ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1993.tb00217.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1993.tb00218.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1993.tb00219.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTRecent reviews of the development of palaeomagnetic techniques in geology have shown that palaeomagnetism has contributed significantly to the acceptance of new concepts in global tectonics. The present paper extends Frankel's (1987) review of palaeomagnetic work on the basalt lava sequences of Iceland in the early 1950s. These lavas provide some of the most complete series of reliable ‘spot readings’ of the geomagnetic field available. It is shown that research on Icelandic basalts has continued to be of importance in improving our understanding of palaeomagnetism and its applicati

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1993.tb00220.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTA geochronological investigation of two rocks with an eclogitic assemblage (omphacite‐garnet‐quartz‐rutile) from the High Himalaya using the Sm/Nd, Rb/Sr, U/Pb and Ar/Ar methods is presented here. The first three methods outline a cooling history from the time of peak metamorphism at 49±6 Ma recorded by Sm/Nd in garnet‐clinopyroxene to the closure of Rb/Sr in phengite at 43±1 Ma and U/Pb in rutile at 39–40 Ma. The Sm/Nd isotopic system was fully equilibrated during eclogitization and has not been disturbed since; its mineral ages may date the peak metamorphic conditions (650±50°C at 13–18 kbar: Pognante and Spencer, 1991). The Ar/Ar data reveal the presence of substantial amounts of excess40Ar in hornblende, and yield a statistically acceptable but geologically meaningless phengite plateau age of 81.4±0.2 Ma, inconsistent with Sm/Nd, Rb/Sr and U/Pb. This questions the use of such a chronometer for the dating of high‐pressure assemblages. The results imply a Late Palaeocene or Early Eocene subduction of the northern Indian plate margin in NW Himalaya. The fact that eclogites are restricted to NW Himalaya may be the result of a peculiar p‐T‐t path associated with a high convergence rate during the first indentation, in contrast to the later and slow subduction in Centra

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1993.tb00221.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTBorehole temperature logs have been inverted to infer ground temperature histories (GTH) in eastern and central Canada. Regional ground temperature histories were obtained by simultaneous inversion of several temperature profiles from the same areas. Simultaneous inversion of 21 temperature logs sampled across all of eastern and central Canada yielded an average solution for this region. All but three of the studied sites show signs of warming in the last 150 years. This period of warming, which started after 1800 AD, was found throughout this part of Canada. The warming followed a cooler period corresponding to the little Ice Age. The inferred ground temperature histories exhibit long‐term trends similar to those obtained from treering growth indices in nearby regions and stable isotope data in the southern hemisphere. The modern warming appears correlated with the atmospheric concentration of CO2as measured in ice core

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1993.tb00222.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTFlume experiments show that current ripples on very fine sand surfaces always develop towards a linguoid shape with constant height and wavelength provided that sufficient time is allowed for their formation. Straight and sinuous current ripples only reflect intermediate stages in ripple development and may be regarded as non‐equilibrium bedforms. The time period which current ripples require to reach linguoid equilibrium morphology is related to an inverse power of flow velocity. In the transitional stage from current ripples to upper stage plane bed (i.e. washed‐out ripple stage) only the equilibrium wavelength remains constant, whereas equilibrium height rapidly decreases to zero.Our observations imply that bed‐roughness parameters in sediment transport calculations can be simplified when equilibrium conditions are attained, and that inferences about flow energy from the dimensions of current ripples in very fine sand need to be regarded with ca

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1993.tb00223.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTThe Palaeocene alluvial‐fan succession in central Anatolia, Turkey, contains three isolated zones of beekite‐encrusted clasts. Each zone is 3–5 m thick andc. 1 km long. Beekite is developed as thin (1–5 mm), concentric silica rings formed by replacement and encrustation of carbonate clasts. The formation of beekites requires a relatively long time, with non‐deposition and fluctuating arid/semiarid conditions. The significance of beekite is thus analogous to that of silcrete. The presence of beekite encrustation within the Palaeocene alluvial‐fan succession indicates considerable breaks in sedimentation in the eastern part of the basin, and this inference is supported by the lower thickness of the alluvium

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1993.tb00224.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTThe Kodiak Shelf Basin is one of the undeveloped basins in Alaska. In surrounding areas, hydrocarbon accumulations have been found. It is of interest to investigate whether there might also be hydrocarbons generated in the Kodiak Shelf Basin.Using both one‐ and two‐dimensional fluid flow/compaction models, together with geological, geophysical and geochemical data from the Kodiak Shelf Basin, we simulate the development with time and depth of sedimentary compaction, porosity, permeability, pressure, palaeoheat flux variation and palaeotemperature variation. We then evaluate quantitatively the maturation of organic matter in the basin. The model results indicate that there was only a low likelihood of hydrocarbon generation in the basin due mainly to the combined effects of a cool palaeothermal history and low organic carbon cont

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1993.tb00225.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTThe mobilities of major and trace elements have been investigated in hydrothermally altered volcanic rocks of Pliocene age on the island of Lesvos. One type of hydrothermal assemblage, montmorillonite‐K‐feldspar‐goethite, was recognized in outcrops north of Mytilini town.The alteration occurs a few metres under the surface of unaltered volcanic rocks of high‐K andesite and high‐K basaltic andesite composition. The unaltered latite‐andesite consists of plagioclase, clinopyroxene, sanidine and magnetite with quartz, calcite, pumpellyite and montmorillonite as subordinate secondary minerals.The main control over major oxide distribution appears to be the extent of conversion of plagioclase to K‐feldspar.Three distinct groups of elements were revealed to be positively correlated:1Si, Mg, Ca, Na, Mn, Sr and Ba, which are leached during the alteration process. This group of elements reflects their association with plagioclase feldspar and clinopyroxene.2Fe, K, Ti, P, Nb, Zr, Y, Rb, Ni, Cr, V. This group includes elements which are enriched relative to the unaltered volcanic rock, reflecting their association with K‐feldspar, iron hydroxides and Ti and P‐bearing minerals.3Ti, P, Nd, La, Ce. From this group of elements it is indicated that Nd, La and Ce are associated with Ti‐ and P‐bearing minerals.There is no evidence of Cu or Zn addition to the system by hydrothermal fluids since there are no correlations between these base metals and elements enriched in the K‐feldspar, montmorillonite‐rich zone. This potassic rich zone may be connected with hidden epithermal sulphide

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1993.tb00226.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY