Our planet’s lithosphere is broken into several tectonic plates. Their order is always changing, as continents fold and break apart, and oceans form, expand, and then begin to close in what is known as the Wilson cycle.

In the Wilson cycle, when a continent like Pangea breaks up, an inland ocean forms. In the case of Pangea, the interior ocean is the Atlantic Ocean. This ocean has a rift in the middle, and a passive margin on one side, which means there is no seismic or volcanic activity along its shores. Intended to continue expanding, the Atlantic-type ocean will eventually become the outer ocean of the next subcontinent. Currently, Earth’s outer ocean is the Pacific Ocean. The Pacific also has a rift in the middle, but it is surrounded by subduction zones and thus will eventually close. Along its margin, earthquakes and eruptions abound — a pattern known as the Ring of Fire.

The oceanic closure phase of each Wilson cycle requires a transition from passive to active (subducting) margins at the margins of the interior ocean. The oceanic crust along the Atlantic coast is older and heavier, so it is primed to subduct, but before it can do so, it must break and bend. The only force of nature that can break such oceanic plates apart is slab pull from another subduction zone.

But it does not happen spontaneously. So how does subduction begin around inland oceans?

There are currently two sub-zones in the Atlantic Ocean: the Lesser Antilles and Scotia. But none of them. created Spontaneous in the Atlantic Ocean; They were forced into the Pacific during the Cretaceous by subduction zones and then spread along the transform margin, where the continent is narrow and barely has a land bridge. They jumped the oceans.

Today, in Gibraltar, on the east coast of the Atlantic Ocean, we have the opportunity to witness the early stages of a process called subduction invasion, when the jump occurs from a different basin — in this case, the Mediterranean. .

This is an incredibly valuable opportunity because the chances of witnessing the beginning of any tectonic process are limited. And subduction initiation is difficult to observe because it leaves no trace behind. Once subduction begins, it erases the record of its early stages. Subducted into the mantle, it will never be exposed to the surface again (except in the rare case of ophiolites).

The activity of the Gibraltar subduction zone in the Mediterranean Sea has been hotly debated. The Gibraltar arc formed in the Oligocene as part of the western Mediterranean subduction zone. While we can see a hump plate in the mantle below it, there is currently almost no further movement.

Duarte et al. A new paper has just been published. Geologysuggests that Gibraltar is active — it’s experiencing a slow-motion phase right now because the subduction slab is so narrow, and it’s trying to pull the entire Atlantic plate down.

“[These are] Some of the oldest pieces of crust on Earth, very strong and rigid — if it were any smaller, the subducting plate would just break and subduction would stop,” Duarte explains. “Even then, it’s barely strong enough. to make it, and so moves very slowly.”

A new computational, gravity-driven 3-D model, developed by the authors, shows that this slow phase will continue for another 20 million years. The Gibraltar subduction zone will then strike the Atlantic Ocean and accelerate. This would be the beginning of recycling of the crust on the eastern edge of the Atlantic Ocean, and the Atlantic Ocean may begin to close, starting a new phase in the Wilson cycle.

Broadly, this study shows that subduction invasion, the process by which a new subduction zone forms in an outer ocean and then migrates to the inner ocean, is a mechanism for initiating subduction in Atlantic-type oceans. is the normal mechanism, and thus plays a key role in Geological evolution of our planet.

Locally, the finding that the Gibraltar subduction is still active has important implications for seismic activity in the region. Recurrence intervals are expected to be very long during this slow phase, but high-magnitude events such as the Lisbon earthquake of 1755 remain possible and require preparation.

Much remains to be known about the future of the Gibraltar Arch. One of the next aspects Duarte will focus on is determining the exact geometry of subduction, which will require estimating the relative strength of nearby continental margins.