Journal of Earth Science最新文献

Jiuzhaigou is situated on a mountain-canyon region and is famous for frequent tectonic activities. An abundance of loose co-seismic landslides and collapses were produced on gullies after the Jiuzhaigou Earthquake on August 8, 2017, which was served as material source for debris flow in later years. Debris flow appears frequently which are seriously endangering the safety of people’s lives and properties. Even the earliest debris flow appeared in areas where no case ever reported before. The debris flow susceptibility evaluation (DFSE) is used for predicting the areas prone to debris flow, which is urgently required to avoid hazards and help to guide the strategy of preventive measures. Therefore, this work employs debris flow in Jiuzhaigou to reveal the characteristics of disaster-pregnant environment and to explore the application of machine learning in DFSE. Some new viewpoints are suggested: (i) Material density factor of debris flow is first adopted in this work, and it is proved to be a critical factor for triggering debris flows by sensitivity analysis method. (ii) Deep neural network and convolutional neural network (CNN) achieve relatively good area under the curve (AUC) values and are 0.021–0.024 higher than traditional machine learning methods. (iii) Watershed units combined with CNN-based model can achieve more accurate, reliable and practical susceptibility map. This work provides an idea for prevention of debris flow in mountainous lands.

In order to decipher element mobility in UHP meta-sedimentary rocks in the continental subduction zone, major and trace element compositions are investigated for a continuous profile from a representative UHP region in the Dabie Mountains. Among the lithologic contact zone, contents of K, Ca, LREE, and LILE exhibit varying degrees of downward trends in both marble and eclogite toward the contact zone, indicating that marble and their associated eclogite can release a large amount of K, Ca, and a small number of LILEs and LREEs. Titanite is the main Ti phase in both marble and eclogite. Titanite rims around rutile can occasionally be seen in eclogite. Contents of Ti and HFSE exhibit a well-coupled relation among marble and eclogite, indicating that substantial Ti and HFSEs were migrated from eclogite to marble, in accord with the capacity of a melt medium. Rutiles and titanites in marble exhibit a relatively limited variation in Nb/Ta ratios (12.9–16.2), similar to those of titanites in eclogite (14.2–16.7), which demonstrates that rutiles and titanites in marble were sourced from eclogite because of short-distance migrations of Ti and HFSEs. According to the P-T path and the temperature and pressure conditions of the peak metamorphism reported by previous studies, the eclogite associated with marble may not form supercritical fluids in the subduction zone because of the addition of carbonate minerals.

On September 5, 2019, a moderate earthquake of Mw5.4 unexpectedly occurred in the apparently quiescent central basin of the South China Sea. We immediately carried out a seismicity monitoring experiment around the epicenter by using broadband ocean bottom seismometers (OBS) for the following three scientific targets. The first is knowing the earthquake seismogenic mechanism, fault structure and further development. The second is finding the role of the residual spreading ridge playing in earthquake processes and further revealing the deep structures of the ridge directional turning area. The third is confirming the existence and significance of the so called “Zhongnan fault”. This paper reports the preliminary results of the first phase experiment. Five OBSs were deployed for seismicity monitoring with a duration of 288 days, but only three were recovered. Micro-earthquakes were firstly detected by an automatic seismic phase picking algorithm and then were verified by analyzing their seismic phases and time-frequency characteristics in detail. A total of 21, 68 and 89 micro-earthquakes were picked out from the three OBSs respectively within the distance of 30 km. The dominant frequency of these micro-earthquakes is 12–15 Hz, indicating tectonic fracturing. During the first two months after the mainshock the seismicity was relatively stronger, and micro-earthquakes were still occurring occasionally till the end of observation, indicating the epicenter area is active. We used Match&Locate method to locate 57 micro-earthquakes preliminarily. Their spatial distribution shows that the seismicity is developed mainly along the NE direction roughly parallel to the residual ridge with depth variations between 10–20 km.

