Spatial-temporal Variation Characteristics of Water Quality in the Lower Reaches of the Nenjiang River

As an important river in the western part of Jilin Province,the lower reach of the Nenjiang River is an important wetland water source conservation area in Jilin Province.Within the watershed,it governs the Momoge Wetland,the Xianghai Wetland,and the Danjiang Wetland in Jilin Province.The main problem in the lower reaches of the Nenjiang River is the uneven distribution of water resources in time and space,and the intensification of land salinization.Zhenlai County and Da an City in the Nenjiang River Basin have sufficient surface water resources,with surface water as the drinking water source.Baicheng City and Tongyu County have scarce surface water resources,and both use groundwater as their domestic water source.The main polluted section in the basin is the Xianghai Reservoir,and the annual water quality evaluation is Class V.However,the water quality of the Tao er River,the main stream of the Nenjiang River,is significantly better than that of the Xianghai Reservoir.In order to better study the water environmental pollution situation in the Nenjiang River basin,monitoring data from five sections of non seasonal rivers in the basin from 2012 to 2021 were selected for studying water quality.This in-depth exploration of the water pollution status and river water quality change trends in the Nenjiang River basin is of great significance for future rural development,agricultural pattern transformation,and the promotion of water ecological civilization construction.

Precipitation cycles in the middle and lower reaches of the Yellow River （1736-2000）

Based on the long-term precipitation series with annual time resolution in the middle and lower reaches of the Yellow River and its four sub-regions during 1736-2000 reconstructed from the rainfall and snowfall archives of the Qing Dynasty, the precipitation cycles are analyzed by wavelet analysis and the possible climate forcings, which drive the precipitation changes, are explored. The results show that： the precipitation in the middle and lower reaches of the Yellow River has inter-annual and inter-decadal oscillations like 2-4a, quasi-22a and 70-80a. The 2-4a cycle is linked with El Nino events, and the precipitation is lower than normal year in the occurrence of the El Nino year or the next year; for the quasi-22a and the 70-80a cycles, Wolf Sun Spot Numbers and Pacific Decadal Oscillation （PDO） coincide with the two cycle signals. However, on a 70-80a time scale, the coincidence between solar activity and precipitation is identified before 1830, and strong （weak） solar activity is generally correlated to the dry （wet） periods; after 1830, the solar activity changes to 80-100a quasi-century long oscillation, and the adjusting action to the precipitation is becoming weaker and weaker; the coincidence between PDO and precipitation is shown in the whole time series. Moreover, in recent 100 years, PDO is becoming a pace-maker of the precipitation on the 70-80a time scale.

Community Structure of the Phytoplankton and Its Relationship with Environmental Factor in Lower Reaches of the Yellow River

[ Objective] The research aimed to study community structure of the phytoplankton and its relationship with environmental factor in lower reaches of the Yellow River. [ Method] From April to June in 2011, phytoplankton was conducted qualitative and quantitative researches in Lijin in lower reaches of the Yellow River. By combining population characteristics of the phytoplankton, its＇relationship with environmental factor was dis- cussed. [ Result] 114 species and 5 phylum of phytoplankton were observed in the whole river section. The abundance of phytoplankton was 0.90 ×104 -303.35×104 ind/m3, and mean was 27.20 ×104 ind/m3. The biomass of phytoplankton was 0.008-5.890 mg/m3, and mean was 0.641 mg/m3. Shannon-Weaver diversity index, Pielou evenness index and Simpson＇s diversity index were respectively 1.59, 0.36 and 0.55. SPSS data analysis indicated that transparency was the uppermost environmental factor affected phytoplankton abundance. Phytoplankton abundance had posi- tive correlation with transparency, and the regression equation was y= -393.8 ＋115.5×-11 .lx2 ＋0.35x3（n=18, R2 =0.952, F=92.665, P〈0.01 ）. [ Conclusion] The community structure of phytoplankton was unstable, and transparency was the uppermost environmental factor affected phyto- plankton abundance in lower reaches of the Yellow River.

Dike Breach Risks of Different Flood Conditions in the Lower Yellow River

Based on the analysis of hanging rivers' actuality in the lower Yellow River and researches related to the evaluation of dike breach risks,it is put forward that the influencing factors of dike beach risks in the lower Yellow River should involve four aspects,the flow and sediment movement,the regional crustal stability,the variation of river regime and the stability of river dikes.With this,the evaluation indexes system of dike breach risks is established,and with the support of geographic information systems technology,the model of multi-hierarchical fuzzy comprehensive judgment is applied to estimate the dike beach risks of the hanging rivers in the lower Yellow River under different flood conditions.The evaluation results of dike breach risks show the following distributing regularities of dike breach risks in the lower Yellow River:(1) Dike breach risks increase with the increase of the flood.(2) Dike breach risks decrease with the changes of river patterns along the channel.(3) There are great risks of dike breach in the wandering reaches,and it is relatively higher in the south bank than in the north in wandering reaches.(4) There is a higher dike breach risk in the north bank than in the south in winding reaches.Simultaneously,the evaluation results manifest that the evaluation indexes system established from the flow and sediment movement,the regional crustal stability,the variation of river regime and the stability of river dikes can represent the actual situation of the lower Yellow River more comprehensively.The application of multihierarchical fuzzy comprehensive judgment can preferably resolve the problem of hanging river dike breach,which has numerous influencing factors and complicated functionary mechanisms.The applications of geographic information systems technology with powerful spatial analysis functions make dike beach risks quantificationally displayed in different spatial positions,and reflect the differences of dike beach risks in different spatial positions of the channel in the lower Yellow River.

Exploring Regional Ecological Cultures in the Upper Reaches of the Yellow River in the Ming and Qing Dynasties from the Perspective of "Top Eight Views"

By studying typical "Top Eight Views" in cities like Xining,Lanzhou and Yinchuan in the upper reaches of the Yellow River,mountain and water landscapes in the study area were analyzed in terms of natural ecology;bridges,canals,ferries,pavilions,mansions,towers,temples,historical sites,production and living landscapes were analyzed in terms of humanistic ecology.On this basis,"Top Eight Views" in the upper reaches of the Yellow River were compared and the following conclusions obtained:mountain landscapes were distributed in the whole region,waterscapes unevenly valued in different cities,bridges,canals and ferries mostly located in Ningxia,temples commonly found in all cities,garden arts introduced into cities,traditional cultures carried forward by historical relics.Then the relationship between "Top Eight Views" and regional eco-cultures was analyzed,and it was proposed that humanistic and cultural connotations of these "Top Eight Views" contributed a lot to beautifying urban environment,satisfying needs of the locals' spiritual life,carrying forward local history and culture,enriching urban cultural connotations,expanding living spaces of local residents and improving integrated functions of cities."Top Eight Views" culture complied with the construction gist of urban ecological cultures,and was of great referential value for the construction of urban ecological cultures in the upper reaches of the Yellow River,and also the healthy,scientific and sustainable development of local cities.

Creation of "Regional Characteristic" in Planning and Design of Farmers' Residential Area at the South of Lower Reaches of the Yangtze River

After analysis of location feature of the south of lower reaches of Yangtze River and its construction of urban and rural integration,the paper pointed out harmonious combination between natural and artificial factors had been neglected in planning and design of farmers' residential area at the south of lower reaches of Yangtze River,"regional characteristic" losing,residential area in the form of "city community" and buildings in European style.In view of these problems,relevant planning and design thoughts and methods had been proposed as to how to create "regional characteristic" from the perspective of planning,architecture and landscape design.It discussed with emphasis the importance of construction base type and combination of environment with residential area construction;inspirations and design methods obtained from traditional architectures;and the content of landscape overall planning and specific design.It was hoped to enlighten designers to shoulder social and historical responsibility,make exploration unremittingly,and construct beautiful homelands for people.

Natural Disasters of the Lower Reaches of Yellow River during the Longshan Period——The Natural Background of Dayu's Flood Control

It is very important to study the archaeological culture and origin of civilization in ancient China.The changes of the channels in the lower reaches of the Yellow River in the prehistoric period are part of the natural environmental background of the development of the ancient civilization in that area to be explored.This paper presents a series of legends,indications,scientific evidence,and macroscopic geographical background information of the evolution in the lower reaches of the Yellow River during the Longshan period.At first the river flowed from Northern Henan and Hebei to southwestern Shandong Province and Northern Anhui–Jiangsu provinces,and the mainstream of the Yellow River changed from the southeast to return to the north and flowed into the Bohai Sea in the late Longshan Period.During this period,floods were frequent.Various ethnic groups in the northern China plains suffered natural disasters and made great migrations which also contributed to the ethnic exchanges and integration.The people of the Central Plains made more dynamic adjustments in the relationship between mankind and the land by primitively escaping from the water and self–defensively controlling the rivers then to maintaining the local ecological environment by large–scale flood control measures,which promoted the settlement of Shandong,Henan,Jiangsu and Anhui provinces,the urban cultural development,and social evolution.Based on these events,the culture symbol of Dayu's Flood Control could be formed.

Changes in groundwater levels and the response of natural vegetation to transfer of water to the lower reaches of the Tarim River

Restoration and reconstruction of the degraded Tarim River ecosystem is an important challenge. A goal of an ecological water conveyance project is to protect and restore the natural vegetation in the lower reaches of Tadm River by transferring water from Bosten Lake, through the river channel, to the lower reaches. This study describes the changes in groundwater depth during the water transfer and the respondence of riparian vegetation to alterations in groundwater levels. The results indicate that groundwater depth along the Tarim River channel has a significant spatial-temporal component. Groundwater levels closest to the river channel show the most immediate and pronounced changes as a response to water transfer while those further away respond more slowly, although the observed change appears to be longer in duration. With a rise in the groundwater level, natural vegetation responded with higher growth rates, biomass and biodiversity. These favorable changes show that it is feasible to protect and restore the degraded natural vegetation by raising the groundwater depth. Plant communities are likely to reflect the hysteresis phenomenon, requiting higher water levels to initiate and stimulate desired growth than what may be needed to maintain the plant community. Because different species have different ecologies, including different root depths and densities and water needs, their response to increasing water availability will be spatially and temporally heterogenous. The response of vegetation is also influenced by microtopography and watering style. This paper discusses strategies for the protection and restoration of the degraded vegetation in the lower reaches of the Tarim River and provides information to complement ongoing theoretical research into ecological restoration in add or semi-arid ecosystems.

Landslide Developmental Characteristics and Response to Climate Change since the Last Glacial in the Upper Reaches of the Yellow River, NE Tibetan Plateau

The upper reaches of the Yellow River in northeastern Tibetan Plateau are geohazards areas.The evolution of the Yellow River,chronology of some landslides,and spatiotemporal distribution characteristics of super large scale and giant landslides within the region are summarized using paleoclimate evidence,and the relationship between the intensive landslide period and climatic changes since the Last Glacial period is analyzed.It is concluded that （1） Super large scale and giant landslides are distributed widely within the region,particularly in the Qunke-Jianzha basin.（2） The chronological sequence of landslides is established by dating the slip zones of landslides and analyzing the relations between landslides and their overlying or underlying loess formations.Five landslide development periods are determined：53-49 ka BP,33-24 ka BP,10-8 ka BP,5-3.5 ka BP,and the present.（3） These correspond closely to warm and wet periods during the last 100,000 years,i.e.,two weak paleosol development stages of Malan loess deposited during the last Glacial period in the Chinese loess Plateau,L1-4 and L1-2 that belong to the marine oxygen isotope stage 3,the last deglaeial period,the Holocene Optimum,and the modern global warming period.（4） Landslide triggers may be closely linked to warm and wet periods related to rapid climatic transitions.

Research on Methods of Ecological Security Assessment of the Middle and Lower Reaches of Liaohe River Based on GIS

Ecological security assessment and early warning research possess the attributes of spatiality, non-linearity and randomicity, so we must process much spatial information. Spatial analysis and data management are the advantages of GIS, which can define distribution trend and spatial relations of environmental factors, and show ecological security pattern graphically. Spatial differences of ecological security assessment based on GIS are discussed in this paper, of which the middle and lower reaches of the Liaohe River is taken as a study case. First, to work out pressure-state-response (P-S-R) assessment indicators system, and investigate in person and gather information; second, to digitize the watershed; third, to quantize and calculate by the fuzzy method; last, to construct GIS grid database, and expound spatial differences of ecological security by GIS interpolation and assembly analysis.

Organic Phosphorus in Shallow Lake Sediments in Middle and Lower Reaches of the Yangtze River Area in China

Thirteen sediment core samples(0-10 cm) were taken from the seven lakes in the middle and lower reaches of the Yangtze River to determine the contents and distributions of organic phosphorus(P) fractions in the sediments of the shallow lakes in the area.The organic P fractions in the sediments were in the order of moderately labile organic P(MLOP) > moderately resistant organic P(MROP) > highly resistant organic P(HROP) > labile organic P(LOP),with average proportional ratios of 13.2:2.8:1.3:1.0.LOP,MLOP,and MROP were significantly related to the contents of total organic carbon(TOC),water-soluble P(WSP),algal-available P(AAP),NaHCO3-extractable P(Olsen-P),total P(TP),organic P(OP),and inorganic P(IP).However,HROP was significantly related to OP and weakly correlated with TOC,WSP,AAP,Olsen-P,TP or IP.This suggested that organic P,especially LOP and MLOP in sediments,deserved even greater attention than IP in regards to lake eutrophication.In terms of organic P,sediments were more hazardous than soils in lake eutrophication.Although OP concentrations were higher in moderately polluted sediment than those in heavily polluted sediment,LOP and MLOP were higher in the heavily polluted sediment,which indicated that heavily polluted sediment was more hazardous than moderately polluted sediment in lake eutrophication.

Evaluation of oasis stability in the lower reaches of the Tarim River

Oasis stability in the lower reaches of the Tarim River was assessed at the landscape scale（including changes in landscape pattern and land use in the oasis） and at the regional scale（including the cold-island effect and NDVI of the oasis） based on the methods of landscape ecology,GIS and Remote Sensing（RS）.Thematic Mapping（TM） remote sensing images from 1990,2000 and 2009 were employed along with the related meteorological and hydrological data pertaining to the reclamation area of the oasis in the lower reaches of the Tarim River.The results indicated that landscape heterogeneity and oasis complexity increased between 1990 and 2000.The comprehensive index of the degree of land use,the average index of the cold-island effect,NDVI value and oasis stability all decreased.However,the change trends in the indices referred to above were opposite between 2000 and 2009,and oasis stability was enhanced to some extent.

Summertime temperature variations in the middle and lower reaches of Yangtze River and their related circulation anomalies in the past five decades

Using the daily data of temperature from China Meteorological Administration and the NCEP/NCAR reanalysis from 1960 to 2005, we have analyzed the relationships between the summertime high/low temperature events in the middle and lower reaches of the Yangtze River （MLRYR） and the related circulation anomalies in the Eastern Hemisphere. Our results have demonstrated that a significantly increasing trend is observed in daily minimum temperature in the past 50 years. And in some regions in the Northern Hemisphere, the opposite scenarios are observed in circulation anomalies in lower and upper parts of the troposphere in the years when the temperatures are higher than normal, as compared to those in the years when the temperatures are lower than normal in the middle and lower reaches of the Yangtze River （MLRYR）. Additionally, the anomalous circulation structure in vertical direction in both the high and lower temperature years are barotropic. It is found that the emergence and maintenance of the aforementioned anomalous circulations are related to three kinds of wave train teleconnection patterns. Further more, influences of the long wave surface radiation on the air temperature are stronger in the nighttime than that in the daytime. While both the maximum and minimum temperatures have negative relationships with the sensible heat flux but positive relationships with the latent heat flux. To some extent, the anomalous dynamic heating （cooling） caused by the vertical thermal advection as well as the diabatic heating （cooling） caused by diabatic processes can explain the formation of the high （low） temperature events in the middle and lower reaches of the Yangtze River （MLRYR） in boreal summer.

Impact of vegetation restoration on hydrological processes in the middle reaches of the Yellow River,China

Sediment discharge from the Yellow River originates mainly from the drainage area between Hekouzhen and Longmen, i.e., the Helong area. Spatial-temporal variations of the vegetation cover in this area during the 1981-2007 period have been investigated using GIMMS and SPOT VGT NDVI data. We have also analyzed the interannual variations in vegetation cover and changes in annual runoff and sediment discharge, the consequences from precipitation change and the Grain for Green Project （GGP）. The results show that vegetation cover of the Helong area has increased during the 1981-2007 period. The northwestern part the Helong area, where the flat sandy lands are covered by grass, has experienced the largest increase. The region where the vegetation cover has declined is largely found in the southern and southeastern Helong area, which is a gullied hilly area or forested. Although precipitation was relatively low during the 1999-2007 period, the vegetation cover showed a significant increase in the Helong area, due to the implementation of the GGP. During this period, the most significant improvement in the vegetation cover occurred mainly in the gullied hilly areas of the Loess Plateau, such as the drainage basins of the Kuyehe and Tuweihe rivers and the middle and lower reaches of the Wudinghe and Yanhe rivers. A comparison of the average annual maximum NDVI between the earlier （1998-2002） stage and the next five years （2003-2007） of the GGP indicates that the areas with increases of 10% and 20% in NDVI account for 72.5% and 36.4% of the total area, respectively. Interannual variation of annual runoff and sediment discharge shows a declining trend, especially since the 1980s, when the decrease became very obvious. Compared with the 1950-1969 period, the average runoff during the 1980-2007 period was reduced by 34.8 × 10^8 m3 and the sediment discharge by 6.4 ×10^8 t, accounting for 49.4% and 64.9% of that in the 1950-1969 period, respectively. There is a positive correlation between the annual maximum NDVI and annual runoff and sediment discharge. This correlation was reversed since the implementation of the GGP in 1999 and vegetation cover in the He- long area has increased, associated with the decrease in runoff and sediment discharge. Less precipitation has been an important fac- tor driving the decrease in runoff and sediment discharge during 1999 2007. However, restoration and improvement of the vegetation cover may also have played a significant role in accelerating the decrease in annual runoff and sediment discharge by enhancing evapotranspiration and alleviating soil erosion.

Formation Mechanisms and Geomorphic Evolution of the Erlian Mudflow Fans, Eastern Guide Basin of the Upper Reaches of Yellow River

Several argillaceous platforms lie along the Yellow River（YR） of the eastern Guide Basin, northeastern Tibetan Plateau, and their compositions, formation processes, and geomorphic evolution remain debated. Using field survey data, sample testing, and high-resolution remote sensing images, the evolution of the Erlian mudflow fans are analyzed. The data show significant differences between fans on either side of the YR. On the right bank, fans are dilute debris flows consisting of sand and gravel. On the left bank, fans are viscosity mudflows consisting of red clay. The composition and formation processes of the left bank platforms indicate a rainfall-induced pluvial landscape. Fan evolution can be divided into two stages： early-stage fans pre-date 16 ka B.P., and formed during the last deglaciation; late-stage fans post-date 8 ka B.P.. Both stages were induced by climate change. The data indicate that during the Last Glacial Maximum, the northeastern Tibetan Plateau experienced a cold and humid climate characterized by high rainfall. From 16–8 ka, the YR cut through the Erlian early mudflow fan, resulting in extensive erosion. Since 8 ka, the river channel has migrated south by at least 1.25 km, and late stage mudflow fan formation has occurred.

Basement Characteristics and Crustal Evolution of the Copper-Gold Metallogenic Belt in the Middle and Lower Reaches of the Yangtze River:Some Isotope Constraints

Studies of the Pb, Sr and Nd isotopic composition of Mesozoic intrusive rocks indicate that the basement of the copper-gold metallogenic belt of the middle and lower reaches of the Yangtze River has “two-layer structure” and partly has “multi-layered structure”, and is inhomogeneous and shows the distinct feature of E-W provincialism. The calculated model lead ages (t1) are mostly greater than 2600 Ma, and the model neodymium ages (TDM) vary from 953 to 2276 Ma and concentrate in two time intervals: 1800–2000 Ma and 1200–1600 Ma. It is concluded that the basement of the MBYR is composed of the Late Archaeozoic to Middle Proterozoic metamorphic series and that the crust was initiated in the Archaean and continued to grow in the Early and Middle Proterozoic, and the proportion of new crust formed by mantle differentiation during the Late Proterozoic is low.

The Background Features and Formation of Chemical Elements of Groundwater in the Region of the Middle and Lower Reaches of the Yangtze River

The paper deals with the background features, formation and distribution of the chemical el-ements K, Na, Ca, Mg, Si, Fe, Mn, Cr, Ni,V, Co, Ti, Mo, Cu, Pb, Zn, As, Hg, Cd, Be, Li, Sr, B,F, Cl, Br and I in the groundwater in the region of the middle and lower reaches of the YangtzeRiver and their relations to the composition of the water-bearing media, properties of the overly-ing rocks and soils, redox environment, and groundwater flow condition, mineralization and pHof groundwater.

Assessment and exploitation of the waterfront resources in the middle and lower reaches of the Yangtze River

Waterfront resources are important and special kind of natural resources in the marginal area between land and wat er. The Yangtze River, the longest river in China, is not only rich in waterfront resources, but also has favorable development conditions with gr eat potentiality. Aided by large-scale underwater topographic map, the maj or factors of the waterfront resources in the middle and lower reache s of the Yangtze River, such as the stability, the water depth and the natural conditions for port construction, are assessed in this pap er respectively on the basis of the overall investigations. The result s show that: (1) the waterfront resources are abundant in the middle and lower reaches of the Yangtze River, but lack of perfectly combined high grade waterfront; (2) there exists an obvious regional difference in the natural quality of the waterfront along the Yangtze; (3) the fore-bank water depth and waterfront stability are the main natural fa ctors related to the waterfront quality in the middle and lower reach es of the Yangtze River; (4) the waterfronts along the Yangtze are mainl y used for port, warehouse and industrial purposes; and (5) the waterfronts near important cities are highly used, especially the high-quality waterfronts. In addition, some suggestions for the development and utilization of the waterfron t resources are presented in this paper.

Structural Characteristics of Lycium ruthenicum Community & Soil Properties on Different Types of Desert Rangeland in the Lower Reaches of Shiyang River

[Objectives] To study structural characteristics of Lycium ruthenicum community and soil properties on different types of desert rangeland in the lower reaches of Shiyang River, and provide theoretical basis for ecological protection and management in the lower reaches of Shiyang River. [Methods] One-way ANOVA analysis and correlation analysis were carried out with the aid of SPSS19.0 software for different types of desert rangeland soils. [Results](i) Among the communities surveyed, there are 31 species belonging to 9 families and 28 genera, including 12 shrub plants, 8 perennial herbs and 11 annual herbs. There are more single genus, occupying 50% of total genus. L. ruthenicum occupies an important position in the community with relatively higher importance value. It plays a dominated role in the community.(ii) In the soils with depths of 0-10 cm, 10-20 cm, 20-40 cm and 40-60 cm in different types of desert rangeland, the organic carbon contents in the saline alkali soil were the highest, with the values of 1.08%, 0.98%, 0.89% and 0.972%, respectively. Compared with that in sandy land, soil organic carbon in saline alkali land(0-10 cm) had the significant difference(P<0.05), available soil potassium in saline alkali land(except the depth of 20-40 cm) and the total phosphorus content in saline alkali land(0-10 cm) had the significant difference(P<0.05), the total soil nitrogen, alkali-hydrolyzable nitrogen, available phosphorus and the total potassium content had no significant difference(P>0.05). Soil organic carbon, available soil phosphorus in saline alkali land, total soil potassium on sand dune and total soil phosphorus in saline alkali land and gravel in different soil layers of the same type of desert rangeland had distinct aggregation effect.(iii) The soil microbial biomass in different types of desert rangeland displayed from high to low as saline alkali land > sandy land > fixed and semi fixed sandy land > gravel. The discrepancy of soil microbial biomass varied in different types of desert rangeland. The four soil enzyme activities in salinization was the highest. Soil invertase activity in saline alkali land in different soil layers were the highest. For different soil layers in the same type of desert rangeland, soil microbial biomass and invertase decreased gradually with the deepening of soil layer.(iv) There was a significant positive correlation between soil organic matter and microbial biomass carbon, microbial biomass nitrogen, phosphatase and sucrase(P<0.01), and a significant positive correlation with urease and catalase(P<0.05). Phosphorus content was significant positively correlated with urease, phosphatase and invertase(P<0.01), and positively correlated with microbial biomass carbon(P<0.05). Soil available potassium content was significant positively correlated with microbial and phosphatase(P<0.01). [Conclusions] The species of L. ruthenicum community and soil properties on different types of desert rangeland in the lower reaches of Shiyang River clearly shows that most species belong to a few families, and a few species belong to most families, and many species are single genus or single family. Soil organic carbon, soil available phosphorus, total phosphorus, soil microbial biomass and enzyme activity in salinization land had distinct surface aggregation effects, so salinization land provides excellent growth environment for the growth of L. ruthenicum, which is of great significance for the protection of L. ruthenicum population.

Three-dimensional P-wave Velocity Structure Modelling of the Middle and Lower Reaches of the Yangtze River Metallogenic Belt: Crustal Architecture and Metallogenic Implications

In this study,we compiled and analyzed 69310 P-wave travel-time data from 6639 earthquake events.These events(M≥2.0)occurred from 1980 s to June 2019 and were recorded at 319 seismic stations(Chinese Earthquake Networks Center)in the study area.We adopted the double-difference seismic tomographic method(tomo DD)to invert the 3-D P-wave velocity structure and constrain the crust-upper mantle architecture of the Middle and Lower Reaches of the Yangtze River Metallogenic Belt(MLYB).A 1-D initial model extracted from wide-angle seismic profiles was used in the seismic tomography,which greatly reduced the inversion residual.Our results indicate that reliable velocity structure of th e uppermost mantle can be obtained when Pn is involved in the tomography.Our results show that:(1)the pattern of the uppermost mantle velocity structure corresponds well with the geological partitioning:a nearly E-W-trending low-velocity zone is present beneath the Dabie Orogen,in contrast to the mainly NE-trending low-velocity anomalies beneath the Jiangnan Orogen.They suggest the presence of thickened lower crust beneath the orogens in the study area.In contrast,the Yangtze and Cathaysia blocks are characterized by relatively high-velocity anomalies;(2)both the ultra-high-pressure(UHP)metamorphic rocks in the Dabie Orogen and the low-pressure metamorphic rocks in the Zhangbaling dome are characterized by high-velocity anomalies.The upper crust in the Dabie Orogen is characterized by a low-velocity belt,sandwiched between two high velocity zones in a horizontal direction,with discontinuous low-velocity layers in the middle crust.The keel of the Dabie Orogen is mainly preserved beneath its northern section.We infer that the lower crustal delamination may have mainly occurred in the southern Dabie Orogen,which caused the mantle upwelling responsible for the formation of the granitic magmas emplaced in the middle crust as the low-velocity layers observed there.Continuous deep-level compression likely squeezed the granitic magma upward to intrude the upper crustal UHP metamorphic rocks,forming the'sandwich'velocity structure there;(3)high-velocity updoming is widespread in the crust-mantle transition zone beneath the MLYB.From the Anqing-Guichi ore field northeastward to the Luzong,Tongling,Ningwu and Ningzhen orefields,high-velocity anomalies in the crust-mantle transition zone increase rapidly in size and are widely distributed.The updoming also exists in the crust-mantle transition zone beneath the Jiurui and Edongnan orefields,but the high-velocity anomalies are mainly stellate distributed.The updoming high-velocity zone beneath the MLYB generally extends from the crust-mantle transition zone to the middle crust,different from the velocity structure in the upper crust.The upper crust beneath the Early Cretaceous extension-related Luzong and Ningwu volcanic basins is characterized by high velocity zones,in contrast to the low velocity anomalies beneath the Late Jurassic to Early Cretaceous compression-related Tongling ore field.The MLYB may have undergone a compressive-to-extensional transition during the Yanshanian(Jurassic-Cretaceous)period,during which extensive magmatism occurred.The near mantle-crustal boundary updoming was likely caused by asthenospheric underplating at the base of the lower crust.The magmas may have ascended through major crustal faults,undergoing AFC(assimilation and fractional crystallization)processes,became emplaced in the fault-bounded basins or Paleozoic sequences,eventually forming the many Cu-Fe polymetallic deposits there.