Control Effect Of Tectonic Framework On Coal Measures In Northern Qinghai-tibet Plateau

Based on multidisciplinary theoretical knowledge of structural geology, sedimentology, coal geology, geochemistry etc., use the methods of data analysis, field geological investigation and sample testing, this paper in detail studied northern Qinghai-Tibet Plateau(Southern Qinghai) 's regional tectonic setting and characteristics of coal bearing formation. It proposed tectono-sedimentary patterns of coal measures forming in Qinghai-Tibet Plateau, established tectonic evolution patterns of coal basins with different coal-forming periods in northern Qinghai-Tibet Plateau(Southern Qinghai). After that, this paper divided coal occurrence tectonic units of research area, summarized the characteristics of coal occurrence tectonic units and the occurrence regularities of coal resources.Qinghai-Tibet Plateau is mainly composed of five terranes, and five suture zones separated these terranes. These suture zones from north to south are West Kunlun-Altyn Tagh-North Qilian suture zone, Muztag-South Kunlun-A'nyemaqen suture zone between East Kunlun-Qaidam terrane and Songpan-Ganzi-Hoh Xil terrane, Jinshajiang suture zone between Songpan-Ganzi-Hoh Xil terrane and Qiangtang terrane, Bangonghu-Nujiang suture zone between Qiangtang terrane and Lhasa terrane, Yarlung Zangpo suture zone in the southern margin of neo-Tethyan tectonic domain. The main structures of northern Qinghai-Tibet Plateau(Southern Qinghai) are the southern of East Kunlun-Qaidam terrane, Songpan-Ganzi Hoh Xil terrane and the northern of Qiangtang terrane.The main coal forming periods in northern Qinghai-Tibet Plateau(Southern Qinghai) are Carboniferous, Permian, Triassic and Jurassic. The under coaly clastic formation of early Carboniferous zaduo group(C1zd) coal-bearing strata mainly distributed in Jinai-Qiyong of Zaduo area in eastern Tanggula mountain coal occurrence belt of southern Qinghai. The under sandy slate formation of late Carboniferous Jiamainong group(C2jm) coal-bearing strata distributed in Jiezha and Nangqian area. The late Permian Nayixiong group(P3n) coal-bearing strata mainly distributed in Wuli-Kaixinling area of the northwest Tanggula mountain area in northern Qiangtang terrane, Zaduo-Nangqian area in southeast, also has sporadically outcrop.The late Triassic Bagong group(T3bg) coal-bearing strata mainly distributed in the core of the synclinorium in western Bashiwudanban, Bangkeqin, Jiushidaoban, and Zhasu etc. The early Jurassic Nianbao group(J1n) coal-bearing strata distributed in dot around the Suohurima, Sangrima and Nianbao area. The middle Jurassic Yangqu formation(J2y) coal-bearing strata mainly distributed in intermountain fault basin of Xueshanfeng, Kuhai, Hongtupo and Jishishan area.The study on distribution characteristics, coal-bearing properties and lithology changes of every era coal measures showed that, the best coal-bearing property of coal-bearing strata in research area are Jurassic Yangqu group and Nianbao group. Then followed by late Permian Nayixiong group and early Carboniferous Zaduo group. The worst is late Triassic Bagong group coal-bearing strata.The two coal forming periods in Carboniferous are similar and both in good and stable coal-forming environment. The late Carboniferous coal-forming environment and coal-bearing property in general worse than early Carboniferous, Its coal seams varies greatly.The thickness of late Permian coal seam is large. The stability of late Permian coal-forming environment and coal-bearing property are well.The thickness of Triassic strata is the biggest, but its gathered numbers of coal seams and average coal seam thickness are the smallest, which indicate the Triassic coal-forming environment and coal-bearing property are not good.The average thickness of Jurassic strata is not very, but its gathered numbers of coal seams is the most, average numbers of coal seams is less, and the variation range of coal seam thickness is the biggest. All of this indicate that the coal-bearing property of Jurassic is good, the numbers of coal seam are large and the thickness of coal seams changes wide.The coal samples testing results of every coal-forming era show that coal measures in northern Qinghai-Tibet Plateau(Southern Qinghai) are generally experienced intense metamorphism. The values of RO and the ash are generally high. The ashes in coal are mainly middle or middle-high due to the strongly weathering denudation of high Plateau climate caused shallow coal weathering. The sulfur in coal is generally low, and the calorific value is middle-high or high. The coal of early Jurassic Niaobao group(J1n) has the largest ash value and the lowest calorific value; also, its values of silicon and aluminum are generally higher than other periods of coal measures.Tethys is the predecessor of Qinghai-Tibet Plateau. The study of the relations in Qinghai-Tibet Plateau tectonic framework and coal measures, are inseparable from the discussion of the Tethys tectonic evolution. The Qinghai-Tibet Plateau belongs to the eastern Tethyan tectonic domain and in the temporal and spatial evolution; it can roughly be divided into four stages of proto-Tethys(Z2-S), Paleo-Tethys(D-T2), meso-Tethys(T3-K1) and neo-Tethys(K2-E).The Paleo-Tethys tectonic evolution stage is mainly from Devonian to Middle Triassic, including the expansion and subtractive processes of the Paleo-Tethys Ocean. The Meso-Tethys tectonic evolution stage is mainly from late Triassic to early Cretaceous, including the closure of the Paleo-Tethys Ocean and the expansion and subtractive processes of the Neo-Tethys Ocean. This stage is a transition from Paleo-Tethys Ocean to Neo-Tethys Ocean, and the mainly tectonic evolution stage of Bangonghu-Nujiang Neo-Tethys oceanic basin. Neo-Tethys tectonic evolution stage is mainly from late Cretaceous to Paleogene, including the subduction and closure processes of the Neo-Tethys oceanic basin.The deposition of coal measures in Qinghai-Tibet Plateau controlled by the tectonic evolution of the Tethys Ocean. In geological period of coal forming, coal measures formed in some of the stability regions within terranes and