To predict the occurrence of the collapse disaster in toppling perilous rock under the action of bidirectional earthquakes, the dynamic stability and fuzzy reliability calculation method of toppling perilous rock under the action of bidirectional earthquakes is proposed. First, the mass viscoelasticity model is used to simulate two main control surfaces of toppling perilous rock, the seismic dynamic response model and motion equation of toppling perilous rock are established based on the D’Alembert principle, and the Newmark-β method is used to solve the dynamic motion equation. Then, the instability event of toppling perilous rock is considered a fuzzy event, the membership function expression of the stability coefficient of toppling perilous rock is determined based on the fuzzy failure criterion, the calculation equations of the toppling perilous rock dynamic stability coefficient and fuzzy reliability are established, and the fuzzy reliability evaluation method based on the probability distribution of reliability is proposed. Finally, the influence of different superposition modes of seismic excitation on the fuzzy reliability of toppling perilous rock is analyzed. The calculation results of toppling perilous rock in the engineering case show that the fuzzy reliability calculated after considering the fuzzy failure criterion is reduced by 10.73% to 25.66% compared with the classical reliability. Considering the bidirectional seismic excitation, the fuzzy reliability of toppling perilous rock is reduced by 5.46% to 14.89%. Compared with using the acceleration peak time encounter mode to superpose the seismic excitation, the fuzzy reliability of toppling perilous rock is reduced by 3.4% when the maximum action effect time encounter mode is adopted.

Understanding the spatial and temporal distribution of different paleontological communities in the southern South China Sea (SCS) is fundamental to explore its paleoclimatic and paleoenvironmental changes. In this study, foraminifera, pollen and diatoms from 100 surface sediment samples covering 40 000 km² of sea floor in the southern SCS were comprehensively investigated in terms of their assemblage and distribution. The results showed the existence of abundant foraminifera and pollen in most of the samples, although diatom communities were relatively scarce. Foraminifera were dominated by G. sacculifer, G. menardii, G. ruber, while diatoms were characterized by T. simonsenii, T. nitzschioides and T. longissima, indicating a typical tropical marine environment. The pollen assemblages showed a better reflection of montane rainforest, tropical rainforest and mangrove. The spatial differences among foraminifera assemblages may indicate the effect of water depth and a warmer environment in the southeast part of the study area, while the spatial patterns of diatom and pollen assemblages imply the influence of coastal current. Our study also noted that the compositions of paleontological communities in the SCS can vary significantly in a short distance, and synthesized studies on multiple biological groups are needed to reconstruct the Quaternary climate and the oceanographic environment.

Voluminous Early Cretaceous granitoids and associated large-scale ore deposits are distributed within the southern Great Xing’ an Range (SGXR), NE China. Based on previously published geochronology and zircon Hf-isotope data, Hf isotope mapping is undertaken to improve our understanding of crustal architecture and its controls on ore deposits. The ore-related Early Cretaceous granitoids were sourced predominantly from juvenile crust, with the involvement of variable proportions of ancient crustal materials. The crustal architecture, as inferred from Hf isotopic contour maps, indicates two distinct Hf isotopic domains in SGXR, including (1) a higher-ε_Hf (+7 to +11) juvenile crust containing minor ancient crustal material, and (2) a lower-ε_Hf (+2 to +6) juvenile crust containing a greater proportion of ancient crustal materials. The Hf isotopic maps identify links between crustal architecture and regional metallogeny. Copper deposits and other deposits with significant Cu production are restricted mainly to the higher-ε_Hf juvenile crustal regions in the northern and eastern SGXR. Deposits dominated by other metals (e.g., Mo, Sn, W, Pb, Zn, and Ag) occur mainly in the lower- ε_Hf juvenile crustal regions in the southern and western SGXR. Interaction between juvenile crust-derived melts and ancient crustal components played an important role on the distribution of various ore metals.

