The Evolution Of The Yanshanian Basins In Western And Northern Beijing,North China,and Its Implication

The “Yanshanian Movement”(Yanshanian tectonism),as the most important tectonic events in the Mesozoic in eastern C hina,has widely attracted the scholars' attention.However,those are still under debate about the deformational process,tectonic paleogeography and the Mesozoic volcanic-sedimentary basins evolution,and ages of its filling sequences.With the accumulation of geological data,the Mesozoic tectonic transformation plays an important role in understanding the regional tectonic evolution,while the timing of tectonic transition is controversial.Those geological problems need to be solved as soon as possible for understanding the Mesozoic tectonic evolution of North China Craton.As a classical area for studying the Yanshanian tectonism,the basin evolution and its filling sequences ages,deformational characteristics and magmatic activity in Beijing area are of great tectonic significance for understanding the Yanshanian tectonism and regional tectonic evolution.Two Yanshanian basins were selected in this study,including a typical inherited basin in western Beijing and a newly formed Q ianjiadian basin in northern Beijing.The paper focus on the Jurassic filling sequences in the Yanshanian basin in western Beijing,and the filling sequences,conglomerate clasts and basin marginal faults analysis in the Qianjiadian basin in northern Beijing,and the LA–ICP–MS zircon U–Pb dating of pyroclastic and volcanic rocks in the basin is carried out to constrain the basin evolution and the age of filling sequences.The paper aims to demonstrate the relationship between basin evolution and “Yanshanian tectonism”,and to constrain tectonic evolution during the Late Jurassic to Early Cretaceous.The main achievements of this paper are as followed.1)The new LA–ICP–MS zircon U–Pb age(138 ± 3 Ma)shows that the inherited basin in western Beijing ended its evolution to the Late Tuchengzi phase in the Early Cretaceous in the early Cretaceous,namely it records the evolutional process during the deposition of Tuchengzi Formation.In summary,it shares a similar tectonic history in age with typical Yanshanian basins in the Yanshan Fold-Thrust belt(YFTB)during the Late Jurassic to Early Cretaceous.2)In the Q ianjiadian basin in northern Beijing,O ur new zircon U–Pb ages(160–156 Ma)for the pyroclastic and volcanic rocks of the Tuchengzi Formation suggest that formation of the QJB commenced at ~160 Ma,almost consistent with the lower limit age of Tiaojishan Formation in the YFTB.The results of paleocurrents and conglomerate clasts reveal that the “Tuchengzi” sediments were derived from the unroofing and erosion on the hanging wall of the Pingquan–Gubeikou thrust fault along the northern of the Q ianjiadian basin.The Q ianjiadian basin underwent a rapid depositional process and was formed under the control of the syn-depositional PGF in the northern and reformed by the post-depositional Qianjiadian thrust fault in the western.3)The Yanshanian basins in western and northern Beijing recorded the intense deformed event,which is considered as the main episode of the “Yanshanian tectonism”.The inherited Yanshanian basin in western Beijing ended its evolution in early Early Cretaceous,while in northern Beijing,the newly born Q ianjiadian basin developed rapidly during the Yanshanian tectonism.4)The Q ianjiadian basin evolutional process and marginal faults indicate that the tectonic regime is dominated by the N–S compression resulting in the uplifting of Pingquan-Gubeikou falut from 160 to 156 Ma.While a tectonic transition from N–S to NW-SE shortening occurred from 156 to ca.135 Ma,which is related to the northwestern subduction of the Paleo-Pacific plate beneath the eastern Asian.