The Creston,California, meteorite fall and the origin of L chondrites

It has been proposed that all L chondrites resulted from an ongoing collisional cascade of fragments that originated from the formation of the ~500 Ma old asteroid family Gefion, located near the 5:2 mean‐motion resonance with Jupiter in the middle Main Belt. If so, L chondrite pre‐atmospheric orbits should be distributed as expected for that source region. Here, we present contradictory results from the orbit and collisional history of the October 24, 2015, L6 ordinary chondrite fall at Creston, CA (here reclassified to L5/6). Creston's short 1.30 ± 0.02 AU semimajor axis orbit would imply a long dynamical evolution if it originated from the middle Main Belt. Indeed, Creston has a high cosmic ray exposure age of 40–50 Ma. However, Creston's small meteoroid size and low 4.23 ± 0.07° inclination indicate a short dynamical lifetime against collisions. This suggests, instead, that Creston originated most likely in the inner asteroid belt and was delivered via the ν₆ resonance. The U‐Pb systematics of Creston apatite reveals a Pb‐Pb age of 4,497.1 ± 3.7 Ma, and an upper intercept U‐Pb age of 4,496.7 ± 5.8 Ma (2σ), circa 70 Ma after formation of CAI, as found for other L chondrites. The K‐Ar (age ~4.3 Ga) and U,Th‐He (age ~1 Ga) chronometers were not reset at ~500 Ma, while the lower intercept U‐Pb age is poorly defined as 770 ± 320 Ma. So far, the three known L chondrites that impacted on orbits with semimajor axes a <2.0 AU all have high (>3 Ga) K‐Ar ages. This argues for a source of some of our L chondrites in the inner Main Belt. Not all L chondrites originate in a continuous population of Gefion family debris stretching across the 3:1 mean‐motion resonance.     

Intelligent identification of remnant ridge edges in region west of Yongxing Island,South China Sea

Edge detection enables identification of geomorphologic unit boundaries and thus assists with geomorphical mapping. In this paper, an intelligent edge identification method is proposed and image processing techniques are applied to multi-beam bathymetry data. To accomplish this, a color image is generated by the bathymetry, and a weighted method is used to convert the color image to a gray image. As the quality of the image has a significant influence on edge detection, different filter methods are applied to the gray image for de-noising. The peak signal-to-noise ratio and mean square error are calculated to evaluate which filter method is most appropriate for depth image filtering and the edge is subsequently detected using an image binarization method. Traditional image binarization methods cannot manage the complicated uneven seafloor, and therefore a binarization method is proposed that is based on the difference between image pixel values; the appropriate threshold for image binarization is estimated according to the probability distribution of pixel value differences between two adjacent pixels in horizontal and vertical directions, respectively. Finally, an eight-neighborhood frame is adopted to thin the binary image, connect the intermittent edge, and implement contour extraction. Experimental results show that the method described here can recognize the main boundaries of geomorphologic units. In addition, the proposed automatic edge identification method avoids use of subjective judgment, and reduces time and labor costs.     

广州“5·21”新冠肺炎疫情时空演化与防控管理

广州“5·21”新冠肺炎疫情是境内首起由新冠病毒“德尔塔”（Delta）变异毒株引发的本土疫情。以广州“5·21”疫情感染者为对象,采用统计分析、文本分析和社会网络分析方法,分析本土境外输入关联疫情的统计特征、扩散网络与防控管理。研究结论主要有：1）广州“5·21”疫情感染者以老年人居多,感染者潜伏期较短,病毒传播速度快。核酸检测筛查可将病毒传播的风险精准控制在已知范围;行程轨迹可以辅助及时厘清病毒传播链条,切断传播途径。2）感染者所在区和镇街、重点防控区域、分级分类防控管理区域呈片状和点状分布,感染者在少数镇街呈集聚分布特征,疫情传播以接触扩散为主、兼具跳跃扩散的特征。3）疫情扩散传播网络以“簇状”和“链状”为主,网络整体呈现一定“小世界”特征;网络节点层面指标值较高的节点处于疫情接触扩散传播网络的关键路径,是疫情分级分类防控管理的重点对象。4）疫情分级分类防控管理应做好出入管理、社区管控、居家隔离、需求保障、生活垃圾处理、环境消杀、健康监测和服务、核酸检测、宣传引导和管控解除等工作。     

