Jianchuan Basin Research Articles

Rutile and zircon U-Pb dating as a constraint on sedimentary provenance of the Jianchuan Basin: Implications for paleo-drainage evolution

The Yangtze River is the largest river in Asia, and its evolution has been a subject of debate for more than a century. The formation of the First Bend is widely recognized as a key drainage reorganization event that led to the birth of the modern Yangtze River. In this study, we use new detrital rutile UPb ages to constrain the sedimentary provenance of the Cenozoic deposits in the Jianchuan Basin. Our results show that detrital rutile UPb ages of the Eocene sandstones from the Jianchuan Basin are similar to those of the Yalong River and Jinsha River, indicating that the upper Yangtze is likely to have flowed through the Jianchuan Basin in the late Eocene. Moreover, the characteristic zircon age signature of the Yalong and Jinsha Rivers also appear in the Jianchuan Basin, which is consistent with the rutile data. Our new rutile data and previous detrital zircon data suggest that the upper Yangtze River is likely to connect the Red River via the Jianchuan Basin since the late Eocene.

Formation of the Great Bend and Enhanced Quaternary Incision of the Upper Yangtze River: New Insights from Low-Temperature Thermochronology and Tributary Morphology

Abstract The occurrence of a sharp turn along the upper course of the Yangtze River is referred to as the “Great Bend” and represents a large-scale drainage reorganization in response to the surface rise of the Tibetan Plateau. However, the timing and mechanism of the formation of the Great Bend remain disputed. In this paper, we report new (U–Th)/He and apatite fission track thermochronological data from the deep river valley in the Great Bend area of the southeastern margin of the Tibetan Plateau. Compared with the adjacent Jianchuan Basin, two phases of younger rapid cooling for the Great Bend area are identified based on thermal-history modeling, namely, Miocene (ca. 17 to 11–8 Ma) and Quaternary, with the former phase being contemporaneous with the formation of the anticline in the Tiger Leaping Gorge. Progressive increases in the normalized channel steepness (ksn) and the degree of river-valley incision with increasing distance downstream for tributaries of the Yangtze River in the Tiger Leaping Gorge indicate that river rerouting and formation of the Great Bend occurred during the Miocene. Samples located at the bottom of the Tiger Leaping Gorge also reveal a phase of rapid cooling since ca. 1.9 Ma, with an exhumation rate of 1.5 ± 0.2 mm/year. We hypothesize that enhanced Quaternary exhumation in the southeastern margin of the Tibetan Plateau occurred mainly within the narrow region between the Sichuan Basin and the Eastern Himalayan Syntaxis, corresponding to an episode of widespread extensional deformation superimposed above middle- to upper-crustal flexure in this region.

Northward expansion of Cenozoic Asian humid climate recorded by sporopollen

The distribution pattern of arid–humid climate in the modern northwest–southeast confrontation in Asia evolved from the zonal distribution pattern of the Paleogene. However, this evolutionary process remains poorly constrained by geological evidence. In this paper, we analyze the characteristics of sporopollen assemblages recorded in Late Eocene strata of the Jianchuan Basin in southwest China. We show that vegetation patterns exhibited a remarkable shift from tropical–subtropical sparse forest to subtropical–temperate mixed broad-leaved–coniferous forest at ∼40.6 Ma; this shift is further indicated by the rapid decrease in drought-tolerant plants, mostly Ephedra and Chenopodiaceae, evidence that the previous hot–dry climate was replaced by a warm–humid climate at this time. This evidence suggests that the Asian monsoon began to affect the study area at ∼40.6 Ma. We then comprehensively integrated sporopollen results from several basins across East Asia, revealing that the frontal edge of the humid monsoon climate moved intermittently to the northwest after the Late Eocene, and may have advanced to the Lunpola–Lanzhou line by the early Late Oligocene, this being similar to the position of the leading edge of the modern monsoon. Moreover, the frontal edge of the humid monsoon climate may have extended as far as the Qaidam Basin during the Middle Miocene, but then gradually retreated to the position of the modern monsoon front. The uplift of the Tibetan Plateau and global climate change may have played dominant roles in this evolution.

