Ancient Volcanic Arc Discovered Beneath South China
Recent research has revealed a vast fossilised volcanic arc beneath South China. This 400-mile-long chain of extinct volcanoes dates back about 800 million years to the early Neoproterozoic era. It formed during the breakup of the supercontinent Rodinia. The discovery reshapes understanding of ancient tectonic and volcanic activity in the region.
Geological Context and Formation
Around 800 million years ago, the Yangtze Block of South China lay at Rodinia’s northwestern edge. Tectonic shifts caused it to drift towards the China Ocean plate. The denser oceanic crust subducted beneath the continental crust. This subduction generated magma as the oceanic plate heated and released water. The magma rose, forming a long volcanic arc along the subduction zone.
Unique Features of the Volcanic Arc
Unlike typical narrow arcs, this volcanic chain is unusually broad. It stretches 430 miles (700 km) long and 30 miles (50 km) wide, extending 550 miles (900 km) inland. This broadness is due to flat-slab subduction, where the oceanic plate moves horizontally at a shallow angle before descending deeper. This process creates two volcanic ridges – one near the plate boundary and one further inland.
Detection and Research Methods
Fossil volcanoes are usually buried under thick sediment layers, making detection difficult. Researchers used airborne magnetic sensing to map magnetic minerals beneath the Sichuan Basin’s sediments. They found iron-rich rocks with strong magnetic signals about 4 miles (6 km) below the surface. Deep borehole samples showed volcanic arc geochemical signatures. Uranium-lead dating confirmed these rocks formed between 770 and 820 million years ago.
Significance in Earth’s Crustal Evolution
Volcanic arcs contribute to crust formation and mountain building. This discovery suggests ancient arcs were more extensive than previously thought in South China. The volume and spread of magmatic activity indicate crustal growth during the Neoproterozoic. Some geologists propose the two volcanic belts may have formed separately and joined later, but the data offers new vital information about ancient tectonics.
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