Diagenesis or dire genesis? The origin of columnar spar in tufa stromatolites of central Greece and the role of chironomid larvae     

JULIAN E. ANDREWS  ALAN C. KENDALL 《Sedimentology》2011,58(5):1283-1302

Two Greek Pleistocene tufa stromatolites were examined petrographically and with stable isotope geochemistry to determine whether calcite spar is of primary or diagenetic origin. The younger (ca 100 ka) tufa from Zemeno primarily is micritic, with primary columnar calcite spar restricted to areas immediately above chironomid larval tubes. This relationship suggests that chironomid larval feeding behaviour is responsible for the development of Zemeno tufa columnar calcite, probably involving biological substances smeared onto the tufa surface. Most micritic crystals are not suitably oriented to allow later post‐depositional growth resulting in columnar fabrics. The older (ca 1 Ma) predominantly sparry tufa from Nemea contains some chironomid tubes and organic cyanobacterial filaments preserved in crystal fans but also contains many fabrics found in primary speleothem spar. Columnar spar here is unlikely to be the result of post‐depositional crystal growth. A comparison of stable isotopic trends between the two tufa deposits suggests that both contain interpretable seasonal trends and implies little or minor post‐depositional alteration of either tufa. Consequently, there is no basis for the common assumption that sparry tufa fabrics must be of diagenetic origin.  相似文献   

Convection forced by a descending dry layer and low-level moist convergence     

ANDREW RUSSELL  GERAINT VAUGHAN  EMILY G. NORTON  HUGO M. A. RICKETTS  CYRIL J. MORCRETTE  TIM J. HEWISON  KEITH. A. BROWNING  ALAN M. BLYTH 《地球，A辑:动力气象学与海洋学》2009,61(2):250-263

A narrow line of convective showers was observed over southern England on 18 July 2005 during the Convective Storm Initiation Project (CSIP). The showers formed behind a cold front (CF), beneath two apparently descending dry layers (i.e. sloping so that they descended relative to the instruments observing them). The lowermost dry layer was associated with a tropopause fold from a depression, which formed 2 d earlier from a breaking Rossby wave, located northwest of the UK. The uppermost dry layer had fragmented from the original streamer due to rotation around the depression (This rotation was also responsible for the observations of apparent descent—ascent would otherwise be seen behind a CF). The lowermost dry layer descended over the UK and overran higher  θ _w   air beneath it, resulting in potential instability. Combined with a surface convergence line (which triggered the convection but had less impact on the convective available potential energy than the potential instability), convection was forced up to 5.5 km where the uppermost dry layer capped it. The period when convection was possible was very short, thus explaining the narrowness of the shower band. Convective Storm Initiation Project observations and model data are presented to illustrate the unique processes in this case.  相似文献   

Distributed, right-lateral strike-slip in Prins Karls Forland, western Svalbard     

ALAN MORRIS 《Polar research》1989,7(1):79-82

The 'Scotiadalen Fault'appears on many maps but has not been identified as a single fault in the field. In addition, the sense of motion on the fault has been an open question. Here I show that this structure is a zone of distributed dextral strike-slip that is probably the result of Tertiary plate motion as the North Atlantic opened. As such it is one of the very few fault zones documented to show direct evidence of dextral, presumably Tertiary, strike-slip.  相似文献   

Geostrophic sand ridge, dune fields and associated bedforms from the Northern KwaZulu-Natal shelf, south-east Africa     

PETER J. RAMSAY  ALAN M. SMITH†  THOMAS R. MASON 《Sedimentology》1996,43(3):407-419

Subaqueous dunes are formed on the KwaZulu-Natal outer-shelf due to sediment transport by the Agulhas Current (geostrophic current). These dunes occur within two dune fields at depths of ? 35 to ? 70 m. The net sediment transport direction is south, but short-period reversals form northward-migrating bedforms. The dune fields are physically bounded by late Pleistocene beachrock and aeolianite ledges. A bedform hierarchy has been recognized in the dune fields comprising a system of three generations of climbing bedforms. The outer dunefield has given rise to a sand ridge (H=12 m; L=4 km; W=1.1 km; and an 8° lee slope) whereas the inner dune fields have achieved large-scale dune status. Bedload parting zones within the dune fields occur where the sediment transport direction switches from north to south due to reversals in the geostrophic flow; these zones occur at depths of ? 60, ? 47 and ? 45 m. An interpretative stratigraphic model is presented on what such geostrophite deposits would look like in the ancient sedimentary record.  相似文献   

Geophysical profile of the Roter Kamm impact crater,Namibia     

DION BRANDT  WOLF UWE REMOLD  ALAN J. FRANZSEN  CHRISTIAN KOEBERL  LUTZ WENDORFF 《Meteoritics & planetary science》1998,33(3):447-453

Abstract— New gravity and magnetic data were obtained along ground profiles across the Roter Kamm impact crater in the southern Namib desert of Namibia. As the traverses of previous studies did not extend sufficiently beyond the crater rim, it had not been possible to adequately determine the regional background values. The gravity results of this study are similar to those obtained by Fudali in 1973, in that a negative, near-symmetrical anomaly was obtained over the crater center. This anomaly conforms to the results expected for a sediment and impact breccia-filled, simple bowl-shaped crater. The magnetic results of this study, however, are different to those previously reported, which is most probably as a result of the longer profiles used in this new study. A slight positive magnetic anomaly was obtained over the crater interior. Short-wavelength, high-amplitude anomalies observed in the vicinity of the crater rim reflect magnetization contrasts that are probably related to brecciation and block rotation. Modelling of the positive magnetic anomaly indicates the possibility of a small magnetic body or lining at the crater floor-breccia interface in the interior of the crater. Also presented is a 10 m contour digital elevation model of the crater and its environs.  相似文献   

