The Formation Age Of Altyn Tagh Fault Zone And The Contribution Of Its Strike-Slipping To The Uplifting Of North Qinghai-Tibet Plateau

Through the detailed studies of deformed structure, metamorphic petrology, geochemistry, isotopicchronology and mineral inclusion on mylonites outcropped in the middle part of Altyn Tagh fault zone, theauthor gives the formation age of Altyn Tagh fault zone. The author also discusses its maximalaccumulated strike-slip displacement and carries out the quantitative research on the subsequent upliftingin the course of strike-slipping and that on the coupling relationship between the active strike-slipping andthe obduction as well as the coordinate relationship between the active strike-slipping and extending. Thusthe contribution of strike-slipping to the uplifting of North Qinghai-Tibet plateau is preliminarily pointedout in this paper.In the middle part of Altyn Tagh fault zone, there outcrops a set of granitic and hornblende mylonites,and mylonited rocks, which is shown to be products of syntectonic anatexis during the left-handed strike-slip shearing and to be results of ductile transformed compression. The author for the first time discoversthe anateetic orientated long columnar zircon formed during shearing in mylonites, whose crystal long axisis accordant with the direction of stretching lineation, representing the orientation of structural stressduring strike-slip shearing. This discovery not only enables us to determine the structural age by U-Pbisotopic dating on orientated zircon, but also provides clue to determine the formation age of Altyn Taghfault zone. According to dating by single grained zircon ion microprobe (SHRIMP), the age of theorientated zircon is 239 to 244Ma while the age of orientated hornblende by ⁴⁰Ar-³⁹Ar is 233 to 226Ma onthe same sample of mylonite. It indicates that the syntectonic anatex happened in Indo-Chinese epoch andthat a strong strike-slip movement ever took place in Altyn Tagh fault zone in the same epoch.In the area of Beidayao-Hongliuxia, the east part of Altyn Tagh fault zone, there outcrops a set ofdorgalite from upper mantle. In Hongliuxia, the age of volcanic rock by K-Ar is 106 to 112Ma while theage of the volcanic vein cutting the volcanic rock by ⁴⁰Ar-³⁹Ar is 83Ma. In Beidayao, the age of dorgalitegrown in the fault zone by K-Ar is 99 to 105Ma. This set of volcanic rocks in form of lava layer isintercalated in cretaceous strata, so the age is cretaceous. In this set of banded volcanic rocks, there existhomoclinal folds. In the eastern side of Altyn Tagh fault zone and the north of Qilian mountain, a largeamount of rootless syntectonic obducted magma is found within the obducted superimposed microlithonformed during the subsequent Strike-slipping. All indicate the crust increases in thickness and happens tomelt during the strike-slipping of Altyn Tagh fault zone, and then the existence of 83 to 112Ma volcanicrocks is the evidence for re-action of Altyn Tagh fault zone.According to TM & SPOT satellite data and outdoor & indoor studies on geomorphology, structuralgeology, petrology, sedimentology, chronology, it is found that there exists a special extended valley in themiddle part of Altyn Tagh fault zone. Its ratio of length and width is about more than 50 and the two sides'boundaries are controlled by the straightened normal fault with strike-slipping. Within the valley, theCenozoic strata are distributed. And Altyn Tagh fault zone passes through it and forms a series ofstrike-slip geomorphological terrace. Centered on the valley, the two sides are antithetic obducted faults,making the two sides' geological blocks composed of old metamorphic rocks being squeezed vertically, constituting extended hills. We call this special geomorphological terrace strike-slip fault valley (basin).This kind of macroscopic extended strike-slip fault valley began to form in Pliocene epoch and basicallycame into being in Holocene epoch, which is a special typed geomorphological terrace formed during thedeformation of strike-slipping in Altyn Tagh fault zone and joint result of transpression and uplifting. Thisis of great importance to understand the genetic relations of the valley (basin) and mountain range, and theorigin of early old geological blocks' quick uplifting and the formation of the mountain range.Altyn Tagh fault zone ever happened to strongly uplift subsequently during its strike-slip deformation,but its way and scale are different. The uplifting during the fault zone's strike-slipping is taken on as theuplifting of the old geological blocks. The uplifting pattern generally focuses on "obducted fault + strike-slip fault". According to the characteristics of structure pattern, the uplifting pattern can be divided intofour types. Mount Danghenan is formed during the strike-slipping of Altyn Tagh fault zone.In Subei and Akesai in Altyn Tagh fault zone, the rate of the left-handed strike-slipping is 20 to22mm/yr since Holocene epoch and the subsequent uplifting rate is 5 to 11mm/yr. In Altyn Tagh fault zone,the strike-slipping, obduction and stretching can be coordinated. In Mount Danghenan of Subei, the rate ofstrike-slipping changes from 22.5mm/yr to 17mm/yr, part of which is transformed to be uplifting rate5mm/yr forming the range (Mount Danghenan) and to be stretching rate 3mm/yr forming Subei basin.On the basis of predecessors' research, through the comparative studies on deformed structuralgeometry, petrology and regional structure, the author thinks that West Kunlun, South Altyn and NorthQaidam are situated in the same structural background in Early Paleozoic era. Altyn eclogitic zonepossibly extends west to West Kunlun. And North Qaidam ophiolitic zone in the west is connected withSouth Altyn ophiolitic zone and towards west continually being attached to Kudlibei ophiolitic zone ofWest Kunlun which is cut by Altyn Tagh fault afterwards. Therefore, it comes that the maximalaccumulated left-handed strike-slip displacement of Altyn Tagh fault is about 900km.During the strike-slipping of Altyn Tagh fault zone, part of energy is transformed to that of obductedfault which made the pre-Cenozoic geological bodies uplift to form hills. Hence the strike-slipping formsand controls the distribution of Mounts Qimantage, Danghenan, Qilian, and also controls the growth andextension direction of North Qinghai-Tibet plateau, coordinating and constraining the coupling relations ofbasin and hill.Research on the formation age of Altyn Tagh fault zone together with the coupling relations of activestrike-slipping and obduction as well as the contribution of the strike-slipping to the uplifting of NorthQinghai-Tibet plateau is not only of great importance to reconstruct the evolutionary history of Altyn Taghfault zone, but also provides first-hand basic data on establishing the formation pattern of Qinghai-Tibetplateau, especially establishing the uplifting & extension model for North Qinghai-Tibet plateau.