Hotspot origin of the Mississippi embayment and its possible impact on contemporary seismicity

Previous authors have related the Late Cretaceous/early Tertiary subsidence of the Mississippi embayment to the opening of the Gulf of Mexico, but the Gulf opened earlier in Triassic/Jurassic time. We offer an alternative hypothesis that development of the embayment was coeval with the passage of the Mississippi Valley graben system over the Bermuda hotspot about 90 Ma. Several lines of evidence of significant uplift of the embayment axis accompanying mid-Cretaceous magmatism and prior to Late Cretaceous subsidence support this proposal.

First, reactivation of the Pascola arch in the northern embayment is recorded by flanking deposits of basal Upper Cretaceous gravel. Second, beneath a regional mid-Cretaceous unconformity, subcrops of Jurassic and Early Cretaceous strata define a pronounced southwest-plunging arch in the southern embayment. This arch is collinear with an arch revealed in Paleozoic rocks after restoration to mid-Cretaceous structural geometries. Third, a deep weathering profile on mid-Cretaceous alkalic plutons along the western embayment margin is nonconformably overlain by Paleocene sediments, and rapid mid-Cretaceous cooling of these intrusions has been interpreted from apatite fission tracks. Moreover, exploratory holes along the embayment axis encountered similar weathered alkalic intrusions nonconformably overlain by basal Upper Cretaceous strata. Fourth, there was an anomalous influx of clastic sediment into the northern Gulf of Mexico during mid-Cretaceous time, and subsequent clastic facies patterns suggest the Mississippi River drainage began to enter the Gulf in the Late Cretaceous.

Passage of the Mississippi Valley graben over the Bermuda hotspot during elevated hotspot activity of Cretaceous time may have significantly weakened the previously rifted lithosphere. Rifted continental margin at Charleston, South Carolina, also passed over this hotspot in latest Cretaceous time. Similarly, the St. Lawrence rift system passed over the Great Meteor hotspot during the Cretaceous. It is important to note that these rift systems are the principal loci of strong seismicity in eastern North America, and thus weakening by increased Cretaceous hotspot activity may be an important common factor for these seismic rift zones.     

迈向城乡共治：改革开放以来城乡关系演变解读

城乡关系历来是经济社会发展的重要关系及党和政府关注的重大问题。为探究城乡关系及其治理逻辑,论文借助CiteSpace软件分析改革开放以来城乡关系的研究热点,结合时代背景解读城乡关系的演变历程,进而揭示城乡关系的脉络特征与治理变革的内在逻辑。研究表明：改革开放以来城乡关系的演变经历了4个阶段,形成了“城乡关系向好发展—城乡关系再度失衡—城乡关系调整完善—城乡关系一体化发展”的演变脉络;折射出“打破城乡分割”“偏向城市领域”“统筹城乡发展”“推进城乡一体化”的治理逻辑;十九大以来,城乡关系向着城乡融合的方向发展,迈向城乡共治是新时代城乡融合发展的内在要求与治理趋势。最后,文章从废除城乡二元体制、建立新的配套体制机制,打破学科界限、融合跨学科知识和实践交叉应用,新型城镇化和乡村振兴两大战略有机结合等方面对城乡融合发展与城乡共治进行了研究展望。     

Geology of Macdonald Seamount region,Austral Islands: Recent hotspot volcanism in the south Pacific

