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Diapir and dome structures on a scale of meters to kilometers are widespread in Earth's continental crust and represent an important tectonic element of cratons, orogenic belts, and sedimentary basins. These structures advect heat from lower to higher crustal levels, often producing pronounced prograde contact metamorphic aureoles. This standard thermal situation is violated by the up to 8 km in diameter migmatitic domes and diapirs of metasedimentary rocks that penetrate the world's largest layered intrusion, the Bushveld Complex. These domes and diapirs rose with an average rate of about 8 mm/yr and were characterized by an unusual inverted thermal structure, with the cores of the structures 200–300 °C colder than the rims. Numerical modeling supports the interpretation that the process was triggered by the emplacement of an 8-km-thick, hot, dense mafic magma over a cold, less dense sedimentary succession, resulting in a dramatic lowering of the viscosity of the sediments during contact metamorphism and partial melting. Dome and diapir nucleation is interpreted to have been defined by the formation of initial anticline-shaped disturbances related to fingered lateral injection of the Lower Zone of the Bushveld intrusion between the felsic roof and sedimentary floor sequence. The partially molten, mobile, but relatively cold domes and diapirs promoted cooling of the giant magma chamber, rapidly bringing cooler material into higher crustal levels, and freezing the surrounding magmas. We argue that our work has a more general significance as similar thermal structures should be a widespread feature associated with partially molten mantle diapirs (“cold plumes”) generated in the proximity of subducting slabs. These structures are likely responsible for rapid upward melt transport above subduction zones and for the associated volcanic activity. The exposed structures observed in the Bushveld Complex provide a unique opportunity to study the “cold” diapir/plume phenomenon, thus leading to a broader recognition and understanding of this geological process.

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