Eocene dry eolian system in the Jianchuan Basin, southeastern Tibetan Plateau: Implications for regional wind regime and paleoclimate

East Asia experienced a climatic transition from a zonal pattern (climate zones defined by latitude) to a monsoon-dominated pattern during the Cenozoic. However, hitherto little consideration has been paid to the atmospheric circulation of the planetary wind system that prevailed during the early uplift of the Tibetan Plateau, due to the lack of reliable proxies. Here, we conduct a study on the eolian successions from the Eocene Baoxiangsi Formation in the Jianchuan Basin, southeastern Tibetan Plateau to reconstruct regional atmospheric circulation and paleoclimate. Two facies associations are recognized in the eolian deposits of the Baoxiangsi Formation: a dominant eolian dune association and a subordinate interdune association. Eolian dune deposits are characterized by large-scale cross-stratified sets of strata composed internally of packages of grainflow, wind-ripple strata and pinstripe lamination. These packages of laminae and cross-stratified eolian dune sets are bounded internally by superimposition and reactivation surfaces and interdune surfaces, respectively. Interdune deposits are uncommon and composed dominantly of subcritical climbing translatent strata to form sets of strata and these are themselves bounded by interdune surfaces to form cosets of strata. The association of dominant eolian dune deposits, widely distributed interdune surfaces of great lateral extent (> 450 m) and that climb at a shallow but positive angle (< 1°) is consistent with the climbing dry eolian system. The commonly occurring superimposition surfaces indicate the presence of compound dunes (megadunes, draas). Based on the analysis of 715 dip–azimuth readings of foresets of eolian cross-beds, we reconstruct the prevailing wind directions: the paleowind in the northern basin flowed toward the northeast; in the southern basin it flowed toward the southwest. This wind regime indicates that the Jianchuan Basin was located at the zonal boundary between the westerlies and the trade winds, and the construction of the dry eolian system was controlled by the subtropical high-pressure belt during the middle–late Eocene. Following the accumulation of the dry eolian system, a monsoon climate regime gradually established itself and prevailed in the Yunnan Province; this expanded northward over time. This was closely related to the uplift of the central Tibetan Plateau and the retreat of the Paratethys ocean in the late Eocene.

Sedimentary recycling in Jianchuan Basin, SE Tibetan Plateau: A solution to the debate on the formation age of the First Bend (Yangtze River)

The sedimentary archive of the Jianchuan Basin provides a critical constraint on the formation of the “First Bend”, a major geomorphological feature of the Yangtze River, involving an abrupt 150° change in flow direction. Despite extensive detrital zircon provenance studies, no consensus has been reached on the timing of this event. Prior to the formation of the First Bend, the Yangtze is thought to have flowed southwards through the Jinchuan Basin, entering the South China Sea via the Red River. In this study, we constrain the timing of the First Bend using detrital K-feldspar Pb isotopic and detrital zircon U-Pb age datasets from Jianchuan Basin sandstones. Provenance data from late Eocene detrital zircon and K-feldspar populations concur in suggesting that they were fed by the southward flowing paleo-upper Yangtze River. Distinct K-Feldspar Pb isotopic data indicate that the overlying later Eocene and Pliocene deposits were fed by local Eocene syenites, granites and tuffs. Stratigraphic constraints on this change in provenance indicate that the southward flowing paleo-Yangtze River was cut off in the late Eocene at or before ∼37 Ma, thereby dating the formation of the First Bend. In contrast, detrital zircon U-Pb geochronology from the youngest units within the Jianchuan Basin is similar to those of the older sandstones and upper Yangtze River sands. This evidence indicates that the majority of zircons deposited in the upper part of the Jianchuan Basin were recycled from older sedimentary rocks. In our view, much of the disagreement relating to the formation time of the First Bend can be attributed to the failure to recognize sedimentary recycling. This study highlights the need to consider the possible presence of recycled detrital zircons when investigating paleo-drainage systems.

