Intracontinental superimposed tectonics— A case study in the Western Hills of Beijing,eastern China

摘要

Eastern China underwent structural evolution from N-S contraction to NW-SE contraction and then ESE-ward crustal thinning during Mesozoic-Cenozoic time; however, the mechanisms are still poorly understood. The Western Hills of Beijing are located at the junction among the E-W-trending Yinshan-Yanshan tectonic belt, the NNE-trending Taihang Mountain belt, and the Late Cretaceous-Cenozoic North China Basin. During Jurassic-Cretaceous time, three deformational events, including one com-pressional and two extensional deformation stages, were superimposed on Middle-Early Triassic south-vergent thrusts and folds in this region. These include (1) WNW-vergent thrusting and fold deformation, (2) ESE-ward crustal thinning with 110°-120°ESE stretching lineations, and (3) E-dipping, high-angle normal faulting. During these deformation stages, muscovite and sericite were produced as synkinematic minerals, and most of these micas are aligned parallel to the stretching lineation or deformed surfaces associated with contractional/extensional deformation. U-Pb zircon dating, as well as~( 40)Ar/~(39)Ar hornblende, muscovite, biotite, K-feldspar, and sericite dating, clearly provides the following age constraints in the Western Hills: (1) N-S contraction at ca. 250-200 Ma; (2) WNW-vergent compression at ca. 170-150 Ma; (3) granodiorite and diorite dike intrusion at ca. 136-135 Ma, followed by intrusion of the Fangshan granitic pluton at ca. 130-128 Ma; and (4) ESE-trending stretching lineation at ca. 130-110 Ma. East-dipping, high-angle normal faults developed at ca. 70-60 Ma. The N-S contraction overlaps temporally with the formation and evolution of the Inner Mongolia orogenic belt during late Paleozoic-early Mesozoic time. WNW-vergent thrusting and folding, similar to that in the Taihang Mountain belt, was controlled by the initial formation of the west Pacific plate and its westward subduction beneath the Eurasian plate. ESE-ward crustal thinning in the area was probably controlled by eastward extrusion of the northern margin of the North China craton, driven by final closure of the Mongol- Okhotsk Ocean. The E-dipping, high-angle normal faulting was related to ongoing eastern Asian detachment since Late Cretaceous time. We propose that the main mechanism :for the multiple deformation events resulted from the closure of the paleo-Asian Ocean, initial formation of the paleo-Pacific plate, and its subduction, combined with closure of the Mongol-Okhotsk Ocean.