New ‘embryonic’ subduction zone found

A new subduction zone forming off the coast of Portugal heralds the beginning of a cycle that will see the Atlantic Ocean close as continental Europe moves closer to America.

Published in Geology, new research led by Monash University geologists has detected the first evidence that a passive margin in the Atlantic ocean is becoming active. Subduction zones, such as the one beginning near Iberia, are areas where one of the tectonic plates that cover the Earth’s surface dives beneath another plate into the mantle – the layer just below the crust.

Lead author Dr João Duarte, from the School of Geosciences said the team mapped the ocean floor and found it was beginning to fracture, indicating tectonic activity around the apparently passive South West Iberia plate margin.

“What we have detected is the very beginnings of an active margin – it’s like an embryonic subduction zone,” Dr Duarte said.

“Significant earthquake activity, including the 1755 quake which devastated Lisbon, indicated that there might be convergent tectonic movement in the area. For the first time, we have been able to provide not only evidences that this is indeed the case, but also a consistent driving mechanism.”

The incipient subduction in the Iberian zone could signal the start of a new phase of the Wilson Cycle – where plate movements break up supercontinents, like Pangaea, and open oceans, stabilise and then form new subduction zones which close the oceans and bring the scattered continents back together.

This break-up and reformation of supercontinents has happened at least three times, over more than four billion years, on Earth. The Iberian subduction will gradually pull Iberia towards the United States over approximately 220 million years.

The findings provide a unique opportunity to observe a passive margin becoming active – a process that will take around 20 million years. Even at this early phase the site will yield data that is crucial to refining the geodynamic models.

“Understanding these processes will certainly provide new insights on how subduction zones may have initiated in the past and how oceans start to close,” Dr Duarte said.

Scientists aboard Iberian coast ocean drilling expedition report early findings

Ending a successful expedition, the JOIDES Resolution arrives in Lisbon, Portugal. -  Fernando Barriga, ECORD Portugal
Ending a successful expedition, the JOIDES Resolution arrives in Lisbon, Portugal. – Fernando Barriga, ECORD Portugal

Mediterranean bottom currents and the sediment deposits they leave behind offer new insights into global climate change, the opening and closing of ocean circulation gateways and locations where hydrocarbon deposits may lie buried under the sea.

A team of 35 scientists from 14 countries recently returned from an expedition off the southwest coast of Iberia and the nearby Gulf of Cadiz. There the geologists collected core samples of sediments that contain a detailed record of the Mediterranean’s history. The scientists retrieved the samples by drilling into the ocean floor during an eight-week scientific expedition onboard the ship JOIDES Resolution.

The group–researchers participating in Integrated Ocean Drilling Program (IODP) Expedition 339: Mediterranean Outflow–is the first to retrieve sediment samples from deep below the seafloor in this region.

Much of the sediment in the cores is known as “contourite” because the currents that deposit it closely follow the contours of the ocean basin.

“The recovery of nearly four kilometers of contourite sediments deposited from deep underwater currents presents a superb opportunity to understand water flow from the Mediterranean Sea to the Atlantic Ocean,” says Jamie Allan, program director at the National Science Foundation (NSF), which co-funds IODP.

“Knowledge of this water flow is important for understanding Earth’s climate history in the last five million years.”

“We now have a much greater insight into the distinctive character of contourites, and have validated beyond doubt the existing paradigm for this type of sedimentation,” says Dorrik Stow of Heriot-Watt University in the United Kingdom and co-chief scientist for Expedition 339.

The world’s oceans are far from static. Large currents flow at various depths beneath the surface. These currents form a global conveyor belt that transfers heat energy and helps buffer Earth’s climate.

Critical gateways in the oceans affect circulation of these major currents.

The Strait of Gibraltar is one such gateway. It re-opened less than six million years ago.

Today, deep below the surface, there is a powerful cascade of Mediterranean water spilling out through the strait into the Atlantic Ocean.

Because this water is saltier than the Atlantic–and therefore heavier–it plunges more than 1,000 meters downslope, scouring the rocky seafloor, carving deep-sea canyons and building up mountains of mud on a little-known submarine landscape.

The sediments hold a record of climate change and tectonic activity that spans much of the past 5.3 million years.

The team found evidence for a “tectonic pulse” at the junction between the African and European tectonic plates, which is responsible for the rising and falling of key structures in and around the gateway.

This event also led to strong earthquakes and tsunamis that dumped large flows of debris and sand into the deep sea.

At four of the seven drill sites, there was also a major chunk of the geologic record missing from the sediment cores–evidence of a strong current that scoured the seafloor.

“We set out to understand how the Strait of Gibraltar acted first as a barrier and then a gateway over the past six million years,” says Javier Hernandez-Molina of the University of Vigo in Spain and co-chief scientist for Expedition 339. “We now have that understanding and a record of a deep, powerful Mediterranean outflow through the Gibraltar gateway.”

The first drill site, located on the west Portuguese margin, provided the most complete marine sediment record of climate change over the past 1.5 million years of Earth history.

The sediment cores cover at least four major ice ages and contain a new marine archive to compare against ice core records from Greenland and Antarctica, among other land-based records.

The team was surprised to find exactly the same climate signal in the mountains of contourite mud they drilled in the Gulf of Cádiz.

Because these muds were deposited much faster than the sediments at the Portuguese margin site, the record from these cores could prove to yield even richer, more detailed climate information.

“Cracking the climate code will be more difficult for contourites because they receive a mixed assortment of sediment from varying sources,” Hernandez-Molina says.

“But the potential story that unfolds may be even more significant. The oceans and climate are inextricably linked. It seems there is an irrepressible signal of this nexus in contourite sediments.”

The team also found more sand among the contourite sediments than expected.

The scientists found this sand filling the contourite channels, deposited as thick layers within mountains of mud, and in a single, vast sand sheet that spreads out nearly 100 kilometers from the Gibraltar gateway.

All testify to the strength, velocity and duration of the Mediterranean bottom currents. The finding could affect future oil and gas exploration, the researchers believe.

“The thickness, extent and properties of these sands make them an ideal target in places where they are buried deeply enough to allow for the trapping of hydrocarbons,” Stow explains.

The sands are deposited in a different manner in channels and terraces cut by bottom currents; in contrast, typical reservoirs form in sediments deposited by downslope “turbidity” currents.

“The sand is especially clean and well-sorted, and therefore very porous and permeable,” says Stow. “Our findings could herald a significant shift in future exploration targets.”