Involvement of Crustal Material in Delamination of the Lithosphere after Continent-Continent Collision

Abstract

Deep submersion of crustal material into the mantle, evidenced for the Variscan orogen at about 340 Ma, is explained by delamination of the continental lithosphere. The cause for such delamination is thickening of the continental crust and the underlying mantle lithosphere due to the collision of Gondwana and Laurussia. Scientific information on this continent-continent collision is scrutinized to test if the evolutionary model for the Variscan orogeny proposed here is compatible with the vast geoscientific data. It is then hypothesized that the evolution of an orogen, resulting from continent-continent collision since the late Neoproterozoic, can, in general, be described by six stages: (1) approach of continental plates; (2) beginning of continent-continent collision; (3) lateral extension of continental crust that was thickened to 60 km or more, for tens of million years; (4) delamination of the continental lithosphere involving material from the continental crust; (5) continuation of continent-continent collision without significant thickening of continental crust lasting again several tens of million years; and (6) cessation of continent-continent collision. These stages are characterized by the formation of different sedimentary, igneous, and metamorphic rock types. For instance, the origin and the beginning of exhumation of high- and ultrahigh-pressure rocks are related to stages 2 and 4. Thus, the corresponding rocks are good relative time markers. The above model was applied to the Cretaceous to Quaternary evolution of the orogenic chains in the Mediterranean region. It is concluded that this region underwent extended lithospheric delamination (stage 4) in the late Paleogene. Finally, the consequences of lithospheric delamination with continental crust involved are discussed. It is speculated that this process has resulted in an enormous loss of continental crust over the last 600 m.y. This loss could have also significantly influenced the climatic and biologic evolution on Earth.