Northern Qilian Research Articles

Chemical and isotopic characteristics and origins of thermal springs in the Jiangcang Basin, China

Thermal springs in the Jiangcang Basin occur in a typical geothermal zone along the northeastern edge of the Qinghai-Tibet Plateau in the northern Qilian orogenic belt. The thermal spring water (TSW) in this region can be divided into three groups on the basis of temperature and salinity, but the origin and recharge-discharge mechanism of the geothermal fluid remain unclear. The chemical and isotopic characteristics of these thermal springs were studied, and the genesis of the thermal system hosted by deep-seated faults was assessed. The results indicate that the springs in this region are of meteoric origin and heated via deep circulation through well-developed faults. The TSW in the Datong River washland and in the southern part of the basin are derived from carbonate rocks recharged from Datong Mountain, whereas the TSW in the northern part of the basin originates from silicates recharged from Tuolai Mountain. Differences in the circulation depth, flow pathway, residence time, and mixing relationship with cold water lead to different TWS characteristics. The results are used to establish a conceptual model of the origins and circulation of the Jiangcang thermal springs. These findings provide a geological and scientific basis for the development and use of geothermal resources in the northeastern Qinghai-Tibet Plateau.

Pre-cenozoic evolution of the northern Qilian Orogen from zircon geochronology: Framework for early growth of the northern Tibetan Plateau

The northern Qilian Shan, located along the northeastern margin of the Tibetan Plateau, experienced multiple phases of orogeny throughout the Phanerozoic, culminating in fold-thrust and strike-slip deformation associated with the Cenozoic India-Asia collision. Earlier phases of deformation in the Qilian Shan have been severely reactivated or transformed by the most recent Cenozoic strain. To better understand the tectonic evolution of this complex region, we conducted an integrated investigation of field observations, U–Pb dating of igneous and detrital zircons, and a synthesis of previously published data to constrain and reconstruct the pre-Cenozoic history of the northern Qilian Shan. This effort reveals five major age populations important to the history of this region: 2550–2350, 1850–1750, 1050–950, 500–435, and 320–240 Ma. Using this dataset, we identified three major depositional shifts and/or variations in drainage patterns that affected sediment provenance of the northern Qilian Orogen. Observed regional geologic constraints, the magmatic history, and new geophysical observations of the deep structure allow us to propose a coherent tectonic model for the pre-Cenozoic evolution of the northeastern margin of the Tibetan Plateau. (1) Late Neoproterozoic to Cambrian rifting opened the Qilian Ocean. (2) Early Cambrian subduction initiation along the margins of Qaidam and North China–Tarim continents resulted in Cambrian-Ordovician bivergent subduction, arc magmatism within these two continents, and consumption of the Qilian Ocean. (3) Final ocean closure and continental collision occurred at ca. 440 Ma and was associated with syn- and post-orogenic magmatism. (4) Collisional orogeny variably eroded the basement rocks, reorganized drainage networks, and altered sedimentary provenance for the lower Paleozoic to upper Paleozoic deposits. (5) Mesozoic extension led to the development of thick Jurassic–Cretaceous terrestrial basins. This pre-Cenozoic history resulted in preexisting weaknesses and/or low-friction detachment horizons that played a decisive role in controlling the pattern, distribution, and timing of Cenozoic deformation across the northern Tibetan Plateau.

Contemporary crustal deformation of Northeast Tibet from geodetic investigations and a comparison between the seismic and geodetic moment release rates

