Délamination (géologie)

In geodynamics, delamination refers to the loss and sinking (foundering) of the portion of the lowermost lithosphere from the tectonic plate to which it was attached. The outer portion of the Earth is divided into an upper, lithosphere layer and a lower, asthenosphere layer. The lithosphere layer is composed of two parts, an upper, the crustal lithosphere and lower, the mantle lithosphere. The crustal lithosphere is in unstable mechanical equilibrium because the underlying mantle lithosphere has a greater density than the asthenosphere below. The difference in densities can be explained by thermal expansion/contraction, composition, and phase changes. Negative buoyancy of the lower continental crust and mantle lithosphere drive delamination. Delamination occurs when the lower continental crust and mantle lithosphere break away from the upper continental crust. There are two conditions that need to be met in order for delamination to proceed: The lower lithosphere must be denser than the asthenosphere The intrusion of more buoyant asthenosphere making contact with the crust and replacing dense lower lithosphere must occur. The metamorphic transition from mafic granulite facies to the denser eclogite facies in the lower portion of the crust is the main mechanism responsible for creating negative buoyancy of the lower lithosphere. The lower crust undergoes a density inversion, causing it to break off of the upper crust and sink into the mantle. Density inversions are more likely to occur where there are high mantle temperatures. This limits this phenomenon to arc environments, volcanic rifted margins and continental areas undergoing extension. The asthenosphere rises until it comes into contact with the base of the lower crust, causing the lower crust and lithospheric mantle to start to peel away. Slumping, cracking, or plume erosion facilitates the intrusion of underlying asthenosphere. Potential energy that drives the delamination is released as the low density, hot asthenosphere rises and replaces the higher density, cold lithosphere.