Early Uplift History of the Tibetan Plateau:Records from Paleocurrents and Paleodrainage in the Hoh Xil Basin

Paleocurrent indicator data collected in field work were used to study the early Cenozoic regional paleodrainage patterns in the Hob Xil basin in northern Tibetan plateau. The paleocurrent directions of the Eocene Fenghuoshan Group obviously show that the flows were northward with a unidirectional dispersal pattern. This probably reflects the uplift of the Qiangtang terrain during the initial basin deposition period and indicates that the Tanggula Moutains occurred as topographic highlands at least in the Eocene. Paleoflows of the Oligocene Yaxicuo Group were dominantly oriented to the north and then flowed eastwards during its late deposition. This regional variability of paleodrainage patterns of the Yaxicuo Group is interpreted to record the dispersal style of sediments from transverse rivers to longitudinal river systems. It is inferred that the Oligocene uplift of the Kunlun Mountains obstructed by northward paleoflows and created longitudinal river systems parallel to the orogenic belts. The temporal and spatial changes of the paleodrainage patterns suggest that the northern boundary of the Tibetan plateau during the early Cenozoic was situated in the Hoh Xil area and its uplift has progressed northwards through time.

THE PALEOCURRENT PATTERN OF EARLY TERTIARY REDBEDS IN THE HOH XIL BASIN,NORTHERN TIBET PLATEAU:IMPLICATIONS FOR AN EARLY UPLIFT AND UNROOFING

We focus on early Cenozoic uplift, erosion and growth of northern Tibet plateau by reconstructing paleo\|drainage pattern in the Hoh Xil basin. Stratigraphical infilling sequences and lithic type orogenic basins are determined largely by contemporaneous tectonic activity. It is therefore reasonable to infer that paleoflow variability record give useful information about regional tectonic events. We now report paleocurrent data from outcrop measurement of 4 expedition profiles across the basin during 1997—1998.The west\|east trending Hoh Xil basin is the largest redbeds basin in northern Tibet plateau. It is 400km in axial length and has maximum width of 200km, which extends along Jin zone, separated the Qiangtang from the Bayankara block. It accommodates more than 5000m of terrestrial sediments during the Eocene to Oligocene dated by preliminary magnetostratigraphy and minor fossils.Detailed sedimentological observation and lithic facies associations analysis of the Eocene Fenghuoshan Group show to have evolved vertically from alluvial fan system in the lowest part, into stacked megasequences of coarsening\|upward fan\|delta system in the middle and upper part. The braided fluvial conglomeratic deposits occur at intervals along the basin margin and grade basin ward interbedded sandstone and mudstone. The Oligocene Yaxicuo Group is locally present and forms as laterally elongate sub\|basins, which is of a maximum thickness of 2000m, as seen in the Wudaoliang and the Hantaishan area. Alluvial fan associations in this sub\|basin in the proximal part adjacent to the southern faulted border and prograded northward to floodplain in distal part.

Paleocurrent Analysis on the Basal Conglomerate of the Model Changzhougou Formation in Qian'an

The paleoflow direction of the basal conglomerate of the Changzhougou formation is obtained by the paleocurrent analysis of certain sedimentary structures,and the ancient sedimentary environment of the Qian’an Region is determined by observing the psephicity and gradation of gravel in conglomerate.The results show that the paleoflow direction is from northwest to southwest,and the paleocurrent data manifest a single-peak style in a rose diagram.The average paleoflow direction in the Qian’an region is 265°.The basal conglomerate is of littoral facies,and the sediments of conglomerates are mainly sourced from Shanhaiguan.

Paleocurrent of the tidal sand body in Subei coastal plain

Based on analysis of magnetic febric and magnetic relict, the paleocurrents recorded in drilling cores in Subei coastal plain are bidirectional. NE_SW in the northern core and NW_SE in the southern core. The paleocurrent pattern in the area is similar to that of the tidal current system in the South Yellow Sea. So it is inferred that the convergent_divergent tidal current system existed during stage of sand body formation, and they should be radial in plane.

Sedimentology and depositional environment of the Kazhdumi Formation Sandstones in the northwestern area of the Persian Gulf

Kazhdumi is one of the Bangestan Group formations aging late early Cretaceous (Albian). Three subsurface sections of the sandstone bearing part of Kazhdumi in the studied area located at northwest of the Persian Gulf are assessed for determining sedimentary facies and depositional environment. Based on the recognized facies, the sandstone bearing zones of Kazhdumi and their relevant paleoenvironments can be addressed as: first one is the B sandstone zone containing thick sandstone layers alternating with shale and siltstone thin layers which are deposited by river influenced conditions because of the characteristics like: cross bedding, oxidized constituents and palesol. The second one, the A sandstone zone which is deposited under a key limestone layer, is composed of sand/sandstone layers with marine features and presents the interaction of marine-river condition in deltaic channels. This zone, is a transition interval showing the changes of river-dominated condition to marine one. The third one is the A sandstone zone deposited above the key limestone layer containing fine grained laminated siliciclastic and chemical sediments. Alternation of distal channel facies and green marine shale of delta containing glauconite, pyrite and organic matter residuals, all confirm the reduction condition in estuarine environment. Applying facies analyses results, isopach maps, porosity distribution maps and sandstone grain size study, the paleocurrent pattern is determined from the southwest to northeast.

Typical basin-fill sequences and basin migration in Yanshan, North China ——Response to Mesozoic tectonic transition

Basin-fill sequences of Mesozoic typical basins in the Yanshan area, North China may be divided into four phases, reflecting lithosphere tectonic evolution from flexure (T3), flexure with weak rifting (J1+2), tectonic transition (J3), and rifting (K). Except the first phase, the other three phases all start with lava and volcaniclastic rocks, and end with thick coarse clastic rocks and/or conglomerates, showing cyclic basin development rather than simple cyclic rift mechanism and disciplinary basin-stress change from extension to compression in each phase. Prototype basin analysis, based on basin-fill sequences, paleocurrent distribution and depositional systems, shows that single basin-strike and structural-line direction controlling basin development had evidently changed from east-west to northeast in Late Jurassic in the Yanshan area, although basin group still occurred in east-west zonal distribution. Till Early Cretaceous, main structural-line strike controlling basins just turned to northeast by north in the studied area.

Paleomagnetic and Fission-Track Dating of a Late Cenozoic Red Earth Section in the Liupan Shan and Associated Tectonic Implications

The north-trending Liupan Shan （六盘山） is an important tectonic boundary between the Tibetan Plateau and the Ordos platform. The Late Cenozoic red earth deposits of the Liupan Shan record its tectonic history and environmental effects. In this article we report a new Late Cenozoic red earth section from an intermontane basin in the southern part of the Liupan Shan. Lithofacies analysis, paleomagnetic and fission-track chronologies, and paleocurrent analysis have been employed to identi- fy the tectonic uplift events of the Liupan Shan. Based on the age constraints of mammal fossils, the pa- leomagnetic polarity zones of the Huating （华亭） Section can be approximately correlated with the standard polarity zones that lie between C3An.2n and C5n.ln of the Geomagnetic Polar- ity Timescale; the bottom age of this section is approximately 10 Ma. Based on this and the previous studies, we infer that a tectonic event commenced in the southern Liupan Shan in this interval between 8.3 and 8.7 Ma, accompanied by a remarkable increase in sediment accumulation rate. Field observations, fission-track dating, determinations of grain-size frequency distribu- tions and the vertebrate fossils found there suggest that the red earth deposits were reworked by water and mainly transported by fluvial-alluvial processes from the adjacent area.

Provenance of Uranium Mineralization of the Yuqia Area,Northwest China:Constraints from Detrital Zircon U-Pb Geochronology and Hf Isotopes

Many sandstone-type uranium mineralization sites have been identified along the northern margin of the Qaidam Basin in North China.Intense tectonism and multistage magmatism that occurred there have restricted the use of conventional techniques(petrogeochemistry)to deduce the sources of detritus and uranium in these sediments,and to further explore the sandstone-type uranium deposits.In this study,U-Pb geochronological and Hf isotopic analyses were conducted on detrital zircon grains collected from Jurassic to Paleogene sandstones exposed in the Yuqia area of the Qaidam Basin.The results indicate that the U-Pb ages of the analyzed zircon grains are clustered into four groups,285–229,498–401,999–806,and 2520–2305 Ma.Theε_(Hf)(t)of the analyzed zircon grains ranges from-21.9 to+10.5,with two-stage Hf model ages(T_(DM2))concentrated between 2.1 and 1.3 Ga.Based on paleocurrent studies and results of petrological,detrital zircon geochronology,and Hf isotopic analyses,the Jurassic sediments in the Yuqia area were likely derived from the tectonic belt along the northern margin of the Qaidam Basin,such as the Qilian Mountains and the Quanji Block.In contrast,the Paleogene sediments were probably sourced from the northern margin of the Qaidam Basin and the Qilian Mountains.The uranium and thorium content and Th/U value of the rock mass and sedimentary strata in the source area show that the Indosinian Early Paleozoic uranium-rich granites and the Middle Jurassic uranium-rich strata of the northern margin of the Qaidam tectonic belt in the source area provide dual uranium sources for the Yuqia sandstone-type uranium mineralization.Therefore,the study area appears to be favorable for sandstone-type uranium mineralization and may have potential for uranium prospecting.