Research On Uplift Process And Uplift Mechanism Of Lesser Khingan Range In Cenozoic

The studied area strides the three geotectonic units named respectively Eerguna-Khingan Massif,Songnen-Zhangguangcailing Massif and Jiamusi Massif. Since lateMesozoic,the continental rift system composed of the Songliao Graben,the Lower LiaoheRift,Yitong-Yilan Rift,Fushun-Mishan Rift and the adjacent faulted basins has been formed inNortheast China.To the south,the continental rift system extends into the North China throughBohai Rift and is connected with the Tanlu Rift and the rift system in North China. To thenorth,it extends into the Zeya River Basin in Russia.The continental rift system forms a hugerift zone in the East Asia.Since late Cenozoic, the Greater Khingan Range,the Lesser KhinganRange and the Zhangguangcai Ridge have been uprising and the Songliao Basin has beendepressing by faulting-block forming the recent structural landfom framework.In this paper,using sedimentology, structural geology,stratigraphy,Quaternary chronologyand statistics,the characteristics of the various landforms along the middle Heilong River arecomprehensively studied. The ages of the floodplains, terraces and planation surfaces alongthe middle Heilong River and the uplift times of the Lesser Khingan Range are determined bythe methods of thermoluminescence dating and stratigraphy.According to the data obtained inthe field investigation,we find that there are five river terraces and two planation surfacesalong the middle Heilong River.According to the heights of the fluvial landforms,theempirical formulas of the height changes of the floodplains and terraces along the middleHeilong River are calculated by linear regression analysis. Using the empirical formulas,the height of river terrace or floodplain at any point can be calculated. According to the heightdifference between higher and lower terraces and the uplift time of the higher terrace, theuplift rate of the Lesser Khingan Range during different period can be calculated.There are two planation surfaces and five river terraces along the middle HeilongRiver.Of them,the first planation surface was formed in Oligocene,the second planationsurface at the end of Pliocene and1~5terraces in Quaternary.During the whole Oligocene,theLesser Khingan Range was greatly uprised by373.7m at the maximum.During Neogene,theLesser Khingan Range was leveled and it became a peneplain.After the uprising and localfault-block-depressing from the end of Paleogene to the beginning of Neogene,the LesserKhingan Range was stable for a long time and the middle Heilong River shifted left and rightaccumulating fluvial deposits which were extensively distributed and composed of riverbedconglomerates and floodplain mudstone(N1-2s).Since Quaternary, the Lesser Khingan Rangehas been uprised intermittently.The lift magnitude was274.5m in early EarlyPleistocene,106.9m in middle Early Pleistocene,64.9m in late Early Pleistocene,38.9m inmiddle Middle Pleistocene,25.0m in early and middle Late Pleistocene and6.0m in earlyHolocene.The tectonism in early Early Pleistocene caused the crust pressing stress to be in nearlyeast-west.Under the acrion of the stress field,the strike-slip faults extending in northeast andnorthwest slipped nearly horizontally.At the same time,the slips of the faults also causedvertical movements which could reach several ten metres.The accumulated vertical-slipmagnitudes could be several hundred metres causing the intermittently fault-block-uprising ofthe Lesser Khingan Range.Through the erosion of flowing water and the wearing of otherexternal agents, the Lesser Khingan Range occurred as the recent landforms.