A new fossil record of Palaeosinomenium (Menispermaceae) from the Upper Eocene in the southeastern margin of the Tibetan Plateau and its biogeographic and paleoenvironmental implications

Palaeosinomenium Chandler (Menispermaceae) was established to accommodate the fossil endocarps that share similar morphological characteristics with the tribe Menispermeae in Menispermaceae. Previous fossil records suggested that Palaeosinomenium was distributed in North America, Europe, and East Asia before the Middle Eocene. A new fossil species, Palaeosinomenium hengduanensis Meng-Xiao Wu et Zhe-Kun Zhou sp. nov., was established based on an endocarp impression fossil from the Upper Eocene (35 ± 1 Ma) Shuanghe Formation, Jianchuan Basin, southwestern China. The new species is characterized by a horseshoe-shaped endocarp, an excavated central area, surrounded by a slightly asymmetrical C-shaped lateral ridge, and an elliptic aperture located near the longer endocarp limb. The fossil site is located in the modern distribution area of the living species Sinomenium acutum (Thunb.) Rehd. et Wils, a potential nearest living relative of the new species. The finding of P. hengduanensis supports that the divergence within the tribe Menispermeae might have occurred by the Late Eocene and the species similar to modern S. acutum appeared in the southeastern margin of the Tibetan Plateau as early as in the Late Eocene.

Eocene mammal tracks from the southeastern Tibetan Plateau

ABSTRACT Late Eocene artiodactyl and creodont tracks have been reported from the Jianchuan Basin, southeastern Tibetan Plateau, representing the first and the oldest reported mammal tracks in Yunnan Province, China. Sedimentary facies analysis indicates that tracks from the upper Baoxiangsi Formation and the lower Shuanghe Formation occurred in lake shore and flood plain environments, respectively. Based on the morphological analysis, the artiodactyl tracks from the Baoxiangsi Formation are attributed to Anoplotheriipus zeuctus. Combined with fossil records and morphological characteristics, the artiodactyl and creodont tracks were probably produced by entelodonts and mesonychids, respectively. The superimposition of artiodactyl tracks with varying sizes indicates coexistence of juveniles and adults as well as their gregarious behaviour. The co-occurrence of artiodactyl tracks and creodont tracks suggests the predator–prey relationship and ecological chain during the late Eocene. The presence of mammal tracks in the Jianchuan Basin coincides with the climate change from dry to wet in the late Eocene, implying the climatic and ecological effects of the uplift of the Tibetan Plateau.

Late Oligocene-early Miocene Origin of the First Bend of the Yangtze River explained by thrusting-induced river reorganization

The origin of the First Bend of the Yangtze River is key to understanding the birth of the modern Yangtze River. Despite considerable efforts, the timing and mechanism of formation of the First Bend remain highly debated. Inverse river-profile modeling of three tributaries (Chongjiang, Lima, and Gudu) of the Jinsha River, integrated with regional tectonic and geomorphic interpretations, allows the onset of incision at the First Bend to be constrained to 28–20 Ma. The spatio-temporal coincidence of initial river incision and activity of Yulong Thrust Belt in southeastern Tibet highlights thrusting to be fundamental in reshaping the pre-existing stream network at the First Bend. These results enable us to reinterpret a change in sedimentary environment from a braided river to a swamp-like lake in the Jianchuan Basin south of the First Bend, recording the destruction of the hypothesized southwards-flowing paleo-Jinsha and Shuiluo Rivers at ~36–35 Ma by magmatism. During the late Oligocene-early Miocene, the paleo-Shuiluo River was diverted to the north by focused rock uplift due to thrusting along the Yulong Thrust Belt, which also led to exhumation of the Jianchuan Basin. Diversion of the paleo-Shuiluo River can be explained by capture from a downstream river in the footwall of the Yulong Thrust Belt. Subsequent rapid headward erosion, that was caused by thrusting-induced drop of local base level, is recorded by upstream younging ages for the onset of incision and led to the formation of the First Bend. The combination of new ages for the onset of incision at 28–20 Ma at the First Bend and younger ages upstream indicates northwards expansion of the Jinsha River at a rate of 62 ± 18 mm/yr. Our results suggest that the origin of the First Bend was likely triggered by thrusting at 28–20 Ma, after which the Yangtze River formed.

