Pre-Miocene Birth of the Yangtze River

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. performed 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. S2) are given at the 2-σ level and are calculated using the unambiguously indicating the eruption ages of these lava flows, mean of all of the plateau steps, each weighted by the inverse with the 21.7 Ma age being consistent with the 22.9 Ma age from variance of their individual analytical error and must define Guizishan. The trapped 40Ar/36Ar compositions of the Guizishan a probability of fit(P) > 0.05. Ages were calculated using the decay and Xiaopanshan samples (Fig. S2 D and F) are indistinguishable

constant of Renne et al. (21) and the Fish Canyon sanidine stan- from an atmospheric ratio of ∼299 (22) suggesting that no excess EARTH, ATMOSPHERIC, dard for which an age of 28.305 Ma (±0.13%) was adopted. 40Ar is present in these samples. No isochron could be obtained for AND PLANETARY SCIENCES Uncertainties on the decay constants and the age of the standard sample Lingyanshan (Fig. S2E) because of a clustering of the data are not included in the age calculation. near the radiogenic axis (and thus the absence of inﬂuence of any The sample from Guizishan (Fig. S2A) yielded a complex age minute trapped Ar on the age calculation). spectrum where no straightforward eruption age could be calculated because of the sudden drop in apparent age of three steps in U–Pb Zircon Dating SCIENCES the middle of the spectrum, which was likely caused by late-stage Zircon grains from sands in the Yangtze gravels were separated ENVIRONMENTAL alteration phases. Nevertheless, assuming alteration is the cause of and dated by the U–Pb method to constrain their provenance. the disturbance, a minimum age can be derived by using all but the This method dates the age at which each zircon crystal cooled three discordant steps in an age spectrum. Because the individual below ∼750 °C (20) and is a widely used and accepted sediment step ages are a mixture between an older crystallization component proxy method based on the concept that different crustal blocks

A B (a) (b) (c) (d) LYS XPS GZS JHB 0 0 0 0 21.7 22.9 Guizishan 32°30' N+Q I 5 10.3 5 5 1 E3 36.5 Xiaopanshan 10 10 10 2 37.7 ﬁ Depth (m) 15 15 15 3 39.2 Fig. 2. (A) Simpli ed geological map of Nanjing Luhe

32°20' area showing the locations of studied sections. (B) Depth (km) 20 20 E2 4 Sedimentary logs from the studied Yangtze gravel Lingyanshan 43.0 sections in the lower reaches of the Yangtze basin in 36.5 47.0 25 Basalt Age (Myr) 5 comparison with the lithostratigraphy of Jianghan 56.0 Basin. (a) Lingyanshan; (b) Xiaopanshan; (c) Guizishan; Halite Gypsum Basalt Petrified wood E1 6 32°10' 65.0 Gravel Sandstone Mudstone Sample K2 and (d) Jianghan Basin. Locations of sediments from which zircons were dated are shown together with I Yangtze I’ C Xiaopanshan Lingyanshan 160 River the age controls. The y axis in the sedimentary logs is I’ 80 in depth; N+Q, Neogene and Pleistocene; E3, Oli- Nanjing 0 masl gocene; E2, Eocene; E1, Paleocene; K2, Late Creta- Yangtze River 32°00' 0 10 20 30 40 50 60 km ceous. (C) Sketch showing a cross section (black line For A and C: Paleozoic Mesozoic Neogene Quaternary Basalt 108°30' 108°40' 108°50' 109°00' in A) of the geology of Nanjing area.

Zheng et al. PNAS | May 7, 2013 | vol. 110 | no. 19 | 7557 Downloaded by guest on September 25, 2021 have different ages of formation and that erosion of the bedrock and North China Craton) (28). We assess the potentially different in a large and diverse drainage basin will result in a unique fin- provenance of each of the sands considered in this study by con- gerprint for the zircon age population in any given river. Because sidering the relative abundance of each of these age groups be- of the complexity of the zircon age population in sands from tween different samples. Because individual age ranges are not large river basins, samples with identical age populations are unique to a single source terrain, and indeed because sources likely derived from the same river system eroding the same contain grains of different ages, it is not possible to assign in- bedrocks. Because zircon is relatively resistant to abrasion during dividual grain to a single source based only on the U–Pb age, al- transport, grains eroded from sources in the headwaters are though it is possible to eliminate possible sources. Furthermore, communicated to the lower reaches. As a result all sources in the a relatively small number of grains do not fall within these groups, catchment, not just those located close to the delta, contribute to but because little data are lost by removing these and, because the bedload found in the lower reaches. The zircon age spectrum these grains are not source diagnostic, they are not helpful in is controlled by the source exposure area and the concentration demonstrating how the sediment load of the river has changed of zircon in the bedrock (23), although focused erosion may through time. We do not try to separate grains associated with the result in certain parts of the basin yielding more sediment than Emeishan flood basalt province because the age range of that others. The result is that the unique combination of bedrock age, province is narrow (29) and cannot be unequivocally resolved from relative abundance, and erosion intensity patterns makes it very the 200–250 Ma range of the Indosinian Orogeny using the in- unlikely that two large rivers would have the same spectra of ductively coupled plasma mass spectrometry (ICP-MS) method zircon U–Pb ages. used here. After excluding grains that do not fall within the di- The location of the zircon samples within each stratigraphic agnostic groups the budgets were recalculated to 100% and section are shown in Fig. 2. The sediments are of clear fluvial plotted as pie charts to show the overall character and source of facies and form cross-bedded sandstone complexes 5 to 10 m the detrital zircons (Fig. S3). These figures also allow the three thick, which suggests deposition in a significant channel complex. age populations younger than 500 Ma to be better resolved into The spectrum of the detrital zircon grain ages (Dataset S3) from 100–200 Ma (Mesozoic arc magmatism/Yanshanian Orogeny), each of the considered samples is shown in Fig. 3 using the 200–250 Ma (Indosinian Orogeny), and 400–500 Ma (“Cale- kernel density estimation (KDE) of Vermeesch (24), which plots donian” Orogeny) (30). As we note above, these age ranges are the detrital ages as a set of Gaussian distributions, but does not not unique to single sources, but rather to tectono-thermal explicitly take into account the analytical uncertainties. Vermeesch events that affected eastern China but which are heterogeneous argues that this is a more statistically robust method for looking at across the Yangtze drainage basin, so that changes in erosion detrital ages when the number of grains and analytical precision are patterns might be expected to change the age spectrum of the both high. This method allows the age ranges and abundances of zircon sand grains in the river sediment. the different age populations to be graphically assessed. In addition to the modern river and Yangtze gravel samples – Detailed comparison is possible by dividing the zircons up into we plot all of the post 3.2-Ma detrital zircons from the Yangtze population groups associated with the major tectono-thermal Delta (7) in an attempt to provide a general image of the sedi- events that have affected the crust of eastern Asia. Specifically ment reaching the delta since the Late Pliocene (Fig. 3). Jia et al. we assign grains to groups with the following age ranges: 100– (7) have shown that the provenance of the sediment in the area 200 Ma (Dabieshan and lower reaches); 200–250 Ma (Indo- of the modern delta has been approximately constant and similar – to the modern river for 3.2 Ma. Our intention is to provide sinian/Qiangtang Block) (25); 400 500 Ma (Songpan Garze) (26); fl 700–1,000 Ma (Yangtze Craton and Songpan Garze) (27); 1,700– a more time-integrated image of recent ux to the delta because 2,100 Ma (Songpan Garze); and 2,400–2,600 Ma (Songpan Garze there are moderate variations over short time periods, as dem- onstrated by the differences between the modern samples from Nanjing and Wuhan that show the river is not of a completely Modern Yellow River -1, N = 84 Modern Yellow River -2, N = 99 constant and homogeneous character but that the degree of variation is limited within modest bounds (Fig. 3). Although we accept that provenance can change over different timescales because of changing climatic patterns and evolving rock uplift, it Modern Yangtze, Nanjing is clear from the work of Jia et al. (7) that the Yangtze has been N = 95 relatively stable in its provenance, and thus catchment size, for at Modern Yangtze, Wuhan least 3.2 Ma. This alone is somewhat surprising given the po- N = 97 tential influence of the Yellow River in the delta region (Fig. 1). To test for the influence of the Yellow River we plot zircon Yangtze delta average, 3.2 Ma N = 526 U–Pb ages from this system using two samples taken from north of the Qinling-Dabie belt (31) (Fig. 1) and which might reflect the >10.3 Ma, Lingyanshan expected composition of the Yangtze gravels if they had actually N = 97 been deposited from that stream and not the Yangtze River. Two Relative probability Relative >21.7 Ma, Xiaopanshan - 2 samples are plotted because each is taken at a potential capture N = 93 point for flow into the Yangtze, and this also allows us to see some of the natural variation within the modern Yellow River. In the >21.7 Ma, Xiaopanshan - 1 N = 91 recent historical past the lower reaches of the Yellow River have switched between two routes, one in the north to the Bohai Sea (as >22.9 Ma, Guizishanshan is the case today) and the other to the south to the Yellow Sea (32). N = 99 This has led to speculation that a switch of the routes has occurred 0 500 1000 1500 2000 2500 3000 in the geological history, which would have influenced the sedi- Crystallization age (Ma) ments of the lower Yangtze system. Fig. 3 shows how the Yellow Fig. 3. KDE diagrams of zircon populations from Yangtze River gravels River age spectra are markedly different from any of the samples showing the similarity of the provenance to the modern river at Nanjing, at taken from the modern and Pleistocene Yangtze River (although Wuhan, and the average composition of the delta 3.2 Ma. Note the different they are quite similar to one another), most notably in having spectrum provided by the lower reaches of the Yellow River. Locations of the a small number of grains dating to 700–1,000 Ma and a high sampled outcrops are shown on Figs. 1 and 2. abundance of grains dating to 1,700–2,100 Ma. The complexity of

7558 | www.pnas.org/cgi/doi/10.1073/pnas.1216241110 Zheng et al. Downloaded by guest on September 25, 2021 the zircon age population demonstrates that the Yangtze gravels could not represent the deposits of a locally sourced tributary be- Qaidam North China cause this would not have sufﬁcient diversity in its bedrock to Qinling-DabieQinling-Da SBSYB Songpan bie generate a complicated age spectrum of this variety. Sichuan ﬁ Qiangtang The pie charts (Fig. S3) represent a simpli ed version of the 30˚ JHB KDE diagrams shown in Fig. 3 and are consistent with the visual Ba ngong-Nuji SB ang s impression that all of the Yangtze gravel and river samples have uture ECECSB similar zircon populations. Not surprisingly the 700–1,000 Ma ? South China Lhasa grains associated with the Yangtze Craton dominate in all these Paleo-Red River

samples. Some variations with time since 23 Ma are to be ASRR Ya expected because of active tectonics in the headwaters (32, 33) rlung-Tsa PPRBRB 20˚ ngpo sut ? YYBB and changing monsoon intensities since that time, which changes ure the patterns of erosion within the basin. Nonetheless, it is ap- ? parent that all of the Yangtze gravel samples have received zir- Indochina cons from the same array of sources as the modern river and that although the proportion of any particular age group varies, the India extent of that variation is limited. We conclude that at least since ? A ∼ fl 10˚ 23 Ma, a river similar to the modern Yangtze owed in the ~32 Ma vicinity of Nanjing and that this must have connected to the middle and upper reaches of the present drainage, as the river Qaidam North China does today Fig. 4). The Yangtze gravels are not the product of Qinling-DabieQinling-Dab SBSYB sedimentation from the Yellow River or a single, local Yangtze Songpan ie Qiangtang Sichuan tributary, which would be dominated by erosion from the Dabie LMS 30˚ B Shan where rocks dating at 100–200 Ma old dominate (34). angong-Nuji ang s JHB To quantify the degree of similarity we here perform additional uture LLhasahasa Yangtze ECSB statistical tests to assess the degree to which the Miocene sedi- r Ya lung-Tsang South China po su HimalayaHi ture ments differ from the modern river at Nanjing, at Wuhan, and from malaya the 3.2-Ma-to-present record at the delta. Statistics suggest that Red River ? ∼100 grains are required to generate a reliable provenance record ASRRASR R PRB in a large, complicated drainage like the Yangtze (24). Modern 20˚ sediments from the Yangtze Delta, not surprisingly, contain zir- IIndiandia cons from all major source terrains, including some reworked from YB older sedimentary rocks. We compare that population with the SSCSCS zircon U–Pb ages from the Yangtze gravels and find that there is Indochina striking degree of similarity. Kolmogorov–Smirnov (K–S) statistical testing cannot prove that two sands are identical, but it does 10˚ B EARTH, ATMOSPHERIC, indicate when there are significant differences between samples. ~16 Ma AND PLANETARY SCIENCES Our analysis (SI Text and Dataset S4) indicates that the Miocene 90˚ 100˚ 110˚ 120˚ sands are indistinguishable from the modern sediments or from Fig. 4. Simplified maps showing the development of the Yangtze River in those in the post–3.2-Ma delta, but are different from the Yellow response to tectonic evolution in East Asia, based on the data presented in River. We argue that this provides proof of a river flowing through this paper as well as associated studies discussed in the text. (A)32and(B) the lower reaches since 23 Ma, and that this stream was collecting 16 Ma. Names of basins as in Fig. 1. Maps reflect the progressive extrusion of sediments from all of the terrains that now feed the Yangtze. The Indochina and the opening of the South China Sea, but recognize that the SCIENCES

fact that the same age populations can also be seen before 22.9 Ma continental blocks east of Longmenshan have remained relatively rigid since ENVIRONMENTAL – (Fig. 3) suggests that this river has been in a state of continuous the Eocene (59). ASRR, Ailao Shan Red River Fault; LMS, Longmenshan. flow from that time until the present day. It appears that the Shaded river segment indicates the reversed section of the Yangtze River. river evolved close to its modern state at least by the Miocene– ∼ Oligocene boundary ( 24 Ma). It could be argued that the composition of the Yangtze was able Drainage Evolution in East Asia to achieve a modern character by 23 Ma as a result of sediment delivery from the Yalongjiang (Fig. 1) rather than from the We argue that although sedimentation in the region of the upper Yangtze, which could still have been connected to the Red modern delta only dates from the Pliocene (4, 5) this does not River. However, the zircon age spectrum in the sediment in the preclude an earlier delta located farther north in the Subei–South Yalongjiang is quite different from that at the First Bend on the Yellow Sea Basin. Moreover, an Early Miocene age for the onset Yangtze, despite the fact that both rivers have major sources in of Yangtze River flow is consistent with Nd isotopic data from the the Songpan Garze terrane. The Yalongjiang has a very high Red River system that indicate loss of drainage from the middle – Yangtze into that river close to the Oligocene–Miocene bound- abundance of 600 1,000 Ma grains compared with the First Bend ary (15) (ca. 24 Ma; Fig. 4). Pb isotope characteristics of (76% versus 19%) and is relatively depleted in grains dated at – – K-feldspar grains in the Red River delta show a connection with 300 600 Ma and 1,700 2,000 Ma (5% and 3% compared with the middle Yangtze in the Eocene, a link that had been lost by the 23% and 25%; Fig. S3). As a result it is doubtful whether the middle Miocene (35), but little firm evidence for a link with the Yangtze 23 Ma would show the same zircon age spectrum as it upper reaches of the Yangtze to the Red River. Similarly, U–Pb does now without a connection to the Yangtze upstream of the zircon provenance data from the lower Red River indicates that First Bend. In any case, data from the Red River itself indicate this river had achieved its present geometry before 12 Ma (36). We little definitive evidence for a connection between the Red River follow studies such as Clark et al. (16) in advocating capture and and the upper Yangtze at any time (36). In theory there could have flow reversal of the middle Yangtze (2,140 km between the First been an early Miocene phase with the upper Yangtze connected Bend and the Three Gorges; ∼33% of the total trunk stream to the Red River and the Yalongjiang connected to the rest of the length) by connection to the lower reaches before 23 Ma. Yangtze, although it would be unlikely that the zircon population

Zheng et al. PNAS | May 7, 2013 | vol. 110 | no. 19 | 7559 Downloaded by guest on September 25, 2021 of the Yangtze load would not have been disturbed by the capture alternative view is that the Yangtze and other Southeast Asian of the upper reaches. rivers originated outside the Tibetan Plateau and then cut into this Although Richardson et al. (9) favored a connection of the flat, high-standing massif as a result of headward retreat, rather Yangtze through the Three Gorges before 40 Ma the presence of than incising into a low-altitude peneplain that was then uplifted evaporites in the Jianghan Basin indicates that the exhumation (52). Our data would also be consistent with this model but would of the Three Gorges at 40–45 Ma could not have been caused by require most of the head retreat to be complete by 23 Ma. initiation of a throughgoing river flowing in that direction. Al- If the current geometry of the Yangtze reflects the evolving though a major river could (and indeed does) flow through a lake topography, it is hard to understand how this river could post- system, it is impossible for a lake in the Jianghan Basin to ever date the large-scale late Miocene uplift of eastern Tibet. By become so desiccated that evaporites, with a thickness up to dating the Yangtze as being older than 23 Ma we now synthesize 2 km, would form because this would be inconsistent with sig- our understanding of the river with what is known of topographic fi ni cant discharge from the river. Although some rivers may form growth in the headwaters (Fig. 4). Furthermore, as Tibet con- fl following a process of alternating uvial and lacustrine phases we tinued to grow, sedimentary basins across eastern China expe- know that if this process had been operating in the Yangtze it rienced the initiation of enhanced early-to-middle Miocene must have ceased before 23 Ma because the provenance ob- subsidence (53), which are connected by throughgoing fluvial served in the lower reaches requires a fully connected river of the systems, as the case of Jianghan and Subei–South Yellow Sea type we see today. A river of this scale is unlikely to be seasonal basins today. Within-plate magmatism testifies to a sharp change even during arid climatic conditions, which in any case are not in the geodynamic regime at that time, likely linked to rollback of characteristic of the early and middle Miocene in East Asia (37). the subducting Pacific plate (54). Initial uplift of Tibet and re- Sedimentation rates in the East China Sea show a steady in- – crease during the Oligocene (38), consistent with our model, gional strike slip faulting driving uplift coupled with subsidence however there is no sharp jump in accumulation rate, probably of basins across eastern China opened a path for the Yangtze to because of sediment buffering in depocenters onshore, mostly the east, diverting the upper and middle reaches away from the notably the Jianghan and Subei basins (39). Red River and South China Sea. We further note that the pre-Miocene uplift of the Tibetan Tibetan Tectonics and the Yangtze River Plateau implied by our reconstruction and by recent thermo- Birth of the Yangtze River around the start of the Miocene can also chronology (43) is also consistent with recent revisions concerning be understood in terms of a progressive or stepwise Paleogene uplift the time of intensification of the East Asian summer monsoon of the Tibetan Plateau, because recent paleoaltitude studies sup- close to the start of the Miocene (37, 55) rather than during the port the idea of central and southern Tibet being close to modern late Miocene ∼8 Ma as had previously been believed (56). These elevations no later than the middle Miocene (∼16 Ma), rather than new dates for monsoon strengthening are based on environ- indicating a late-stage rapid uplift in the Pleistocene (40). mental and floral proxies and are not model dependent and more Although (U–Th)/He thermochronometry from the Long- closely linked to precipitation than had been the case for earlier menshan of western Sichuan indicates that major topographic uplift monsoon indicators. Prell and Kutzbach (57) are among several predated 8–11 Ma (41, 42), a recent study using a variety of ther- studies that predict a correlation between increasing plateau al- mochronology proxies from this same area indicates significant ex- titude and monsoon intensity. Although it is clear that monsoon humation starting during the Oligocene–early Miocene and the intensity is controlled by more than simply Tibetan elevation, presence of some topography in eastern Tibet even predating India– recent climate models still show that the presence of a plateau Eurasia collision (43). These data are broadly consistent with tectonic intensifies summer rainfall in the Asian region (58). Thus, the models indicating a stepwise growth of the plateau with southeastern timescale of uplift implied by the start of Yangtze throughflow Tibet/southwest China uplifting in Oligocene times (44). before 23 Ma is consistent with the more recent climate recon- – Strike slip tectonism strongly affected the geology and topog- structions that indicate strengthening around the end of the raphy of the eastern plateau margin and must have been in- Oligocene (37, 55). strumental in helping to guide the reorganization of rivers in the region. The southeastern flank of the Tibetan Plateau is deformed Conclusions – by large strike slip faults the activity of which is likely linked to the In this study we constrain the age of initiation of the Yangtze River surface uplift. Dating of the Red River fault zone shows that mo- through study of the Yangtze gravels in the region of Nanjing. The ∼ tion started 34 Ma and was most rapid after 27 and before 17 Ma 40Ar/39Ar dating of basalts interbedded within these sediments (45, 46) and other faults in this region (e.g., Xianshui He Fault) shows that some of these were deposited before 23 Ma, and are not share similar ages of motion and exhumation (47, 48). Lacassin Pleistocene as previously assumed. We further analyzed the et al. (49) note strong shearing and metamorphism of the moun- provenance of the gravels through application of U–Pb dating to tains immediately north of the Yangtze First Bend after 36 Ma, zircon sand grains extracted from these deposits. Statistical anal- followed by uplift driven by folding around 17 Ma. This date ysis shows that lower Miocene sediments are indistinguishable requires that the river was flowing through that region before 17 Ma, although it need not necessarily have been connected to from sediments collected from the modern river or from the delta the present day Yangtze. It can be imagined that surface uplift during the last 3.2 Ma. This requires that a river, much like the driven by this deformation and related to the progressive growth modern one, was in existence before 23 Ma. Our data do not allow of Tibetan topography toward the southeast might trigger major us to date the oldest possible age of the Yangtze, but we infer that reorganization of the rivers flowing through this area and that the the river did not start before the late Eocene because of the initial reorganization of the rivers would occur before major sur- presence of evaporites in the Jianghan Basin before that time. face uplift (16, 50), i.e., predating 17 Ma. Those sediments lie just downstream of the Three Gorges and We emphasize that headwater capture does not require major preclude major river flow until after 36.5 Ma. A pre–23-Ma surface uplift in the First Bend area, but only a gentle retilting of Yangtze River is consistent with both recent estimates for the the topography toward the east, because water will flow downhill timing of eastern Tibetan uplift, the onset of major strike–slip even if the slope is not very steep. Indeed, the entrenchment of the deformation, and with the age of headwater capture of the middle Yangtze and its tributaries into deep canyons during the Late Yangtze reaches away from the Red River. Our study confirms Miocene (51) makes further capture after that time very difficult a close relationship between the evolving tectonically driven to- because the topography prevents lateral channel migration. An pography and drainage systems in East Asia during the Cenozoic.

7560 | www.pnas.org/cgi/doi/10.1073/pnas.1216241110 Zheng et al. Downloaded by guest on September 25, 2021 ACKNOWLEDGMENTS. The authors thank Philippe Leloup, Zhenyu Yang, Program” of the Chinese Academy of Sciences (XDB03020301), the Na- Lin Ding, and Xiumian Hu for fruitful discussions. P.C. thanks the Hanse tional Science Foundation of China (NSFC 40830107, 41111140016, and Wissenschaftskolleg for providing financial support to work in Asia. This 41102104) and United Nations Educational, Scientific, and Cultural work was financially supported by the “Strategic Priority Research Organization IGCP581.

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