The southeastern extension of the Austral Islands volcanic chain terminates near 29°S, 140°W at the active Macdonald Seamount. The hotspot region near Macdonald consists of at least five other volcanic edifices each more than 500 m high, included in an area about 50–100 km in diameter. On the basis of the sea-floor topography, the southeastern limit of the hotspot area is located about 20 km east of the base of Macdonald, where it is defined by the 3950 m isobath. At the edge of the hotspot area, there is a marked deepening of the seafloor from c.3900 m down to 4000–4300 m. The deeper sea-floor is faulted and heavily sedimented. The Macdonald volcano itself stands 3760 m above the surrounding seafloor, and has a basal diameter of 45 km. Its summit in January 1987 was 39 m below sea level, and it seems likely that Macdonald will emerge at the surface in the near future.Recent (March and November 1986) phreatic explosions on Macdonald Seamount erupted fragments of ultramafic and mafic plutonic blocks together with basic lapilli (volcaniclastic sand). The plutonic blocks have been variably altered and metamorphosed, and in some cases show signs of mineralisation (disseminated sulphides). The blocks presumably come from deeper levels in the volcanic system. The volcanics so far dredged from Macdonald consist of olivine and clinopyroxene cumulus-enriched basalts, evolved basalts, and mugearite. On the basis of incompatible element variations, simple crystal fractionation seems to be controlling the chemical evolution of Macdonald magmas.     

Geoid anomalies over the Canary Islands Group

The Canary Islands geoid anomaly shows only a short-wavelength high associated with island topography. The predicted anomaly best matches the observed geoid when we assume that the lithosphere is a very stiff plate. We find no evidence for shallow reheating or a mantle plume, since there is no evidence of a midplate swell and the corresponding elastic thickness is appropriate for the age of the seafloor on which the island group rests.     

农业经济类期刊中土地问题评述——基于国内257篇核心期刊论文的统计分析

基于文献计量学、共词分析法,通过运用Excel, Ccitespace V软件,对中国学术期刊数据库(CNKI)中2002-2018年间发表于农业经济类核心期刊中的257篇土地科学领域的文献进行计量分析。通过整理分析文献发表年度、期刊来源、研究机构分布、作者分布、关键词主题聚类和关键词共现,探讨了我国土地科学领域的研究现状、研究热点,初步推断了土地学科未来的发展趋势。     

热点作用背景下的洋中脊跃迁和扩展作用:印度洋盆地张开过程探讨

在《印度洋底大地构造图》的基础上,分析了印度洋盆构造格局和洋盆演化重大事件序列,并从印度洋盆初始裂解机制、扩张中心跃迁与热点作用、洋中脊扩展作用等方面讨论了印度洋盆的张开过程,提出以下几点认识:(1)现今印度洋洋中脊可分为两个系统:东南印度洋中脊-中印度洋中脊-卡斯伯格洋脊系统(东支)和西南印度洋中脊系统(西支),前者是太平洋洋中脊扩展作用的产物,后者是太平洋-东南印度洋中脊与大西洋中脊之间构造调节的产物;(2)印度洋盆最初裂解受地幔柱垂向挤压-水平伸展作用控制,沿前寒武造山带等地壳薄弱带发育;(3)印度洋盆经历两次扩张中心的跃迁,其趋向性跃迁方向与热点相对板块的运动方向具有一致性,显示两者存在内在联系。(4)大西洋和太平洋洋中脊在印度洋交汇,于古近纪连通,末端伴随陆块持续发生碎裂化、裂解化,可称为鱼尾构造模式,表明印度洋盆衔接和调节了三大洋盆的发育和演化过程,具有全球洋盆枢纽的关键意义。     

生计脆弱性研究热点与前沿的可视化

以Web of science核心数据库中1991~2015年间收录主题为“livelihood vulnerability”的文献为基础,应用可视化分析软件CiteSpace Ⅲ,采取关键词共现分析、文献共被引分析、突现词分析等方法对生计脆弱性研究现状进行可视化分析。研究发现：① 生计脆弱性研究文献呈现逐年上升趋势,主要分布在生态、环境和管理等学科领域;美、英、澳、加、德5国及东英格兰大学、哥本哈根大学、麦吉尔大学、利兹大学、萨塞克斯大学等研究机构具有较强的研究实力。② Ellis F、Smit B、Adger W N、Kelly P M、Davies S和Turner B L等学者及其代表作是生计脆弱性研究的知识基础。③ 饥荒、社会生态系统、生计策略、贫困、减贫等是生计脆弱性的研究热点与前沿领域。