Crustal eclogitization and lithosphere delamination in orogens

Abstract

Many orogens, including the Central Andes and Himalayas, are characterized by wide areas that have undergone upper crustal shortening and surface uplift. The behaviour of the deep lithosphere is poorly constrained, and in some mountain belts, lower crust and mantle lithosphere appear to have been removed through delamination during orogen development. Thermal–mechanical numerical models demonstrate that as crust thickens during shortening, the lowermost crust may undergo metamorphic eclogitization, which increases its density. Even a small density increase (7% or more) causes shortening to localize above the eclogitic crustal root, promoting the development of thick lithosphere in this area which is then prone to gravitational removal. Complete removal of orogen mantle lithosphere occurs if the eclogitized lower crust is weak enough to allow full detachment of negatively buoyant mantle lithosphere; this can occur even if the lower crust is less dense than the mantle. The onset of delamination may be determined by the hydration state of the lower crust, as the presence of water promotes eclogitization and significantly reduces rock strength. Two distinct styles of delamination are observed: (1) retreating delamination in which weak mantle lithosphere rolls back and peels away from the crust, producing a contemporaneous migration of crustal thickening, surface uplift and magmatism, and (2) stationary delamination in which strong lithosphere separates from the weak lower crust and slides into the deep mantle at a stationary detachment point, followed by widespread crustal deformation and magmatism.