Degradation stage effects on vegetation and soil properties interactions in alpine steppe

In recent decades,overgrazing and the warming and drying climate have resulted in significant degradation of alpine grasslands in the source region of the Yellow River.However,research into the relationships between vegetation and soil properties has mainly focused on an overall degradation stage,and few studies have investigated which soil properties can impact vegetation change at different stages of degradation.Vegetation and soil properties were observed in the field and measured in the laboratory for different predefined stages of degradation for alpine grassland in Maduo County in the source region of the Yellow River.Results show that:1)the aboveground and belowground biomass,soil organic carbon,total nitrogen,nitrate,and ammonia content did not decrease significantly from the intact to moderate degradation stage,but decreased significantly at severe and very severe stages of degradation;2)dominant plant species shifted from gramineaes and sedges to forbs;3)the species richness and Pielou evenness indices decreased significantly at the very severe degradation stage,the Shannon-Wiener diversity index increased at the slight and moderate degradation,but decreased at the severe and very severe degradation stages;4)soil bulk density was the strongest soil driver for changes in the plant biomass and community diversity at the intact,slight and moderate degradation stages,whereas soil organic carbon and nitrate nitrogen content were the main driving factors for changes in plant biomass and diversity at the severe and very severe degradation stages.Our results suggest that there may be different interactions between soil properties and plants before and after moderate stages of degradation.     

利用磁异常解析方法试验研究城市断裂

文中选取北京地区作为城市断裂研究试验场,对航磁资料运用解析延拓、功率谱分析、正则化滤波、方向导数、小波分析、分形维数、视磁化强度反演及磁性界面反演等一系列磁异常解析方法进行了数据处理。在处理数据分析与对比研究的基础上,对磁法勘探的各种解释方法在城市断裂研究中所发挥的作用、功效及适应性进行了评估,对今后城市断裂探测研究工作中的磁法勘探技术方面提出了指导性意见。文中对于揭示深部断裂、浅层断裂、基底直立断裂、以及预定方向的断裂和构造两侧磁性差异较大的断裂构造所运用的方法、功效及注意事项均做了较好的说明。另外还对功率谱用于计算不同尺度磁性体场源深度的效果进行了分析     

基于密集匹配点云的DEM生产可行性研究

无人机倾斜摄影直接生产的成果通常包括三维模型、TDOM、DSM等,然而规划设计通常不能直接利用倾斜数据输出的DEM,需要辅以人工编辑。作为倾斜摄影影像处理的过程成果,密集匹配点云未得到充分利用。其与激光雷达点云具备相似的结构,且点云密度可自由选择,在不考虑数据量的情况下,密集匹配点云的点密度可数倍于激光雷达点云。此外,密集匹配点云无需单独赋色,即具有纹理信息,对人工目视编辑自动分类后的地面点具有一定的辅助作用。本文对比分析了同一测区的密集匹配点云与激光雷达点云,验证了密集匹配点云用于房屋建筑区及稀疏林区地面点滤波并生产DEM的可行性。     

基于绕月单卫星和双星测量的月球重力场恢复仿真分析

月球重力场是月球科学研究的重要部分,是进一步了解月球内部结构和构造的基础,也是我国探月计划“嫦娥工程”一个重要的科学研究内容.本文针对“嫦娥工程”,利用GSFC/NASA/USA的GEODYNII/SOLVE轨道分析软件,分析计算了利用单星跟踪数据恢复月球重力场的能力,同时模拟计算了双星跟踪数据在恢复月球重力场方面的能力,结合可能的工程环境我们对两种情况分别给出了30天、60天和90天的计算结果.计算表明双星相比于单星对月球重力场的中低阶位系数有比较显著的改善.     

