Continental Collision - Deep Subduction of Continental Crust

Deep Subduction of Continental Crust

The continental crust on the downgoing plate is deeply subducted as part of the downgoing plate during collision, defined as buoyant crust entering a subduction zone. An unknown proportion of subducted continental crust returns to the surface as ultra-high pressure (UHP) metamorphic terranes, which contain metamorphic coesite and/or diamond plus or minus unusual silicon-rich garnets and/or potassium-bearing pyroxenes. The presence of these minerals demonstrate subduction of continental crust to at least 90–140 km deep. Examples of UHP terranes are known from the Dabie–Sulu belt of east-central China, the Western Alps, the Himalaya of India, the Kokchetav Massif of Kazakhstan, the Bohemian Massif of Europe, the Western Gneiss Region of Norway, and Mali. Most UHP terranes consist of an imbricated sheets or nappes. The fact that most UHP terranes consist of thin sheets suggests that much thicker, volumetrically dominant tracts of continental crust are more deeply subducted.