We processed the data recorded by 32 continuous GPS stations from the Crustal Movement Observation Network of China and combined them with published GPS observations in a common geodetic reference frame to better depict the ongoing crustal velocities and deformation field of Northeast Tibet. The derived velocity field indicates that the deformation of Northeast Tibet is characterized mainly by crustal shortening coupled with the eastward extrusion of crustal material and clockwise block rotation. We further used the GPS velocity field to evaluate the strain rate tensors by the method of Sandwell and Wessel (2016). The strain rate field shows an obvious transition of the contraction rate from NE-SW to NEE-SWW, and a decrease in the maximum shear strain rate from west to east was observed along the East Kunlun fault and Haiyuan fault, indicating that the block rotation and lateral extrusion therein may be partitioned by NE-SW-oriented compression to some extent. The inhomogeneity of the rotation strain rate implies that Northeast Tibet does not behave like a single rigid block but instead resembles a left-lateral shear zone, supporting the hypothesis that the northward transfer of motion along the East Kunlun fault may play a significant role in the deformation style of this area. The dilatation rate verified that most of Northeast Tibet experiences crustal thickening, which coincides with the average uplift rate of ~1 mm/y revealed by GPS velocities and channel steepness indices. We also estimated the geodetic moment rates and compared them with the seismic moment rates estimated using the truncated Gutenberg-Richter earthquake distribution. The ratio of the seismic strain rate to the geodetic strain rate in the regions near the Altyn Tagh fault, the Qaidam Basin adjoining the Qilian Mountains, the Haiyuan fault and the central part of the East Kunlun fault are near unity, indicating that the deformation in these areas is dominated by seismic release and may represent fully coupled seismogenic zones. The occurrence of large earthquakes in the past (estimated magnitudes M ≥ 6) coupled with a high geodetic deformation rate in these regions could indicate overdue M ≥ 6 earthquakes. However, other factors, including the occurrence of creep and the long recurrence time of Mw ≥ 8.0 earthquakes, should be taken into consideration along the Haiyuan fault. By contrast, in other regions, including the southern and eastern parts of the Haiyuan fault, along the Elashan fault, and the northern Qilian Mountains, the geodetic moment rate is higher than the seismic moment rate by a factor between 3 and 9. We propose that this phenomenon is related to the undersampling of long-term earthquake rates within the seismic catalogue, long-term aseismic crustal deformation, or a combination of these factors. Our study has implications for better comprehending the current seismotectonic pattern of Northeast Tibet.

The Active Modern Faults of the Western Segment of the Qilian Mountains (North Tibet)

The Qilian Mountains, at the northeastern border of the Tibetan Plateau, absorbed crustal shortening and accommodated it with a left-lateral displacement along the Altyn Tagh fault. A detailed geomorphological study of river valleys on the northern edge of the Qilian Mountains showed that since the late Pleistocene the rate of crustal uplift in the northern spurs of the Qilian Mountains has been greater than in their central part. Due to the extension of the Tibetan plateau between the Changma and Yumen faults, a new fault-and-fold belt formed on the northern edge of the Qilian Mountains. As a result of studying the changes in heights of river terraces over the years last 60 ka, it has been found that the vertical displacement rate along the Changma fault was 0.31 ± 0.06 mm/yr, and the rate of horizontal crustal shortening was 0.11 ± 0.02 mm/yr. The vertical displacement rate along the northernmost Yumen fault has been determined to be 0.33 ± 0.02 mm/yr and the rate of the horizontal crustal shortening is 0.53 ± 0.03 mm/yr. Active faults in the western segment of the northern Qilian Mountains account for 12% of the total crustal shortening in the entire Qilian Mountains. In addition, the rate of the crustal shortening in the northern fault zone is much greater than the rate of shortening within these mountains. This indicates that since the Late Pleistocene the rate of uplift for the northern Qilian Mountains has been greater than that of the central Qilian Mountains.

Active present faults of the Western segment of the Qilian Mountains (Northern Tiber)

The Qilian Mountains, as the northeastern margin of the Tibetan Plateau, absorbed the crustal shortening and accommodated the left-lateral displacement of the Altun Tagh fault. Detailed geomorphologic study of river valleys on the northern margin of the Qilian Mountains showed that since the late Pleistocene the crustal uplift rate of the northern Qilian Mountains has been greater than the central part. Due to the extension of the Tibetan plateau between the Changma fault and the Yumen fault the latest belt of faults and folds was formed on the northern margin of the Qilian Mountains. The study of the height of river terraces over the past 60 thousand years shows that the rate of vertical displacement along the Changma fault is 0,31±0,06 mm/a and its horizontal crustal shortening rate is 0,11±0,02 mm/a. The rate of vertical displacement along the northernmost Yumen fault is 0,33±0,02 mm/a and its horizontal crustal shortening rate is 0,53±0,03 mm/a. Active faults in the western segment of the northern Qilian Mountains account for 12% of the total crustal shortening in the Qilian Mountains. In addition, the crustal shortening rate of faults in the northern Qilian Mountains is much greater than the crustal shortening rate of faults inside the Qilian Mountains, which further indicates that since the Late Pleistocene the crustal uplift rate of the northern Qilian Mountains has been greater than the central Qilian Mountains.

