ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00763.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00764.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTGlaciers and volcanoes have been the trademark of Iceland for centuries. The glaciers now cover ≅10% of the country and the volcanic regions are covered by lava flows every 5–10 kyr on average. Naturalists have concentrated on these two aspects, making notes on volcanic eruptions and traveling along the glacier margins ever since the first settlement of Iceland in the ninth century in some of the earliest episodes of geological research. Systematic studies of the glaciers began in the latter half of the 18th century. Even earlier, features such as striations and moraines were discovered and described in locations remote from the contemporary glacier margins. These features were interpreted as the effect of a much more extensive ice sheet on the island. At the beginning of the 20th century the discussion of the origin of sediments was unseparable from the age problem. From 1910 to 1950 very few new data were presented on the glaciation history of Iceland. A few sporadic publications either accepted or rejected previous conclusions. Systematic geological mapping of the country started in the wake of World War II encouraged by the acceptance of the plate tectonic theory in the late sixties. One of the most important results of this revived mapping effort was the identification of interbedded sediments interpreted as glacial deposits. Around 1975 Iceland was known in the geological literature for preserving more numerous glacial deposits than found elsewhere on land in the northern hemisphere. Over the last few years, systematic lithofacies analysis of sediments interbedded within lava flows has been conducted with the aim of delineating the earliest glaciation of Iceland and the periodicity of glaciation through the Tertiary and Quaternary. The results show a gradual growth of ice from south‐east towards the north and west during the Pliocene‐Pleistocene transition interrupted by periods of recessions and intermittent ice‐free c

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00765.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTResults of electron microprobe and microthermometric studies of samples collected from the Bouvet Triple Junction Region (BTJR) during a joint Russian‐Italian geological expedition on the R/VAcademician Nikolaj Strakhov(1994) have revealed new data on the composition of basaltic magmas and oceanic hydrothermal fluids connected with magmatic processes. Detailed analysis of basaltic glasses shows that the modem Mid‐Atlantic Ridge (MAR) rift valley is composed of normal mid‐ocean ridge basalts with low concentrations of K2O and TiOz(N‐MORB), while its flanks are more enriched with these components approaching E‐MORB. A marked influence of the Bouvet hot spot volcanism on magma generation on the South‐West Indian Ridge (SWIR) near Bouvet Island is observed. Basaltic melts in this area belong to alkalic and transitional series and have maximum contents of K2O, TiO2, H2O.Microthermometric analyses of fluid inclusions in the samples from the BTJR have revealed major differences in the oceanic hydrothermal fluid systems on the MAR and near SWIR, which depends on the peculiarities of magma. In the area of the MAR (with dry melts) only H2O solution inclusions in quartz were found; thus, seawater is probably the only primary source of hydrothermal fluids (NaCl + MgCl2+ H2O;T= 170–200°C). In the SWIR area (with the high content of water in melts) syngenetic liquid CO2and H2O solution inclusions in quartz indicate the influence of the magmatic fluid component on the ore‐forming water/carbon dioxide solutions (NaCl + CaC12+ H2O + CO2;T= 200–310

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00766.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTDetrital micas are moderately abundant components in lithic sandstones. Examples from the Törtel and Szolnok Formations (Pannonian basin) and from the Lower Freshwater Molasse and Upper Marine Molasse (Swiss Molasse basin) show that the response of micas to increasing effective stress due to burial is substantially different from that of true ductile particles: the mica plates bend, but their thickness remains essentially constant. Based on this observation, a new model is proposed to describe the relationship between porosity loss and the ratio of mica‐thickness and grain size. The results of calculations emphasize the importance of this latter parameter, as opposed to porosity loss from deformation of true ductile grains which is independent of the relationship between the grain sizes of deforming and rigid componen

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00767.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTArtificial radionuclides (60Co,106Ru,134Cs and137Cs) produced in the nuclear power plants along the Rhǒne valley have been detected in most of the 35 sediment samples collected from the shores of the Rhǒne river in southern France. In one sample traces of241Am were found, suggesting the presence of transuranic elements. The wide use of river water for irrigation and the occasional big inundations of the Rhǒne have spread the artificial radionuclides over huge areas, quite far behind the river shores. Although artificial radionuclides are present in fields where food crops are collected, their activity is currently considerably lower than the activity of the natural radionuclides present in the river sediment.

