Late Miocene rapid exhumation in the West Kunlun range: Insights into Tibetan Plateau growth and India-Asia lithospheric collision

The convergence of India and Eurasia, which began in the early Cenozoic, established the Tibetan Plateau and the Circum-Tibetan Plateau Basin and Orogen System. When and how the convergence-driving strain propagated into this system is important in deciphering the growth processes of the Tibetan Plateau. We conducted a structural analysis of the West Kunlun–southern Junggar transect along the NW margin of the Tibetan Plateau to establish the propagation of deformation and, through this, to determine the plateau growth processes. Our results suggest a two-phase deformation mode. The first-stage features deformation confined to pre-existing weak zones (e.g. the West Kunlun orogen, the Buchu Uplift and the Tian Shan orogen) during the Paleogene, when the intracontinental strain is speculated to be mainly consumed by shortening of these weak zones. The second stage is characterized by deformation propagating into the foreland regions since the early Miocene, where shortening along the foreland fold–thrust belts on a scale of tens of kilometres and decreasing basinwards had a key role in absorbing intracontinental strain. We suggest that this two-phase deformation mode may reflect a shift in the governing mechanism of the expansion of the Tibetan Plateau from a rigid block to a critical wedge taper style. Thematic collection: This article is part of the Fold-and-thrust belts collection available at: https://www.lyellcollection.org/cc/fold-and-thrust-belts

Abstract. The important roles of the Tibetan Plateau (TP) atmospheric boundary layer (ABL) in climate, weather, and air quality have long been recognized, but little is known about the TP ABL climatological features and their west–east discrepancies due to the scarce data in the western TP. Based on observational datasets of intensive sounding, surface sensible heat flux, solar radiation, and soil moisture from the Third Tibetan Plateau Atmospheric Scientific Experiment (TIPEX-III) and the routine meteorological-operational-sounding and ground-based cloud cover datasets in the Tibetan Plateau for the period 2013–2015, we investigate the west–east differences in summer ABL features over the TP and the associated influential factors for the first time. It is found that the heights of both the convective boundary layer (CBL) and the neutral boundary layer (NBL) exhibit a diurnal variation and a west–east difference in the TP, while these features are not remarkable for the stable boundary layer (SBL). Moreover, the ABL shows significant discrepancies in the amplitude of the diurnal variation and the persistent time of the development between the eastern and western TP. In the early morning (08:00 BJT, Beijing time), the ABL height distribution is narrow, with a mean height below 450 m a.g.l. (above ground level) and a small west–east difference. The SBL observed at this moment accounts for 85 % of the total TP ABL. There is a wide distribution in the ABL height up to 4000 m a.g.l. and a large west–east difference for the total ABL height at noon (14:00 BJT), with a mean height above 2000 m a.g.l. in the western TP and around 1500 m a.g.l. in the eastern TP. The CBL accounts for 77 % of the total TP ABL at this moment, with more than 50 % of the CBL above 1900 m a.g.l. In the late afternoon (20:00 BJT), the CBL and SBL dominate the western and eastern TP, respectively, which results in a larger west–east difference of 1054.2 m between the western and eastern TP. The high ABL height in a cold environment over the western TP (relative to the plain areas) is similar to that in some extreme hot and arid areas such as Dunhuang and Taklimakan deserts. In general, for the western (eastern) TP, there is low (high) total cloud coverage, with large (small) solar radiation at the surface and dry (wet) soil. These features lead to high (low) sensible heat flux and thus promote (inhibit) the local ABL development. This study provides new insights for west–east structures of the summer ABL height, occurrence frequency, and diurnal amplitude over the TP region and the associated reasons.

Multi-stage exhumation history of the West Kunlun orogen and the amalgamation of the Tibetan Plateau

The lack of sounding observations in the western Tibetan Plateau (TP), the highest terrain in the world, has resulted in few efforts to evaluate the quality of the atmospheric reanalysis results in this region. Using the sounding observations from the Third Tibetan Plateau Atmospheric Scientific Experiment during 2013–2015, the NCEP and ERA-Interim reanalysis temperature and humidity fields in the TP are evaluated and the characteristics of the reanalysis Atmospheric Boundary Layer (ABL) height are utilized to explain the reasons for the differences in temperature and humidity between the western and eastern TP. The results show that the NCEP and ERA-Interim reanalysis temperature and humidity products generally have larger errors at low level (such as 500 hPa) in the western TP (WTP) than in the eastern one (ETP) at 12:00 UTC. However, this difference is small at 00:00 UTC. Further analysis reveals that the temporal and spatial variations of temperature and humidity errors at low level are closely associated with the differences in the terrain and ABL between the western and eastern TP. In the early morning when the ABL height is low over the TP, the 500 hPa pressure level in both the WTP and ETP is significantly above the top of ABL, with weak spatial variations of temperature and humidity errors. However, in the late afternoon when there is a larger increase in ABL height over the WTP than over the ETP, the 500 hPa pressure level is located inside the ABL in the WTP and is still above the ABL in the ETP, which causes significant regional differences in these errors.

Rapid warming over the Tibetan Plateau (TP) has been associated with an increasing trend in atmospheric water vapor content, which is critical for recharging the Asian water tower. However, the mechanism associated with the wetting phenomenon remains unclear. Long‐term changes in moisture balance and precipitation (PRE) recycling processes are investigated using the ERA5 reanalysis from 1979 to 2019. The increasing trend over TP is mainly due to the summer water vapor trend with significantly increases over the western TP. Based on the moisture balance analysis, it is found that PRE, evaporation, and the convergence of moisture are all increasing over the western TP but decreasing over the eastern TP. Based on the dynamical PRE recycling model, the results suggest that both internal and external cycles contributes to the wetting over TP, with 63.87% and 36.13% contributions respectively. Further analysis found that the atmospheric heating source is also increasing over the western TP, which could shift the moisture transportation from east to west at the southern boundary of TP. The increasing moisture convergence could enhance PRE, and the enhanced latent heating in the mid‐atmosphere can further induced moisture convergence, which forms a positive feedback. However, an opposite situation occurred over the eastern TP. The internal and external cycle of the water cycle can stimulate (suppress) each other through PRE over the western (eastern) TP. This mechanism linked the changes in the PRE recycling and atmospheric circulation, and induced the increasing trend over TP in summer.

Proto-Tethys oceanic slab break-off: Insights from early Paleozoic magmatic diversity in the West Kunlun Orogen, NW Tibetan Plateau

A tectonic facies investigation carried out in the West Kunlun, China allows us to have worked out a tectonic model of orogen. The tectonic facies, from the north to the south, are composed of the following: 1. Southern Tarim tectonic realm; 2. North Kudi magmatic arc; 3. Kudi melange; 4. Kudi micro-continent; 5. main shear zone; 6. Xianan Bridge calc alkaline complex; 7. Mazar-Kangxiwar melange-accretion complex; and 8. Tianshuihai foreland fold-thrust belt. The tectonic facies 1->5 recorded the history of the northward subduction of the Prototethys and southward accretion of Eurasia in the Late Proterozoic-Early Paleozoic time, while the tectonic facies 6->8 recorded the history of the northward subduction of the Paleotethys and southward accretion of Eurasia in the Late Paleozoic-Early Mesozoic time, that of the tectonic evolution of the passive margin of the Qiangtang block, and that of the docking and the final amalgamation of the Qiangtang block to the Eurasian continent. The tectonic facies investigation has indicated that a complicated archipelago-accretion orogenesis took place in the West Kunlun orogen, which was the important character of southward growth of the Eurasian continent.

Simulation of sensible and latent heat fluxes on the Tibetan Plateau from 1981 to 2018

The spatial distribution and temporal variation of land surface sensible heat (SH) flux on the Tibetan Plateau (TP) for the period of 1981–2018 were studied using the simulation results from the Noah-MP land surface model. The simulated SH fluxes were also compared with the simulation results from the SEBS model and the results derived from 80 meteorological stations. It is found that, much larger annual mean SH fluxes occurred on the western and central TP compared with the eastern TP. Meanwhile, the inter-annual variations of SH fluxes on the central and western TP were larger than that on the eastern TP. The SEBS simulation showed much larger inter-annual variations than did the Noah-MP simulation across most of the TP. There was a trend of decrease in SH flux from the mid-1980s to the beginning of the 21st century in the Noah-MP simulations. Both Noah-MP and SEBS showed an increasing SH flux trend after this period of decrease. The increasing trend appeared on the eastern TP later than on the western and central TP. In the Noah-MP simulation, the western and central TP showed larger values of temperature difference between the ground surface and air (Ts–Ta) than did the eastern TP. Both mean Ts–Ta and wind speed decreased from the mid-1980s to approximately 2000, and then increased slightly. However, the Ts–Ta transition occurred later than that of wind speed. Changes in mean ground surface temperature (Ts) were the main cause of the decreasing and increasing trends in SH flux on the TP. Meanwhile, changes in wind speed contributed substantially to the decreasing trend in SH flux before 1998.