Cenozoic multi-phase deformation in the Qilian Shan and out-of-sequence development of the northern Tibetan Plateau

Uplift of the Tibetan Plateau and the distribution of deformation across it are the result of India-Asia collision, which bring an opportunity of understanding intracontinental tectonics in the context of continent-continent collision. The Tibetan Plateau is bound on the northern margin by the Qilian Shan thrust belt and the strike-slip Haiyuan fault. These Cenozoic fault systems play a critical role in accommodating continental convergence, yet the initiation age, deformation sequence and mechanisms of deformation are debated. In this study, integrated geologic mapping, field observations, and apatite fission track thermochronology were conducted to constrain the initiation ages of the localized thrust faults and the exhumation history of the central and northern Qilian Shan, northern Tibet. Our analyses reveal the central and northern Qilian Shan underwent rapid cooling during the Cretaceous as a result of a far-field tectonic event. In the Eocene-Oligocene, a period of thrust-related cooling occurred along the Shule Nan Shan, Tuolai Nan Shan and Tuolai Shan faults. Reactivation of the proximal thrust faults and initiation of the western segment of the Haiyuan fault occurred at ca. 16 Ma and drove final accelerated Miocene cooling and denudation to the surface. We argue that the Qilian Shan thrust belt has persisted as the stationary and internally deformed northern boundary of the Himalayan-Tibetan orogen since the early Cenozoic, involved overprinting out-of-sequence development starting by Eocene related to initiation of India-Asia collision, and the basins and ranges across the northern Tibetan Plateau have since experienced multi-phase of growth.

Petrogenesis and tectonic setting of Early Paleozoic granites and high-Mg diorites in the Northern Qilian Orogen, China

The Northern Qilian Orogenic Belt (N-QOB) was formed by oceanic subduction and collision between the Alashan and Central Qilian Belt in the Early Paleozoic. New zircon U–Pb ages, Hf isotopic and whole-rock elemental data of the intrusive rocks in the central N-QOB have been conducted to constrain the tectonic evolution of N-QOB. Zircon dating results revealed that the Ladonggou, Suyougou granitic plutons and Xichagou dioritic pluton were emplaced at 505 ± 3 Ma, 471 ± 3 M and 425 ± 3 Ma, respectively. The Ladonggou monzogranite displays depletion of Nb, Ta, Ti, P and negative εHf(t) values (−6.5 to −0.3), and old Hf crustal model ages (1.5–1.9 Ga), suggesting generation from melting of ancient crust above subducted North Qilian oceanic slab. The Suyougou granite possesses geochemistry of I-type granite. The relative enrichment of LILEs, depletion of HFSEs and negative Eu anomalies, as well as prominent Hf isotope heterogeneities and old Hf crustal model ages (2.1–2.5 Ga) suggest a magma source of ancient crust mixed with minor mantle-derived melts in a subduction-related setting. Diorites from the Xichagou plutonare characterized by high Mg# values, enrichments in Cr, Ni, Y, Yb and high Sr/Y ratios, and convincing subduction-related features, suggesting generation from partial melting of subducted sediments mixed with mantle components. Taking all these geochemistry and geochronology together with the regional geology, we interpreted the Ladonggou monzogranite, Suyougou granite and Xichagou diorite were related to the long-term northward subduction of North Qilian oceanic lithosphere during ca. 505 and 425 Ma.

