Hydrothermal activity on the southern Mid-Atlantic Ridge: Tectonically- and volcanically-controlled venting at 4–5°S     

C.R. German  S.A. Bennett  D.P. Connelly  A.J. Evans  B.J. Murton  L.M. Parson  R.D. Prien  E. Ramirez-Llodra  M. Jakuba  T.M. Shank  D.R. Yoerger  E.T. Baker  S.L. Walker  K. Nakamura 《Earth and Planetary Science Letters》2008,273(3-4):332-344

We report results from an investigation of the geologic processes controlling hydrothermal activity along the previously-unstudied southern Mid-Atlantic Ridge (3–7°S). Our study employed the NOC (UK) deep-tow sidescan sonar instrument, TOBI, in concert with the WHOI (USA) autonomous underwater vehicle, ABE, to collect information concerning hydrothermal plume distributions in the water column co-registered with geologic investigations of the underlying seafloor. Two areas of high-temperature hydrothermal venting were identified. The first was situated in a non-transform discontinuity (NTD) between two adjacent second-order ridge-segments near 4°02′S, distant from any neovolcanic activity. This geologic setting is very similar to that of the ultramafic-hosted and tectonically-controlled Rainbow vent-site on the northern Mid-Atlantic Ridge. The second site was located at 4°48′S at the axial-summit centre of a second-order ridge-segment. There, high-temperature venting is hosted in an  18 km² area of young lava flows which in some cases are observed to have flowed over and engulfed pre-existing chemosynthetic vent-fauna. In both appearance and extent, these lava flows are directly reminiscent of those emplaced in Winter 2005−06 at the East Pacific Rise, 9°50′N and reference to global seismic catalogues reveals that a swarm of large (M 4.6−5.6) seismic events was centred on the 5°S segment over a  24 h period in late June 2002, perhaps indicating the precise timing of this volcanic eruptive episode. Temperature measurements at one of the vents found directly adjacent to the fresh lava flows at 5°S MAR (Turtle Pits) have subsequently revealed vent-fluids that are actively phase separating under conditions very close to the Critical Point for seawater, at  3000 m depth and 407 °C: the hottest vent-fluids yet reported from anywhere along the global ridge crest.  相似文献   

Catastrophic middle Pleistocene jökulhlaups in the upper Susquehanna River: Distinctive landforms from breakout floods in the central Appalachians     

R. Craig Kochel  Richard P. Nickelsen  L. Scott Eaton 《Geomorphology》2009,110(3-4):80-95

Widespread till and moraines record excursions of middle-Pleistocene ice that flowed up-slope into several watersheds of the Valley and Ridge Province along the West Branch of the Susquehanna River. A unique landform assemblage was created by ice-damming and jökulhlaups emanating from high gradient mountain watersheds. This combination of topography formed by multiple eastward-plunging anticlinal ridges, and the upvalley advance of glaciers resulted in an ideal geomorphic condition for the formation of temporary ice-dammed lakes. Extensive low gradient (1°–2° slope) gravel surfaces dominate the mountain front geomorphology in this region and defy simple explanation. The geomorphic circumstances that occurred in tributaries to the West Branch Susquehanna River during middle Pleistocene glaciation are extremely rare and may be unique in the world. Failure of ice dams released sediment-rich water from lakes, entraining cobbles and boulders, and depositing them in elongated debris fans extending up to 9 km downstream from their mountain-front breakout points. Poorly developed imbrication is rare, but occasionally present in matrix-supported sediments resembling debris flow deposits. Clast weathering and soils are consistent with a middle Pleistocene age for the most recent flows, circa the 880-ka paleomagnetic date for glacial lake sediments north of the region on the West Branch Susquehanna River. Post-glacial stream incision has focused along the margins of fan surfaces, resulting in topographic inversion, leaving bouldery jökulhlaup surfaces up to 15 m above Holocene channels. Because of their coarse nature and high water tables, jökulhlaup surfaces are generally forested in contrast to agricultural land use in the valleys and, thus, are readily apparent from orbital imagery.  相似文献   

Crustal structure of a super-slow spreading centre:a seismic refraction study of Mohns Ridge, 72° N     

F. Klingelhöfer  L. Géli  L. Matias  N. Steinsland  & J. Mohr 《Geophysical Journal International》2000,141(2):509-526

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Gakkel洋中脊基底隆起的非对称地壳结构          下载免费PDF全文

凌子龙  高金耀  赵俐红  杨春国  管清胜  张涛 《地球物理学报》2019,62(5):1755-1771

超慢速扩张的北冰洋Gakkel洋中脊具有六个沿扩张方向的线性基底隆起(本文编号为A—F).这些线性基底隆起在中轴两侧的地球物理场和地壳结构呈现不同程度的非对称性.本文利用Gakkel洋中脊的地形、空间重力异常(FAA)和航空磁力数据,计算了它的扩张速率、剩余地幔布格重力异常(RMBA)、地壳厚度和非均衡地形.根据中轴两侧地形和地壳厚度的对称关系,我们将六个基底隆起分为对称型和非对称型两种类型.整体上,B、D和F区基底隆起在中轴两侧的地形和地壳厚度的非对称幅值(两侧差值的绝对值)较小,其中地形的非对称幅值分别为～157m、～125m、～208m,地壳厚度的非对称幅值分别为～1km、～0.06km、～0.3km;而A、C和E区的非对称幅值较大,其中地形的非对称幅值分别为～510m、～410m、～673m,地壳厚度的非对称幅值分别为～2km、～2.5km、～1.1km.我们因此推断B、D和F区具有相对对称的地壳结构,而A、C和E区具有非对称的地壳结构.根据A、C和E区中轴两侧非均衡地形的对称关系和非对称地形的补偿状态,推测A区的非对称性可能是由岩浆分配不均所导致;而C区和E区的非对称性可能是由构造断层作用使断层下盘向上抬升变薄所导致.我们进一步推测洋中脊走向的改变可能使得构造作用更易集中于基底隆起的一侧.  相似文献   

对雅鲁藏布江结合带形成演化的再探讨          下载免费PDF全文

张鑫全  张振利  王金贵  王硕  杨鑫朋  专少鹏  侯德华  张泽国  张立国  程洲 《中国地质》2020,47(4):944-970

通过1∶5万区域地质调查和收集相关资料的综合研究,本文对雅鲁藏布江结合带的形成演化作了进一步的探讨。雅鲁藏布江特提斯洋具有弧后扩张洋盆的性质,在早三叠世至中三叠世中期洋盆初步形成,中三叠世晚期至晚三叠世洋盆全面形成,从早侏罗世至晚白垩世洋盆逐步萎缩,到古新世至始新世关闭。南带的蛇绿岩主要为洋中脊扩张型(MORB型),形成于中三叠世晚期至晚三叠世。北带的蛇绿岩主要为与洋内俯冲相关的俯冲带上盘型(SSZ型),形成于早中侏罗世。带内侏罗纪至白垩纪其他岩浆岩主要为前弧玄武岩类(FAB型)。显示雅鲁藏布江特提斯洋从早侏罗世开始发生了洋内俯冲,并同步向北向冈底斯带之下主动俯冲消减和向南向喜马拉雅地块之下被动俯冲消减,持续发展到晚白垩世,在古新世至始新世俯冲碰撞消亡转化为结合带。  相似文献   

