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Priv.-Doz. Dr. Karlheinz Schäfer 《International Journal of Earth Sciences》1972,61(3):942-960
In Iceland the mid-Atlantic ridge rises above sea-level for a north-south-extension of 350 km. One or two east-west-directed transform faults may be expected on land in Iceland, since the mid-Atlantic ridge is offset by transform faults every 100–200 km.Two of these faults, the Reykjanes Fracture Zone in southern Iceland, and the Tjörnes Fracture Zone in northern Iceland, have been located by recent seismological activity (Ward, 1971).There are no strike-slip faults within the Tjömes Fracture Zone that support transform faulting on land. En echolon faults may indicate a status nascendi of lateral offset for the Reykjanes Fracture Zone. A third transform fault, the Snaefellsnes-Vatnajökull Fracture Zone, however, was already active about 3 m.y. ago, when the rift system in southern Iceland began to spread faster than in northern Iceland. The features resulting from this process can be observed in (1) a change in strike from a northerly trend in northern Iceland to a north-easterly direction in southern Iceland; (2) the separation of the rift axis in southern Iceland into two branches; (3) right lateral strike-slip movements along the Snaefellsnes-Vatnajökull Fracture Zone with an offset of about 100 km.Field mapping and observations from air photographs have enabled the construction of a detailed tectonic pattern for the latter zone. The Pleistocene to Postglacial volcanic activity, the offset of magnetic anomalies, and recent active seismicity support the mechanism of transform faulting.At least 3 m.y. ago the west-wandering Icelandic Rift System started with higher spreading rates, revealing a trend to connect directly the Reykjanes- and the Iceland-Jan Mayen Rift Systems. Since that time three crustal plates proceeded from the Vatnajökull Triple Junction with different drift velocities, inducing the activity of the Snaefellsnes-Vatnajökull Transform Fault, which separates the north-western from the south-western Icelandic plate.
Zusammenfassung In Island erhebt sich der zentrale Teil des Mittelatlantischen Rückens auf einer N-S-Erstreckung von 350 km über den Meeresspiegel. Das regelmäßige Auftreten E-W-gerichteter transform faults, die im Abstand von 100–200 km die Mittelatlantische Schwelle queren, läßt auf die Ausbildung einer oder mehrerer transform faults im Bereich der Insel schließen.Zwei in E-W-Richtung und damit rechtwinklig zur isländischen Riftzone verlaufende, seismologisch aktive Zonen wurden im S der Insel (Reykjanes Fracture Zone) und im N (Tjörnes Fracture Zone) als transform faults beschrieben (Ward, 1971). Jedoch keines der beiden Systeme gibt einen lateralen Versatz von geologischen Einheiten und magnetischen Anomalien zu erkennen. In gleicher E-W-Streichrichtung wie die beiden vermuteten transform faults erstreckt sich zwischen der Snaefellsnes-Halbinsel und dem Vatnajökull eine rechtsdrehende Blattverschiebung mit einem Horizontalversatz von etwa 100 km. Die Störung entstand vor etwa 3 Millionen Jahren, als das südisländische zentrale Riftsystem gegenüber dem nördlichen eine höhere Driftgeschwindigkeit entwickelte, und war aktiv bis in die jüngste Zeit.Der Richtungswechsel von durchweg N-S-streichenden geologischen und tektonischen Strukturen im N der Insel und NE-SW-gerichteten Einheiten, sowie die Aufspaltung der Riftzone in zwei getrennte Äste im S, können als Ergebnis dieses Vorganges angesehen werden.Die tektonisch interessantesten Abschnitte der Snaefellsnes-Vatnajökull-Transform Fault wurden kartiert. Kartierung und vorliegende Luftaufnahmen ermöglichten eine detaillierte Darstellung des tektonischen Musters.Gleichzeitig mit der Snaefellsnes-Vatnajökull-Transform Fault entstand der Vatnajökull-Tripel-Punkt, von dem sich drei Krustenplatten mit unterschiedlichen Driftgeschwindigkeiten entfernen. Seit dieser Zeit zeigt das W-wandemde isländische Rift-system den Trend, eine unmittelbare Verbindung zwischen Reykjanes- und Island-Jan-Mayen-Rücken herzustellen.