Spatial variation in the present-day stress field and tectonic regime of Northeast Tibet from moment tensor solutions of local earthquake data

SUMMARYFocal mechanism solutions and their predicted stress pattern can be used to investigate tectonic deformation in seismically active zones and contribute to understanding and constraining the kinematic patterns of the outward growth and uplift of the Tibetan Plateau. Herein, we determined the focal mechanisms of 398 earthquakes in Northeast Tibet recorded by the China National Seismic Network (CNSN) by using the cut-and-paste method. The results show that the earthquakes predominately exhibited thrust and strike-slip faulting mechanisms with very few normal events. We then combined the derived focal mechanisms with global centroid moment tensor (GCMT) catalogue solutions and previously published solutions to predict the regional distribution of the stress field through a damped linear inversion. The inversion results show that most of region is dominated by a thrust faulting regime. From the southern East Kunlun fault in the west to the northern Qilian Mountains along the Altyn Tagh fault (ATF), the maximum compression axis rotates slightly clockwise; farther to the south of the Haiyuan fault in the east, there is an evident clockwise rotation of the maximum compression axis, especially at the eastern end of the Haiyuan fault. In the Qilian Mountains, the axis of the compressive stress orientation approximately trends NE–SW, which does not markedly differ from the direction of India–Eurasia convergence, emphasizing the importance of the compressive stress in reflecting the remote effects of this continental collision. The overall spatial pattern of the principal stress axes is closely consistent with the GPS-derived horizontal surface velocity. A comparison of the stress and strain rate fields demonstrated that the orientations of the crustal stress axes and the surface strain axes were almost identical, which indicates that a diffuse model is more suitable for describing the tectonic characteristics of Northeast Tibet. Additionally, the compressive stress orientation rotated to ENE–WSW in the northern Qilian Mountains along the ATF and to ENE–WSW or E–W along the eastern part of the Haiyuan fault and its adjacent area to the south, highlighting the occurrence of strain partitioning along large left-lateral strike-slip faults or the lateral variation of crustal strength across these faults. Combining geodetic, geological and seismological results, we suggest that a hybrid model incorporating both the diffuse model associated with shortening and thickening of the upper crust and the asthenospheric flow model accounting for the low-velocity zone in the middle-lower crust may reflect the primary mode of crustal deformation in Northeast Tibet.

10Be and OSL dating of Pleistocene fluvial terraces along the Hongshuiba River: Constraints on tectonic and climatic drivers for fluvial downcutting across the NE Tibetan Plateau margin, China

Accurate geochronologic constraints of fluvial surfaces along tectonically active mountain fronts can help clarify river responses to tectonic uplift and rapid climate change. In this study, we re-constrained abandonment ages of four fluvial terraces along the Hongshuiba River on the northern margin of the Qilian Shan, NE Tibet using 10Be depth profiles to date fluvial deposits, and OSL dating of capping loess. Comparison of chronological ages of the same surface derived from both boulders and depth profiles provides an opportunity to evaluate the suitability of different sampling methods for determining landform ages in arid climate. Correlating terrace ages with regional climate proxies suggests that terraces formed during an interglacial stage or during glacial to interglacial transitions, or during a period of strong summer monsoon or monsoonal weakening. The initial period of fluvial incision may primarily result from climate changes (e.g, monsoon evolution), and other factors that influence the ratio of sediment flux to water discharge. Similarities between fluvial incision rates and incremental tectonic uplift rates suggest a tectonic control on continuous river downcutting before ∼26 ka. In summary, climate forcing drives the initial abandonment of fluvial terraces in this region, while the amplitude of fluvial incision was mainly controlled by progressive tectonic uplift. However, comparison of regional fluvial incision rates (e.g. ∼6mm/a of the Hongshuiba River), exhumation rates from the Qilian Shan (∼0.5mm/a), denudation rates from the river catchments (0.3-0.8mm/a), and vertical slip rates of the northern Qilian Thrust (∼1mm/a) all suggest climate change driven forcing of incision since the Late Pleistocene to Holocene.

Variation of the Relative Soil Moisture of Farmland in a Continental River Basin in China

The reduction of grain production caused by drought is one of the most serious problems caused by natural disasters. The relative soil moisture of farmland is the most important monitoring indicator for agricultural drought. This study investigated the relative soil moisture of farmland data from 38 agrometeorological stations in a continental river basin area in China from 1992 to 2012. Spatial and temporal variations of the relative soil moisture of farmland were studied using geostatistical analysis. The results show that, from 1992 to 2012, the average annual relative soil moisture of farmland in the continental river basin ranged from 62.5 to 86.1%, and the relative soil moisture of farmland was high in the marginal areas of basins and low in the central areas of basins and plateau areas. The relative soil moisture of farmland was high in the Tarim Basin and the Hexi Corridor, which are located in the northern Tianshan Mountains and the southern and northern Qilian Mountains, and was low from the northern Altun Mountains to the south of Lop Nor, the Turpan Depression, and the Tarbagatai Mountains. From 1992 to 2012, the annual average relative soil moisture of farmland in the continental river basins showed an increasing trend, with a growth rate of 0.57% yr−1. The variation tendency of the relative soil moisture of farmland was different in different river basins; the relative soil moisture showed a decreasing trend in the Mongolian Plateau and an increasing trend in other basin areas. The relative soil moisture of farmland increased in summer, spring, and winter, and decreased in autumn. The change in relative soil moisture of farmland was due to a combination of climatic factors, such as precipitation and temperature, as well as topography and glacial meltwater.

Miocene Range Growth Along the Altyn Tagh Fault: Insights From Apatite Fission Track and (U‐Th)/He Thermochronometry in the Western Danghenan Shan, China

AbstractThe left‐lateral strike‐slip Altyn Tagh fault that defines the northern margin of the Tibetan Plateau plays a crucial role in accommodating the Cenozoic deformation related to the growth of plateau. However, the slip history along the fault remains highly debated. Here we report new 14–16 Ma apatite fission track (AFT) and 9–11 Ma apatite (U‐Th)/He (AHe) data in the western Danghenan Shan, north Tibet. Age‐elevation relationships and AFT/AHe age differences suggest a period of rapid exhumation with an average rate of 0.1–0.3 km/Ma from 16 to 9 Ma for this area. Thermal history modeling indicates that this was preceded by accelerated exhumation between the late Oligocene and middle Miocene (~15 Ma). A northward increase in AFT ages and asymmetric topography across the western Danghenan Shan indicate that the uplift and exhumation are mainly controlled by the thrust fault along the southern flank of the western Danghenan Shan. As the thrust fault is a branch of the Altyn Tagh fault, the rapid exhumation probably represents onset of the transition along the Altyn Tagh fault from left‐slip motion to crustal shortening in the Dangnenan Shan region. Our findings show that the middle Miocene deformation is not only recorded in the middle and northern Qilian Shan but also in the southwestern portion of the Qilian Shan, which favors a synchronous middle‐Miocene deformation model for the entire Qilian Shan.

Holocene slip rate of the frontal thrust in the western Qilian Shan, NE Tibetan Plateau

SUMMARY The activities of frontal thrusts in the northern Qilian Shan are critical for understanding the deformation of the Qilian Shan and the northeastern Tibetan Plateau. In this study, we estimate the slip rate of the active Fodongmiao–Hongyazi thrust along the northern margin of the Qilian Shan. High-resolution satellite imagery interpretations and detailed field investigations suggest that the fault displaced late Pleistocene terraces and formed fresh prominent north-facing fault scarps. To quantify the slip rate of the fault, we measured the displacements along the fault scarps using an unmanned aerial vehicle system and dated the displaced geomorphic surfaces using optically stimulated luminescence (OSL) and 14C methods. The vertical slip rate of the fault is estimated at 1.0 ± 0.3 mm yr−1 for the western segment. The slip rates for two branches in the eastern segment are 0.3 ± 0.1 and 0.6 ± 0.1 mm yr−1. Using a fault dip of 40 ± 10°, we constrain the corresponding shortening rates to 1.4 ± 0.5 and 1.2 ± 0.4 mm yr−1, respectively. The rates are consistent with values over different timescales, which suggests steady rock uplift and northeastward growth of the western Qilian Shan. Crustal shortening occurs mainly on the range-bounding frontal thrust.

