Another piece in the tectonic plate mosaic
Forty rocks recovered from the seafloor along the Kermadec Arc have given scientists a clearer picture of subduction processes occurring off the North Island’s east coast.
Analysis of the rocks for about 50 different chemical elements has enabled scientists to build a better model of the way the Pacific plate is being dragged down into the Earth’s mantle and recycled into magma and other volcanic products, some of which end up as mineral deposits on the seafloor.
The multi-year study, involving scientists from New Zealand, Australia, Germany and the United Kingdom, has shown that the chemical makeup of seafloor volcanic chains is strongly influenced by the chemistry of the subducted tectonic plate.
A key part of the study were isotope ratios of various elements in the rocks gathered from the seafloor. Scientists used high precision mass spectrometry to measure ratios of strontium, lead, and neodymium isotopes to give telltale signs of different processes occurring during plate subduction.
The seafloor rocks are all volcanic in origin and were found in or near some of the 30 submarine volcanoes between the Bay of Plenty coast and Monowai volcano, about 1200km northeast of Whakatane. The study was published in the international journal Nature Communications this month.
“We’ve found that the chemistry of the downgoing tectonic plate influences the type of submarine volcanism and possibly the type of mineral deposits that are formed on the seafloor,” said lead author and marine geologist, Christian Timm, of GNS Science.
Dr Timm said a 20km-thick section of the Pacific plate, known as the Hikurangi Plateau, was ‘consumed’ as it descended beneath the Australian plate east of the North Island.
“It’s remarkable that the unique chemical signature of the Hikurangi Plateau and its seamounts are revealed in rocks sitting on the seafloor today millions of years after they have been subducted.
“This enables us to determine exactly where and to what extent the Hikurangi Plateau slid under the Australian plate during the past 10 million years.”
The results contributed to a better understanding of the processes occurring along the Kermadec Arc and subduction zones in general.
“In a nutshell, the chemistry of magma in submarine volcanoes allows us to reconstruct tectonic plate interactions going back millions of years.”
The Hikurangi Plateau is about half the size of France and sits at the bottom of the ocean east and northeast of the North Island.
As well as being unusually thick, the Hikurangi Plateau is fluid-rich. More fluid in the Earth’s mantle beneath volcanic arcs induces more molten rock, so we have very productive part of the Earth’s crust off the New Zealand coast with this highly active chain of seafloor volcanoes. They erupt often and produce a lot of volcanic material.
Dr Timm described the subduction of the Hikurangi Plateau as counter-intuitive. “Nowhere else in the world is such a large and thick block of oceanic crust subducting like this.”
He said the study had highlighted the way that the chemistry of the down going plate and its effects on seafloor volcanism had far-reaching implications for hazards and the economic potential of the submarine volcanoes above the plate interface.