The dynamic effect is a very important issue widely debated by scholars when studying the genetic and disaster-causing mechanisms of earthquake-triggered landslides. First, the dynamic effect mechanism and phenomena of earthquake-triggered landslides were summarized in this paper. Then, the primary types of dynamic effects were further used to interpret the Mogangling landslide in Moxi Town of Luding County, China. A field investigation, remote sensing, numerical calculation and theoretical analysis were carried out to illustrate the failure mechanism of slope rock masses affected by earthquakes. The interaction between seismic waves and slope rock masses and the induced dynamic effect of slope rock masses were primarily accounted for in the analysis. The slope topography, rock mass weathering and unloading characteristics, river erosion, regional seismogenic structure, and rock mass structure characteristics were also discussed. The results showed that the formation of the Mogangling landslide was mainly related to the high amplification effect of seismic acceleration and back slope effects, interface dynamic stress effects, and double-sided slope effects of seismic waves caused by the catastrophic Ms 7.75 Moxi Earthquake in 1786. The principles for the site and route selection of large-scale infrastructure in the planning stage and the scientific prevention of seismic geological disasters were proposed on the basis of the dynamic effect of earthquake-induced landslides.

While acid mine drainage (AMD) issues have become a topic of global concern, few studies have focused on acid drainage problems of non-mining activities. We conducted field research and a series of laboratory experiments to investigate the characteristics, release processes and formation of acid drainage contamination. Spoil rock samples and adjacent surface water, groundwater, soil and sediment samples were collected at a railway tunnel construction site in central China, and various parameters, such as the pH, mineral ion concentrations, and heavy metal concentrations, were measured. Based on the measured concentrations, surface water and sediments were seriously contaminated by acids, sulfate salts and heavy metals. Contamination in surface water showed a decreasing tendency as the distance from the spoils increased, while that in sediments showed a greater influence of coprecipitation and adsorption processes of heavy metal ions. The eluviation experiments of three rock samples indicated that R2 (silty fine sandstone) was the most likely major acid drainage contributor. Thiobacillus ferrooxidans was cultured and isolated from contaminated water to study the oxidation conditions during the release processes. The significant release of acid drainage when air and bacteria were both in the culture container suggested that oxygen and bacteria were necessary to produce acid drainage from spoils.

Arroyo Verde epithermal deposit, in the North Patagonia region of Argentina, is located within the Lower Chon Aike volcanism and is developed between 192.6 ± 2.5 Ma (mineralization host rock) and 189.5 ± 2.6 Ma (overlying lava). Marifil volcanic complex is the host rock for these veins, veinlets and breccias. This particular small deposit has developed low to intermediate sulphidation characteristics, with gangue mineral textures associated with several ore minerals. Electron probe microanalysis determines electrum, silver minerals such as acanthite, and tetrahedrite, related Ag-Cu minerals like jalpaite, mckinstryite, stromeyerite, and base metals like chalcopyrite, sphalerite, galena and pyrite. Fluid inclusions in quartz of two out of five events indicate that these Low Jurassic veins were formed due to subtle boiling and dilution mechanisms and by low-salinity (3.4 wt.% to 6.7 wt.% NaCl Eq) magmatic-hydrothermal fluid at 225 to 310 °C. The fluid inclusion measurements that indicate boiling (Event 3) were plotted, and based on this data, we interpret that the Arroyo Verde deposit has been eroded between 550 and 700 m. This particular character has not been previously recorded for other sectors of the eastern north Patagonian region.

The Bipindi iron ore district is located in the central section of the Nyong Complex at the northwestern margin of the Congo Craton in Southwest Cameroon. This iron district contains numerous iron mineralization hosted by the Mewongo, Bibole, Kouambo, and Zambi banded iron formations (BIFs). These BIFs contain magnetite as the main iron ore mineral associated with pyrite, and gangue minerals are quartz with minor chlorite and amphibole. The origin of iron ore from these BIFs was investigated using a combination of in-situ magnetite and whole-rock chemistry. The studied BIF ore samples have a narrow range of TFe between 30.90 wt.% and 43.20 wt.%, indicating a low-grade ore. The geochemical signatures of magnetite such as low contents of base metals (e.g., Cu, Co, V, and Zn) and low Co/Zn ratios < 0.85 indicate a hydrothermal origin. Combined with the geochemical features of these BIFs, e. g., high Fe/Ti and Fe/Al ratios (mean > 600 and > 75, respectively), we suggest that magnetite was derived from a mixture of seawater and ∼0.1% low-temperature hydrothermal fluids in an oxidizing environment. Collectively, low-temperature hydrothermal and later metamorphic fluids were necessary for the transformation of the protolith Nyong Complex BIFs to iron ore.