How do oceans start to close? New study suggests the Atlantic may ‘soon’ enter its declining phase
IMAGE:
THE Strait of Gibraltar PROVIDES A NATURAL PHYSICAL BARRIER BETWEEN THE COUNTRIES OF Spain (NORTH) AND Morocco (SOUTH). IN GEOLOGIC TERMS, THE 10-MILE (16-KILOMETER) STRAIT THAT SEPARATES THE TWO COUNTRIES, AS WELL AS EUROPE AND AFRICA, IS LOCATED WHERE THE TWO MAJOR TECTONIC PLATES—THE EURASIAN PLATE AND THE AFRICAN PLATE—COLLIDE.
view moreCREDIT: NASA, PUBLIC DOMAIN, VIA WIKIMEDIA COMMONS
A new study, resorting to computational models, predicts that a subduction zone currently below the Gibraltar Strait will propagate further inside the Atlantic and contribute to forming an Atlantic subduction system – an Atlantic ring of fire. This will happen ‘soon’ in geological terms - in approximately 20 million years.
Oceans seem eternal to our lifespan, but they are not here for long: they are born, grow and one day close. This process, which takes a few hundred million years, is called Wilson Cycle. The Atlantic, for example, was born when Pangea broke up, around 180 million years ago, and will one day close. And the Mediterranean is what remains from a big ocean – the Tethys– that once existed between Africa and Eurasia.
For an ocean like the Atlantic to stop growing and start closing, new subduction zones – places where one tectonic plate sinks below another – have to form. But subduction zones are hard to form, as it requires plates to break and bend, and plates are very strong. A way out of this “paradox” is to consider that subduction zones can migrate from a dying ocean in which they already exist – the Mediterranean – into pristine oceans – such as the Atlantic. This process was dubbed subduction invasion.
This study shows for the first time how such a direct invasion can happen. The computational, gravity-driven 3-D model predicts that a subduction zone currently below the Gibraltar Strait will propagate further inside the Atlantic and contribute to forming an Atlantic subduction system – an Atlantic ring of fire, in an analogy to the already existing structure in the Pacific. This will happen ‘soon’ in geological terms – but not before approximately 20 million years.
“Subduction invasion is inherently a three-dimensional process that requires advanced modelling tools and supercomputers that were not available a few years ago. We can now simulate the formation of the Gibraltar Arc with great detail and also how it may evolve in the deep future” explains João Duarte, first author, researcher at Instituto Dom Luiz, at Faculty of Sciences of the University of Lisbon (Ciências ULisboa) (Portugal).
This study sheds a new light on the Gibraltar subduction zone, as few authors considered it to be still active, because it has significantly slowed down in the past million years. According to these results, its slow phase will last for another 20 million years and, after that, will invade the Atlantic Ocean and accelerate. That will be the beginning of the recycling of crust on the eastern side of the Atlantic, and might be the start of the Atlantic itself beginning to close.
“There are two other subduction zones on the other side of the Atlantic – the Lesser Antilles, in the Caribbean, and the Scotia Arc, near Antarctica. However, these subduction zones invaded the Atlantic several million years ago. Studying Gibraltar is an invaluable opportunity because it allows observing the process in its early stages when it is just happening”, adds João Duarte.
Broadly, this study shows that subduction invasion is likely a common mechanism of subduction initiation in Atlantic-type oceans, and thus plays a fundamental role in the geological evolution of our planet.
The finding that the Gibraltar subduction is still currently active has also important implications for seismic activity in the area. Subduction zones are known for producing the strongest earthquakes on Earth. Events such as the 1755 Great Lisbon Earthquake are a threat and require preparedness.
This study results from a collaboration between researchers of the Faculty of Sciences of the University of Lisbon (Portugal) – João Duarte and Filipe Rosas – and researchers of the Johannes Gutenberg University Mainz (Germany) Nicolas Riel, Anton Popov, Christian Schuler and Boris Kaus.
Atlantic Ocean Crustal Age Image with Plates
CREDIT
Mr. Elliot Lim, CIRES & NOAA/NCEI
JOURNAL
Geology
ARTICLE TITLE
Gibraltar subduction zone is invading the Atlantic
ARTICLE PUBLICATION DATE
13-Feb-2024
Early-stage subduction invasion
IMAGE:
MAP HIGHLIGHTING THE ATLANTIC SUBDUCTION ZONES, THE FULLY DEVELOPED LESSER ANTILLES AND SCOTIA ARCS ON THE WESTERN SIDE AND THE INCIPIENT GIBRALTAR ARC ON THE EASTERN SIDE. FROM DUARTE ET AL., 2018.
view moreCREDIT: JOÃO C. DUARTE
Contributed by Arianna Soldati, GSA Science Communication Fellow
Our planet’s lithosphere is broken into several tectonic plates. Their configuration is ever-shifting, as supercontinents are assembled and broken up, and oceans form, grow, and then start to close in what is known as the Wilson cycle.
