The formation of ultradeep sedimentary basins through metamorphism with rock contraction in continental crust

Sedimentary covers are up to 15-20 km thick in ultradeep sedimentary basins. Joint interpretation of seismic reflection sounding and gravimetric data indicates that eclogites are located in the basins under the Moho. In these rocks the velocities of Pwaves are close to those in mantle peridotites. The basins show only moderate crustal stretching and their formation was caused primarily by the transformation of gabbroids into dense eclogites in the lower part of the continental crust. The transformation took place episodically as man� tle fluids infiltrated the lower crust and it was ensured by pressure rise in the lower crust occurring with the accumulation of sediments. Moderate metamorphism developed in silicic upper crust as temperature and pressure increased under thick sedimentary covers. In ironrich metasedimentary rocks, deep metamorphism resulted in the density increase, and Pwave velocities there increased to those characteristic of the oceanic crust.