Mesozoic-Cenozoic Exhumation History Of The Central Yanshan Belt

Yanshan is an east-west mountain ranges in the northern margin of north China.The sedimentary records in the Cretaceous Songliao basin and Jiaolai basin which distributed respectively on the north and south side of the Yanshan belt suggest that in the early Cretaceous,the both sides of the Yanshan belt had warm and wet climate environment.In late Cretaceous,however,south to Yanshan belt turned into hot dry climates whereas the north to Yanshan belt still kept a warm and wet climate.This differing in climate between the south and the north side of the Yanshan belt are inferred to be resulted from the rapid uplift of the paleo Yanshan range in the intersection of the Early and Late Cretaceous.Previous thermochronology studies show that the western and eastern parts of the paleo Yanshan range were exhumated during the Cretaceous period.However,no thermochronology data controls on the exhumation process in the middle part of the Yanshan belt.In this study,a couple of granitic plutons exposed in the north,central and south part of the Yanshan belt are selected for thermochronological studies.They are,late Triassic Dushan pluton(U-Pb: 216 Ma),Jurassic Wangtufang pluton(U-Pb: 191 Ma)and Cretaceous Wulingshan pluton(U-Pb: 129 Ma)as well as Xiangshan pluton(U-Pb: 117 Ma).The zircon(U-Th)/He elevation profiling and apatite fission track(AFT),apatite and zircon(U-Th)/He were employed for lower temperature thermochronology studies.Acquired dating results from these plutons decipher two phases of rapid exhumation events in the Yanshan belt indicated the corresponding rapid cooling rate.The first phase occurred in 160-150 Ma,with exhumation rate of 250.65 m/Myr.The second phase was 120-100 Ma,and the corresponding exhumation rate was 240.73 m/Myr.The earlier rapid exhumation events were confined in region of the Yanshan belt north to the Mengjiazhuang-Jiyuqing-Xiadian-Pingquan fault.The rapid exhumation events appear to propagation from north to south acrossing the central Yanshan belt with the most rapid exhumation in the middle of the central Yanshan belt.Combined with the previous study results from the eastern and western parts of Yanshan,it has been shown that there was a uniform rapid exhumation event in Yanshan area at the turn of Early and the Late Cretaceous(100-80 Ma).However,there was a significant difference in the exhumation rate of the east,central and west segments of Yanshan belt,with the highest exhumation rate in the central segment,lowest in the west and intermediate in east.Although the rapid and unified uplift of Yanshan at the turn of the Early and the Late Cretaceous might make it a geographical barrier separating its north and the south sides,whether it ever reached the height to prevent the migration of water vapor and thus affected the paleoclimatic and environmental changes in the north and the south remains unconstrained.