黄河和长江碎屑物质扩散研究——来自江苏沙脊物源示踪的约束     

林旭  刘海金  吴中海 《地球学报》2024,45(2):165-174

研究源-汇系统对于理解岩石风化、河流输砂、泥砂沉积以及陆地和海洋沉积过程具有重要意义。我国江苏省东海岸具有丰富的沉积物供应,在某些沿海地段形成了一系列沙脊,是进行沉积物源-汇过程研究的理想沉积记录之一。但目前对其物源区来自哪里还未达成共识。我们利用已发表的碎屑锆石U-Pb年龄和钾长石Pb同位素数据,结合江苏沙脊及其周缘区域的数据结果,详细厘定江苏沙脊的物源区,并运用蒙特卡罗反演模型定量评估江苏沙脊潜在物源区对其物质贡献率。结果表明江苏沙脊北部的碎屑物质主要受大别山(45%)和鲁中山区(24%)的共同影响;黄河(29%)、古黄河(27%)和长江(25%)的碎屑物质对江苏沙脊中部的物质组成起重要影响。江苏沙脊南部的物源主要来自长江(68%)。江苏沙脊的出现得益于黄河和长江这样的大陆尺度的大河搬运了丰富的碎屑物质到下游,南黄海西海岸的平坦地貌有利于保存这些碎屑物质,末次冰期后东亚陆架海动力过程(潮汐、海浪和沿岸流等)持续对其改造,这体现了河流-海洋在塑造河口-海岸地貌中的交互作用。  相似文献   

卡尔斯伯格脊 60°~61°E洋脊段多波束后向散射特征及其对构造与岩浆作用强度的指示          下载免费PDF全文

杨驰  韩喜球  王叶剑  李洪林  邱中炎  吴招才 《海洋学研究》2018,36(3):37-49

多波束声纳数据可以有效记录海底地形地貌和底质特征信息。本文利用船载多波束数据对慢速扩张的卡尔斯伯格脊60°~61°E洋脊段的典型构造地貌单元的后向散射强度特征进行了研究,在此基础上,分析了该洋脊段的构造和岩浆作用强度特征。结果表明,洋脊段I以构造拉张作用占主导,脊轴及附近后向散射强度为-29 dB左右,裂谷壁高差可达1 200 m以上,裂谷内断裂发育,裂谷侧翼高度与裂谷宽度的比值为78.7~126.2,裂谷两侧翼部线性构造较少,但轴向正断层面更宽,倾角更小;与洋脊段裂谷中段相比,末端火山活动频率较低但喷发规模较大,火山机构数量和体积也更大,且可发育深大断裂获取深部热源。洋脊段II以岩浆作用占主导,脊轴及附近后向散射强度达-35 dB,裂谷内轴向火山脊发育,裂谷壁高差小于500 m,裂谷侧翼高度与裂谷宽度的比值为77.6~116.8,裂谷两侧翼部线性构造数量众多、长宽比较大且呈近似对称,相邻线性构造之间沉积物广泛分布。通过提取挖掘与底质属性密切相关的多波束后向散射强度数据,结合海底地形地貌的分析,可以为洋中脊的构造和岩浆作用强度的定量研究提供有效的证据。  相似文献   

Canadian expedition to study the Alpha Ridge complex: results of the seismic refraction survey     

I. Asudeh  A. G. Green  D. A. Forsyth 《Geophysical Journal International》1988,92(2):283-301

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How partial melts of mafic lower crust affect ascending magmas at oceanic ridges     

Astri J. S. Kvassnes  Timothy L. Grove 《Contributions to Mineralogy and Petrology》2008,156(1):49-71

We present experiments showing that the lower oceanic crust should melt efficiently and quickly when heated by hot ascending magmas. Average plagioclase–olivine and plagioclase–augite pairs from the lower crust at the Southwest Indian Ridge have melt–mineral saturation boundaries at 1,190 and 1,154°C, respectively, and melt rapidly (>0.01 mm/h) at 50°C or more above these temperatures. Melting experiments performed on olivine–plagioclase and augite–plagioclase mineral pairs from actual oceanic lower crustal rock samples and under conditions applicable to a MOR setting (1,220–1,330°C, 1 atm, quartz–fayalite–magnetite oxygen buffer, 0.25–24 h) indicate that the resulting disequilibrium melts are linear mixes of the mineral compositions. The rates of melting are slower than the rate of heat-diffusion into a sample and are approximated as: Our results indicate that great care must be taken in backward models using basalt chemistry alone to explore mantle-melting processes, assuming only crystallization and fractionation during ascent, as partial melts may mix with intruded hot magma.  相似文献   

Results of a sea-floor electromagnetic survey over a sedimented hydrothermal area on the Juan de Fuca Ridge     

D. C. Nobes  L. K. Law  R. N. Edwards 《Geophysical Journal International》1992,110(2):333-346

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