Geophysical evidence for a large impact structure on the Falkland (Malvinas) Plateau          下载免费PDF全文

Maximiliano C.L. Rocca  Michael R. Rampino  Jaime L. Báez Presser 《地学学报》2017,29(4):233-237

A large, roughly circular structural basin is recognised on the Falkland (Malvinas) Plateau to the NW of West Falkland (Gran Malvina) Island (S 51°00′, W 62°00′). The basin, seen in seismic‐reflection profiles and evident as a large negative gravity anomaly, has a diameter of ~250 km. The age of the basin is estimated to be Late Palaeozoic. It is completely buried by younger sediments and has no topographic expression on the sea floor. We propose that the basin and geophysical anomalies, especially the combination of a large circular negative gravity anomaly with a rim of positive anomalies, and a marked circular series of positive magnetic anomalies in the same area, may be best explained by the presence of a large buried impact structure.  相似文献   

Influence of Water Content on the Mechanical Properties of an Argillaceous Swelling Rock   总被引：1，自引：0，他引：1  

Maximiliano R. Vergara  Theodoros Triantafyllidis 《Rock Mechanics and Rock Engineering》2016,49(7):2555-2568

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Back to the future: reflections and directions of South African marine bioinvasion research     

ME Alexander  CA Simon  CL Griffiths  K Peters  S Sibanda  S Miza 《African Journal of Marine Science》2016,38(1):141-144

Biological invasions continue to increase around the world, with impacts on many coastal marine systems. Here we review the South African marine invasion literature which, despite the field being relatively new, has grown to have significant presence in both the local and international arenas. Of the 79 papers reviewed, 70% focused on the establishment and spread of alien species, with modes of transport and introduction largely overlooked. An emphasis was also apparent towards field studies, in particular survey work, with few experimental studies. The overwhelming majority of papers focused on a single species, the Mediterranean mussel Mytilus galloprovincialis, reflecting the scale of this invasion and the tractable nature of rocky shores as study systems. With the exception of this one species, the impacts of marine alien species have rarely been quantified. We suggest that future research extends the taxonomic coverage of present work and develops a better understanding of the mechanisms of introduction, establishment and spread of marine alien species. Through an experimental approach, the drivers of altered ecological patterns and processes resulting from invasions should be addressed, providing insight into associated impacts. This approach will maintain the local applicability and international relevance of South African marine invasion research.  相似文献   

Impacts of alien ‘ecosystem engineers’ overwhelm interannual and seasonal shifts in rocky-shore community composition on Marcus Island,South Africa     

S Sadchatheeswaran  GM Branch  CL Moloney 《African Journal of Marine Science》2018,40(2):137-147

The South African coastline has been invaded by numerous alien species. Rare pre-invasion (1980) and post-invasion datasets (2001 and 2012) exist for Marcus Island, a small land-tied island in Saldanha Bay, South Africa. These snapshot datasets of the island’s intertidal invertebrate community were complemented with monitoring across seasons, from 2014 to 2016. Invertebrate communities were compared among the summers of 1980, 2001, 2012, 2014, 2015 and 2016 to assess interannual differences, while invertebrates and algae were monitored quarter-annually to assess seasonal changes. In addition, the population dynamics of the alien mussel Mytilus galloprovincialis were monitored. Differences in invertebrate communities between consecutive summers were significant but much smaller than changes induced by the arrival of alien species. Invertebrate and seaweed communities differed among years and shore zones but not among seasons, whereas species diversity differed among years, seasons and shore zones, with zones having the strongest influence. The invasion by M. galloprovincialis, and ensuing spatial and temporal variability in its recruitment, emerged as the most important factor influencing community composition, overshadowing interannual and seasonal changes. This work highlights that the impacts of alien species can be distinguished from natural variability by combining long-term monitoring with surveys at finer temporal scales. This is an important step in extending our understanding of the impacts of marine alien species.  相似文献   

A review of the assessments of the major fisheries resources in South Africa     

CL de Moor  SJ Johnston  A Brandão  RA Rademeyer  JP Glazer  LB Furman 《African Journal of Marine Science》2015,37(3):285-311