From desert to monsoon: irreversible climatic transition at\u2009~\u200936\xa0Ma in southeastern Tibetan Plateau

Although there is increasing evidence for wet, monsoonal conditions in Southeast Asia during the late Eocene, it has not been clear when this environment became established. Cenozoic sedimentary sequences constrained by radiometrically dated igneous rocks from the Jianchuan Basin in the southeast flank of Tibetan Plateau now provide a section whose facies and climatic proxies determine this evolution. Semi-arid conditions had dominated the region since Paleocene controlled by the northern sub-tropical high pressure system, culminating in mid Eocene when desert dunes developed. From 36 Ma, the basin began to accumulate swamp sediments with coals, together with synchronous braided river deposits and diversified pollen assemblages, indicating significant increase in precipitation. This remarkable transition from dry to wet conditions precedes the Eocene/Oligocene boundary at 34 Ma, thus excluding general global cooling as the prime driver. We propose that uplift of Tibetan Plateau might have reached a threshold elevation by that time, operating through thermal and dynamic forcing, causing the inception or significant intensification of monsoonal rains to penetrate into this downwind locality.

Detrital zircon U‐Pb geochronology of the Jianchuan Basin, southeastern Tibetan Plateau, and its implications for tectonic and paleodrainage evolution

AbstractThe Jianchuan Basin is a key region for determining the spatial‐temporal evolution of paleodrainage and tectonic at the southeastern Tibetan Plateau. New detrital zircon U‐Pb geochronology and paleocurrent data from Eocene formations in this basin suggest that the sediment sources switched from the remote sources (including Hoh‐Xil, Songpan‐Ganzi, North Qiangtang, Yidun and western Yangtze terranes) to local lithologies (adjacent Triassic and Eocene igneous rocks and western Yangtze block) from the early‐middle Eocene Baoxiangsi to the late Eocene Shuanghe and Jianchuan Formations. Such provenance changes support that large‐scale drainage reorganization occurred in the late Eocene (prior to ~36 Ma). This significant late Eocene provenance and drainage changes occurred as one of the responses to coeval topographic uplift in the southeastern Tibetan Plateau.

Paleogene Sedimentary Records of the Paleo‐Jinshajiang (Upper Yangtze) in the Jianchuan Basin, Yunnan, SW China

AbstractCenozoic surface uplift of Tibetan Plateau has driven the birth and reorganization of large river systems in Asia. The development of the Yangtze River, closely linked to plateau uplift, has important implications for the regional tectonic‐geomorphology processes, but is still under debate. The key scientific question is whether the Upper Yangtze (upper Jinshajiang) once flowed southeast to the South China Sea. In this study, we carried out provenance analysis on Paleogene sedimentary rocks in the Jianchuan Basin, composed of the Baoxiangsi, Jinsichang, and Shuanghe formations. We compared them with similar deposits in other regional basins, as well as with potential sources. Heavy‐mineral and geochemical data show that sediments from the Jinsichang formation are more mature compared with those from the Shuanghe and Baoxiangsi formations. The Jinsichang formation represents the products of a large fluvial system, whereas the Baoxiangsi formation is mainly dominated by local sources. The Shuanghe formation contains material from both distant sources and locally eroded materials. Detrital zircon U‐Pb age patterns from the Jianchuan Basin are similar to those in sediments in the Gonjo, Simao, and Chuxiong basins, and show great similarity with the modern Jinshajiang River. Statistical analysis of the zircon ages suggests that the Songpan‐Ganzi terrane was the primary source for all sediments, while the Yangtze Craton and northern Qiangtang terrane constitute secondary sources. Taken together these data confirm that a paleo‐Jinshajiang flowed south toward to the South China Sea from the SE Tibetan Plateau in the late Eocene, constraining the age of formation of the First Bend.

