Parallel Extension Tectonics : Mechanism of Early Cretaceous thinning and destruction of the lithosphere of the North China Craton

The mechanism of lithosphere thinning and destruction of the North China Craton has long been controversial. Two popularly discussed models, i. e., the top-down tectonics model and the bottom-up model, based on petrological and geochemical studies that stress the importance of deep processes. The former is exemplified by delamination and foundering of thickened lithospheric root, while the later is represented by processes involving thermo-mechanical and chemical erosions, mantle replacement or metasomatism. How the regional lithospheric extension contributed to thinning of the lithosphere remains not well-understood, although tectonic stresses are generally accepted to be one of the major driving forces during cratonic evolution. It is also debatable if the crustal extension is a response to deep processes or both are parallel products of lithospheric extension. This paper demonstrates, based on comprehensive analysis of Early Cretaceous extensional and magmatic activities in East Asia that there are obvious differences in temporal, spatial and genetic relationships between extensional tectonic deformation and magmatic activities in different area, especially in the eastern part of the North China Craton. Extensional tectonic activities predominted over the generation, up-welling and emplacement of syntectonic magmas. At different stages of the tectonic extension, there are early syn-extensional, syn-extensional and post-extensional magmatic activities. The characteristics of magmatic activities and their source attributes vary significantly with time at different tectonic domains. Meanwhile, there are obvious differences in magmatic sources at different stages of tectonic extension in a specific tectonic domain, in spite of a general tendency from mixed sources with dominant ancient crust contributions to separate mantle or crustal sources. Further more, different tectonic domains may have different tendencies of source region variations. Taking the Jiaodong and Liaodong peninsulas at the southeastern North China Craton as an example, mixed sources of ancient crust, juvenile crust and lithospheric mantle have important contributions to magma evolution in the Jiaodong area, but evolving magma sources played key roles in the generation of magmas in the Liaodong Peninsula. The present study suggests that the Early Cretaceous tectono-magmatic activities in the North China Craton and adjacent areas can be reasonably explained by Parallel Extension Tectonics (PET). In the PET model , both crust and mantle detachment occurs in response to regional lithosphere extension. Detachment faulting in the crust and in the lithospheric mantle can be either coupled or decoupled, which may have different tectono-magmatic responses. Three different modes of crustal detachment and exhumation of metamorphic core complexes are shown to have occurred during the Early Cretaceous tectonic extension of the North China Craton: co-magmatim (hot) mode extension, amagmatism (cold) mode extension and multi-mode extension.