Northern Qilian Mountains Research Articles

Finer topographic data improves distribution modeling of Picea crassifolia in the northern Qilian Mountains

Finer topographic data improves distribution modeling of Picea crassifolia in the northern Qilian Mountains

A Glyptagnostus reticulatus trilobite faunule from the Cambrian of the Northern Qilian Mountains, northwest China, and its paleogeographical implications

AbstractA Glyptagnostus reticulatus (Angelin, 1851)–bearing trilobite assemblage has been found from an unnamed Cambrian formation in the Northern Qilian Mountains area, of which geographical placement in the Cambrian is contested. Glyptagnostus reticulatus is a biostratigraphic indicator of the Furongian Series and Paibian Stage, and three agnostoid and six polymerid taxa from the Changgou section, Daliang area are described herein, along with conspecific forms from the nearby Chuancigou section. This well-preserved assemblage shows strong taxonomic affinity with northwestern Queensland, Australia, and western Hunan–eastern Guizhou, China, and likely comes from deep outer-shelf to slope setting associated with the Northern Qilian arc. It is consistent with other arguments that during the Cambrian, the Northern Qilian arc, along with the Hexi Corridor of the Alxa terrane, were more closely allied to South China than to North China.

Lake-level history of Qinghai Lake on the NE Tibetan Plateau and its implications for Asian monsoon pattern - A review

The paper presents a comprehensive review on lake-level history of Qinghai Lake since marine isotope stage (MIS) 5, after re-examining the sedimentary and geomorphological nature of highstand deposits and re-evaluating the published dating results (318 luminescence ages and 17 14C ages) of shorelines, near shoreline, lacustrine, alluvial, and aeolian deposits. It is concluded that 1) Three lake shoreline sets were classified in the Qinghai Lake Basin with the corresponding elevations of 3220–3260, 3206–3215 and 3194–3205 m. 2) 14C dating on lake sediments is restricted by the upper dating limit of ∼35 thousand years (ka) BP in practice, availability of dating materials and reservoir effect, and that quartz optically stimulated luminescence (OSL) ages older than 80–60 ka may be underestimated while feldspar post-infrared infrared stimulated luminescence (pIRIR) ages could be reliable for dating these older samples. 3) Four highstands were detected by ages. The highest lake-levels at elevations of 3220–3260 m with 26–66 m above the present lake-level occurred in MIS 5, and highstands also occurred in MIS 3 (∼42–38 ka), the last deglaciation (∼16–12 ka), and the Holocene (∼6–5 ka) with similar heights at elevations of ∼3200–3209 m. During the Holocene, lake-levels were relatively low in the early Holocene (∼11–8 ka), increasing substantially after ∼8 ka and reaching the highest lake-levels (elevation of 3203.1 m) at ∼6–5 ka, then decreasing with an interruption by a highstand (elevation of 3202.4 m) at ∼1.8 ka. 4) Shoreline-based lake-level variations are consistent with major climatic proxies (e.g., δ13Corg and %thaum) of drilling cores from the lake, regional aeolian activity/pedogenesis and climate change. 5) Compared with the representative lake-levels and moisture records in the Westerlies and East Asia summer monsoon (EASM) dominances, it is suggested that lake-level variations in the southern Qilian Mountains exhibited simultaneity with changes in the intensity of EASM, while those in the northern part were synchronous with the moisture records in arid central Asia, suggesting that EASM did not extend further to the northern Qilian Mountains where the Westerlies prevailed.

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.

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.

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.

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.

The verification of millennial-scale monsoon water vapor transport channel in northwest China

Long-term changes of the Asian summer monsoon water vapor transport play a pivotal role in the variability of monsoon precipitation. Paleo-climate simulations have shown that there is an important monsoon vapor transport channel in western China. Previous studies mostly focused on the correlation between monsoon precipitation and intensity. Little research has been done on the verification of the water vapor channel. Compared with speleothem and lacustrine systems, the hydrological cycle of land surface sediments is more directly related to the monsoon water vapor. In this study, we used carbonate δ18O and organic matter δ13C of the surface eolian sediments from the piedmont of the northern Qilian Mountains to verify the monsoon water vapor on the Holocene millennial-scale. Two surface sedimentary sections were selected to study paleo-monsoon water vapor transport. Proxy data, including carbonate δ18O and organic matter δ13C of surface eolian sediments, as well as total organic matter and carbonate content were obtained from the two eolian sections. We also synthesized transient simulations of the CCSM3 and the Kiel climate models. The PMIP 3.0 project and TRACE isotopic simulations were also compared with the reconstructed monsoon water vapor transport. Our findings indicate that the strength of the Holocene Asian summer monsoon is consistent with the water vapor transport in western China that has significant impacts to long-term monsoon precipitation in northern China. This study verifies a significant millennial-scale correlation between the monsoon strength and monsoon water vapor transport intensity along the eastern Qinghai–Tibet Plateau.