Constraints of new apatite fission-track ages on the tectonic pattern and geomorphic development of the northern margin of the Tibetan Plateau

The northern margin of the Qilian Shan is the northernmost edge of the Tibetan Plateau. The deformation timing and geomorphic development along the northern margin of the Qilian Shan are critical to understanding the dynamics of plateau growth. Although previous studies have suggested that thrusting along the middle and westernmost sections of the northern margin of the Qilian Shan began in the middle Miocene, the >600 km mountain range front may have experienced differential thrusting and exhumation history. We collected 11 apatite fission-track samples along an elevation transect to the south of Jiayuguan to unravel the cooling history and erosion variation of the northern margin of western section of the Qilian Shan. The apatite fission-track ages, fission-track lengths, and thermal modeling suggest a period of rapid exhumation initiated at ∼15–10 Ma along the northern margin of the Qilian Shan. The new fission-track ages can be divided into three domains with different mean ages. As the distribution of the fission-track ages is tightly confined by tectonic structures among them, we infer that tectonics plays the first-order control on erosion variations. Meanwhile, the strong linkage between erosion rates and glacier distribution may indicate that glacial buzzsaw exerts an important role in mountain building in the northern Qilian Shan. Additionally, combining this result with published data, we suggest that the northern margin of the Qilian Shan experienced a phase of synchronous deformation in the middle Miocene.

The distribution of active rock uplift in the interior of the western Qilian Shan, NE Tibetan Plateau: Inference from bedrock channel profiles

The distribution of rock uplift rates in the interior of the western Qilian Shan is not yet fully resolved, which limits our knowledge about the uplift pattern and landscape evolution in the northeastern Tibetan Plateau. The stream power incision model suggests that the spatially differential rock uplift rate usually can exert a primary control on channel gradient, thus providing a powerful approach to map the rock uplift pattern of active orogens. Here we analysed the river longitudinal profiles in the western Qilian Shan and found a spatially differential pattern with higher steepness indices along the parallel ranges (e.g. the Danghe Nanshan, Daxue Shan, Tuolai Shan, and the Northern Qilian Shan) but lower values in the intramontane basins. Comparison of steepness indices to variations in lithology and annual precipitation revealed limited correlation of channel gradient with lithologic resistance and climate in this landscape. We also observed no systematic relationship between steepness indices with drainage area, sediment flux, or channel concavities. We argued that the systematic changes in channel steepness indices are tectonically controlled rather than the consequence of variable sediment flux, climate, or lithology. Thus, the normalized channel steepness pattern of the western Qilian Shan could reflect that the rock uplift is largely restricted to mountain ranges bounded by active thrust faults and imply distributed Cenozoic crustal shortening. By comparing the NNE-SSW-directed elevation swath profiles and the channel steepness pattern, we concluded that tectonic uplift rate should be higher in the Northern Qilian Shan than in the internal ranges. Thus, our study provides geomorphic evidence to favor both distributed crustal shortening and Asian mantle lithosphere underthrusting as the primary mechanisms to explain the landscape evolution in the NE Tibetan Plateau.

Neogene Expansion of the Qilian Shan, North Tibet: Implications for the Dynamic Evolution of the Tibetan Plateau

AbstractThe Qilian Shan, at the northeastern frontier of the Tibetan Plateau, is a key area for studying the expansion mechanism of the Tibetan Plateau. Although previous thermochronology and paleomagnetic studies indicate Neogene northward expansion of the northern Qilian Shan, there is a distinct temporal gap in knowledge relative to the tectonic history of the southern Qilian Shan. This has hindered a complete understanding of the Cenozoic deformation pattern of the entire Qilian Shan. To study the growth history of the southern Qilian Shan, apatite fission track (AFT) data have been acquired from Zongwulong Shan and the Huaitoutala section. AFT thermal history modeling from the former shows a rapid cooling episode occurred at ~18–11 Ma, which is interpreted as marking the onset of intensive exhumation in the southern Qilian Shan. Within the Huaitoutala section, detrital grain up‐section shows progressively decreasing peak AFT ages followed by an age increase from midsection, implying that a sediment‐recycling event occurred at approximately 7 ± 2 Ma. Together with a shift in paleocurrent directions, this change marks the onset of Late Miocene deformation of the northern Qaidam Basin. Combined with previous studies on the deformation time of the Qilian Shan, our findings suggest that both the northern and southern Qilian Shan region grew outward synchronously in opposite directions during the Neogene. This resulted in the formation of a flower structure, which had an important impact on the deformation pattern of north Tibet. The synchronous outward expansion may have been triggered by the removal of mantle beneath north Tibet.

