture event,’ which they argue is similar to a massive, super-slow earthquake. The researchers believe the event resurrected ancient subduction faults that had lain dormant for many millions of years.
“We don’t know where or why,” says Sutherland, “but something happened that locally induced movement, and when the fault started to slip, like in an earthquake the motion rapidly spread sideways onto adjacent parts of the fault system and then around the western Pacific.” But unlike an earthquake, Sutherland says, the subduction rupture event may have taken more than a million years to unfold. “Ultimately,” he says, “Zealandia’s sedimentary record should help us determine how and why this event happened and what the consequences were for animals, plants, and global climate.”
The process has no modern analogue, according to Sutherland, and because the subduction rupture event is linked to a time of rapid, global plate tectonic change, other instances of such change in the geologic record may imply that comparable events have occurred in the past. “Geologists generally assume that understanding the present is the key to understanding the past,” he says. “But at least in this instance, this may not hold.”