In the Wilson cycle, when a supercontinent like Pangea is broken up, an interior ocean is formed. In the case of Pangea, the interior ocean is the Atlantic. This ocean has a rift in the middle, and passive margins on the side, which means no seismic or volcanic activity occurs along its shores. Destined to keep expanding, an Atlantic-type ocean will eventually become the exterior ocean of the next supercontinent. Currently, Earth’s exterior ocean is the Pacific. The Pacific also has a rift in the middle, but it is bounded by subduction zones and thus will eventually close. Along its margins, earthquakes and eruptions abound—a pattern known as the ring of fire.
The ocean-closing phase of each Wilson cycle requires the transition from passive to active (subducting) margins at the edges of the interior ocean. The oceanic crust along the coast of the Atlantic is old and heavy, so it is primed to subduct, but before it can do so, it must break and bend. The only force in nature that can break oceanic plates like these is slab pull from another subduction zone.
But this doesn’t happen spontaneously. So how does subduction initiate around interior oceans?
There currently are two subduction zones in the Atlantic: the Lesser Antilles and Scotia. But neither of them formed spontaneously in the Atlantic; they were forced by subduction zones in the Pacific during the Cretaceous and then propagated along transform margins, where the continent is narrow and there is barely a land bridge. They jumped oceans.
Today, on the eastern shore of the Atlantic, in Gibraltar, we have the opportunity to observe the very earliest stages of this process, known as subduction invasion, while the jump occurs from a different basin—in this case, the Mediterranean.
This is an incredibly valuable opportunity because the chances of observing the very start of any given tectonic process are limited. And subduction initiation is difficult to observe because it leaves almost no traces behind. Once subduction starts, it erases the record of its initial stages; the subducted plate ends up in the mantle, never to be exposed at the surface again (except in the rare case of ophiolites).
The activity of the Gibraltar subduction zone in the Mediterranean has been hotly debated. The Gibraltar arc formed in the Oligocene as a part of the Western Mediterranean subduction zones. While we can see a subducted plate in the mantle underneath it, almost no further movement is currently happening.
A new paper by Duarte et al., just published in Geology, suggests that Gibraltar is active—it is just currently experiencing a slow movement phase because the subducting slab is very narrow, and it is trying to pull down the entire Atlantic plate.
“[These are] some of the oldest pieces of crust on Earth, super strong and rigid—if it were any younger, the subducting plate would just break off and subduction would come to a halt,” explains Duarte. “Still, it is just barely strong enough to make it, and thus moves very slowly.”
A new computational, gravity-driven 3-D model, developed by the authors, shows that this slow phase will last for another 20 million years. After that, the Gibraltar subduction zone will invade the Atlantic Ocean and accelerate. That will be the beginning of the recycling of crust on the eastern side of the Atlantic, and might be the start of the Atlantic itself beginning to close, initiating a new phase in the Wilson cycle.
Broadly, this study shows that subduction invasion, the process whereby a new subduction zone forms in an exterior ocean and then migrates to an interior ocean, is likely a common mechanism of subduction initiation in Atlantic-type oceans, and thus plays a key role in the geological evolution of our planet.
Locally, the finding that the Gibraltar subduction is still currently active has important implications for seismic activity in the area. Recurrence intervals are expected to be very long during this slow phase, but the potential for high-magnitude events, such as the 1755 Lisbon earthquake, remains and requires preparedness.
Much remains to be figured out about the future of the Gibraltar arc. One of the next aspects that Duarte will focus on is determining the exact geometry of the subduction, which will require assessing the relative strength of the nearby continental margins.
Maps showing the evolution of the Gibraltar subduction zone from 30 million years ago to 50 million years into the future. From Duarte et al., 2024.
CREDIT
João C. Duarte
FEATURED ARTICLE
Gibraltar Subduction Zone Is Invading the Atlantic
João C. Duarte, Nicolas Riel, Filipe M. Rosas, Anton Popov, Christian Schuler, Boris J.P. Kaus
https://doi.org/10.1130/G51654.1
Contact: João Duarte, University of Lisbon, jdduarte@fc.ul.pt
GEOLOGY articles published ahead of print are online at https://pubs.geoscienceworld.org/geology/early-publication. Representatives of the media may obtain complimentary copies of articles by contacting Katie Busser. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.
###
ARTICLE TITLE
Gibraltar subduction zone is invading the Atlantic
ARTICLE PUBLICATION DATE
13-Feb-2024
No comments:
Post a Comment