A secret geological corridor beneath Panama may explain why rocks from the Earth’s mantle have been found more than 1,000 miles (1,609 km) from where they originated.
This corridor, located about 62 miles (100 km) below the Earth’s surface, could allow mantle material to flow all the way from beneath the Galapagos Islands to the bottom of Panama.
This previously undiscovered form of transmission may help explain why there are so few active volcanoes in Panama. On the western coast of Central America, the Cocos tectonic plate is sinking and pushing oceanic crust under the continental crust of the North American, Caribbean and Panama tectonic plates, a process called subduction.
This subduction zone creates a line of volcanoes called the Central American Volcanic Arc as lava rushes across the boundary. The volcanoes stop in western Panama, which lies on the Panama Plate, said David Beckert, a postdoctoral researcher in marine chemistry and geochemistry at Woods Hole Oceanographic Institution in Massachusetts.
This relative peace has long been a mystery. Now, Beckert and colleagues report in a new study published November 23 in the journal Proceedings of the National Academy of Sciences, that the culprit may be a window-like opening in the Cocos tectonic plate that is being pushed toward the center of the Earth.
Beckert and colleagues are trying to understand more about how subduction near Central America works. Cocos plate subduction under North America has the potential to cause major earthquakes, including the 2017 Chiapas earthquake, an 8.1 Richter scale earthquake that killed dozens.
To learn more, the researchers delved into the region’s geochemistry, collecting volcanic rock samples as well as gas and liquid samples from hot springs. They were interested in looking at molecular isotope ratios, which are variations of the same atoms with different numbers of neutrons in their nuclei. In this case, the researchers focused specifically on the isotopes of helium and lead.
“Different sources of geological material usually have different compositions, so we can trace the contribution from different regions of the mantle,” Beckert said.
The mantle consists mostly of silicate rocks, which are rocks with a specific structure of silicon and oxygen atoms. But the exact composition can vary greatly even over small distances. The researchers found that there are some strange anomalies under Central America.
“We found that in certain places in Central America, specifically western Panama and behind the volcanic arc in Costa Rica, we have some strange signatures,” Beckert said. [للكيمياء الجيولوجية] Which is really similar to what you have in the Galapagos.”
This was strange, because there was no clear way to explain how mantle elements from the Galapagos could reach Panama, Pickart said. The researchers then turned to seismic imaging of the mantle, which uses earthquake waves to map what lies beneath the surface, and computer modeling to try to explain what might happen.
They found that buried fragments of the Cocos Plate may hold the answer deep in Panama. And when one tectonic plate slides under another tectonic plate during subduction, that merging plate not only disappears; It retains its structure when rolled into the mantle, only gradually heating and warping.
“Just below Panama, there is a crater, a window through the slab, that allows this mantle component to flow,” Beckert said.
This window may be the result of a pre-existing natural fracture in the subducted Cocos crust, or it may be a place where the crust breaks during subduction. Either way, it allows materials to pass – from one side of the board to the other – like a breeze through an open window.
The researchers found two possibilities: First, the material was moving through the Panama Rift Zone, a fracture zone in the crust and upper mantle that connects the Galapagos to Panama.
The researchers found that the most likely scenario is that a typical large-scale rotation of the mantle simply drives material through the hole in the sloping plate.
The presence of the mantle window could also explain the lack of active volcanoes in Panama, Beckert said. Water trapped in the crust of subducted plates tends to promote volcanic formation because water lowers the melting point of rocks, which leads to magma formation. The hole in the slab below Panama means there is a hole in the water-rich crust at that spot, which in turn means that molten magma is difficult to flow there.
The mantle flux that the team discovered has not been adequately studied, Beckert said, but that there are unexplained anomalies in mantle chemistry around the world. The team hopes to conduct a similar analysis in Chile next, but ultimately wants to expand the method worldwide.
Source: Live Science
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