Deformation of Zealandia, Earth's Hidden continent, linked to forging of the Ring of Fire

image: 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.

Image:

International Ocean Discovery Program, JOIDES Resolution Science Operator

Boulder, Colo., USA: 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 today 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.”