Mesozoic asthenospheric upwelling orogeny in eastern China

Abstract: Mesozoic orogens in eastern China are different from continental-margin subductional and continent-continent collisional orogens and are not the product in a particular stage of development and evolution of continental-margin and continent-continent collisional orogens either. They are a new type of orogen resulting from orogeny due to asthenospheric upwelling at depth and may be distinguished from plate-margin and continent-continent collisional orogens by their tectonic setting, tectonic framework, tectonic deformation and metamorphism, magmatism and sedimentation and tectonic evolution. We consider that such orogeny caused by asthenospheric upwelling is another orogenic mechanism that is as important as continent-continent collisional and plate-margin subductional orogenies. Therefore the process of formation of orogens by asthenospheric upwelling is termed by us asthenospheric upwelling orogeny or East Asia-type orogeny. Viewing the Da Hinggan orogen, Yanshan orogen and other orogens in eastern China from the lithospheric-asthenospheric deep process, we think that the formation of these orogens is an orogenic event caused by vertically upward dynamic processes originating by asthenospheric upwelling at depth as well as a new orogenic mechanism. The evidence is as follows: (1) the lithosphere-asthenosphere dos not show a typical horizontal layered structure but asthenospheric material commonly occurs as mushroom-shaped upwelling plumes penetrating and intruding into the lithospheric mantle, thus causing the older lithospheric mantle to be extensively dismembered and delaminated and forming a steep contact pattern with the relics of the older mantle; (2) the chronological structure of the lithosphere has the nature of uncoupling, i.e.: the mantle is younger, while the crust is older; (3) mafic-ultramafic rocks generated by underplating and ascending of Mesozoic asthenospheric material along lithospheric fractures are found; (4) magmatism of Mesozoic volcanic-plutonic rocks is different from that of classic subduction-type and continent-continent collision-type magmatic rocks but is related to upwelling of the asthenosphere; and (5) the process of asthenospheric upwelling orogeny has the unique character of evolution from the deep level to shallow level. Asthenospheric upwelling orogeny in eastern China is a global important tectonothermal event and its orogenic process is as follows: (1) before and in the initial orogenic phase in the Early Mesozoic (230-180 Ma), upwelling and underplating of asthenospheric material at depth resulted in linear fracturing and local delamination of the cold and strong continental lithosphere; (2) in the main orogenic phase in the Mid-Late Jurassic (180-140 Ma), due to large-scale upwelling of asthenospheric material and its lateral intrusion and extension along the crust～mantle boundary at the base of the lithosphere, vertical differential movement was transformed into horizontal compression, and thus extensive fold and nappe structures occurred at the crustal surface, causing the continental crust to be thickened into the crustal root and the lithospheric root to be delaminated; and (3) in the late orogenic phase in the Cretaceous (140-80 Ma), with thickening of the continental crustal root and extensive delamination of the lithospheric root, the lithosphere entered a completely new stage of transition from compression to extension and substantial thinning and the asthenosphere upwelled to form mountains.