Rupture process and aftershock focal mechanisms of the 2022 M6.8 Luding earthquake in Sichuan

According to the China Earthquake Networks Center, a strong earthquake of M6.8 occurred in Luding County, Ganzi Tibetan Autonomous Prefecture, Sichuan Province, China (102.08°E, 29.59°N), on September 5, 2022, with a focal depth of 16 km. Rapid determination of the source parameters of the earthquake sequence is vital for post-earthquake rescue, disaster assessment, and scientific research. Near-field seismic observations play a key role in the fast and reliable determination of earthquake source parameters. The numerous broadband seismic stations and strong-motion stations recently deployed by the National Earthquake Intensity Rapid Report and Early Warning project have provided valuable real-time near-field observation data. Using these near-field observations and conventional mid- and far-field seismic waveform records, we obtained the focal mechanism solutions of the mainshock and M ≥ 3.0 aftershocks through the waveform fitting method. We were further able to rapidly invert the rupture process of the mainshock. Based on the evaluation of the focal mechanism solution of the mainshock and the regional tectonic setting, we speculate that the Xianshuihe fault formed the seismogenic structure of the M6.8 strong earthquake. The aftershocks formed three spatially separated clusters with distinctly different focal mechanisms, reflecting the segmented nature of the Xianshuihe fault. As more high-frequency information has been applied in this study, the absolute location of the fault rupture is better constrained by the near-field strong-motion data. The rupture process of the mainshock correlates well with the spatial distribution of aftershocks, i.e., aftershock activities were relatively weak in the maximum slip area, and strong aftershock activities were distributed in the peripheral regions.     

Tectonic evolution of convergent plate margins and its geological effects

Science China Earth Sciences - Oceanic lithosphere is generated at divergent plate boundaries and disappears at convergent plate boundaries. Seafloor spreading and plate subduction together...     

Exhumation history and preservation of the Jiaojia giant gold deposit,Jiaodong Peninsula

Science China Earth Sciences - The Jiaojia giant gold deposit is the largest gold deposit in China, with a total gold reserve of approximately 1200 t. Until now, the knowledge of the exhumation...     

重庆云阳地区中侏罗世新田沟组底部细粒碎屑岩地球化学特征及意义

重庆云阳地区中侏罗统新田沟组是四川盆地新发现恐龙化石的重要埋藏层位之一.野外地质调查表明,研究区内新田沟组下部岩性以深灰色薄层泥质粉砂岩及粉砂质泥岩为主,夹中层状岩屑石英砂岩,偶含紫红色粉砂岩,向上砂岩增多.本次研究连续采集重庆云阳地区新田沟组底部细碎屑岩,通过主、微量元素地球化学分析,探究沉积物物源特征及沉积时期古风化和古气候条件.分析结果显示:样品w(SiO2)为57.41％～66.53％,w(A12O3)介于15.66％～20.01％;全碱含量w(Na2O+K2O)约3.21％～4.85％,地化特征说明新田沟组底部细碎屑岩主要属富石英岩屑杂砂岩.样品轻稀土元素(LREE)含量为163.6～216.3×10-6,重稀土元素(HREE)含量为18.1～25.0×10-6,轻重稀土比值(LREE/HREE)为7.71～10.10,说明样品轻重稀土分异明显.各项岩石地球化学比值表明其源岩为长英质火山岩,源区构造背景为大陆岛弧环境.剖面底部4件样品的化学蚀变指数(CIA)均在80以上,向上逐渐减小(平均73),指示中侏罗纪早期气候条件存在由炎热潮湿到温暖湿润的变化趋势,化学风化作用经历了由极强到中等风化的转变.     

沿江丘陵平原工程地质分区研究——以安庆地区为例

长江中下游地区沿江丘陵平原地貌分布广泛且地质背景相似,开展单个地区的工程地质分区方法研究,对同类区域具有一定的借鉴意义。以安庆地区为例,提出一级分区以地貌类型,二级分区以岩土体类型、结构和重要工程地质层分布为划分原则的工程地质分区方法,将该区划分为5个工程地质区和14个工程地质亚区,并对各分区工程地质特性进行论述,旨在为同类区域水工环地质调查、城市地质调查研究提供参考。     

Investigation on Snow Characteristics and Their Distribution in China

The background, scientific objective, investigation contents and scheme of project “Investigation on snow characteristics and their distribution in China” was introduced in this paper. The general objective of the investigation is to build comprehensive and systematic database of snow characteristics in China, at the service of providing data for the climate change, water resource and snow disaster studies. The investigation will be performed on the three fields including the compilation of historical data, in situ measurement of snow characteristics in the typical regions, and investigation of snow characteristics using remote sensing methods. For the compilation of historical data, the historical snow data from the meteorological stations and research institutes will be firstly collected, and then they will be compiled based on a standard rule. In situ observation will be performed at point, line and area-scale on the typical regions which include Northeast region, Xinjiang Degion, and Qinghai-Tibet Plateau. The observation content will contain snow depth, snow density, snow water equivalent, snow particle shape, hardness of snowpack surface, liquid water content, grain size, snow temperature, snow/soil temperature, dielectric constant, and some chemical parameters. These snow characteristics are the priority information used for the modification of retrieval algorithm on snow parameters. Remote sensing methods will be used to build long-time series of snow cover, snow albedo and snow water equivalent datasets based on these modified algorithms. Finally, the snow characteristics from both in situ and remote sensing investigation will be used to classify snow types in China, and produce distribution maps of snow characteristic and other thematic maps.     

Trace Element Record of Pyrite from Diverse Deposits-Examples from the Dabaoshan Polymetallic Deposit of Northern Guangdong,South ChinaEI北大核心CSCD

The Dabaoshan polymetallic deposit, located in the middle Nanling metallogenic belt, is an important and representative W-Mo-Cu-Pb-Zn deposit in the northern Guangdong. The deposit is characterized by a complex combination of ore-forming elements, containing 0.86 Mt Cu, 0.85 Mt Zn, 0.31 Mt Pb, 0.14 Mt WO3 at grades of 0.86%, 12%, 1.77% and 0.12%, respectively. Due to different mineral association and occurrence, mineralization at Dabaoshan can be defined as three types: porphyry W-Mo mineralization, skarn Mo-W mineralization and strata-bound sulfide mineralization. Pyrite from the three-mineralization types has different textures and element compositions. EMPA mapping analysis suggested that distribution of Co, Ni, and As is nearly homogeneous, whilst As in pyrite from the stratabound sulfides shows weak irregular zoning. LA-ICP-MS analyses of pyrite suggested that the three type have distinctive trace element characteristics even though the concentrations of the trace elements are relatively low. Cobalt, Ni, As and Se substitutes Fe and S in pyrite respectively while Cu, Zn, Sn Sb are mainly represented by chalcopyrite, sphalerite, cassiterite and stibnite inclusions enclosed in pyrite. Ni and Ag are rich in the pyrite from the stratabound mineralization. The results suggested that the trace element characteristic of pyrite can be used to: a) study the occurrence of trace element in pyrite; b) imply the chemical-physical conditions; c) trace the regional ore-forming substance and ore forming process. The stratabound mineralization is formed during the volcano eruption during the Devonian then and overprinted by the deep source material and multiple ore-forming fluids in the Yanshanian period. © 2018, Science Press. All right reserved.     

Late Cretaceous basalts and rhyolites from Shimaoshan Group in eastern Fujian Province,SE China: age,petrogenesis, and tectonic implications

ABSTRACT

Southeastern China is characterized by an extensive Late Mesozoic (Yanshanian) tectono-magmatic-metallogenic event. Although Late Cretaceous volcanism gradually weakened during the epilogue of the Yanshanian event, its petrogenesis and geodynamic processes remain unclear. In this study, we present new zircon U–Pb–Hf isotopic, whole-rock elemental, and Sr–Nd isotopic compositions data, for volcanic rocks from the Zhaixia Formation of the Shimaoshan Group in Fujian Province. The lower member of the Zhaixia Formation consists of basalts and rhyolites, and the upper member is only rhyolites. These volcanic rocks erupted in the early stage of Late Cretaceous, with basalts erupting earlier (ca. 99–98 Ma) than rhyolites (ca. 98–94 Ma). These basalts record high-K calc-alkaline to shoshonitic, light rare earth element (LREE)- and LILE-enrichment, high field strength element (HFSE)depletion with negligible Eu anomalies, and uniform whole-rock ε_Nd(t) (–3 to –6) and zircon ε_Hf(t) (–3.3 to –14.1) values. The overlying rhyolites record peraluminous and high-K calc-alkaline characteristics, LREE- and LILE-enrichment with negative Eu anomalies, and Nb–Ta depletion. The whole-rock ε_Nd(t) and zircon ε_Hf(t) values of these rhyolites both increase from the lower member (ε_Nd(t), –1.5 to –4.7; ε_Hf(t), –5.1 to –16.1) to the upper member (ε_Nd(t), –0.5 to 0.1; ε_Hf(t), –0.3 to –4.3). The features imply that these basalts were derived from the partial melting of the enriched lithospheric mantle and the overlying rhyolites from the melting of the crustal components, respectively. Data from the rhyolites in the upper member indicate that more juvenile, Nd–Hf isotopically depleted materials were injected into their source. During the Late Cretaceous, the new, fast rollback of the subducting slab triggered lithospheric extension and asthenospheric upwelling beneath the coastal regions, which induced the melting of lithospheric mantle and crustal components. As continued, the new round of basaltic underplating provided necessary heat to cause partial melting of the deep crust, including the younger, juvenile, and isotopically depleted crustal components.     

Evolution of the mid-channel bars in the middle and lower reaches of the Changjiang (Yangtze) River from 1989 to 2014 based on the Landsat satellite images: impact of the Three Gorges Dam

The mid-channel bars have long been identified as essential landforms in the large rivers of the world, and the significance of connectivity between morphology and flow-sediment dynamics has been intensively emphasized. In this study, remote sensing images and associated hydrological data from 1989 to 2014 were used to explore mid-channel bars evolution in the middle and lower reach of the Changjiang and their responses to the Three Gorges Dam (TGD), the world’s largest hydrological engineering. The results indicated that mid-channel bars, respectively, exhibited deposition and erosion in the flood and dry season in pre-TGD period, while mild deposition in flood season and deposition in dry season were found in post-TGD period. As a consequence, mid-channel bars area was characterized by ‘remarkable seasonal differences in pre-TGD period, mild seasonal pattern in post-TGD period’. The obvious shift in seasonal features could be attributed to the TGD operation in 2003. Specifically, flood duration decrease and sediment load reduction following TGD regulation suppressed the bars growth in flood season. TGD-induced variations in differences between sediment carry capacity and suspended sediment concentration resulted in the bars transformation in dry season. Meanwhile, the change trends of downstream mid-channel bars became weak as their locations’ distance to TGD increases because of the river adjustment and tributaries supplement. Moreover, mid-channel bars in different river patterns presented various change trends with the most remarkable variation being detected in goose-head-shaped river pattern. The results of this paper provide a theoretical basis for the river channel improvement in the middle and lower reaches of the Changjiang River.     

渤海湾盆地大歧口凹陷新生代构造演化与盆地原型

大歧口凹陷位于渤海湾盆地黄骅坳陷中北部,是中新生代叠合盆地中新生界沉积最厚的凹陷。本文基于大量二维、三维地震剖面和钻井资料,从凹陷主控断裂活动性、沉积沉降中心分布和构造格架等方面,侧重对大歧口凹陷新生代构造演化过程和原型盆地类型进行研究。认为:1)大歧口凹陷构造单元有4级,凹陷具有"东西分带、南北分块"的平面构造格局和北断南超箕状断陷的剖面结构。2)凹陷内断裂有4级,主要断裂的规模、活动期次和断裂活动具有演化的阶段性和空间的迁移性,典型构造带活动性具有由北往南迁移的特点。3)新生代期间整个大歧口凹陷的沉积沉降中心均在东部海域的歧口主凹内,但整体具有往南迁移的趋势。4)凹陷经历了拓展裂谷、拉分断陷、箕状断陷和碟状坳陷4个构造演化阶段,断陷阶段在整个构造演化时期意义重大,且具有走滑拉分特点。故认为大歧口凹陷原型盆地是一种"板内拉分盆地"。