New Research Reveals Violent Birth of Continent Zealandia
Zealandia — Earth’s seventh continent — experienced dramatic elevation changes between about 50 million and 35 million years ago, according to a new analysis of samples collected during the International Ocean Discovery Program (IODP) Expedition 371 in 2017; 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.
Zealandia has 4.9 million km2 (1.9 million miles2) of landmass and once made up approximately 5% of the area of the supercontinent Gondwana, the ancient supercontinent that included Antarctica and Australia. Roughly 94% of the area of Zealandia currently is submerged.
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 around 85 million years ago.
“After the tectonic fireworks, this model has Zealandia doing nothing but gently cooling and subsiding,” said Dr. Rupert Sutherland, a geophysicist at the Victoria University of Wellington.
But fossils in the drillcores collected by the IODP Expedition 371 indicate that during the early Cenozoic, portions of northern Zealandia rose 1-2 km (0.6-1.2 miles) while other sections subsided about the same amount before the entire continent sank another kilometer deep underwater.
The timing of these topographic transformations 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, scientists don’t yet understand how it starts,” Dr. Sutherland said.
“One of the amazing things about our observations is that they reveal the early signs of the Ring of Fire were almost simultaneous throughout the western Pacific,” he added.
“Because this timing predates the global tectonic plate reorganization, scientists need to find an explanation for how subduction began across such a broad area in such a short time.”
The study authors propose a new mechanism: a ‘subduction rupture event,’ which they argue is similar to a massive, super-slow earthquake.
They believe the event resurrected ancient subduction faults that had lain dormant for many millions of years.
“We don’t know where or why, 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,” Dr. Sutherland said.
“But unlike an earthquake, the subduction rupture event may have taken more than a million years to unfold.”
“Ultimately, 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 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,” Dr. Sutherland said.
“Geologists generally assume that understanding the present is the key to understanding the past. But at least in this instance, this may not hold.”
The findings are published in a paper in the journal Geology.
R. Sutherland et al. Continental-scale geographic change across Zealandia during Paleogene subduction initiation. Geology, published online February 6, 2020; doi: 10.1130/G47008.1