The waters off South Africa's coastline boast a rich mix of commercially fished species. Quantitative assessments of these marine resources have developed from simple methods first applied in the 1970s, to models that encompass a wide range of methodologies. The more valuable resources have undergone regular assessments in recent decades, with frequencies closely related to the management approach employed for each fishery. Many of these assessments form the operating models used to simulation-test candidate management procedures. This paper provides a comprehensive review of the assessments of 11 of the most important fisheries resources in South Africa. Some assessments use simple biomass dynamics models, whereas others are a hybrid of age- and length-based models, each designed to model the specific characteristics of the resource and fishery concerned. Many of the assessments have been disaggregated by species/stock and/or area as related multispecies/stock/ distribution hypotheses have arisen. This paper explores the similarities and differences in the data available and the methods applied. The review indicates that, whereas the status of three of these resources cannot be estimated reliably at present, the status of six resources is considered to be reasonable to good, whereas that of abalone Haliotis midae and West Coast rock lobster Jasus lalandii remains poor.  相似文献   

赣南营前岩体的年代学、地球化学、Sr-Nd-Hf同位素组成及其地质意义   总被引：6，自引：1，他引：5  

郭春丽  毛景文  陈毓川 《岩石学报》2010,26(3):919-937

赣南营前岩体由早期似斑状花岗闪长岩和晚期细粒黑云母花岗岩组成,在前者中普遍发育暗色基性包体。SHRIMP锆石U-Pb测年显示似斑状花岗闪长岩为172.2±3.1Ma,细粒黑云母花岗岩为168±3Ma,包体为167.6±3.3Ma。所有的岩石均为钙碱性系列,稀土元素显示LREE富集,HREE亏损,Eu负异常并不明显的特征,Nb-Ta负异常显示壳源物质的特征。晚期细粒黑云母花岗岩、早期似斑状花岗闪长岩、暗色包体的(87Sr/86Sr)i和εNd(t)值分别为0.70885～0.70965、0.71058～1.46393、0.70788～0.70923和-6.38～-5.31、-6.51～-5.17、-5.47～-4.31;锆石εHf(t)值分别为-12.4～-5.7、-9.2～-4.2、-9.4～+0.1。结合岩相学、地球化学研究,初步认为营前岩体的两期花岗质岩石源于地壳重熔,地幔底侵导致地壳熔融形成营前岩体,基性岩浆加入到未冷却的酸性岩浆中形成了目前所见到的暗色包体。结合区域上的研究资料,认为中侏罗世期间,沿"十杭"断裂带及与其相平行的一些深断裂发生了板块的撕裂,这些缝合带是岩石圈相对薄弱的地带,有利于幔源物质底侵上升以及壳幔相互作用。  相似文献   

15,000-yr pollen record of vegetation change in the high altitude tropical Andes at Laguna Verde Alta, Venezuela     

Valentí Rull  Mark B. Abbott  Pratigya J. Polissar  Alexander P. Wolfe  Maximiliano Bezada  Raymond S. Bradley 《Quaternary Research》2005,64(3):308-317

Pollen analysis of sediments from a high-altitude (4215 m), Neotropical (9°N) Andean lake was conducted in order to reconstruct local and regional vegetation dynamics since deglaciation. Although deglaciation commenced 15,500 cal yr B.P., the area around the Laguna Verde Alta (LVA) remained a periglacial desert, practically unvegetated, until about 11,000 cal yr B.P. At this time, a lycopod assemblage bearing no modern analog colonized the superpáramo. Although this community persisted until 6000 cal yr B.P., it began to decline somewhat earlier, in synchrony with cooling following the Holocene thermal maximum of the Northern Hemisphere. At this time, the pioneer assemblage was replaced by a low-diversity superpáramo community that became established 9000 cal yr B.P. This replacement coincides with regional declines in temperature and/or available moisture. Modern, more diverse superpáramo assemblages were not established until 4600 cal yr B.P., and were accompanied by a dramatic decline in Alnus, probably the result of factors associated with climate, humans, or both. Pollen influx from upper Andean forests is remarkably higher than expected during the Late Glacial and early to middle Holocene, especially between 14,000 and 12,600 cal yr B.P., when unparalleled high values are recorded. We propose that intensification of upslope orographic winds transported lower elevation forest pollen to the superpáramo, causing the apparent increase in tree pollen at high altitude. The association between increased forest pollen and summer insolation at this time suggests a causal link; however, further work is needed to clarify this relationship.  相似文献   

Cretaceous deformation of the southern Central Andes: synorogenic growth strata in the Neuquén Group (35° 30′–37° S)          下载免费PDF全文

Lucas M. Fennell  Andrés Folguera  Maximiliano Naipauer  Guido Gianni  Emilio A. Rojas Vera  Germán Bottesi  Victor A. Ramos 《Basin Research》2017,29(Z1):51-72

The Neuquén Group is an Upper Cretaceous continental sedimentary unit exhumed during the latest Miocene contractional phase occurred in the southern Central Andes, allowing a direct field observation and study of the depositional geometries. The identification of growth strata on these units surrounding the structures of the frontal parts of the Andes, sedimentological analyses and U–Pb dating of detrital components, allowed the definition of a synorogenic unit that coexisted with the uplift of the early Andean orogen since ca. 100 Ma, maximum age obtained in this work, compatible with previous assignments and constrained in the top by the deposition of the Malargüe Group, in the Maastrichtian (ca. 72 Ma). The definition of a wedge top area in this foreland basin system, where growth strata were described, permitted to identify a Late Cretaceous orogenic front and foredeep area, whose location and amplitude contrast with previous hypotheses. This wedge top area was mostly fed from the paleo‐Andes with small populations coming from sources in the cratonic area that are interpreted as a recycling in Jurassic and Lower Cretaceous sections, which contrasts with other analyses performed at the foredeep zone that have mixed sources. In particular, Permian sources are interpreted as coming directly from the cores of the basement structures, where Neopaleozoic sections are exposed, next to the synorogenic sedimentation, implying a strong incision in Late Cretaceous times with an exhumation structural level similar to the present. The maximum recognised advance for this Late Cretaceous deformation in the study area is approximately 500 km east of the Pacific trench, which constitutes an anomaly compared with neighbour segments where Late Cretaceous deformations were found considerably retracted. The geodynamic context of the sedimentation of this unit is interpreted as produced under the westward fast moving of South America, colliding with two consecutive mid‐ocean ridges during a period of important plate reorganisation. The subduction of young, anhydrous, buoyant lithosphere would have produced changes in the subduction geometry, reflected first by an arc waning/gap and subsequently by an arc migration that coexisted with synorogenic sedimentation. These magmatic and deformational processes would be the product of a shallow subduction regime, following previous proposals, which occurred in Late Cretaceous times, synchronous to the sedimentation of the Neuquén Group.  相似文献   

DIRECT LOCATION OF OIL AND GAS BY THE SEISMIC REFLECTION METHOD*     

POH-HSI PAN  J. CL. DE BREMAECKER 《Geophysical Prospecting》1970,18(Z1):712-727

The seismic velocity of sedimentary rocks is determined by the porosity, the velocity in the matrix and that in the fluid. The last two are known experimentally. The velocity can be measured from the surface. By applying the time average equation we can find the porosity. If the only difference at two points is the nature of the fluid in one layer, the ratio of the reflectivities of the layer at these points determines the nature of the fluid; this ratio is equal to the ratio of the amplitudes at these points. The size of the reservoir can then be found. The method can be applied to both oil exploration and production. It is more accurate in the latter case because more information is available than in the former case. The method is also more effective in developing discovered fields than the conventional method of relying only on the geological structural configurations.  相似文献   

Preliminary results of a finite-element,multi-scale model of the Mahakam Delta (Indonesia)   总被引：1，自引：1，他引：0  

Benjamin de Brye  Sébastien Schellen  Maximiliano Sassi  Bart Vermeulen  Tuomas Kärnä  Eric Deleersnijder  Ton Hoitink 《Ocean Dynamics》2011,61(8):1107-1120

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