Résumé En Islande, la partie centrale de la dorsale Atlantique médiane est exondée sur une longueur de 350 km en direction N-S. La présence régulière de failles transformelles de direction E-W traversant le seuil médian de l'Atlantique à des distances de 100 à200 km, permet de conclure à la formation d'une ou de plusieurs failles transformelles sur l'étendue de l'île.Deux zones de direction E-W, et par conséquent d'allure perpendiculaire au rift islandais (la zone de fractures de Reykjanes au S de l'île et la zone de fractures de Tjörnes au N), ont été décrites comme failles transformelles à cause de leur activité séismique (Ward, 1971). Mais aucun de ces deux systèmes ne montre de décrochement des unités géologiques pas plus que des anomalies magnétiques. Avec la même direction E-W que celle des deux failles transformelles présumées, s'étend un autre décrochement dextrogyre entre la presqu'île du Snaefellsnes et le glacier Vatnajökull. Cette faille est apparue il y a trois millions d'années, lorsque le rift central de la partie S de l'Islande se développait avec une vitesse supérieure à celle du Nord de l'Islande; elle fut active jusque récemment.Le changement de direction des structures géologiques et tectoniques d'allure N-S dans le N de l'île et des unités dirigées NE-SW, comme aussi l'ouverture de la zone de rift en 2 branches séparées dans le S, peuvent être considérés comme le résultat de ce processus. La cartographie de ces sections tectoniquement très intéressantes liées à la faille transformelle de Snaefellsnes-Vatnajökull a été levée.La cartographie et les vues aériennes présentées ont permis une figuration détaillée de cette manifestation te tonique.En même temps que la faille transformelle Snaefellsnes-Vatnajökull, il se développait le triple point du Vatnajökull, à partir duquel trois plaques crustales divergent avec des vitesses différentes. Depuis cette époque le compartiment islandais possède l'allure d'une liaison immédiate entre la dorsale de Reykjanes et celle de l'Islande-Jan Mayen.
, N-S , 350 . , E-W 100–200 - , , . , E-W . . : ( Reykjanes Fracture), ( Tjörnes Fracture). , (Ward, 1971). , . , Snaefellsnes Vatnajökull . 3 , - . N-S NE-SW , — , . Snaefellsnes Vatnajokull. . Snaefellsnes-Vatnajökull Vatnajökull-Tripel, , . , aW, Reykjanes-Island-Jan-Mayen.相似文献
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Friedrich Schüler 《Ocean Dynamics》1954,7(3-4):140-143
Summary Echo records from the Atlantic west of the Faroe Islands are presented which, for the first time, give evidence of the influence exerted by day on the vertical movement of pelagic fishes by a considerable reduction of luminous intensity (solar eclipse). This phenomenon corresponds with the fact well known since some years from investigations in the North Sea which prove that by night pelagic fishes move in a vertical direction according to the variation in luminous intensity.
Sur des enregistrements ultra-sonores caractérisant le comportement des poissons pélagiques pendant l'éclipsé de soleil au 30 juin 1954
Résumé On présente des enregistrements ultra-sonores faits en océan Atlantique à l'ouest de l'archipel de Færoé. Ces enregistrements font, pour la première fois, preuve de l'influence exercée de jour sur le mouvement vertical des poissons pélagiques par une réduction sensible de l'intensité lumineuse (éclipse de soleil). Ce phénomène correspond avec le fait bien connu depuis quelques ans par des recherches dans la mer du Nord. Suivant ces recherches, les poissons pélagiques se meuvent nuitamment dans la direction verticale suivant la variation de l'intensité lumineuse.相似文献
47.
Ohne Zusammenfassung 相似文献
48.
Detailed facies analysis of hyaloclastites and associated lavas from eight table mountains and similar "hyaloclastite volcanoes"
in the Icelandic rift zone contradict a rapid and continuous, "monogenetic", entirely subglacial evolution of most volcanoes
studied. The majority of the exposed hyaloclastite deposits formed in large, stable lakes as indicated by widespread, up to
300-m-thick, continuous sections of deep water, shallow water and emergent facies. Salient features include extensively layered
or bedded successions comprising mainly debris flow deposits, turbidites, base surge and fallout deposits consisting of texturally
and compositionally variable, slightly altered hyaloclastites, as well as sheet and pillow lavas. In contrast, chaotic assemblages
of coarser-grained, more poorly sorted and more strongly palagonitized hyaloclastite tuffs and breccias, as well as scoria
and lava are interpreted to have formed under sub- or englacial conditions in small, chimney-like ice cavities or ice-bound
lakes. Irregularly shaped and erratically arranged hyaloclastite bodies produced at variable water levels appear to have resulted
mainly from rapid changes of the eruptive environment due to repeated build-up and drainage of ice-bound lakes as well as
the restricted space between the ice walls. We distinguish a "deep water" facies formed during high water levels of the lake,
a hydroclastic shallow water and emergent facies (leakage of the lake or growth of the volcano above the water surface). Our
model implies the temporary existence of large, stable lakes in Iceland probably formed by climatically induced ice melting.
The highly complex edifices of many table mountains and similar volcanoes were constructed during several eruptive periods
in changing environments characterized by contrasting volcanic and sedimentary processes.
Received: 10 June 1997 / Accepted: 28 July 1998 相似文献
49.
To decipher the geodynamic significance of Cenozoic magmatism along the Red River shear belt, geochemical analyses, U-Pb and Rb-Sr dating, and Pb-Sr-Nd isotope tracing were undertaken. Zircon, monazite, titanite, and a Ti-U-oxide from foliated granitoid intrusions in the shear belt gneisses yield U-Pb emplacement ages of 33.1?±?0.2 (2σ), 31.9?±?0.3, 25.8?±?0.2 and 24.7?±?0.2?Ma, and an age of 35.0?±?0.3?Ma was obtained for the roughly 100?km long, adjacent Jinping (Phan Si Pang) alkali granite. Together with our previous data the new ages suggest that magmatism and left-lateral strike-slip movements occurred coevally during latest Eocene–Oligocene times from 33 to 22?Ma. The Rb-Sr dating of muscovite and biotite from the northernmost gneisses indicates that cooling to 500?°C occurred at 52.6?±?1.1?Ma, pre-dating the onset of magmatism, whereas further cooling to 300?°C took place at 28.9?±?0.6. This shows that unroofing in the north took place almost 9?million years earlier than in the central gneiss segments of the shear zone. Geochemical data substantiate two types of magmas: (1) amphibole-bearing intrusions of alkaline trend which are derived from sources with Isr: 0.7065–0.7089 and i Nd: ?3.7 to ?6.6; (2) leucogranitic layers and bodies having Isr: 0.7084–0.7354 and i Nd: ?3.3 to ?13.4. The former type of intrusion is found in both the gneisses and the adjacent unmetamorphosed cover rocks, whereas leucogranites are restricted to the shear belt gneisses. Source signatures of the alkaline intrusions lie adjacent to the those of OIB, plotting at the lower end of the Mantle Array. Contamination of these melts by continental material seems to be very limited. On the other hand, the leucogranitic layers are essentially crustal derived but none of the them has country rock isotope signatures, requiring melting of crust different from the actually exposed gneisses. Magma sources similar to those of ocean island basalt indicate magmatism to involve melting of light rare earth element and large ion lithophile element enriched mantle domains, most likely present in the lithosphere underneath the region. Since lithospheric thickening or subduction can be ruled out to produce both types of magmas, the presence of an important thermal anomaly is required, which is coevally active with left-lateral strike-slip shear. Adiabatic decompression and melting within the rising anomaly is the most plausible mechanism to produce the mantle magmas, which successively migrate through the crust to induce anatectic melting at 20–15?km crustal depth. Alkaline magmas largely dominate the volume of magmatism along the belt, being continuously present in the shear zone for millions of years. Such lubrication potentially explains how very large amounts of displacement can be absorbed in surprisingly narrow shear zones such as the Red River belt, possibly also playing a rôle for where and when zones of plate-scale lateral extrusion develop. 相似文献
50.
Reinhard Beer 《Icarus》1976,29(2):193-199
The “cosmic” abundance of boron is one of the least well known of all the 92 natural elements even thought it can appear in substantial concentrations on the Earth's surface. It is suggested that Jupiter might be a suitable place to search for boron because it forms numerous stable, volatile compounds with hydrogen and other abundant elements whose presence might be sought either spectroscopically or by entry devices. 相似文献