No connection between the Yangtze and Red rivers since the late Eocene

The unusual drainage pattern in the southeastern Tibetan Plateau has been proposed to result from the breakup of a single paleo-Red River system. However, whether there or not was a major early Cenozoic “paleo-Red River” flowing from southeastern Tibet to the South China Sea is still highly debated. In this study, we have used Pb isotopic analyses of detrital K-feldspar to constrain the sedimentary provenance of the Cenozoic deposits from the onshore Jianchuan Basin and offshore Yinggehai and Qiongdongnan basins, and further to decipher the drainage evolution of the upper-middle Yangtze and Red Rivers. Detrital K-feldspar from the Baoxiangsi Formation in the Jianchuan Basin shows an apparent mixture of sand delivery from both the upper Yangtze and Yalong rivers, indicating that there was a river draining the Qiangtang Block and Songpan-Ganzi that used to flow through the Jianchuan Basin during the Eocene. In contrast, K-feldspar Pb isotopes from the Eocene to Pliocene deposits of the Yinggehai and Qiongdongnan basins are different from those of the Jianchun Basin and Yangtze, Salween, and Mekong rivers, but similar to that of the modern Red River. These lines of evidence suggest that major rivers draining southeastern Tibet have not been tributaries of the paleo-Red River and hence, there has been no fluvial connection between southeastern Tibet and the South China Sea since at least the late Eocene.

Upper Eocene travertine‐lacustrine carbonate in the Jianchuan basin, southeastern Tibetan Plateau: Reappraisal of its origin and implication for the monsoon climate

AbstractThe Asian monsoon is one of the largest climatic systems in the world, but age of its onset has been estimated differently ranging from the late Eocene to the Quaternary. We investigated the sedimentology and stable isotopic compositions of the upper Eocene Jiuziyan Formation, a terrestrial limestone unit in the Jianchuan basin, Yunnan Province in China. This limestone formation is restricted in several localities in the central part of the basin. Previously, this has been characterized as palustrine carbonate and the transition to the sublacustrine deposit of the overlying Shuanghe Formation was interpreted as the appearance of wetter climate during the late Eocene. Our observations of macro‐ and microfacies revealed sedimentary fabrics indicating rapid CaCO3 precipitation, such as dendritic calcite and calcified reed stems, which are unlikely to develop in a simple lacustrine setting. High carbonate content (mostly &gt;90 %) and restricted distribution of the Jiuziyan limestone indicate a depositional setting spatially limited and isolated from clastic influx. These findings, together with clearly higher δ13C values (−0.7 ‰ to +6.9 ‰) and lower δ18O values (−14.6 ‰ to −10.5 ‰) than those of the Shuanghe Formation, indicate that the limestone was mainly travertine, carbonate formed from endogenic spring water. The elevated δ13C resulted from a large amount of CO2 degassing from spring water with high pCO2. In addition, the occurrence of centimeter‐scale lamination coupled with cyclic changes in δ13C and δ18O is almost identical with the modern annually‐laminated travertine reported from Baishuitai in northern Yunnan Province, implying comparable amplitude of seasonal temperature and precipitation changes to the record of the modern travertine at Baishuitai. Our results do not contradict the previous interpretation of late Eocene wetting and additionally suggest the existence of the late Eocene monsoon climate in the Jianchuan basin.

