Uplift And Exhumation History Of The Middle Greater Khingan Mountains Since Late Mesozoic

The Greater Khingan Mountains in Northeast China are located in the northeastern part of the Inner Mongolia Autonomous Region.The terrain is high in the west and low in the east,located at the junction of the second and third staircases.The Basin and Songliao Basin form a typical basin-mountain landform.Since the Late Mesozoic,this area has experienced the superposition of the subduction and closure of the MongoliaOkhotsk Ocean tectonic domain and the multi-stage continuous action of the PalaeoPacific tectonic domain.The structure is active and the geological body is uplifted and denuded strongly.Chronological methods reveal ideal regions for intracontinental tectonic activity.At present,a large amount of fission track chronology data has been accumulated in the Daxing'anling area,but there is still considerable controversy over the period of episodic exfoliation experienced in the middle part of the Greater Khingan Mountains since the Late Mesozoic.The fission track data processing method is single,and its rich geological information still has a lot of room for excavation.In this paper,apatite fission track analysis of the Upper Jurassic Murui Formation in Chaihe area and the Upper Ordovician Naked River Formation,the middle part of the Greater Khingan Mountains was carried out,the central age distribution of detrital apatite is between 260 and 62 Ma,and the single particle The ages are concentrated around100 Ma,and the fitting component ages after the decomposition of the single particle age of the sample are three peaks: P1(16-42Ma),P2(108-159Ma)and P3(378Ma).According to the number of single particle and confined track length of the sample,three samples were selected for thermal history simulation using hefty software.The results show that the main body of the study area has experienced cooling events from early Cretaceous to late Cretaceous(130-94Ma),regional warming events from Paleocene to Eocene(64-45Ma),and cooling events since Eocene(since 45Ma).Combined with regional geological data,the thermal history of each stage is discussed in detail.It is concluded that the early Cretaceous to late Cretaceous cooling events occurred under the background of regional extensional orogeny.During the Period of Paleocene-Eocene,the extensive volcanic activities in the Greater Khingan Mountains led to the rise of paleotemperature and regional warming.Since Eocene,the study area was affected by the subduction of the Pacific plate,and the cooling exhumation occurred under the background of back-arc extension.Three-dimensional thermodynamic modeling of apatite fission track data in the study area was carried out based on the results of two-dimensional thermal history simulation.,different scenarios were simulated to verify the cooling of the Early Cretaceous-Middle-Late Cretaceous(130-94Ma)in the middle part of the Greater Khingan Mountains revealed by the 2D thermal history simulation.The stripping start and stripping end of the stripping event.The inversion results of the 3D model(steadystate terrain)show that the best transition times are at 128.72 Ma and 98 Ma,and the maximum probability values of the fast dew rate(E2)are 0.147 km/Myr and 0.137km/Myr(the geothermal gradient is 22.2 °C/km).The published fission track data of the Greater Khingan Mountains and Hailar Erlian Basin and Songliao Basin on both sides are statistically analyzed.The statistical results show that the peak age of fission track between the Greater Khingan Mountains and Hailar Erlian basin is similar between 160-100 Ma,indicating that the evolution of basin mountain system in the west of the Greater Khingan Mountains is affected by the same tectonic domain;During the period of 100-42 Ma,the Greater Khingan Mountains,Hailar Erlian Basin and Songliao Basin show three groups of similar peak ages at the same time.It is considered that the Greater Khingan Mountains and its adjacent areas began to evolve together as a whole during this period.Based on the above-mentioned low-temperature thermochronological analysis,combined with regional geological data,etc.,we propose the tectonic evolution process since the late Mesozoic in the study area.We believe that during the period of 130-94 Ma,the extensional orogeny of the Greater Khingan Mountains was the result of the closure of the Mongolia-Okhotsk Ocean leading to the thickening and demolition of the lower crust of the Greater Khingan Mountains.This caused regional extension,upwelling of the asthenosphere,large-scale magmatic activity,and rapid mountain uplift.Since 45 Ma,the subduction rate of the Pacific plate to the Eurasian plate has shown an overall increasing trend.The Greater Khingan Mountains and the basins on both sides are affected by the Pacific plate.Subduction influence,in the tectonic background of back-arc extension.