Early Holocene environment at a key location of the northwest boundary of the Asian summer monsoon: a synthesis on chronologies of Zhuye Lake, Northwest China

The intensified monsoon increases summer rainfall and creates wet conditions in the Asian summer monsoon region during the early Holocene. Along with millennial-scale changes of the monsoon intensity, it is still unclear whether the boundary of the monsoon region changes according to monsoon variability. Investigations into the early Holocene environment in monsoon marginal zones are crucial for understanding the monsoon boundary changes. Zhuye Lake is located at the northwest edge of the Asian summer monsoon, the northern Qilian Mountains, which are less affected by modern summer monsoon water vapor. Previous studies have reached different conclusions regarding the early Holocene climatic and environmental changes based on different dating methods (14C and OSL (optically stimulated luminescence)) and materials (shells, carbonate, pollen concentrates and bulk organic carbon). In this study, we synthesized 102 14C dates and 35 OSL dates from ten Holocene sedimentary sections and ten paleo-shorelines in the lake basin. A comparison between ages from different dating methods and materials generally shows that carbon reservoir effects are relatively slight in Zhuye Lake while the disordered chronologies are mainly related to the erosion processes and reworking effects. In addition, proxy data, including lithology, pollen, total organic carbon and carbonate, were collected from different sites of Zhuye Lake. According to the new synthesis, the early Holocene environment was relatively humid, associated with high runoff and lake water levels. The result indicates that the monsoon boundary moves to the north during the period of the intensified monsoon. A typical arid-area lake was formed during the mid-Holocene when carbonate accumulation and high organic matter contents were the main features of this period. The lake retreated strongly during the late Holocene, showing a drought trend. Overall, the lake evolution is generally consistent with the Holocene Asian summer monsoon change, showing the monsoon influence to monsoon marginal zones.

Spatial pattern of soil organic carbon in desert grasslands of the diluvial-alluvial plains of northern Qilian Mountains

The soil properties in arid ecosystems are important determinants of vegetation distribution patterns. Soil organic carbon (SOC) content, which is closely related to soil types and the holding capacities of soil water and nutrients, exhibits complex variability in arid desert grasslands; thus, it is essentially an impact factor for the distribution pattern of desert grasslands. In the present study, an investigation was conducted to estimate the spatial pattern of SOC content in desert grasslands and the association with environmental factors in the diluvial-alluvial plains of northern Qilian Mountains. The results showed that the mean values of SOC ranged from 2.76 to 5.80 g/kg in the soil profiles, and decreased with soil depths. The coefficients of variation (CV) of the SOC were high (ranging from 48.83% to 94.67%), which indicated a strong spatial variability. SOC in the desert grasslands of the study region presented a regular spatial distribution, which increased gradually from the northwest to the southeast. The SOC distribution had a pattern linked to elevation, which may be related to the gradient of climate conditions. Soil type and plant community significantly affected the SOC. The SOC had a significant positive relationship with soil moisture (P<0.05); whereas, it had a more significant negative relationship with the soil bulk density (BD) (P<0.01). However, a number of the variations in the SOC could be explained not by the environmental factors involved in this analysis, but rather other factors (such as grazing activity and landscape). The results provide important references for soil carbon storage estimation in this study region. In addition, the SOC association with environmental variables also provides a basis for a sustainable use of the limited grassland resources in the diluvial-alluvial plains of northern Qilian Mountains.

