Pre-Miocene Birth of the Yangtze River
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Pre-Miocene birth of the Yangtze River Hongbo Zhenga,1, Peter D. Cliftb, Ping Wanga, Ryuji Tadac, Juntao Jiad, Mengying Hee, and Fred Jourdanf aSchool of Geography Science, Nanjing Normal University, Nanjing 210023, China; bDepartment of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803; cDepartment of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, Japan; dSchool of Geosciences, China Petroleum University, Qingdao 266580, China; eSchool of Earth Science and Engineering, Nanjing University, Nanjing 210093, China; and fWestern Australian Argon Isotope Facility, Department of Applied Geology and John de Laeter Centre, Curtin University, Perth, WA 6845, Australia Edited by Paul Tapponnier, Earth Observatory of Singapore, Singapore, and approved March 22, 2013 (received for review September 19, 2012) The development of fluvial systems in East Asia is closely linked to Geological Setting the evolving topography following India–Eurasia collision. Despite Downstream of the Three Gorges, the river crosses the Jianghan this, the age of the Yangtze River system has been strongly debated, Basin (JHB; Fig. 1 and Fig. S1), entering the East China Sea along with estimates ranging from 40 to 45 Ma, to a more recent initiation the southern margin of the Subei–South Yellow Sea Basin. The 40 39 around 2 Ma. Here, we present Ar/ Ar ages from basalts interbed- Jianghan Basin begun rifting in the Late Cretaceous, as did the ded with fluvial sediments from the lower reaches of the Yangtze Subei–South Yellow Sea Basin, and became a well-developed together with detrital zircon U–Pb ages from sand grains within extensional basin during the Paleogene (10, 11). Sedimentary se- these sediments. We show that a river containing sediments indis- quences, up to 7 km thick and spanning the Late Cretaceous to ∼ tinguishable from the modern river was established before 23 Ma. present, preclude passage of the Yangtze through this region We argue that the connection through the Three Gorges must post- before 36.5 Ma (12). This conclusion is based on the presence of date 36.5 Ma because of evaporite and lacustrine sedimentation in lacustrine and, especially, evaporite sediments (up to 2 km thick) the Jianghan Basin before that time. We propose that the present whose depositional age is controlled by well-dated, intercalated Yangtze River system formed in response to regional extension volcanic rocks (12, 13). The presence of evaporites and organic- – throughout eastern China, synchronous with the start of strike slip rich lacustrine sediments is incompatible with flow of the Yangtze tectonism and surface uplift in eastern Tibet and fed by strengthened fi through that basin at that time because evaporate require a weak rains caused by the newly intensi ed summer monsoon. supply of water. If the middle and upper Yangtze existed before 36.5 Ma, then they must have drained in a different direction, Asian monsoon | drainage capture | provenance | Subei Basin | Yangtze gravel likely toward the southeast and into the Red River (14–16). During Neogene time, eastern China entered a postrifting phase ajor river systems are responsible both sculpting the land- characterized by thermal subsidence, forming regional down- Mscape over large areas of the continental crust, as well as warping depressions with basin fills onlapping over the earlier controlling the development of offshore geology along continental fault-bounded rift sequences (6). The Yangtze gravels, which are margins. Despite this significance it is often unclear why major observed along the lower reaches all of the way from the Three riversystemsareinitiallyformedandwhatprocessesinfluence Gorges to the delta, are of fluvial nature and were deposited their development. The Yangtze River is one of the largest in East during this postrift phase. Asia and has been the subject of debate for more than a century Basin formation has been linked to volcanic activity, which is (1). Despite disagreement on the timing of its establishment most tholeiitic and basaltic during the Paleocene–Eocene in northern geologists agree that the development of this river represents China, but became more intense in eastern China in the middle to a response to the evolving topography and climate of East Asia (2). late Miocene when the volcanism switched to alkaline and per- alkaline compositions (17). The Yangtze gravel sediments are Uplift of the Tibetan Plateau and subsidence in eastern China, fl following the cessation of Cretaceous arc magmatism within the predominantly sands of uvial facies and have widely been inter- preted as Pleistocene deposits (18), although scattered fossil wood Cathaysia block during the Cenozoic (3) have acted to reverse an fragments of Miocene age have also been discovered (19). In this earlier Cretaceous southeast-to-northwest regional topographic fl study, we examine sections close to Nanjing (Fig. 2 and Dataset S1) gradient and allow the Yangtze to ow eastward into the East where the fluvial sediments are overlain by and interbedded with China Sea (Fig. 1). Continued debate about the age of high basaltic lavas that provide the opportunity to accurately date the elevation in Asia means that constraining when the modern sediments. Sedimentary sequences underlying the dated basalts rivers achieved their present geometry is important because of were also selected for provenance studies. their sensitivity to the changing regional topographic gradient. A proposed Pleistocene age for the Yangtze largely hinges on 40Ar/39Ar Dating of Basalts evidence that the modern delta had only been active since that The groundmass approach has been widely used to date the rapid time (4, 5). However, extensive petroleum exploration in the cooling young volcanic rocks immediately after their eruption and Subei–South Yellow Sea Basin (Fig. 1) to the north of the modern has been successfully applied worldwide (20), because of the delta has revealed the stratigraphy of Late Cretaceous–Cenozoic general freshness of young rocks that results in groundmass 40 39 sedimentation, indicating that large fluvial systems must have been Ar/ Ar ages that are internally concordant and fully consistent feeding the basin since a much earlier time (6). In addition, U–Pb with other geological constraints and isotopic methods. Dating of dating of zircons from the modern delta region shows that a river basaltic lavas was performed at the Western Australian Argon indistinguishable from the modern stream was supplying sediments to that area before 3.2 Ma (7). Further constraints on the initiation of flow come from upstream where the modern Yangtze is con- Author contributions: H.Z. designed research; H.Z., P.W., R.T., J.J., M.H., and F.J. per- formed research; H.Z., P.D.C., and R.T. analyzed data; and H.Z., P.D.C., and P.W. wrote trolled by flow through the First Bend (FB) and the Three Gorges the paper. (TG; Fig. 1). The age of incision of the Three Gorges has variously The authors declare no conflict of interest. been argued to postdate 750 ka (8) at one extreme and to be as This article is a PNAS Direct Submission. – early as 40 45 Ma at the other (9). Here we address this debate by 1To whom correspondence should be addressed. E-mail: [email protected]. study of sediments in the lower reaches of the river in an attempt This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. to determine when the river first achieved its present character. 1073/pnas.1216241110/-/DCSupplemental. 7556–7561 | PNAS | May 7, 2013 | vol. 110 | no. 19 www.pnas.org/cgi/doi/10.1073/pnas.1216241110 Downloaded by guest on September 25, 2021 RiverRiver t Tarim aul h F 40° Tag low yn- l BBHBBHBBBHB Altyn-TaghAlt Fault e Bohai YellowY Sea t YR-1 Faul Yellow Sea u Qinling-DabieQin Belt YR-2 l an ling- n XianshuiXianshuihe Fault a sh Da TanluT Fault SPG en bie B SYSYSYSYSSSYYYY QB m el YaloYalongh e ong t FaultLLongmenshanSB Wuhan East n T I B E T g TG Nanjing China RRiver Yichang Sea er B i iv v JHB RiverR SBSB er angtze CSSS 30° YangtzeY ECSECSECS Fig. 1. Topographic map of East Asia, showing ma- FB Pearl jor rivers and the locations mentioned in the text. SBSYB, Subei–South Yellow Sea Basin; BHB, Bohai RReded River Fault RiverRiver Red River Basin; ECSB, East China Sea Basin; PRB, Pearl River RiverR iv Fault Mouth Basin; YB, Yinggehai Basin; SPG, Songpan er PRB Garze; QB, Qiangtang Block. Red circles show loca- YBYBYB South PCCIFICF CFIAPP tions of Yellow River samples (31). Major faults 20° ChiChna marked are taken from Replumaz and Tapponnier Sea (59). The black dashed box indicates the location of 90° 100° 110° 120° 130° Fig. 2. Isotope Facility at Curtin University, operated by a consortium and a younger alteration component, an absolute age cannot be consisting of Curtin University and the University of Western obtained for this sample. Nonetheless, we can say that the crys- Australia (Dataset S2). Each sample was step heated in a double tallization age must be older than the age given by the flat sections vacuum high-frequency Pond Engineering furnace. This approach defined by the oldest steps. This yielded a minimum age of 22.9 ± is designed to look at the gas released from sites of increasing argon 0.3 Ma (mean square weighted deviation = 1.5; P = 0.16) for the retentivity. When a number of consecutive steps, carrying a sub- eruption of this lava flow. Samples from Lingyanshan (Fig. S2B) stantial amount of the total argon released, give the same age, the and Xiaopanshan (Fig. S2C) yielded 100% plateau ages of 10.32 ± resulting average value carries geological significance. Plateau ages 0.13 Ma (P = 0.15) and 21.71 ± 0.17 Ma (P = 0.10), respectively, (Fig.