volcanic arcs and basins that formed in the process of Tethys ocean evolution with suitable paleoclimate, paleogeography and settling velocity conditions. According to the characteristics of geotectonic position of the coal measures, this paper divided the formation of the coal measures in Qinghai-Tibet Plateau into two primary, five secondary, and four tertiary tectono-sedimentary patterns. The two primary tectono-sedimentary patterns are Intra-terrane deposition and Marginal-terrane deposition. The five secondary tectono-sedimentary patterns are Piedmont depression deposition, intermontane basin deposition, Stable marginal-terrane deposition, Subduction marginal-terrane deposition and Conversion marginal-terrane deposition. The four tertiary tectono-sedimentary patterns are Suture zone intermontane basin deposition, intra-terrane intermontane basin deposition, fore-arc deposition and back-arc deposition.The Carboniferous is the first coal-forming era in northern Qinghai-Tibet Plateau. The main coal forming period are the late of early Carboniferous and early of late carboniferous. The Carboniferous coal measures deposition occurred in the north margin of the northern Qiangtang terrane with the absence of subduction. Its tectono-sedimentary pattern is Stable marginal-terrane deposition. The late Permian coal measures deposition occurred in the tectonic stable Wuli-Kaixinling area between the internal terrane and the volcanic arcs which formed by the oceanic crust subduction. It controlled by the regional faults in the northern slope area of the Taniantaweng paleo uplift. Its tectono-sedimentary pattern is Subduction marginal-terrane back-arc deposition. The late Triassic coal measures deposition mainly occurred in northeastern of Wuli-Kaixingling area, and distributed banded in the north margin of the late Permian coal basin. The deposition of the late Triassic coal measures mainly occurred in fore-arc basins within the terrane margins where subduction occurred. Its tectono-sedimentary pattern is Subduction marginal-terrane fore-arc deposition.The Jurassic coal measures deposition mainly occurred in the southern of Kunlun arc, southeastern of east Kunlun and Jishishan areas. Unlike other period's coal measures, the Jurassic coal measures formed in the interior of the terrane. Its tectono-sedimentary pattern is Intra-terrane deposition. The coal measures in northern Dawu basin and southeastern Buerhanbuda Mountain is Suture zone intermontane basin deposition, the coal measures in southern Dawu basin northern Buerhanbuda Mountain is Piedmont depression deposition.The coal measures in Northern Qinghai-Tibet Plateau(southern area) strongly effected by later tectonic reconstruction. After the formation of the Carboniferous coal measures, it experience multiperiodic, complex and intense deformational later tectonic reconstructions. The main coal transformation is lateral extrusion from north and south. Because of this, on the one hand, the coal were folded and faulted and become discontinuous; On the other hand, due to the crust differential settlement, the fault uplift area strongly rise cause coal measures serious erosion, the downfaulted area deposits thick Cenozoic strata made the coal measures buried deep.The obvious later reformation movement to late Permian and late Triassic coal measures is Indosinian. The north-south compressional stress made the coal measures folded, faulted, and discontinuous. Coal measures controlled by boundary faults and exposed with fault blocks. The tectonic deformation in south and east are stronger than in north and west.The Jurassic coal measures strongly pressed by SSW-NNE direction stress. A series of thrust faults and thrust sheets formed within the basin. Coal measures in the top of the thrust sheets eroded due to the uplift, and coal measures in the bottom better saved. In Cretaceous, squeezing actions in this area made the basin closed, and coal-bearing areas uplifted again and part of it eroded. From Tertiary to Quaternary, the depth of coal seams in northern coal-bearing area increases relatively, but the distribution and morphology of coal measures changes a little.Northern Qinghai-Tibet Plateau could divided into three secondary and six tertiary coal occurrence tectonic units. Three secondary coal occurrence tectonic units are East Kunlun, Jishishan, and Tanggulashan coal occurrence tectonic unit. Six third coal occurrence tectonic units are Eastern Kunlunshan depression, Southern Buerhanbuda depression, Dawu basin, Bayankela basin, Northern Tanggulashan depression, and Southern Tanggulashan depression.The coal-bearing basins in East Kunlun fault depression coal occurrence tectonic belt are isolated, which mainly are intermontane fault basin, are greatly influenced by middle KunLun fault. With the boundary of Buqingshan Mountain, this area divided into two tertiary coal occurrence tectonic units, which are Eastern Kunlunshan depression and Southern Buerhanbuda depression.The coal-bearing basins in Jishishan fault depression coal occurrence tectonic belt are a series of immature strike slip pull apart basin controlled by southern KunlunA'nyemaqen fracture system. With the boundary of southern Buqingshan fault, this area divided into two tertiary coal tectonic units, which are Dawu basin in north and Bayankala basin in south.Coal bearing strata in Tanggula Mountain fold coal occurrence tectonic belt scattered in intermontane basins. With the boundary of southeastern axis of Wuli synclinorium, this area divided into two tertiary coal occurrence tectonic units, which are northern Tanggulashan depression, and southern Tanggulashan depression.In East Kunlun fault depression coal occurrence tectonic belt, the elements of gallium and germanium in coal of Gamayangqu area have good prospects. The most potential exploration prospect area of coal resources in Jishishan fault depression coal occurrence tectonic belt is Dawu coalfield and Nianbao area. In Tanggula Mountain fold coal occurrence tectonic belt, coal resource potential area are eastern Wuli, Bashiwudaoban, Zhasu, East Kaixinling, Dongba, Baocaogou etc., The shale gas in coal measures also has the potential in this area.