Fingerprinting subducted oceanic crust and Hainan Plume in the melt sources of Cenozoic Basalts from the South China Sea Region

Geochemical data compilation of Cenozoic basalts recovered from the South China Sea tectonic domain shows westward weakening of the influence of a focal zone‐like component in Nd–Hf, Nd–Pb and Sr–Pb, but not in Pb–Pb isotope spaces because the Pb isotopes are dominantly controlled by the high U/Pb component derived from the subducted Pacific oceanic slab. Low Th/U melt generated by recycling of marine carbonates, rather than the subduction‐related enriched mantle (EM2), signals the emplacement of the Hainan Plume at ~25 Ma. Radiogenic Hf in the pre‐existing ancient sub‐continental lithospheric mantle beneath the Cathaysia Block was greatly depleted by early‐stage magmatism influenced by the high U/Pb component. Hence, late Cenozoic basalts associated with the carbonatitic melts display contrasting Nd–Hf isotope covariations, with the Red River–Zhongnan Fault System as a dividing line, implying that the East and Southwest sub‐basins have been developed on the Cathaysia and Indochina Blocks respectively.     

Petrogenesis of lunar meteorite Northwest Africa 2977: Constraints from in situ microprobe results

Abstract– Northwest Africa (NWA) 2977 is an olivine‐gabbro lunar meteorite that has a distinctly different petrographic texture from other lunar basalts. We studied this rock with a series of in situ analytical methods. NWA 2977 consists mainly of olivine and pyroxene with minor plagioclase. It shows evidence of intense shock metamorphism, locally as high as shock‐stage S6. Olivine adjacent to a melt vein has been partially transformed into ringwoodite and Al,Ti‐rich chromite grains have partially transformed into their high‐pressure polymorph (possibly CaTi₂O₄‐structure). Olivine in NWA 2977 contains two types of lithic inclusions. One type is present as Si,Al‐rich melt inclusions that are composed of glass and, in most cases, dendritic pyroxene. The other type is mafic and composed of relatively coarse‐grained augite with accessory chromite, RE‐merrillite, and baddeleyite. Two Si,Al‐rich melt inclusions are heavy rare earth elements (REE) enriched, whereas the mafic inclusion has high REE concentrations and a KREEP‐like pattern. The mafic inclusion could be a relict fragment captured during the ascent of the parent magma of NWA 2977, whereas the Si,Al‐rich inclusions may represent the original NWA 2977 melt. The calculated whole‐rock composition has a KREEP‐like REE pattern, suggesting that NWA 2977 has an affinity to KREEP rocks. Baddeleyite has recorded a young crystallization age of 3123 ± 7 Ma (2σ), which is consistent with results from previous whole‐rock and mineral Sm‐Nd and Rb‐Sr studies. The petrography, mineralogy, trace element geochemistry, and young crystallization age of NWA 2977 support the possibility of pairing between NWA 2977 and the olivine‐gabbro portion of NWA 773.     

云南省威信县地质灾害与防治对策

威信县地质构造较复杂,地质灾害破坏严重。主要地质灾害类型有滑坡、崩塌、泥石流、不稳定斜坡及地裂缝五种。其中以滑坡和不稳定斜坡为主,灾害点分布较广且个体规模小,稳定性较差。调查显示,威信县地质灾害的形成与发生时多种致灾因素相互作用的结果。通过对威信县地质灾害成因分析得出:地质构造、岩土体类型、地形地貌、生态植被等因素是滑坡、泥石流、崩塌及不稳定斜坡形成的基本条件;降雨及人类活动等因素是形成滑坡、泥石流、崩塌、地裂缝及不稳定斜坡的主导因素。对地质灾害的防治应采用以群测群防为基本手段,与搬迁避让、工程防治、生物防治、清除危岩(土)体及排水为主的防治措施相结合的综合治理方法。