Formation of the First Bend in the late Eocene gave birth to the modern Yangtze River, China

AbstractThe First Bend on the Yangtze River (China), the point where the river ceases flowing toward the south and heads toward the northeast, has been one of the most strongly debated geomorphic features in Asia because it holds the key to understanding the history of the Yangtze River and is linked to the tectonically driven surface uplift of the southeastern Tibetan Plateau. Mid- to upper Eocene sedimentary rocks preserved in the Jianchuan Basin located immediately southwest of the First Bend demonstrate that a large river system, presumably the paleo–Jinshajiang River (the upper Yangtze), used to flow south through that region. Provenance analysis of sediments over the wider region, based mostly on U-Pb dating of detrital zircon grains, confirms that the once south-flowing paleo–Yangtze River originated in the Tibetan Plateau and flowed into the South China Sea. Inversion of the Jianchuan Basin, starting in or after the latest Eocene and associated with wider plateau surface uplift at that time, caused the river to be diverted toward the northeast where it was confined along tectonic lineaments associated with strike-slip faulting, giving birth to the First Bend, a major step in the formation of the modern Yangtze River.

Eocene Basins on the SE Tibetan Plateau: Markers of Minor Offset along the Xuelongshan–Diancangshan–Ailaoshan Structural System

AbstractThe offset of geological bodies provides robust evidence of displacement along a fault or ductile shear zone. The amount of displacement along the Xuelongshan–Diancangshan–Ailaoshan structural system, southeastern Tibetan Plateau, is uncertain because of the lack of offset geological markers. This NNW–SSE‐trending system is developed in three isolated metamorphic complexes and interjacent nonmetamorphosed rocks. They are expected to record similar post‐Eocene strain, although their structural patterns should be distinct. Geological mapping in the area between the Xuelongshan and Diancangshan metamorphic complexes has revealed a small Eocene basin, the Madeng Basin, located to the west of the structural system. The sedimentary and volcanic successions of the Madeng Basin are comparable to those of the Jianchuan Basin, which is located to the east of the structural system. Zircon U–Pb geochronological and bulk geochemical data demonstrate that the volcanic rocks of both basins formed during 37–34 Ma and share the same geochemical features. These data suggest that the Madeng and Jianchuan basins previously constituted a single basin, with the distribution of high‐K volcanic rocks in the basins defining an ENE–WSW‐trending volcanic belt that shows a limited dextral offset of ≤20 km across the Xuelongshan–Diancangshan–Ailaoshan structural system. Therefore, the northern segment of the structural system records no evidence of large‐scale lateral movement/displacement. The results suggest that the Indochina block, which is bounded by the Xuelongshan–Diancangshan–Ailaoshan structural system to the east and the Sagaing Fault to the west, has not extruded southward as a whole but rather has been deformed by pervasive crustal shortening.

Thrusting, exhumation, and basin fill on the western margin of the South China block during the India-Asia collision

Abstract The pattern and timing of deformation in southeast Tibet resulting from the early stages of the India-Asia collision are crucial factors to understand the growth of the Tibetan Plateau, but they remain poorly constrained. Detailed field mapping, structural analysis, and geochronological and thermochronological data along a 120 km section of the Ludian-Zhonghejiang fold-and-thrust belt bounding the Jianchuan basin in western Yunnan, China, document the early Cenozoic tectonic evolution of the conjunction between the Lanping-Simao and South China blocks. The study area is cut by two major southwest-dipping brittle faults, named the Ludian-Zhonghejiang fault and the Tongdian fault from east to west. Numerous kinematic indicators and the juxtaposition of Triassic metasedimentary rocks on top of Paleocene strata indicate thrusting along the Ludian-Zhonghejiang fault. Similarly, structural analysis shows that the Tongdian fault is a reverse fault. Between these structures, fault-bounded Permian–Triassic and Paleocene rocks are strongly deformed by nearly vertical and upright southwest-vergent folds with axes that trend nearly parallel to the traces of the main faults. Zircon and apatite (U-Th)/He and apatite fission-track data from a Triassic pluton with zircon U-Pb ages of 237–225 Ma in the hanging wall of the Ludian-Zhonghejiang fault, assisted by inverse modeling, reveal two episodes of accelerated cooling during 125–110 Ma and 50–39 Ma. The Cretaceous cooling event was probably related to crustal thickening during the collision between the Lhasa and Qiangtang terranes. The accelerated exhumation during 50–39 Ma is interpreted to record the life span of the fold-and-thrust belt. This timing is corroborated by the intrusive relationship of Eocene magmas of ca. 36–35 Ma zircon U-Pb age into the fold-and-thrust belt. Early Cenozoic activity of the deformation system controlled deposition of alluvial-fan and braided-river sediments in the Jianchuan basin, as evidenced by eastward and northeastward paleoflows and terrestrial clasts derived from the hanging wall of the Ludian-Zhonghejiang thrust. Since 39 Ma, decreasing cooling rates likely reflect cessation of activity on the fold-and-thrust belt. Early Cenozoic compressive deformation on the western margin of the South China block together with geological records of contraction in central, northern, and eastern Tibet document Eocene upper-crustal shortening located in the Himalaya, Qiangtang terrane, and northern plateau margins together with contractional basin development in the intervening Lhasa, Songpan-Garze, and Kunlun terranes, coeval with or shortly after the onset of the India-Asia collision. This suggests that moderate crustal shortening affected a large part of Tibet in a spaced way, contrary to models of homogeneous crustal thickening soon after the collision, and prior to the main crustal thickening, propagating progressively from south to north. This complex deformation pattern illustrates the complexity of Asian crustal rheology, which contrasts with assumptions in existing geodynamic models.