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00768.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe Tyrrhenian rift zone has been the site of widespread magmatism since late Tortonian times. A pronounced asymmetrical distribution, reflecting the tectonic structure, characterizes Italian magmatism. Sodic basalts occur on the western Tyrrhenian flank and transitional‐MORB basalts occur in the Tyrhenian Sea. The eastern flank, however, is characterized by K‐alkaline and HK‐ to ultra‐alkaline (e.g. carbonatites and melilitites) rocks. Major trace elements and isotopic compositions allow two major magmatic lineages to be identified: one relating to a non‐radiogenic basaltic end‐member and the other to a mantle end‐member enriched in Ca, with high LILE/HFSE ratio and high Sr isotopic ratios. Their mantle sources are located within the lithosphere thermal boundary layer (TBL) and the metasomatized phlogopite‐carbonate asthenosphere at the base of the TBL, respectively. The composition and spatial distribution of volcanism and relative mantle sources tend to map the geometry of the lithospheric mantle and to define a pronounced increase in depth from less than 60 km to about 100 km across the boundary between the thinned Tyrrhenian lithosphere and the Adriatic lithosphere. A mechanism of intra‐continental passive rifting, which drives mantle upwelling, is sufficient to satisfy the petrological and geochemical constraints and the observed tectonic environment without requiring a

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00769.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe Burdekin River is an example of a class of tropical streams which experience two to four orders of magnitude variation in discharge, in response to seasonal but erratic monsoonal rainfall. Floods of the Burdekin rise abruptly, reaching peak discharges of up to 40,000 m3s‐1in less than 24 h; maintain peak flow for up to a few days, and recede exponentially. The geomorphology and deposits of these rivers reflect the extreme discharge fluctuations, and have not previously been described.A stretch of the upper Burdekin River comprising four bends and one straight reach was examined near the town of Charters Towers. The river bed is largely exposed for most of any year, with a small, misfit perennial channel carrying low stage flow. Major geomorphic elements of bends include point bars with ridge‐and‐swale topography, three distinct types of chute channels, avalanche slipfaces up to 5 m or more high around the downstream edges of bars, and on the outer part of one point bar an elevated, vegetated ridge. Straight reaches are flat or gently inclined, sand‐ and gravel‐covered surfaces. Much of the river bed is covered by well sorted, in places gravelly, coarse to very coarse‐grained sand with local accumulations of pebble to boulder gravel. Lower parts of the river bed are periodically draped by mud which is desiccated on exposure. Dunes and plane beds are the most commonly occurring bedforms, with local development of gravelly antidunes. Most bank tops and upper, vegetated bars are covered by silt and fine‐grained sand. The river bed also hosts a low‐diversity but locally high‐abundance, flood‐tolerant flora dominated by the paperbark treeMelaleuca argentea, which plays an important role in controlling the distribution of sediment.The gross geomorphology of the river bed and most of the sedimentary features are interpreted as having formed during major (bankfull or near bankfull) flows, which have a recurrence of about 18 years (based on 65 years hydrographic data). The initial rapid drop in discharge following flood peaks appears to preserve flood peak features on upper bars more or less intact, whereas lower areas are subjected to variable degrees of modification during falling stage and by more frequent, non‐bank

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00770.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTFission track analyses of apatites from sediments, Precambrian gneisses and Caledonian to Tertiary intrusive rocks from the Kangerlussuaq region reveal its post‐Caledonian thermotectonic history. Inland the history involves cooling to temperatures within the high temperature part of the apatite annealing interval and slow cooling (or reheating) continued in Cretaceous times through this interval. Apatites from coastal areas between Kangerlussuaq and Tasiilaq reveal only Tertiary cooling. In Tertiary times cooling accelerated after the main intrusive phase in the Tertiary. The evolution is taken as evidence for a general uplift/erosion since Caledonian times probably disturbed by basin formation and sedimentation and reheating due to magmatic activity. Thermal subsidence of the rift shoulders following the opening of the adjacent oceanic basin is not indicated. Annealing patterns inland of the plume centre in the Kangerlussuaq provide no evidence for the earlier movement of the plume from a westerly directio

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00771.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe Umbria‐Marche fold‐and‐thrust belt (Northern Apennines, Italy), provides excellent opportunities to evaluate the structural heritage of the opening of the Mesozoic Tethys Ocean in the 3D geometry of the Neogene compressional structures related to the Alpine Orogeny. The structure and evolution of a portion of the southernmost belt, between the Nera River and the Rieti Basin, are described as a field example, and the kinematics along well‐exposed Mesozoic extension structures are provided. Cross‐section restoration shows a close coincidence between these extension structures and the Neogene thrust ramps, thus suggesting that the geometry of the latter was controlled by the map distribution of the former. Sequential balancing also allows for the definition of the geometrical pattern of pre‐existing normal faults, which were produced in response to a unidirectional or a two‐directional extension stress field. The inferred direction of principal extension, corrected for the effects of late deformation, is consistent with that proposed for the northern margin of the Adria Promontory in global‐scale plate tectonic

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00772.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY