Late Cretaceous drainage reorganization of the Middle Yangtze River

The middle reach of the Yangtze River experienced significant drainage reorganization during the Mesozoic to Cenozoic tectonic evolution of South China. A continental-scale, westward-flowing axial river, or paleo–Middle Yangtze, formed following the Mesozoic collision between the North China and South China blocks. The river later flowed to the east as a result of the Cenozoic uplift of the Tibetan Plateau, but its reversal history remains largely unknown. Changes in detrital zircon U-Pb ages of Mesozoic to Cenozoic strata in the Three Gorges region identify a vital provenance shift in the sediments of the paleo–Middle Yangtze. Combined with paleocurrent measurements and petrography, our results indicate that the paleo–Middle Yangtze initially reversed during the late Cretaceous, followed by progressive westward capturing in the Eocene. This conclusion provides a paleogeographic explanation for the limited exposures of Upper Cretaceous to Cenozoic rocks in the Sichuan Basin and refutes the “drainage divide” hypothesis for the Three Gorges.


INTRODUCTION
The middle reach of the Yangtze River, or the so-called Middle Yangtze, flows east from the Sichuan Basin to the Jianghan Basin and drains most of the South China block. The South China block was accreted to the North China block during the middle-late Triassic and is marked today by the Qinling-Dabie Shan mountains (Ratschbacher et al., 2003). Sediment provenance, paleocurrent direction, and volumetric estimates of the Middle to Upper Triassic flysch in the Songpan-Ganzi remnant ocean basin consistently support the existence of a large axial river system originating from the Dabie Shan ( Fig. 1; Nie et al., 1994;Bruguier et al., 1997;Weislogel et al., 2006;Enkelmann et al., 2007). This ancient river system is envisaged to have flowed parallel to the Qinling-Dabie suture with a dominantly North China block source (Weislogel et al., 2006), and it likely contributed significant amounts of sediment to the Sichuan Basin during the Jurassic (She et al., 2012).
Here, we call this proposed Mesozoic drainage the "paleo-Middle Yangtze" because of its comparable size but generally reversed flow direction relative to the present-day Middle Yangtze River ( Fig. 1). While recent studies have constrained the birth of the integrated Yangtze River to pre-Miocene time , the timing and mechanisms by which the drainage was reorganized remain open questions. One model for Middle Yangtze drainage reorganization proposes that reversals initiated in the Three Gorges region and expanded westward through successive capture events (Clark et al., 2004), accounting for ~2 km of post-Eocene erosion in the Sichuan Basin (Richardson et al., 2008). Although low-temperature thermochronology studies can provide time constraints for basin denudation (Richardson et al., 2008(Richardson et al., , 2010, it is difficult to characterize the drainage evolution using these data alone. In this paper, we synthesized sandstone petrography, detrital zircon provenance, and paleocurrents to infer the drainage reorganization of the paleo-Middle Yangtze from Mesozoic through Cenozoic time. We focused on the detrital record preserved in the Three Gorges region, which exposes more than 5000 m of largely fluvial, late Triassic to Jurassic sediments that are stratigraphically equivalent to the fluvial sediment of the Sichuan Basin. Moreover, the Three Gorges region was a foreland area structurally confined by two coeval, noncoaxial orogens, the NW-trending Qinling-Dabie Shan and SW-trending Xuefeng Shan (Fig. 1). The NW-directed shortening of the Xuefeng Shan led to the development of the thin-skinned Eastern Sichuan fold belt (ESFB; Yan et al., 2003), while the SW-directed shortening of the Qinling-Dabie Shan led to Dabashan fold belt (Shi et al., 2012). River flow along the axis of this foreland basin must have been westward across the structural low between these two fold belts (Fig. 1). This inference and the observed correlation between Triassic to Jurassic fluvial deposits in the Three Gorges and Sichuan Basin regions (Liu et al., 2005) suggest that the Three Gorges area is likely the best candidate for recording the paleo-Middle Yangtze flow.
The present-day Yangtze River flows through the Three Gorges region and cuts across the dome-like Huangling anticline in its eastern margin before flowing into the Jianghan Basin (Fig. 1). Early studies proposed that the Huangling anticline served as a drainage divide prior to incision of the Three Gorges (Barbour, 1936), while others suggested that the preexisting gorges may have formed as early as the Cretaceous (Lee, 1934). To resolve this long-standing controversy, we collected sandstone samples from Mesozoic and Cenozoic sedimentary rocks on both sides of Late Cretaceous drainage reorganization of the Middle Yangtze River | RESEARCH the Huangling anticline and present 734 new U-Pb detrital zircon analyses coupled with paleocurrent measurements and detrital composition analyses. Our results provide new insights into the timing of drainage reorganization of the paleo-Middle Yangtze and its link to the tectonic evolution of the South China block.

GEOLOGICAL CONTEXT
The Three Gorges region, located between Sichuan Basin and Jianghan Basin, is best known for well-preserved Precambrian to Phanerozoic rocks of the South China block (Figs. 1 and 2A). The core of the Huangling anticline exposes ~1000 km 2 of crystalline rocks composed of an Archean metamorphic complex and Neoproterozoic South China block granitoids dated to ca. 800 Ma (Gao et al., 2011, and references therein). These batholithic rocks are unconformably overlain by a Precambrian sequence of sandstone and limestone with a total thickness of ~1600 m (Mao and Wang, 1999). The more than 5 km of Paleozoic to Lower Triassic sediments comprising the bulk of the remaining strata are dominated by shallow carbonate platform sediments deposited on the passive margin of the South China block (Hsu et al., 1990).
Shallow-water marine deposition ceased with the collision of the South China block and North China block beginning in the Middle Triassic. The age of suturing between the two blocks decreases from early Triassic in the east to late Triassic in the west, concurrent with deposition of the Songpan-Ganzi flysch ( Fig. 1; Enkin et al., 1992). The thick and vast spatial extent of Middle and Upper Triassic detritus in the Songpan-Ganzi basin has been interpreted as the product of erosional exhumation of the Dabie Shan ultrahigh-pressure (UHP) rocks (Nie et al., 1994;Weislogel et al., 2006). During the latest Triassic, the Songpan-Ganzi basin experienced intense eastward shortening during the formation of the Longmen Shan fold-and-thrust belt (Jia et al., 2006). Meanwhile, convergence between the North China and South China blocks continued by southwestward shortening of the Qinling-Dabie Shan (Ratschbacher et al., 2003). During that same period, the Xuefeng Shan in the central South China block formed as an intercontinental orogen, probably due to northwestward subduction of the Pacific plate ( Fig. 1; Wang et al., 2005). The Sichuan Basin, surrounded by these orogenic "walls" (Carroll et al., 2010), was therefore able to accumulate thick deposits of fluvial sediments derived from its perimeter during late Triassic through early Cretaceous time (Liu et al., 2005).
Deposition resumed in the late Cretaceous in response to the development of the ESFB along the eastern margin of the Sichuan Basin (Yan et al., 2003). All the foreland sequences (Upper Triassic to Lower Cretaceous) were involved in NE-SW-or nearly E-W-trending folds, whereas the late Cretaceous deposits are largely undeformed and mainly exposed in the southeast and northeast corners of the Sichuan Basin (Fig. 1). Some Cretaceous outcrops can also be found as intermontane basins in the ESFB, for example, in Enshi, and have been lithostratigraphically correlated to the well-dated rocks in the Three Gorges region (Lei et al., 1987). To the west, exposure of the Upper Cretaceous is more restricted  X u e f e n g S h a n X u e f e n g S h a n than that of the Jurassic to Lower Cretaceous units, and they are present mainly in the southwestern corner of the Sichuan Basin (Fig. 1). The Jurassic and Lower Cretaceous strata of the southwestern Sichuan Basin are dominated by fluvial and lacustrine facies with a thickness of up to 1000 m (Burchfiel et al., 1995).
Exposures of Cenozoic rocks are rare in the Sichuan Basin, but they comprise much of the Jianghan Bain east of the Three Gorges (Figs. 1 and 2A). Prior work has proposed that the Sichuan Basin has experienced >2 km of denudation since the Eocene as a result of Yangtze River erosion when its lower reach (east of the Sichuan Basin) was integrated with its upper reach (west of the Sichuan Basin) through successive westward capture (Richardson et al., 2008(Richardson et al., , 2010. The westward-flowing paleo-Middle Yangtze should have also reversed during these processes (e.g., Clark et al., 2004). However, geomorphic observations in the eastern Sichuan Basin indicate that at least part of the paleo-Middle Yangtze was already flowing east before these capture events . A regional drainage divide developed along the "midline" of this arc-shape fold belt, such that the northern rivers flowed to the northeast into the Jianghan Basin, whereas the southern rivers flowed south or southwest into the Sichuan Basin (see fig. 15 in Wang et al., 2013). However, it is still unclear when this drainage pattern was established.

METHODS
Approaches to tracing the evolution of the paleo-Middle Yangtze are based on field observation and analysis of the Mesozoic to Cenozoic sedimentary rocks exposed in the Three Gorges region ( Fig. 2A). These strata are in total more than 10 km thick, with vertically varied fluvial, lacustrine, alluvial-fan, and delta facies, as discussed in the following section. Here, we focused on the Jurassic Shaximiao Formation, the Lower Cretaceous Wulong Formation, the Upper Cretaceous Honghuatao Formation, the Paleogene Cheyanghe Formation, and the Neogene Yangtze Gravel. These formations contain fluvial facies with well-developed paleocurrent indicators.  Late Cretaceous drainage reorganization of the Middle Yangtze River | RESEARCH Sections were measured at the meter scale to determine vertical changes through the depositional facies, measure paleoflow indicators, perform clast composition counts, and collect sandstone for detrital zircon analyses. The Jurassic sections are located west of the Huangling anticline at Zigui (Fig. 2). For comparison, we also included previously published Jurassic sections located in the northeastern Sichuan Basin (Qian et al., 2016) and south of the Dabie Shan (Yang et al., 2010;see Fig. 2B). The Lower Cretaceous sections are mainly exposed east of the Three Gorges region at Wulong, with a small but distinguishable relict preserved west of the Huangling anticline at Zhouping (Fig. 2). The Upper Cretaceous sections crop out both east and west of the Huangling anticline, including the western Jianghan Basin at Yichang and the intermontane basin at Enshi, respectively (Fig. 2). Cenozoic strata are limited to the east of the Huangling anticline as the main basin fill of the western Jianghan Basin, including the Paleogene section at Yidu and the Neogene section at Xiaoting (Fig. 2).
Paleocurrents were measured on two-and three-dimensional crossbedding and pebble-cobble imbrications for Jurassic to Paleogene sandstone and Neogene Yangtze Gravel, respectively (Table 1). Orientation data were collected and analyzed with the method described by DeCelles et al. (1983). Sandstone petrography was derived from 48 thin sections with >500 framework grains counted per slide using the Gazzi-Dickinson method (Dickinson, 1985; see also Table DR1 1 ). Clasts were counted on single conglomeratic beds for lithologic composition in addition to the thin sections (Table DR2). Zircons are selected by handpicking under a binocular microscope, cast in an epoxy mount, and polished to section the crystals for detrital zircon analysis. Laser-ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb analyses were performed at Nanjing University, using Agilent 7500a ICP-MS equipped with New Wave 213 nm laser. Data were processed using GLITTER 4.0 software (www.glitter-gemoc.com) and plotted in age probability plots using the Isoplot program (Ludwig, 2001; see Table DR3).

Lower Cretaceous (Wulong Formation)
The Lower Cretaceous interval includes two stratigraphic units, the Shimen and Wulong Formations (Fig. 3B). The Wulong Formation contains abundant fossils, including megaspore, plant, and dinosaur fossils (Lei et al., 1987). The megaspore assemblage is dominated by Trileites, Hostisporites, and Erlansonisporites, with a few Hughesisporites and Maexisporites and sparse Bicharsporites and Dijkstraisporites. This assemblage is similar to those of the Wealden Stage both in the Netherlands and Britain. The sporo-pollen is characterized by an assemblage of Wulongspora reticulata-Zhonghuapollis plicatus-Tricolpites micromunus. The dinosaur fossil cf. Prodeinodon kwangshiensis Hou, Yeh et Zhao and plant fossil Pseudofrenelopsis parceramosa Watson have also been found in the Wulong Formation. All these fossils indicate an age of late Early Cretaceous or early Late Cretaceous.
At the Wulong-Yichang section (Fig. 3B), the Shimen Formation rests unconformably on the eastern flank of the Huangling anticline, and it consists of ~400-m-thick, poorly organized, thick-bedded, pebble-cobble conglomerate that can be interpreted as medial or proximal deposits of an alluvial fan. The Wulong Formation overlies the Shimen Formation with an erosional contact. It consists of gray-green coarse-grained or pebbly sandstone, red medium-grained sandstone, red siltstone, and minor gray-green conglomerate, totaling ~1800 m in thickness. The lower part (~600 m) of the Wulong Formation consists of a series of basal-scoured, upward-fining and stacked sand bodies. Beds show a lenticular geometry

(A) An ~10-m-thick section of upward-thinning and upward-fining sequences composed of thick-to medium-bedded sandstone and thin-bedded mudstone or siltstone in the Shaximiao Formation (Jurassic, Zigui section). (B) An ~3-m-thick outcrop of medium-grained, lenticular sand bodies with low-angle, lateral accretion and cross-stratification in the Shaximiao Formation (Jurassic, Zigui section). (C) An ~20-m-thick section of vertical stacked, lenticular, coarse-grained sand bodies in the lower part of the Wulong Formation (Lower Cretaceous, Wulong-Yichang section). (D) An ~2-m-thick outcrop of sand bodies showing well-developed trough cross-stratification in the lower part of the Wulong Formation (Lower Cretaceous, Wulong-Yichang section). (E) An ~20-m-thick section of upwardfining and upward-thinning sequences composed of mediumgrained sandstone and siltstone in the upper part of the Wulong Formation. (F) An ~5-m-thick interval of fine-grained sandstone and siltstone in the upper part of the Wulong Formation (Lower Cretaceous, Wulong-Yichang section). (Continued on following page.)
Late Cretaceous drainage reorganization of the Middle Yangtze River | RESEARCH

(continued ). (G) An ~1-m-thick outcrop of conglomerate showing well-rounded, imbricated gravels in the Luojingtan Formation (Upper Cretaceous, Wulong-Yichang section). (H) An ~2-m-thick outcrop and (J) line drawing of medium-grained, cross-bedded sandstone in the Honghuatao Formation (Upper Cretaceous, Wulong-Yichang section). (I) An ~1-m-thick outcrop and (K) line drawing of mediumgrained, cross-bedded sandstone in the Honghuatao Formation (Upper Cretaceous, Enshi section). (L) An ~4-m-thick outcrop of several lenticular sandstone beds with well-developed trough crossstratification in the Cheyanghe Formation (Eocene, Yidu section). (M) An ~20-m-thick section of gravel beds with several lenticular sandstone intervals in Yangtze Gravel (Neogene, Xiaoting section).
commonly ~2-3 m in thickness and ~10-20 m in width, indicating the lateral migration of transverse or longitudinal bars (Fig. 4C). Each main sand body is coarse to medium grained with well-developed trough crossstratifications (Fig. 4D). Pebbles usually occur on the base of the sandstone beds, showing sharp, undulate scour surfaces. Some siltstone intervals, on a scale of decimeters, also occur in these stacked sand bodies, representing overbank deposition. We infer that the lower part of the Wulong Formation occurred in a sandy, braided river depositional setting. Upward in the section, the braided river sand bodies are interrupted by an ~10-m-thick interval of conglomerate, followed by an overall upward-fining succession in the middle part. The upper part (~1000 m) is characterized by a series of depositional sequences, each of which begins with a scoured channel base overlain by ~5-8 m of medium-grained sandstone with tabular cross-stratifications or, in places, lateral accretion bedding. These sand bodies are generally overlain by ~3-5-m-thick, rippled, thin-bedded siltstone beds with several fine-grained sandstone intervals (Fig. 4E). We interpret the middle part of the Wulong Formation to have been deposited in a meandering river environment. The uppermost Wulong Formation (~200 m) shows an upward-coarsening succession, which consists of a series of basal scoured and stacked sand bodies, similar to the lower part of this section. Cross-stratified, coarse-grained, and pebbly sandstone beds commonly occur in lenticular geometries with siltstone intervals. Typically, the uppermost Wulong Formation represents a braided river depositional setting and reflects increased sediment input during this period. West of the Huangling anticline, the lower part of Zhouping section is composed of a series of upward-fining units, each of which contains gray-green coarse-grained or pebbly sandstone, reddish fine-grained sandstone, and siltstone, totaling ~100 m in thickness (Fig. 3C). Sandstone beds usually show ~5-8-m-thick lenticular geometry with well-developed basal scours, and low-angle cross-stratifications. These sand bodies are often overlain by ~2-3 m of rippled, thin-bedded siltstone with several fine-grained sandstone intervals (Fig. 4F). This succession represents typical meandering river deposition lithostratigraphically correlative to the upper part of the Wulong Formation east of the Huangling anticline.

Upper Cretaceous (Honghuatao Formation)
The Upper Cretaceous interval contains three stratigraphic units, including the Longjingtan, Honghuatao, and Paomagang Formations, from bottom to top. The Luojingtan and Honghuatao Formation are not rich in fossils. The known fossils are characterized by an ostracode assemblage of Talicypridea-Cypridea, including Talicypridea amoena (Liu), Talicypridea gibbera (Yuan), Cypridea cavernosa Gal., Clinocypris sp., Mongolianella sp., and Cyprois sp. (Lei et al., 1987). This assemblage is widely distributed in East Asia, indicating a middle Late Cretaceous age. The Paomagang Formation contains abundant fossils, such as ostracodes, charophytes, sporo-pollens, estherians, fishes, and dinosaur eggs (Lei et al., 1987). The ostracodes fauna includes 87 species and 22 genera and is characterized by a Talicypridea-Cypridea-Candona assemblage. The charophytes are very common and contain 20 species and 14 genera, termed a Latochara cylindrica-Charites tenuis assemblage. The sporo-pollens are represented by the assemblage Multinodisporites taizhouensis-Ulmoideipites krempii-Jianghanpollis ringens. In addition, a dinosaur egg fossil of Macroolithus cf. yaotunensis Zhao and fish fossil of Knightia yuyanga Liu were also found in the Paomagang Formation. All these fossils indicate an age of late Late Cretaceous.
At the Wulong-Yichang section (Fig. 3B), the Luojingtan Formation is dominated by clast-supported, well-organized, gray to red, pebble-tocobble conglomerate, totaling more than 1000 m in thickness. The contact between the Luojingtan and underlying Wulong Formation is gradational over 50 m at the base of this conglomeratic sequence, where the pebble conglomerate is interbedded with coarse-grained sandstone. The conglomerate coarsens and thickens upward overall, showing roughly horizontal bedding with minor basal scours and sandstone intervals (Fig. 4G). Bed thickness ranges from several meters to tens of meters and consists of tabular bodies with varying assemblages of gravel traction current deposits. This succession is interpreted to have been deposited in a braided river environment with high-energy stream flow. The Honghuatao Formation conformably overlies the Luojingtan Formation, which is mainly composed of thick-bedded, well-sorted medium sandstone, totaling ~300 m in thickness. Cross-bedding is common in these sandstone beds, although it is not often easy to recognized (Figs. 4I and 4K). The uppermost Paomagang Formation mainly consists of purple-red thick-bedded silty mudstone with several gray-green fine-grained sandstone intervals, representing lacustrine deposition.
West of the Huangling anticline, the Upper Cretaceous is also exposed at the Enshi section ( Fig. 3D) with ~200-m-thick conglomerate-dominated Luojingtan Formation at the bottom and ~1000-m-thick sandstonedominated Honghuatao Formation at the top. The conglomerate is poorly organized, poorly rounded, and pebble-to-cobble sized, indicating a proximal fan deposition. The sandstone lithofacies, however, are well sorted and cross stratified (Figs. 4H and 4J), similar to the Honghuatao Formation at the Wulong-Yichang section.
At the Yidu section (Fig. 3E), the Gongjiachong and Yangxi Formations contain interlayered siltstone and limestone with the thickness less than 400 m, representing a continuation of the lacustrine deposition of the late Cretaceous Paomagang Formation. The Cheyanghe Formation is composed by a series of upward-thinning and upward-fining sequences, totaling more than 800 m in thickness. Each sequence contains 4-6 beds of yellow-green, coarse-to medium-grained, cross-stratified sandstone with red siltstone intervals. Sandstone beds usually show lenticular geometry and range from several meters to tens of centimeters in thickness (Fig. 4L). The bases of most sandstone beds are sharp, with undulating scour surfaces and a few conglomerate or mud clasts, indicating river channel deposits. Siltstone is usually thin to medium bedded with massive textures, representing levee, floodplain deposits. These sequences of Cheyanghe Formation are interpreted to represent sandy braided river deposition. The overlying Pailoukou Formation is only ~100 m thick, dominated by interlayered yellow-green, medium-coarse grained sandstone, red siltstone, or mudstone. Scoured surfaces are common at the Late Cretaceous drainage reorganization of the Middle Yangtze River | RESEARCH base of the sandstone beds, with many mudstone clasts. Thin-bedded siltstone or mudstone is interlayered with fine-grained sandstone and usually shows massive or ripple-laminated textures. Calcite nodules are common in these siltstones and mudstones. These fine-grained depositions indicate increased sinuosity of the river with a well-developed floodplain.

Neogene (Yangtze Gravel)
The Neogene "Yangtze Gravel," characterized by unconsolidated gravels, sands, and silty clays, unconformably overlies the Paleogene or Cretaceous rocks in the western Jianghan Basin. The age of the Yangtze Gravel has recently been constrained by the overlying basaltic lavas, for which 40 Ar/ 39 Ar ages range from 22.9 ± 0.3 Ma to 10.3 ± 0.1 Ma . At the Xiaoting section (Fig. 3F), the age control for Yangtze Gravel comes from overlying laterites and pollen fossils. The laterites unconformably overlie the Yangtze Gravel and represent deeply weathered eolian deposits, which are widely preserved in the South China area. Detailed palaeomagnetic studies have revealed that these laterites developed during the Pleistocene (Jiang et al., 1997;Xiong et al., 2002). In addition, palynoflora has been recovered from silty clays, including at least 30 taxa at the generic level, where Pinus, Quercus, Polypodiaceae, and Gramineae are important elements (Wang et al., 2014). Comparison of this palynoflora with the well-dated basin-interior deposits suggests a Neogene age for the Yangtze Gravel (Paleontological Group of Jianghan Oil Bureau, 1976).
At the Xiaoting section (Fig. 3F), the Yangtze Gravel is dominated by pebble-cobble-sized, yellow-gray gravel beds with several yellow-gray coarse-grained sand and minor gray-green silty clay intervals, totaling ~100 m in thickness. Gravel beds are typically a few meters thick, showing sharp contacts or scoured bases, and thickening upward overall. In general, the gravels are clast supported and exhibit good imbrication or crude low-angle cross-stratification. At the top of each bed, the gravels usually fine upward into sands. Sand beds show lenticular geometries with decimeter-scale thickness and can be traced laterally for tens of meters (Fig. 4M). Locally, two to three sand beds are stacked on top of each other and display well-developed cross-stratification. The thick-bedded gravels have been interpreted to represent braided channel deposits by the through-going Yangtze River (Wang et al., 2014).

Paleoflow Direction
Paleocurrent data were compiled for stratigraphic columns east and west of the Huangling anticline to show the change of paleoflow direction through time (Fig. 2B). For the Jurassic Shaximiao Formation, the paleocurrent indicators based on cross-stratified sandstone at the Zigui section indicate westward flow direction, consistent with published paleocurrent measurements northeast of the Sichuan Basin (Yunyang; Qian et al., 2016) and south of the Dabie Shan (Yang et al., 2010). Other data collected from the eastern part of the Sichuan Basin, including at Wanzhou, Lichuan, Liangping, Dazhu, Xuanhan, Changshou, Chongqing, and Fuling ( Fig. 1), all generally indicate east to west paleoflow. The paleoflows of the Lower Cretaceous Wulong Formation are consistent between the Wulong-Yichang and Zhouping sections, showing no difference with that of the Jurassic Shaximiao Formation. By the Upper Cretaceous section, however, paleoflows are largely reversed in the Honghuatao Formation, with northeastward and eastward flow directions at the Enshi and Wulong-Yichang sections, respectively. In contrast, the paleocurrent data from Chengdu and Xishui in the southern Sichuan Basin generally show southwestward flow directions (Fig. 1), reflecting a structurally controlled drainage pattern with a regional drainage divide developed along the "midline" of the ESFB . In the Cenozoic section, the Paleogene Cheyanghe Formation and Neogene Yangtze Gravel consistently exhibit eastward flow direction, similar to the modern Yangtze River.

Petrography
The Jurassic sandstones are arkosic, with lithics dominated by volcanic fragments and cherts (Fig. 5A), and they plot within the magmatic arc field on a quartz-feldspar-lithics (Q-F-L) diagram (Fig. 5B), indicating provenance from an active orogen. The Cretaceous and Cenozoic sandstones are litharenites, with varying amounts of quartz and lithics, mainly sourced from recycling of a sedimentary succession, such as a fold-and-thrust belt (Fig. 5B). The Upper and Lower Cretaceous samples are all rich in limestone fragments, with fewer sandstones and cherts, whereas Eocene samples contain more cherts (Fig. 5A). The Neogene Yangtze Gravel has a conspicuously different lithic composition, which lacks limestone components but contains a considerable amount of locally derived granitoid fragments sourced from the Huangling anticline (Fig. 5A).

Detrital Zircons and Provenance
Six major age populations characterize the detrital zircon grains from the Mesozoic to Cenozoic strata (Fig. 5C). The oldest population, 2.6-2.4 Ga, corresponds to ages in the North China block (Kusky and Li, 2003). Zircons between the ages of 2.0 and 1.7 Ga are widespread in the North China block (Wilde et al., 2002); however, similar ages are also reported as detrital zircon grains (Liu et al., 2008) from Precambrian sedimentary rocks exposed in the Huangling anticline. The 900-700 Ma population corresponds to ages in the South China block , as well as Proterozoic granitoids in the core of the Huangling anticline (Gao et al., 2011). The 500-400 Ma and 300-200 Ma populations are all typical ages of Qinling-Dabie Shan synorogenic granitoids (Ratschbacher et al., 2003), though the latter have also been found in the Xuefengshan belt in the central South China block (Wang et al., 2005). The youngest 200-100 Ma population is most likely derived from the granitic plutons in Qinling-Dabie Shan orogen (Ratschbacher et al., 2003). Although there are also 200-100 Ma granitoid plutons in the southeastern part of the South China block (Li and Li, 2007), the region is not a likely source area because the NE-trending Xuefeng Shan belt may have served as a barrier to northwestward sediment transport.

Evolution of Zircon Ages
The Jurassic sample (ZG0615-25) from the Shaximiao Formation is dominated by zircons from the North China block (2.0-1.7 Ga and 2.6-2.4 Ga), which comprise as much as 66% of the whole age distribution (Fig.  5C). The detrital zircon ages, combined with the west-directed paleoflow of the Shaximiao Formation, indicate a major input from exhumation of the Dabie Shan UHP belt (e.g., Weislogel et al., 2006). The South China block, in contrast, was never a dominant source for the Jurassic basin, as evidenced by the relative scarcity of the 900-700 Ma zircon grains. Lower Cretaceous and Jurassic sandstones have similar zircon populations, although they are petrographically distinct from those of the Triassic. Nearly 50% of zircons dated in samples ZP11182 and WL11231 from Wulong Formation were derived from North China block (2.0-1.7 Ga and 2.6-2.4 Ga; Fig. 5C), indicating a dominant source from the North China block. The remarkable increase in limestone fragments and decrease in volcanic fragments in the Lower Cretaceous interval may represent the propagation of the foreland thrust belt in front of the Dabie Shan.
The decrease in North China block-derived zircons in the Upper Cretaceous samples of the Honghuatao Formation signals an important change in source area. The 2.0-1.7 Ga and 2.6-2.4 Ga age groups account for ≤20% of zircons in samples YC07063-1 and ES0801 (Fig. 5C), whereas the 900-700 Ma zircons account for 32% of the whole age distribution. As the Upper and Lower Cretaceous sandstones show little difference in petrography, the increase in South China block-derived zircons and the eastward flow direction indicate recycling of the Paleozoic to Middle Triassic strata of the ESFB. The Eocene Cheyanghe Formation is characterized by a sharp peak of ca. 1.8 Ga zircons, corresponding to the Precambrian sedimentary rocks of the exhuming Huangling anticline ( Fig. 5C; Wang et al., 2014). However, the Huangling batholith was not exposed until the Neogene, based on the appearance of granitoid fragments and the numerous Huangling granitoid zircons with ages of 900-700 Ma present in the Yangtze Gravel.

Spatial Change in Zircon Ages
The visual similarity in age spectra between sample ZG0615-25 from the Three Gorges region and Jurassic sandstone from south of the Dabie Shan (Yang et al., 2010) indicates consistent North China block sources during the Jurassic. The sample south of the Dabie Shan has a higher abundance of 900-700 Ma grains compared with the Three Gorges sample (14% vs. 6%), but it is depleted in grains dated at 200-100 Ma (0% vs. 6%) (Fig. 5C). The westward decrease in the South China block zircons and increase of the Qinling-Dabie Shan zircons may reflect sediment transport by westward-flowing paleorivers. The Lower Cretaceous samples from west (ZP11182) and east (WL11231) of the Huangling anticline also show similar age spectra (Fig. 5C) and consistently west-directed paleocurrents (Fig. 2B), indicating that the Huangling anticline was not a barrier to sediment transport during the early Cretaceous. The Upper Cretaceous sandstones east (YC07063-1) and west (ES0801) of the Huangling anticline contain nearly all the same age groupings (Fig. 5C) and share similar eastward paleoflows oriented parallel to the strike of the fold belt (Fig. 2B), refuting the presence of a paleo-drainage divide in the Three Gorges region at that time (Barbour, 1936). The Cenozoic deposits were only found in the western Jianghan Basin, indicating overall exhumation of the Sichuan Basin and the Three Gorges region.

DISCUSSION
The provenance and paleoflow results allow for a reconstruction of the paleogeography and drainage evolution of the Middle Yangtze River during the Mesozoic to Cenozoic tectonic evolution of South China.

Late Triassic to Late Cretaceous (Ca. 230 Ma to Ca. 100 Ma)
Subduction of the South China block under the North China block produced the Qinling-Dabie Shan orogen and Sichuan foreland basin (Fig. 6A). The Jurassic sandstone in the Three Gorges region contains many volcanic fragments and North China block-derived zircons corresponding to this orogenic activity. Paleocurrent, sandstone petrography,    and detrital zircon signatures all support a westward-flowing, synorogenic, axial river system, or paleo-Middle Yangtze, during the deposition of the Middle Jurassic Shaximiao Formation (She et al., 2012). To the east, the Jianghan Basin was also filled when it was the eastern extension of the Sichuan Basin and later covered by Cenozoic sediments (Liu et al., 2005). This river system likely existed into the early Cretaceous, as paleoflow direction and zircon populations remained unchanged. Over 1.5 km of overburden was eroded, exposing the UHP rocks of the Dabie Shan (Nie et al., 1994), which dominate the zircon age spectra of Jurassic sediments in the Sichuan Basin with abundant North China block grains. Farther southeast, the Xuefeng Shan in the South China block has been described as an intercontinental orogen due to flat-slab subduction of the Pacific plate during the same time period (Li and Li, 2007). This orogen may also have contributed to the influx of sediment to the Sichuan Basin during the same period (Wang et al., 2005;Liu et al., 2005), but its contribution is less pronounced due to the lack of the South China block zircons in the Jurassic and Lower Cretaceous sandstone.

Late Cretaceous to Eocene (Ca. 100 Ma to Ca. 50 Ma)
A remarkable shift in provenance to South China block-derived detritus occurred in the late Cretaceous. Westward propagation of the ESFB from the Xuefeng Shan into the Sichuan Basin (Yan et al., 2003), combined with initial onset of extension in the Jianghan Basin (Ratschbacher et al., 2000), led to regional drainage reorganization of the paleo-Middle Yangtze (Fig. 6B). Development of the arc-shaped ESFB might have produced a NW-trending, transversal drainage divide: Sediments in the northeastern ESFB were shed into the Jianghan Basin along the ENEtrending, fold-belt strike valleys, whereas sediments from the southern ESFB were drained southward into the Sichuan Basin (Fig. 6B). This drainage pattern was recognized by geomorphic observation , and it can also explain the limited exposure of Upper Cretaceous rocks restricted to the southwestern and southern margins of the Sichuan Basin (Burchfiel et al., 1995).
The Jianghan Basin began rifting and subsequently acted as a sediment trap during the Cenozoic. The tectonic transition from compression to extension has been widely interpreted as a result of slab rollback of the Pacific plate under the South China block (Li and Li, 2007), which had a great impact on the basin evolution of East Asia. In the central part of the Jianghan Basin, Eocene deposits contain hundreds of halite beds , indicating rapid tectonic subsidence with limited clastic sediment input. The local base level of this underfilled basin would promote the capture of adjacent river networks, eventually leading to the reversal of water flow in the Middle Yangtze. Consequently, the Sichuan Basin could have had an east-flowing fluvial system that crossed the Three Gorges into the Jianghan Basin. Comparison between the Upper Cretaceous samples east and west of the Huangling anticline reveals no major differences in provenance, indicating an antecedent precursor connection for the Three Gorges (Lee, 1934). However, the Jianghan Basin at this time was hydrologically closed, as revealed by thick lacustrine and evaporate deposition , suggesting that the initially reversed paleo-Middle Yangtze might have been confined to the area between the Sichuan Basin and the Jianghan Basin, with no evidence for a Lower Yangtze connection.

Eocene to Neogene (Ca. 50 Ma to Ca. 23 Ma)
The sedimentary environment began to change in the Three Gorges region in the middle Eocene (ca. 50 Ma), marked by the occurrence of large-scale braided river deposits in the Cheyanghe Formation.
Simultaneously, uplift of the Tibetan Plateau dominated the regional Cenozoic tectonics and resulted in broad changes in regional topographic gradients and drainage networks (Clark et al., 2004;Stüwe et al., 2008;Fig. 4C). The west-flowing paleo-Middle Yangtze drainage met its final demise due to the progressive westward march of headward river capture, probably along the trajectory of the modern Yangtze River (Clark et al., 2004;Richardson et al., 2010). The sedimentary record in the western Jianghan Basin provides important evidence for these captures, as seen from varied zircon provenance through Cenozoic time (Wang et al., 2014).
Reorganization of the Middle Yangtze not only led to the development of the through-going Yangtze River, but also the erosion of the Sichuan Basin. Erosion might have begun as early as the late Cretaceous, when the westward propagation of the ESFB caused the paleo-Middle Yangtze to be initially reversed, resulting in limited deposition of late Cretaceous sediments over the most of the Sichuan Basin. This erosion was enhanced around 40 Ma, probably due to increased discharge during the progressive capture by the reversed Middle Yangtze (Richardson et al., 2010). In the western Jianghan Basin, the presence of a detrital contribution from the Huangling anticline demonstrates this enhanced erosion and exhumation of the Sichuan Basin and Three Gorges region during the Eocene (Wang et al., 2014). Our paleogeographic reconstruction is consistent not only with the detrital records in the Three Gorges region, but also with the cooling history of the Sichuan Basin (e.g., Richardson et al., 2008).
Our work documents the late Cretaceous to Paleogene reversal and integration of the Middle Yangtze with the Lower Yangtze River. Development of this connection was the first step in the subsequent development of today′s integrated Yangtze River with headwaters in the Tibetan Plateau (Clark et al., 2004). This tectonic-induced river reorganization also had a major effect on the pattern of sediment dispersal in East Asia. Compared to the Mesozoic basins (e.g., Songpan-Ganzi and Sichuan Basins) in western China, the Cenozoic basins were mostly developed around continental margins (e.g., East China and South China Sea Basins), with sediments originating from the Songpan-Ganzi and Tibetan highland at the Yangtze River′s headwaters (e.g., Metivier et al., 1999). Deposition of these sediments on continental margins represents not only potential sources for hydrocarbon exploration, but also climatic records of the Asia Monsoon. Although the development of monsoonal conditions is compatible with the erosional regime and sediment accumulation in regional basins , our new results clearly indicate that, at the scale of continental Asia, the rearrangement of the Cenozoic sediments does not correlate well with climate changes and points toward a stronger tectonic control on drainage reorganization.

CONCLUSION
Age distributions of detrital zircons from the Three Gorges region document changing drainage patterns of the Middle Yangtze in the Mesozoic and Cenozoic. Zircon signatures of the North China block and Qinling-Dabie Shan dominate Upper Triassic through Lower Cretaceous strata, whereas the Upper Cretaceous to Cenozoic strata are dominated by South China block-derived zircons. This provenance change indicates the initial reversal of the paleo-Middle Yangtze in the northeastern Sichuan Basin beginning in the late Cretaceous, as a result of westward propagation of the ESFB. This regional reversal was followed by progressive westward capture during the Eocene, leading to an eastward-flowing Middle Yangtze and enhanced erosion of the Huangling anticline. Upper Cretaceous samples west of the Huangling anticline share similar provenance with those east of Huangling anticline, indicating river connectivity for the Three Gorges by that time.