40 Ar/ 39 Ar mica dating of late Cenozoic sediments in SE Tibet: implications for sediment recycling and drainage evolution

The Indo-Asia collision significantly changed the topography and drainage network of rivers around the Tibetan Plateau. Debate continues as to when and how the current drainage system of the Yangtze River was formed. Here we use 40 Ar/ 39 Ar dating of detrital micas (muscovite and biotite) to constrain provenances of the Pliocene sediments from the Jianchuan and Yuanmou basins in SE Tibet. Muscovite and biotite data of the same Pliocene samples from the Jianchuan Basin suggest contrasting distal v. local sources, respectively. Similarly, muscovite data of the Yuanmou Basin suggest a derivation of sediments from the Yalong River, but the characteristics of the Pliocene cobbles (palaeocurrent and subrounded cobbles) suggest that these sediments are locally sourced. Sediment reworking is proposed as an explanation for the different sediment provenance signals in the Jianchuan and Yuanmou basins that have led to the controversy of an either Pleistocene or pre-Miocene age of formation of the current Yangtze. Based on sediment provenance constraints, the evolution of the Jinsha River is reconstructed. The upper Jinsha River lost its connection with the southward flowing Red River upstream from the Jianchuan basin at least before the Pliocene. At the same time a parallel site in the Yuanmou Basin shows that the Yalong River stopped flowing southward into this basin. Detrital mica from early Pleistocene sediments at the Panzhihua site between the Jianchuan and Yuanmou basins is sourced from the current Jinsha and Yalong rivers. These results would suggest that the current upper Yangtze drainage system should have been established before the Pliocene. Supplementary material: Muscovite 40 Ar/ 39 Ar data, biotite 40 Ar/ 39 Ar data and muscovite geochemistry are available at https://doi.org/10.6084/m9.figshare.c.4821573

Oligocene‐Early Miocene Topographic Relief Generation of Southeastern Tibet Triggered by Thrusting

AbstractThe timing and mechanisms of uplift in southeastern Tibet remain disputed. To address this debate, we conducted structural and morphological analyses of the Yulong thrust belt; we also reconstruct the cooling and exhumation history of the Jianchuan basin in the hanging wall of the thrust system using inverse thermal modeling of apatite fission‐track and (U‐Th)/He thermochronology data. Our results provide evidence for 2.3–3.2 km of rapid exhumation in the Jianchuan basin between ~28 and ~20 Ma, followed by limited exhumation of less than 0.2 km since then. The magnitude of basin exhumation is consistent with the present‐day topographic step of 1.8–2.4 km across the Yulong and Chenghai thrust belts, as shown by morphometric analysis. We thus infer that the present‐day morphology of the southeastern margin of Tibet results partly from thrusting along the Yulong thrust belt during the Late Oligocene‐Early Miocene. This structure may be the southwest continuation of the Longmen Shan thrust belt, offset by the Xianshuihe fault in the Late Miocene. On a regional scale, the approximate synchronicity of exhumation in the hanging walls of the Yalong‐Yulong and Longmen Shan thrust systems indicates that widespread crustal shortening and thickening took place in southeastern Tibet during the Late Oligocene‐Early Miocene.

Eocene-Oligocene potassic high Ba-Sr granitoids in the Southeastern Tibet: Petrogenesis and tectonic implications

Potassic high Ba-Sr granitoids are widely distributed over collisional orogenic belts worldwide, however, their magma sources and petrogenetic processes are still debated. To better understand them, we collected a suite of Paleogene high Ba-Sr granitoids in the Jianchuan Basin, southeast Tibet. Based on the mineralogical compositions, major and trace element geochemistry, and age of formation, we subdivide them into two groups: intermediate series and granitic series. LA-MC-ICP-MS zircon U-Pb analyses show that the intermediate series crystallized at 33.8–36.9 Ma, similar to or slightly earlier than the granitic rocks with crystallization ages of 33.1–34.0 Ma. New geochemical and zircon Hf isotopic results, in combination with previous results, support an enriched mantle source for the high Ba-Sr intermediate rocks, and a mixed source of enriched mantle and lower crust for the granitic ones. We suggest that partial melting of enriched mantle with fractional crystallization of mafic minerals produced the potassic high Ba-Sr intermediate series. Meanwhile, the juvenile heat from such magma induced melting of thickened lower crustal materials which together with magma from mantle accounted for source of the granite series. The high Ba-Sr intermediate rocks have nearly identical major and trace element, and isotopic signatures as Archean sanukitoids, thus offer a potential constraint on the formation mechanism for the sanukitoids. Our study supports that the emergence of the sanukitoids requires a sediment-metasomatized mantle and thus indicates the onset of crustal recycling into the mantle.

Paleoelevations in the Jianchuan Basin of the southeastern Tibetan Plateau based on stable isotope and pollen grain analyses

The southeastern margin of the Tibetan Plateau (TP) is a key region for understanding the region's surface uplift mechanisms. This study focused on the Jiuziyan and Shuanghe Formations (Fms) in the Jianchuan Basin, both of which include lacustrine calcareous mudstones and marls. Ostracods of the genus Austrocypris found within the Jiuziyan and Shuanghe Fms constrain the age of strata to the Late Eocene. This study used two different proxies, i.e. fossil pollen coexistence and the δ18O (VPDB) values of carbonate (δ18Oc), to reconstruct paleoelevation and the extant paleoenvironment from lacustrine calcareous mudstones and marls preserved in the Eocene stratigraphy of the Jianchuan Basin. The coexistence approach (CA) using pollen data from the Shuanghe Fm indicates a paleoelevation of 1.3–2.6 km above sea level (asl), which would most probably have been associated with a vegetation cover consisting of tropical-subtropical, deciduous, coniferous, broadleaf forests. The reconstructed mean annual air temperature (MAAT) had a value of 16.8–21.7 °C, warmer than today's MAAT (~6 °C). Oxygen isotope results from the Jiuziyan Fm, with/without modification between Eocene and modern Myanmar sea level, suggested that the surface of the Jianchuan Basin was at a paleoelevation between 0.5+0.8–0.5 km asl and 2.5 ± 0.7 km asl (δ18Omw: −8.9 ± 1.3‰, 2σ). During the Shuanghe Fm sedimentation the paleoelevation was between 0.9 +0.7–0.7 km asl and 2.9 ± 0.6 km asl (δ18Omw: −9.5 ± 1.1‰, 2σ). Our results suggest that a stepwise uplift of Jianchuan Basin and crustal thickening initiated during the Eocene was the cause of passive surface uplift of the southeastern Tibetan Plateau (TP) rather than Miocene lower crustal flow.