Based on empirical orthogonal function (EOF) analysis, the dominant modes of variations in summer surface sensible heating (SH) over the Tibetan Plateau (TP), as well as the associated atmospheric circulation anomalies, were investigated in this study. The results show that the first dominant mode of summer SH presents a feature of decadal reduction over the whole TP on an interdecadal time scale, and the second dominant mode is characterized by a zonally asymmetric pattern with positive (negative) SH anomalies in the western (eastern) TP on an interannual time scale. The variations of summer SH are dominated by anomalies in downwelling surface shortwave radiation (DSWR), which are associated with atmospheric circulation changes. The first dominant mode of variation in SH is connected to the interdecadal variation of the Silk Road Pattern (SRP). Further analysis reveals that the interdecadal phase shift of the SRP induces anticyclone circulation to the northeast of the TP, leading to enhanced water vapor supply and convergence over the TP. This can lead to an increase in the total cloud cover, and a reduction in DSWR, contributing to the decadal reduction in SH over the TP. The second dominant mode of variation in SH is related to a stationary teleconnection pattern over the Eurasian continent named the North Atlantic-East and North Asia pattern (NAENA). Corresponding to the positive phase of the NAENA, there is a cyclone anomaly to the west TP, leading to anomalous water vapor convergence (divergence) over the eastern (western) TP. This can result in enhanced (decreased) cloud cover, reduced (increased) DSWR, and therefore, an anomalous decrease (enhancement) in SH over the east (west) of the TP. Furthermore, the southwesterly wind anomaly, which is accompanied by the anomalous cyclone to the west TP, leads to positive SH in the western TP.

In recent years, the crust and upper mantle structure of the northern Tibetan plateau has already attracted wide attention from geologists throughout the world. The collision generates not only in the southern Tibetan plateau but also in the northern part. The Tarim basin as a continental block not only has been obstructing the collision from the Indian continent, but also may have been subducting beneath the Tibetan plateau and generating collision, but the scale and deep process have not been fully understood yet. Therefore, probing the deep structure of the collision boundary in the northern Tibetan plateau is of special significance to comprehend the deep process of the intracontinent deformation caused by collision. Since 1993, deep geophysical investigations have been carried out along the northern margin of Tibet across the basinand-range conjectures. They include several deep seismic reflection profiles, wide-angle reflection and refraction profiles as well as broadband regional observation. They revealed the lithospheric structure of the northern margin of the Tibetan plateau at different tectonic layers. Some profiles and interpretations in detail will be discussing in this paper. (1) The reflection image of the southward Tarim block subduction at a steep angle was found both in the West Kunlun (Gao et al. 2000) and Qilian profiles (Gao et al. 1995, 1999). In the Altyn profile, the shearing at a lithosphere scale constrains the subduction of the Tarim crust beneath the Tibetan plateau, but the Tarim mantle has already subducted beneath the plateau (Gao et al. 2001). (2) Many groups of stronger reflections, dipping northwards under the west Kunlun Mts. and southwards under the southern margin of the Tarim basin, constitute the evidence for the collision between the Tarim basin and the Tibetan plateau (Gao et al. 2000, Kao et al. 2001). The image of reflection structure reveals the Vshaped basin-and-range coupling relationship between the west Kunlun Mts. and the Tarim basin on a lithosphere scale. It should be particularly pointed out that a face-to-face collision pattern has not been found under the lithosphere of Tibetan plateau before. Based on the comparison of the face-toface compression model with the deep seismic reflection profile of the Indian continent subduction beneath the southern Tibet and the seismic research of subduction residuals of the Tethys oceanic crust found under Yarlungzangbo suture, the authors consider that the north-dipping reflection under the west Kunlun Mts. should be caused by the subduction of the continental lithosphere. Although it cannot be determined whether it comes from India or Eurasia, a continental lithosphere is thrusting northwards along this thrust fault. (3) The deep process of the normal collision and deformation are different from that of oblique collision. West Kunlun and the Qilian Mts. are both located at the position of collision and deformation, where the lithosphere of the Tarim is subducting southwards. Because West Kunlun is relatively close to the Indian plate, the Tarim lithosphere collided with the northsubducting Indian lithosphere under West Kunlun as the former subducted southwards for a short distance. The Altyn Mts. featuring oblique collision and deformation constrains the deep subduction of the Tarim crust beneath the Tibetan plateau because of strike-slipping and shearing of the lithosphere. However, in the mantle lid, low-angle south inclining reflections may reflect that the Tarim mantle has already subducted beneath the Tibetan plateau and resulted in detachment structure at the bottom of the crust. This may be the deep effect of the oblique collision. (4) In the west Kunlun-Tarim and Qilian profiles, a thrust deformation zone has developed for about 50 km from the piedmont to the basins. And in the Altyn–Tarim profile, the deformation zone is about 120 km in width. This may be related to the angle of subduction and collision. In the Himalayan, the thrust deformation zone is about 200 km in width (Chen et al. 1999). The Indian plate is subducting along the MBT at a low angle. Therefore, deep processes of the collision deformation are different between the northern margin and southern margin of the Tibetan plateau.

The Cenozoic collision between India and Asia promoted the widespread uplift of the Tibetan Plateau, with significant deformation documented in the Pamir Plateau and West Kunlun Mountains. Low-temperature thermochronology and basin provenance analysis have revealed three episodes of rapid deformation and uplift in the Pamir–West Kunlun Mountains during the Cenozoic. However, there is very little low-temperature thermochronology age–elevation relationship (AER) data on fast exhumation events in this area—especially in the West Kunlun Mountains— leading to uncertainty surrounding how these events propagated within and around the mountain range. In this study, we produced an elevation profile across granite located south of Kudi, Xijiang Province, China, to reveal its exhumation history. Apatite fission track AER data show that a rapid exhumation event occurred at ∼26 Ma in the southern West Kunlun Mountains. When combined with published data, we interpret that the initial uplift events related to the India–Asia collision began in the central Pamir, southern West Kunlun, and northern West Kunlun regions during the Late Eocene, Oligocene, and Middle Miocene periods, respectively. Therefore, the Cenozoic northward growth process occurred from south to north around West Kunlun.

<p>        The northern Tibetan Plateau, between the Kunlun and the Altyn Tagh faults, contains high relief topography, such as the Eastern Kunlun Range, the Altyn Tagh Range and the Qilian mountain belt, and plays an important role in researching the tectonic evolution and topographic growth of the Tibetan Plateau. We present new apatite fission track (AFT) and <sup>40</sup>Ar/<sup>39</sup>Ar thermochronologic data from the Subei and Shibaocheng areas near the eastern Altyn Tagh fault. Two Cenozoic exhumation phases have been identified from our AFT thermochronology. The AFT cooling ages of ~ 60–40 Ma farther away from the faults represented a slow widespread denudation surface as response to the Indo-Eurasia collision and signified that the Subei and Shibaocheng areas denudated as a whole in the northern Tibetan Plateau. Another phase with AFT cooling ages between about 20.5 Ma to 13.6 Ma on the hanging walls near the faults, located in the Danghenanshan and Daxueshan Mountains, recorded widespread fault activities resulted from local uplift and exhumation in late Miocene (~ 8 Ma) acquired from AFT thermal history modeling. A Cretaceous exhumation (~ 120–70 Ma) acquired from AFT thermal history modeling may have made great contributions to the growth of the pre-Cenozoic northern Tibetan Plateau.</p>

The western Tibetan Plateau (TP) is characterized by low relief with high elevation separated by deep river valleys. When and how this characteristic topography developed remains ambiguous. Here, we present apatite (U‐Th)/He and fission‐track ages from three groups of samples with different geomorphic and geological settings. Thermal histories and exhumation rates were extracted from the thermochronological data. An early stage of exhumation (ca. 0.1 km/Ma) during 70–40 Ma was recorded by all samples, followed by slow exhumation (0.03–0.06 km/Ma) since the Eocene for the sample farthest away from faults and incised valley. The second stage of faster exhumation (ca. 0.13–0.15 km/Ma) during 30–23 Ma was revealed by samples from the hanging wall of thrust faults, whereas the third stage of exhumation during 15–12 Ma (ca. 0.1 km/Ma) was identified from samples in the Dingzi Zangbo valley. Combined with the regional geological setting, we propose that (a) the Late Cretaceous‐Early Eocene exhumation in the western TP was related to regional thrust‐induced crustal thickening and led to the formation of the proto‐TP; (b) the proto‐TP was modified by local structures and river incision. The Oligocene exhumation might be caused by local thrust activity, whereas the Miocene exhumation might be related to the transition from internal to external drainage. The continuous activity of the Karakoram fault resulted in another reorganization of drainage which led to the slow exhumation since 9 Ma. Our results highlight that tectonic and drainage network reorganization play an important role in shaping the geomorphology of the western TP.

We present new detrital zircon U–Pb chronological and whole‐rock geochemical data for the newly discovered black rock units in West Kunlun Orogen in an attempt to constrain the provenance variations of the siliciclastic rocks and the tectonic history of NW China. Geochemical data indicate these black rock units could have formed in an active continental margin to continental arc setting during the Late Ordovician and source rocks could have undergone modest chemical weathering under a relatively warm and humid climate during the deposition of the black rock units. U–Pb dating of 170 detrital zircons defines five major age populations at 459–545 Ma, 710–900 Ma, 1,300–900 Ma, 1,814–1,641 Ma, and 2,153–2,845 Ma, demonstrating that the black rock units deposited during the Late Ordovician and the tectonic events that occurred in the West Kunlun Orogen were related to the assembly and breakup of the supercontinents of Columbia and Rodinia. Prevalence of Early Palaeozoic magmatic zircons further indicates the black rock units were derived from a proximal source, most likely from the South Kunlun arc.