A fluvial record of fault-propagation folding along the northern Qilian Shan front, NE Tibetan Plateau

The kinematics of thrust fault is key to understand the deformation and uplift of mountain ranges. Along the northern front of the central Qilian Shan mountain range, the NE Tibet Plateau, a flight of terraces developed along the Red Sand River were deformed above a growing fault-propagation fold. Via luminescence and 10Be exposure dating, abandonment ages of the terrace surfaces were determined to be 100.9 ± 5.8 ka BP for T9, 88.7 ± 7.0 ka BP for T8, and 68.8 ± 7.6 ka BP for T7. Terrace profiles measured by differential GPS across the monocline were used to constrain the recent fold deformation, and the trishear model was applied to estimate the fault slip rate and thrust history. We thus derived an average slip rate of 1.3 ± 0.1 mm/a for the Heihekou (HHK) fault since ~90 ka BP. In addition, our results imply that the onset of the HHK fault occurred at 0.8–1.4 Ma, indicating a recent northeastward propagation of the active thrust faulting of the North Qilian Shan Fault (NQF). Our results also indicate uniformly distributed slip along the western, central, and eastern segments of the north Qilian Shan. Due to the different thrust kinematics along the NQF, the uplift rate is not uniformly distributed along the hanging wall of the thrust.

Mid-Miocene uplift of the northern Qilian Shan as a result of the northward growth of the northern Tibetan Plateau

The northern Tibetan Plateau, north of the Qaidam Basin and south of the Hexi Corridor (China), consists of a series of WNW- to NW-trending elongated mountain ranges. Deciphering the time-space deformation pattern of these ranges is central to understanding the mechanism of plateau formation and to the controversial issue of whether Tibet has undergone progressive northward growth or synchronous growth since the India-Eurasia collision. Here, we report new constraints on the timing of accelerated uplift of the Tuolai Shan, one of the elongated mountain ranges in the northern Tibetan Plateau. New apatite fission-track data from an elevation transect in the Tuolai Shan provide a definitive tie to rapid cooling that began at 17–15 Ma. We attribute this rapid cooling to accelerated exhumation resulting from thrusting in the hanging wall of the Haiyuan fault in response to progressive northward growth of the plateau. Combining these fission-track data and the published geologic, sedimentological, and thermochronologic data from the northern Qilian Shan and Hexi Corridor, we propose a progressively north-northeastward growth model for the northernmost part of Tibet, suggesting that deformation in the inner Qilian Shan occurred synchronously in the middle Miocene, and subsequently, increasingly further north.

Provenance analysis of the Yumen Basin and northern Qilian Shan: Implications for the pre-collisional paleogeography in the NE Tibetan plateau and eastern termination of Altyn Tagh fault

Understanding the pre-collisional paleogeography in the NE Tibetan plateau provides insights into the growth mechanisms of the northern portion of the plateau in the Cenozoic. We conducted sandstone petrography analysis and determined U-Pb ages for detrital zircons from Cretaceous sandstone from the Yumen Basin and the northern Qilian Shan. Cretaceous strata in the northern Yumen Basin yield a unimodal age population at 290–240 Ma that indicates primary derivation from Bei Shan. Cretaceous strata in the westernmost Yumen Basin contain zircons of 2.6–2.2 Ga, 2.1–1.7 Ga, 1.4–0.7 Ga, 440–380 Ma and 300–230 Ma, suggesting source derivation from both the Qilian Shan and Bei Shan. Within the northern Qilian Shan, Cretaceous strata yield age populations of 2.8–2.3 Ga, 2.1–1.2 Ga, 480–380 Ma and ca. 270 Ma, indicating derivation from the Qilian Shan. Sandstone composition results show that a sample from the northern Qilian Shan contains more lithic fragments and plots in the recycled orogen field of the quartz-feldspar-lithics (QFL) diagram, while samples from Yumen Basin are more quartz-rich and plot close to the continental block field of the QFL diagram. This compositional difference corresponds to source variation, consistent with the detrital zircon record. Combined with existing sedimentology and low-temperature thermochronology datasets, we suggest the presence of Cretaceous topographic relief in the Bei Shan and Qilian Shan prior to India-Asia collision. Considering >300 km post-Cretaceous left-lateral offset along the Altyn Tagh Fault (ATF) and the consistently similar detrital zircon ages spectra of the samples from the Cretaceous to late Oligocene strata in the Yumen Basin, we infer the paleogeography in the NE Tibetan plateau has been similar from the late Cretaceous to the late Oligocene with ATF termination in the western Yumen Basin instead of having been linked to strike-slip faults in the Alxa or other regions to the east since its initiation.