Climatic and environmental change in Yanchi Lake, Northwest China since the Late Glacial: A comprehensive analysis of lake sediments

Modern climate research has shown that the Asian summer monsoon water vapor transport is limited to the eastern part of the Qilian Mountains. On the Holocene millennial-scale, whether the northwest boundary of the summer monsoon varies according to climate change is a key scientific issue. Yanchi Lake is located in the northern Qilian Mountains and the middle of the Hexi Corridor, where the modern climate is less affected by the Asian summer monsoon. It is a key research area for examining the long-term variations of the Asian summer monsoon. Paleoclimatic data, including AMS 14C dates of pollen concentrates and bulk organic carbon, lithology, grain-size, mineral composition and geochemical proxies were acquired from sediments of Yanchi Lake. The chronological results show that the lower part of the lacustrine section is formed mainly in the Late Glacial and early Holocene period, while the proxies’ data indicate the lake expansion is associated with high content of mineral salts. The middle part of this section is formed during the transitional period of the early and middle Holocene. Affected by the reworking effect, the pollen concentrates AMS 14C dates from the middle part of the section are generally older than those from the lower part. Since the mid-Holocene, Yanchi Lake retreated significantly and the deposition rate dropped obviously. The Yanchi Lake record is consistent with the Late Glacial and Holocene lake records in the Qinghai-Tibet Plateau and the climatic records in typical monsoon domain, which indicate the lake expansion and the strong Asian summer monsoon during the Late Glacial and early Holocene. The long-term monsoonal pattern is different from the lake evolution in Central Asia on the Holocene millennial-scale. This study proves the monsoon impacts on the northwestern margin of the summer monsoon, and also proves the fact that the northern boundary of the summer monsoon moves according to millennial-scale climate change.

Variations of temperature and precipitation of snowmelt period and its effect on runoff in the mountainous areas of Northwest China

Water resources in the arid land of Northwest China mainly derive from snow and glacier melt water in mountainous areas. So the study on onset, cessation, length, temperature and precipitation of snowmelt period is of great significance for allocating limited water resources reasonably and taking scientific water resources management measures. Using daily mean temperature and precipitation from 8 mountainous weather stations over the period 1960–2010 in the arid land of Northwest China, this paper analyzes climate change of snowmelt period and its spatial variations and explores the sensitivity of runoff to length, temperature and precipitation of snowmelt period. The results show that mean onset of snowmelt period has shifted 15.33 days earlier while mean ending date has moved 9.19 days later. Onset of snowmelt period in southern Tianshan Mountains moved 20.01 days earlier while that in northern Qilian Mountains moved only 10.16 days earlier. Mean precipitation and air temperature increased by 47.3 mm and 0.857°C in the mountainous areas of Northwest China, respectively. The precipitation of snowmelt period increased the fastest, which is observed in southern Tianshan Mountains, up to 65 mm, and the precipitation and temperature in northern Kunlun Mountains increased the slowest, an increase of 25 mm and 0.617°C, respectively, while the temperature in northern Qilian Mountains increased the fastest, increasing by 1.05°C. The annual runoff is also sensitive to the variations of precipitation and temperature of snowmelt period, because variation of precipitation induces annual runoff change by 7.69% while change of snowmelt period temperature results in annual runoff change by 14.15%.

River islands, refugia and genetic structuring in the endemic brown frog Rana kukunoris (Anura, Ranidae) of the Qinghai‐Tibetan Plateau

Frequently, Pleistocene climatic cycling has been found to be the diver of genetic structuring in populations, even in areas that did not have continental ice sheets, such as on the Qinghai-Tibetan Plateau (QTP). Typically, species distributed on the plateau have been hypothesized to re-treat to south-eastern refugia, especially during the Last Glacial Maximum (LGM). We evaluated sequence variation in the mitochondrial DNA gene Cytb and the nuclear DNA gene RAG-1 in Rana kukunoris, a species endemic to the QTP. Two major lineages, N and S, were identified, and lineage N was further subdivided into N1 and N2. The geographical distribution and genealogical divergences supported the hypothesis of multiple refugia. However, major lineages and sublineages diverged prior to the LGM. Demographical expansion was detected only in lineage S and sublineage N2. Sublineage N1 might have survived several glacial cycles in situ and did not expand after the LGM because of the absence of suitable habitat; it survived in river islands. Genetic analysis and environment modelling suggested that the north-eastern edge of QTP contained a major refugium for R.kukunoris. From here, lineage S dispersed southwards after the LGM. Two microrefugia in northern Qilian Mountains greatly contributed to current level of intraspecific genetic diversity. These results were found to have important implications for the habitat conservation in Northwest China.

Electrical resistivity structures between the Northern Qilian Mountains and Beishan Block, NW China, and tectonic implications

Controversies exist on the tectonic models at the northern margin of the Qinghai-Tibetan Plateau. New magnetotelluric (MT) data, with a bandwidth of 0.001–5827.5s, were collected along one profile starting from the Northern Qilian Mountains in the southern end, crossing the Hexi Corridor, the Kuantan Shan-Hei Shan uplift, the Huahai Basin, and coming into the Beishan Block. Two-dimensional (2D) and three-dimensional (3D) resistivity models of the data are derived using magnetotelluric inversion codes. The 2D models, and the electrical structures along the profile extracted from the 3D models, are comparable and complementary in defining reliable scales and shapes of anomalies in electrical resistivity. The crust-upper mantle structures beneath the northern plateau are characterized by two imbricated southward underthrusted blocks of continental crust or lithosphere. The Northern Qilian Fault is interpreted as a gently south-dipping décollement in the upper crust along the top of a high resistivity body below the Northern Qilian Mountains. The two high resistivity bodies below the Kuantan Shan-Hei Shan uplift are disconnected; the upper crustal one may connect with the high resistivity basement below the Northern Qilian Mountains in that the attitudes and deformations of the strata at these areas are coherent; whereas the lower crust-upper mantle one may be the cold lithospheric materials of the block north to the Hexi Corridor and that forms a south-dipping boundary between the Hexi Corridor and its northern neighbors. The observed high electrical resistivity beneath the Beishan Block provides further support for the notion that the Beishan Block should be rigid as a whole; only in this manner could it transfer stress from south to north. The northern boundary of the Qinghai-Tibetan Plateau is below the Kuantan Shan-Hei Shan Fault and underthrusting of continental crust or lithosphere is the main cause for the thickening of the plateau crust in the north.

Rainfall history for the Hexi Corridor in the arid northwest China during the past 620 years derived from tree rings

AbstractWe present a 620‐year long ring width record from the middle Qilian Mountains, where it is presently controlled by the Westerlies. The chronology was developed from Qilian juniper (Sabina przewalskii) growing at sites near the western distribution limit of the species in the northern Qilian Mountains, Gansu Province, China. A linear regression model between ring width and annual (July to June) precipitation accounts for 34.9% of the observed instrumental precipitation variance during the period 1952 to 2007. Spatial correlation analyses between the reconstruction and gridded precipitation data shows that the annual precipitation reconstruction captures regional climatic variations over the Qilian Mountains and the nearby Hexi Corridor. We also show the scaled standard chronology adjusted to the mean and variance of the instrumental data. Relatively wet periods are identified for AD 1390–1413, 1425–1450, 1530–1649, 1792–1920, 1937–1949 and 1980–1985. Dry conditions prevailed during AD 1414–1424, 1451–1529, 1650–1791, 1921–1936, 1950–1979 and 1986–2007. In comparison with the regression‐based reconstruction, the scaled reconstruction indicates considerably wetter conditions during 1390–1413, 1425–1450, 1570–1630 and 1790–1920. The interval AD 1451–1529 was the most intense and longest drought epoch in the Hexi Corridor over the past six centuries. This drought was not only recorded in the Qilian Mountains but also occurred in northern and eastern China. It might be caused by a substantial weakening of the Asian summer monsoon induced by the joint effects of solar and volcanic activities at that time. Our results also suggest that the Hexi Corridor was under the control of the Asian monsoon circulation on inter‐decadal to centennial timescales in the past centuries. Copyright © 2010 Royal Meteorological Society

Geology, Geochemistry and Minerogenesis of the Shijuligou Zinc–Copper Deposit in Gansu, China

Abstract:The Shijuligou deposit was separated by an arcuate ductile shear zone cross the center of the deposit region, resulting in the difference between the southern and northern ore bodies. The lead (Pb) isotopic data of ores of the Shijuligou copper deposit have averages of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb in 17.634, 15.444, and 37.312, respectively. It has been shown that ore‐forming metals originated from intrusive and extrusive rocks in the upper part of ophiolites. The sulfur isotopic data of pyrite and chalcopyrite in the northern part change from +7.61‰ to +8.09‰ and +4.95‰ to +8.88‰ in the southern part. Isotopes of δ18O in the Shijuligou copper deposit are between +11.1‰ and +18.6‰, with the calculated δ18OH2O at +0.65‰. It is suggested that the mineralized fluid is a mixture of magma fluid, meteorological water, and seawater through circulating and leaching metals from the volcanic rocks. The zircon uranium‐lead (U–Pb) dating of gabbro is 457.9±1.2 Ma, and the lower crossing age of the discordant and concordia curves of pyroxene spilite of zircon is 454±15 Ma. It is indicated that the Shijuligou deposit formed in a new ocean crust (ophiolite) of the back‐arc basin in the late Ordovician. Mineralization should occur in the intermittence period after strong volcanic activity, and the age should be the late Ordovician. Moreover, the mineralization of ophiolite‐hosted massive sulfide deposits in the ancient orogenic belt of the late Ordovician in the northern Qilian Mountains was controlled by the primary fault/fracture, with the forming of a metallogenic hydrothermal system by a mixture of volcanic magma fluid and seawater, which circularly leached the metallogenic metals from the volcanic rocks, resulting in their accumulation. The ore bodies were transformed with morphology and metallogenic elements. Jasperoid is an important sign for prospecting such deposits. There were many island arcs in the continent of China. This study provides evidence for understanding and exploration of ophiolite‐hosted massive sulfide deposits in western China, especially in the area of northern Qilian Mountains.

Lawsonite blueschist in Northern Qilian, NW China: P–T pseudosections and petrologic implications

The lawsonite-bearing blueschist from the Nine Spring area, Sunan, Northern Qilian Mountains, is a unique well-preserved lawsonite blueschist terrane in China. Based on the dominant Ca–Al hydrosilicate, the lawsonite-bearing blueschists were divided into lawsonite-bearing pumpellyite blueschist, lawsonite blueschist and epidote blueschist with mineral assemblages of Gln (>40%) + Pmp (30%) + Lws (3%) + Chl (10%) + Ab (8%) + Qtz (5%) + Cal (1%), Gln (35–40%) + Lws (35–40%) + Chl (10%) + Ab (10%) + Grt (1–2%) + Zo (<2%) + Qtz (<1%) and Gln (30–35%) + Zo/Czo/Ep (∼30%) + Chl (15%) + Ab (15–15%) + Grt (2%) + Qtz (<2%) and Mag (1%), respectively. P–T pseudosections using internally consistent mineral thermodynamic database and Domino/Theriak software yield P–T conditions of 320–350 °C, 0.75–0.85 GPa for lawsonite-bearing pumpellyite blueschist, 335–355 °C, 0.8–0.95 GPa for lawsonite blueschist and 345–375 °C, 0.75–0.85 GPa for epidote blueschist, respectively. The transition from the lawsonite blueschist facies to epidote blueschist facies is defined by the reaction Lws + Gln = Chl + Czo + Ab + Qtz + H 2O. These high-P rocks of the Northern Qilian Mountains have undergone prograde metamorphic P–T path with no retrograde overprinting suggesting a fast subduction and exhumation which is consistent with channel flow model.

40Ar/39Ar ages of blueschist facies pelitic schists from Qingshuigou in the Northern Qilian Mountains, western China

Abstract Pelitic schists from Qingshuigou in the Northern Qilian Mountains of China contain mainly glaucophane, garnet, white mica, clinozoisite, chlorite and piemontite. Isotopic age dating of these schists provides new constraints on the formation of the high‐grade blueschists at Qingshuigou. White mica 40Ar/39Ar ages range from 442.1 to 447.5 Ma (total fusion age of single grain) and from 445.7 to 453.9 Ma (integrated age of white mica concentrates). These ages (442.1–453.9 Ma) represent the peak metamorphic ages or cooling ages of the blueschists during exhumation shortly after peak metamorphism. The 40Ar/39Ar dates in the present study are similar to ages previously reported for eclogites and blueschists in the area; this suggests that both the eclogites and pelitic sediments underwent high‐grade metamorphism during the same subduction event. From this chronological evidence and the presence of well‐developed Silurian remnant‐sea flysch and Devonian molasse, it is concluded that the Northern Qilian Ocean had closed by the end of the Ordovician, and rapid orogenic uplift followed in the Devonian.