by Mary Caperton Morton Friday, December 21, 2018
Plate tectonics is a fundamental control on how Earth operates and is important for the planet’s habitability, but how this crustal recycling process got started has long been a mystery. A new study examining some uniquely coupled metamorphic and volcanic rocks in Oman is adding some needed clarity about the initiation of subduction zones, a critical component in plate tectonics.
“The driving force of plate tectonics is the lithosphere sinking into subduction zones and pulling other plates at the surface. The million-dollar question is: If subduction zones are what powers plate tectonics, how do you start a subduction zone?” says Robert Stern, a geophysicist at the University of Texas at Dallas.
Subduction zone initiation is thought to occur by one of two mechanisms: spontaneous or forced. In spontaneous subduction, weaknesses and density contrasts in a plate encourage the denser part of the plate to start sinking under the force of gravity. As the downgoing slab sinks, cold rock and sediments formerly on the seafloor are metamorphosed when they encounter hot mantle. At the same time, extensional forces pull on the leading edge of the upper plate, triggering volcanism that creates an area of new crust above the subduction zone called a forearc. “When dated, these metamorphic and volcanic materials will be synchronous,” says Carl Guilmette, a geological engineer at Laval University in Quebec and lead author of the new study in Nature Geoscience.
By contrast, in forced subduction, lateral movement of part of the lithosphere leads to compression, eventually forcing a dense slab downward under an adjacent plate. This convergence takes time to develop, however, such that new metamorphic rock in the downgoing plate will form a few million years before extension and forearc volcanism in the upper plate begins, creating a lag between the ages of the metamorphic and volcanic rocks.
Computer models suggest that Earth’s first subduction zone may have been triggered by spontaneous subduction. “It makes sense that, at some point, a denser zone [of lithosphere] developed in one place that [then] started sinking under its own weight,” Guilmette says. But no clear examples of the early phases of subduction exist on Earth today — although evolving transform boundaries south of New Zealand and west of Gibraltar may offer some tantalizing clues — so geoscientists look for evidence in the rock record of how slabs might begin sinking. Guilmette and his colleagues focused on the Semail Ophiolite of Oman in the southeast corner of the Arabian Peninsula. Here, pieces of both the overlying and underlying plates of a fossil subduction zone are exposed at the surface, enabling scientists to study how the system evolved.
“In Oman, we have the ultimate archetype of a subduction zone that has been very well studied and documented,” Guilmette says. He and his colleagues are the first to date the burial of the lower plate in a fossilized subduction zone, and the results offer some clues about the timing of the events surrounding subduction initiation. They found that the metamorphic rocks in the downgoing slab started subducting 104 million years ago, whereas previous studies determined that the volcanic rocks in the upper slab dated to 96 million years ago.
“The new contribution by this study is this clear understanding of the magnitude of this lag time,” Stern says. “The geochronology seems robust, with the lower metamorphism clearly predating the upper igneous activity.” A study published last April in Geoscience Frontiers reported similar geochronological evidence of forced subduction initiation in Turkey, in an area that would have been geographically connected to the subduction zone forming in Oman. “It looks like a 3,000-kilometer-long convergent plate margin was forming about this time that stretched along the southwest margin of Eurasia from Oman to the island of Cyprus,” Stern says.
Fossil subduction zones that preserve materials from both the upper and lower plates have also been identified in Newfoundland, California and the Himalayas. “I’d like to see more studies using this methodology to determine the absolute timing of subduction initiation settings around the Mediterranean and elsewhere,” Stern says.
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