Differential rock uplift along the northeastern margin of the Tibetan Plateau inferred from bedrock channel longitudinal profiles

Bedrock channel profile analysis is a robust reconnaissance tool for examining the rates and patterns of deformation in active orogens. Here, steepness and concavity indices were extracted from bedrock channels draining the northern Qilian Mountains based on the universal shear stress incision model. The profile analysis reveals a systematic spatial distribution of steepness and concavity indices. The average steepness index for channels draining the western part is approximately 1.6 times higher than that for the eastern part. Through empirical calibration of erosion coefficient K, the influence of channel width, rock strength, discharge and sediment flux are identified, and an approximately 1.1–1.2 times difference in K values is found between the west and east under the hypothesis that lithologic resistance and sediment flux have no contribution. Correlating profile analyses with parameter calibration, an approximate 2–3 times increase in the rock uplift rate is found from the eastern part to the western part of the orogen. The integration of this study with previous work in the eastern part of the mountains indicates that cross-strike (2–4 times) and along-strike (2–3 times) differential rock uplift both exist in the northeastern margin of the Tibetan Plateau. In addition, the abnormal values of concavity are apparently associated with the channels transecting large thrust fault zones. Thus, the shortening deformation may play a dominant role in controlling the differential rock uplift through folding and thrusting of the northeast-directed thrust faults across the northeastern margin of the Tibetan Plateau.

Early Paleozoic arc magmatism and metamorphism in the northern Qilian Block, western China: Petrological and geochronological constraints

The Datong–Menyuan Complex, located in the northern margin of the Qilian Block, is composed dominantly of high‐grade metamorphic rocks covered by a Paleozoic–Mesozoic sedimentary sequence. In this study, we present petrographic observation, conventional thermobarometry and P–T pseudosection modelling, whole‐rock major and trace geochemistry, zircons U–Pb dating and Hf isotopic data together, to reveal the relationship between the magmatism and metamorphism along the northern margin of the Qilian Block. The anticlockwise P–T paths of gneiss and amphibolites are obtained using the garnet isopleths thermobaromety combined with phase equilibria modelling. The P–T pseudosection also shows that the gneiss and amphibolite underwent similar retrograde metamorphism with slightly different peak metamorphic conditions at ~720 °C and ~6.4 kbar for the former and ~700 °C and ~7.1 kbar for the latter. This indicates the Datong–Menyuan Complex recorded an amphibolite–granulite‐facies metamorphism. The zircon U–Pb analyses of 3 intrusive rocks including granitic dike, diorite, and granite are dated at ca. 499.8 ± 4.3, 495.9 ± 3.3, and 505.5 ± 3.2 Ma, and 3 representative metamorphic rocks underwent contemporaneous metamorphism at ca. 498.9 ± 4.1, 504.4 ± 3.9, and 499.3 ± 2.9 Ma. Zircon Hf isotopic analyses show that 505‐Ma zircons from the granitoid have positive εHf(t) values ranging from +8.5 to +12.8, indicating a depleted mantle source. Based on the penecontemporaneous magmatic and metamorphic event, we suggest that the southward subduction of the north Qilian Ocean triggered the activity of mantle‐derived magma and coeval metamorphic event. A subsequent compression is attributed to crustal thickening, indicating counter clockwise P–T paths. Combining these data with previous studies, it suggests paired metamorphic belts: the penecontemporaneous high‐temperature metamorphic belt related to an arc along the northern Qilian Block and high‐pressure/low‐temperature metamorphic belt existed in the north Qilian suture.