The Mesozoic And Cenozoic Tectonic Characteristics And Tectonic Evolution Of The Southern Margin Of Central Qilian,in The Shulehe Area

As the boundary of the northeastern margin of Qinghai-Tibet plateau,Qilian Shan plays an important role in understanding the uplift and northward extension of Qinghai-Tibet plateau because of its special tectonic location,and become a key area of researching the basin-mountain system and lithosphere dynamics mechanism coupling relationship.The Meso-Cenozoic fold-thrust deformation characteristics of Shulehe area in the south margin of cental Qilian can provide field evidence in understanding the spatial and temporal variations of the north-expanding Qinghai-Tibet plateau.This study uses the theory of "structural analysis",to analysis the tectonic characteristics,tectonic period,paleo stress inversion and document the relationship between tectonic evolution of study area and the periodic uplift of the northeastern margin of Qinghai-Tibet plateau.The paper obtains the following achievements:1. The main structure of Shulehe area is NW-SE trending,locally developing nearly SN trending structure,which manifests compressive shear tectonic style and reflects multiple superimposed deformation characteristics.We determined two large scale fold-thrust belt:the Tuolainanshan fold-thrust belt and Bengkungou-Zaermage(BKZM) fold-thrust belt.The general structure of the Tuolainanshan fold-thrust belt is that the Proterozoic basement southward imbricate reverse on the Paleogene and Neogene Baiyanghe formation,Shule River formation.On the other hand,the BKZM fold-thrust belt result in the Devonian-Triassic sedimentary cover layer thrusted northwards,which consists of the BKZM main thrust fault,and its recoil fault and the striker fault.The two fold-thrust belt shows the north-south hedge thrust style in the macroscopic,and refers that they are controlled by the same detachment in a certain depth underground.2. Concerning the time of the two fold-thrust belt:we consider that the BKZM fold-thrust belt was formed at late Indosinian period and activated again in late Yanshan period,but the activity intensity is weaker than the Indosinian period.Qilian area had a strong tectonic shortening since the Oligocene and the BKZM fold-thrust belt has no evidence of activities,mainly show the overall tectonic uplift;opposite TuolaiNanshan fold-thrust belt has strong activity since the Oligocene,which formed Huaerdi nappe structure and the Baiyanghe formation closed-isoclinal folds.3. Though structure profile restoration,we estimate that the north margin of Shulehe Basin has a minimum shorting during the cenozoic of 2.45km(the shortening rate is 60.1%)along the TuolaiNanshan thrust belt because of the TuolaiNanshan Cenozoic thrust belt results from NE-SW trending compressional stress in the region. The south margin of Shulehe Basin has a minimum shorting during the Cenozoic of 0.65km(the shortening rate is 13.4%).The minimum shortening of BKZM fold-thrust belt is considered to be the main contribution of the Mesozoic Indosinian tectonic movement. That Qilian shan-Nanshan thrust belt has a initial activities inherited basal boundary fault in the Indosinian period,and has a strong activity of the Himalayan period,now still at the stage of the northward expansion.Therefore in the assessment of NE-SW trending tectonic shortening in Qilian shan-Nanshan thrust belt,should not ignore the tectonic shortening during Indosinian movement.4. Combining with the study of angular unconformity characteristics and each tectonic layer deformation analysis,the study area has experienced 3 tectonic cycles(the Indosinian movement, Himalayan movement and Himalayan movement),which included 6 stages of tectonic deformation.The tectonic evolution of study area is well corresponding to the periodic uplift of NE margin of Qinghai-Tibet Plateau:(1)Late Indosinian period the collision between Qiangtang block and Kunlun block,the northern margin of the Qinghai-Tibet plateau prototype appeared.The Devonian to Triassic strata in Qilian area formed ejective folds under the NE-SW compression-tectonic stress,which showed the first period of Indosinian movement;during the second period of the Indosinian movement,the BKZM thrust system developed with dextral thrust movement,under the near N-S trending compressional stress field.On the other hand,due to the impact of the dextral strike slip and basement strata,the northern part of BKZM main thrust in the formation of the “z” type folds and North-South tectonic superposition.(2)By the impact of the collision between the Lhasa block and the QiangTang block,in the late Yanshan movement,the BKZM fold-thrust system was reactive again,mainly showing that the reverse thrust of the recoil fault F3 and the striker fault F4 which carved early cretaceous Hekou formation.(3)Since the Indo-Asia initial collision(55~50Ma B.P.) at the Himalayan epoch,Qilian area has occurred multi-stage uplift:the study area happened strong crustal shorting from late Oligocene to early Miocene,which is characterized by congruous fold of Baiyanghe Formation and the angular unconformity with the Shulehe Formation. The block uplift occurred from late Miocene to Pliocene times,which controlled the basin-range tectonic geomorphology pattern,and the study area shows that the thrust belt juxtapose thebasement rocks over the Shulehe Formation,which the formation of fault block uplift and strong tectonic shortening, which laid a foundation for the current basin-mountain tectonic landform pattern.From pliocene to now the study area is characterized by the brittle deformation and tectonic geomorphology uplift.