    

ALAN K. BETTS 《Boundary-Layer Meteorology》1997,83(3):511

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An Introduction to Geophysical Exploration     

ALAN. MUSSETT 《Geophysical Journal International》1997,128(2):499-499

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Secular Geochemistry of Central Puerto Rican Island Arc Lavas: Constraints on Mesozoic Tectonism in the Eastern Greater Antilles   总被引：2，自引：0，他引：2  

JOLLY  WAYNE T.; LIDIAK  EDWARD G.; DICKIN  ALAN P.; WU  TSAI-WAY 《Journal of Petrology》2001,42(12):2197-2214

Island arc volcanism in the Greater Antilles persisted for >70m.y. from Middle Cretaceous to Late Eocene time. During theinitial 50 m.y., lavas in central Puerto Rico shifted from predominantlyisland arc tholeiites (volcanic phase I, Aptian to Early Albian,120–105 Ma), to calc-alkaline basalts (phase II, LateAlbian, 105–97 Ma), and finally to high-K, incompatible-element-enrichedbasalts (phases III and IV, Cenomanian–Maastrichtian,97–70 Ma). Following an island-wide eruptive hiatus, geochemicaltrends were reversed in the Eocene with renewed eruption ofcalc-alkaline basalts (phase V, 60–45 Ma). Progressiveincreases in large-ion lithophile elements (LILE)/light rareearth elements (LREE), LILE/high field strength elements (HFSE),LREE/HFSE, and HFSE/heavy rare earth elements (HREE) characterizethe compositional evolution of the first four volcanic phases.The shift in trace element compositions is mirrored by increasingradiogenic content of the lavas. Pb  相似文献   

Pressure--Temperature--Time Paths of Regional Metamorphism I. Heat Transfer during the Evolution of Regions of Thickened Continental Crust   总被引：5，自引：0，他引：5  

ENGLAND  PHILIP C.; THOMPSON  ALAN BRUCE 《Journal of Petrology》1984,25(4):894-928

The development of regional metamorphism in areas of thickenedcontinental crust is investigated in terms of the major controlson regional-scale thermal regimes. These are: the total radiogenicheat supply within the thickened crust, the supply of heat fromthe mantle, the thermal conductivity of the medium and the lengthand time scales of erosion of the continental crust. The orogenicepisode is regarded as consisting of a relatively rapid phaseof crustal thickening, during which little temperature changeoccurs in individual rocks, followed by a lengthier phase oferosion, at the end of which the crust is at its original thickness.The principal features of pressure—temperature—time(PTt) paths followed by rocks in this environment are a periodof thermal relaxation, during which the temperature rises towardsthe higher geotherm that would be supported by the thickenedcrust, followed by a period of cooling as the rock approachesthe cold land surface. The temperature increase that occursis governed by the degree of thickening of the crust, its conductivityand the time that elapses before the rock is exhumed sufficientlyto be affected by the proximity of the cold upper boundary.For much of the parameter range considered, the heating phaseencompasses a considerable portion of the exhumation (decompression)part of the PTt path. In addition to the detailed calculationof PTt paths we present an idealized model of the thickeningand exhumation process, which may be used to make simple calculationsof the amount of heating to be expected during a given thickeningand exhumation episode and of the depth at which a rock willstart to cool on its ascent path. An important feature of thesePTt paths is that most of them lie within 50 °C of the maximumtemperature attained for one third or more of the total durationof their burial and uplift, and for a geologically plausiblerange of erosion rates the rocks do not begin to cool untilthey have completed 20 to 40 per cent of the total uplift theyexperience. Considerable melting of the continental crust isa likely consequence of thickening of crust with an averagecontinental geotherm. A companion paper discusses these resultsin the context of attempts to use metamorphic petrology datato give information on tectonic processes.  相似文献   

Pressure--Temperature--Time Paths of Regional Metamorphism II. Their Inference and Interpretation using Mineral Assemblages in Metamorphic Rocks   总被引：1，自引：0，他引：1  

THOMPSON  ALAN BRUCE; ENGLAND  PHILIP C. 《Journal of Petrology》1984,25(4):929-955

A companion paper (England & Thompson, 1984a) investigatesthe pressure-temperature-time (PTt) paths followed by rocksundergoing burial metamorphism in continental thickening events.This paper discusses problems involved in inferring such pathsfrom the petrological data available in metamorphic rocks and—oncesuch paths are determined—how they may be interpretedin terms of the thermal budgets of metamorphism. Each of theprincipal facies series (glaucophane-jadeite, andalusite- sillimaniteand kyanite-sillimanite) may be encountered by rocks involvedin the thickening and erosion of continental crust in a regimeof average continental heat flow. The inference of a minimumthermal budget required for a given metamorphism depends stronglyon a knowledge of the PTt paths followed by rocks during themetamorphism. Discrimination between possible thermal regimesis greatly enhanced if portions of PTt paths, rather than singlePT points, are available, and additional constraint is possibleif these paths are supplemented by geochronological, structuraland heat flow data.  相似文献