Geomorphological modelling and nature protection in arctic and subarctic environments

Geomorphological processes of the cold climate zone can be divided into natural and accelerated. In the former group the following processes are being discussed: frost weathering, mass wasting, solifluction, landslides, slopewash and linear erosion and wind action. Processes of accelerated action are due to natural and artificial stimuli. To the former belong for instance heavy rains which sometimes occur in polar zones (as in Spitsbergen in July 1972). More frequent however, are, artificial causes which are connected with man's activities.The action of geomorphological processes is indicated by the denudational balance which can be either positive or negative. The author has determined this balance for some regions of the Arctic and Subarctic, especially for Siberia. The balance is positive (degradation) in the zone of polar desert and in the oceanic part of the Arctic, and also in high mountains of this zone. Negative denudation balance prevails in the continental section of the Subarctic. The areas of positive denudational balance are threatened with nature destruction and therefore particularly deserve to be covered by a nature protection scheme.     

Bubble coalescence in basalt flows: comparison of a numerical model with natural examples

Gas accumulation in magma may be aided by coalescence of bubbles because large coalesced bubbles rise faster than small bubbles. The observed size distribution of gas bubbles (vesicles) in lava flows supports the concept of post-eruptive coalescence. A numerical model predicts the effects of rise and coalescence consistent with observed features. The model uses given values for flow thickness, viscosity, volume percentage of gas bubbles, and an initial size distribution of bubbles together with a gravitational collection kernel to numerically integrate the stochastic collection equation and thereby compute a new size spectrum of bubbles after each time increment of conductive cooling of the flow. Bubbles rise and coalesce within a fluid interior sandwiched between fronts of solidification that advance inward with time from top and bottom. Bubbles that are overtaken by the solidification fronts cease to migrate. The model predicts the formation of upper and lower vesicle-rich zones separated by a vesicle-poor interior. The upper zone is broader, more vesicular, and has larger bubbles than the lower zone. Basaltic lava flows in northern California exhibit the predicted zonation of vesicularity and size distribution of vesicles as determined by an impregnation technique. In particular, the size distribution at the tops and bottoms of flows is essentially the same as the initial distribution, reflecting the rapid initial solidification at the bases and tops of the flows. Many large vesicles are present in the upper vesicular zones, consistent with expected formation as a result of bubble coalescence during solidification of the lava flows. Both the rocks and model show a bimodal or trimodal size distribution for the upper vesicular zone. This polymodality is explained by preferential coalescence of larger bubbles with subequal sizes. Vesicularity and vesicle size distribution are sensitive to atmospheric pressure because bubbles expand as they decompress during rise through the flow. The ratio of vesicularity in the upper to that in the lower part of a flow therefore depends not only on bubble rise and coalescence, but also on flow thickness and atmospheric pressure. Application of simple theory to the natural basalts suggests solidification of the basalts at 1.0±0.2 atm, consistent with the present atmospheric pressure. Paleobathymetry and paleoaltimetry are possible in view of the sensitivity of vesicle size distributions to atmospheric pressure. Thus, vesicular lava flows can be used to crudely estimate ancient elevations and/or sea level air pressure.     

Pan African plate tectonics and its repercussions on the crust of northeast Africa

Pan African belts of the African mainland and the Arabian-Nubian Shield exhibit evolutionary features which are either compatible with intracontinental ensialic development or with plate margin and Wilson cycle tectonics during the time period 1100–500 Ma ago.It is suggested that both the ensialic and plate margin developments were caused by the same fundamental subcrustal forces that are reponsible for present-day lithospheric motion, and the Pan African event is therefore representative of a transition from Precambrian ensialic plate tectonics to Phanerozoic Wilson cycle tectonics.The North African craton between the central Hoggar and eastern Egypt/Sudan is characterized by widespread late Pan African thermal activity, documented by voluminous calc-alkaline granitoid intrusions and alkaline to peralkaline ring complexes, as well as a general resetting of mineral isotopic systems in older basement rocks (Schürmann, 1974;Vachette, 1974). Nagy et al. (1976) interpreted these features as evidence for progressive eastward growth of the West African craton but I suggest that pre-Pan African continental crust including Archaean elements (Ouweinat) was already in existence in northeast Africa. This crust was strongly affected by intraplate stresses resulting from closing of the Pharusian ocean in the western Hoggar and from collisional tectonics in the Arabian shield.Much of the late Pan African deformation in northeast Africa appears to have occurred by strike slip motion and there is a striking analogy with the Cenozoic deformation pattern behind the Himalayan belt in northeast Asia.Following the model ofMolnar andTapponier (1975) the deformation patterns of Asia and northeast Africa are compared, and it is possible to relate the Pan African structures and magmatism between the Hoggar and Egypt/Sudan to continental collision and identation of a semi-infinite hot crustal segment by a cold rigid plate (the West African craton).

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Zusammenfassung Die panafrikanischen Faltengürtel des afrikanischen Kontinentes und des Arabisch-Nubischen Schildes zeigen während des Zeitraumes von ca. 1100 bis 500 M. J. Entwicklungstendenzen, die teilweise im Sinne einer ensialischen Orogenese und teilweise als Folge einer Plattenrand-Evolution (Wilson Zyklus) gedeutet werden können.Trotz ihrer verschiedenartigen Anlage sind sowohl die ensialischen als auch die Plattenrand-Orogene wahrscheinlich auf die gleichen subkrustalen Kräfte zurückzuführen, die auch heute für die Bewegung von Lithosphärenplatten verantwortlich sind. Es ist daher anzunehmen, daß die panafrikanische Episode geotektonisch eine Übergangsphase von vorwiegend ensialischer Orogenentwicklung des Präkambriums zur phanerozoischen Plattentektonik darstellt.Im Bereich des nordafrikanischen Kratons zwischen dem zentralen Hoggar Gebirge und Ost-Ägypten/Sudan wirkte sich die panafrikanische Episode vorwiegend als Thermalphase aus, wobei weitverbreitet voluminöse Granitplutone und alkalische Ringkomplexe in die alte Kruste eindrangen und von einer regionalen isotopischen Rejuvenation der älteren Gesteine begleitet wurden (Schürmann, 1974;Vachette, 1974). Einige Autoren deuten die panafrikanischen Granitintrusionen in Nordostafrika als Resultat einer allmählichen Krustenakkretion, es gibt jedoch genügend Anhaltspunkte für die Annahme einer kontinuierlichen prä-panafrikanischen Kruste vom Zentralhoggar bis nach Ägypten, zu der auch die archaischen Elemente des Ouweinat Massives gehören. Dieser stabile Bereich wurde als Folge der spätpräkambrischen Kollisionstektonik im westlichen Hoggar und im Arabisch-Nubischen Schild starken Spannungen unterworfen und reagierte vor allem durch Bruchtektonik, wobei besonders Horizontalverschiebungen dominierten.Es besteht eine auffallende Ähnlichkeit zwischen dieser Verformung und dem Deformationsschema in Nordost-Asien hinter dem jungen Faltengebirge des Himalaya. Beide Bereiche werden verglichen und es scheint möglich zu sein, die panafrikanische Bruchtektonik und den zugehörigen Magmatismus im Sinne vonMolnar &Tapponier (1975) als Resultat von Plattenkollision zu deuten. Nach diesem Modell kollidierte im Westen der westafrikanische Kraton unter Schließung des pharusischen Mini-Ozeans mit Nordostafrika während im Osten im Bereich des heutigen Arabischen Schildes kontinuierliche Subduktionsvorgänge zur Bildung eines Kontinentalrandes vom Anden-Typus mit voluminösen kalkalischen Vulkaniten führten.

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Résumé Les zones mobiles pan africaines du continent africain et du bouclier arabo-nubien montrent des tendances évolutionnaires qui sont compatibles en partie avec le développement intracontinental et ensialique en partie avec les tectoniques de bord de plaques et du cycle de Wilson pendant la période: 1100–500 Ma.Il y a lieu d'admettre que le développement orogénique-ensialique et celui du bord des plaques sont provoqués part les mêmes activités fondamentales et sous-crustales qui sont responsables pour des mouvements lithosphériques d'aujourd'hui. Pour cette raison l'événement pan-africain peut être considéré comme une transition de la tectonique du plaqueensialique du Précambrien à la tectonique cyclique de Wilson du Phanérozoique.Le craton d'Afrique du Nord, entre le Hoggar central et l'Egypte de l'Est/Soudan, est caractérisé par une activité thermale tardive pendant l'épisode pan-africain. Cette activité est démontrée par des intrusions volumineuses de granite calcoalcalin et des complexes cirenlaires alcalins à peralcalins aussi bien que par une remise générale en place de systèmes isotopiques des minéraux dans les roches plus vieilles du socle. (Schürmann, 1974;Vachette, 1974). Nagy et al. (1976) ont interprêté ces caractères comme une preuve de la croissance progressive en direction de l'Est du craton ouest-africain; mais nous supposons que la croûte continentale pré-pan africaine y compris les éléments archéens (Ouweinat), a déjà existé en Afrique du Nord-Est. Cette partie de la croûte terrestre a été influencée fortement par des pressions internes qui résultent de la fermeture de l'océan pharusien dans l'ouest du Hoggar et de la tectonique de collision dans le bouclier arabe.Beaucoup de déformations pan-africaines tardives en Afrique du Nord-Est semblent être apparues àla suite d'un mouvement de décrochement. Il y a une analogie frappante avec le type de la déformation cénozoique à l'arrière de la chaine himalayenne de l'Asie nord-est. Les types de déformations de l'Asie et de l'Afrique du Nord-Est ont été comparés. Il semble possible de mettre en rélation les structures pan-africaines et le magmatisme de l'Hoggar et de l'Egypte/Soudan avec la collision continentale et l'indentation dune partie chaude de la croûte avec une plaque rigide et froide (le craton ouest-africain) après le modèle deMolnar &Tapponier (1975).

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Chemical classification of iron meteorites—IX. A new group (IIF), revision of IAB and IIICD,and data on 57 additional irons

Structural observations and concentrations of Ni, Ga, Ge and Ir allow the classification of 57 iron meteorites in addition to those described in the previous papers in this series; the number of classified independent iron meteorites is now 535. INAA for an additional six elements indicates that five previously studied irons having very high $\text{Ge}\text{Ga}$ ratios are compositionally closely related and can be gathered together as group IIF. A previously unstudied iron, Dehesa, has the highest $\text{Ge}\text{Ga}$ ratio known in an iron meteorite, a ratio 18 × higher than that in CI chondrites. Although such high $\text{Ge}\text{Ga}$ ratios are found in the metal grains of oxidized unequilibrated chondrites, their preservation during core formation requires disequilibrium melting or significant compositional and temperature effects on metal/silicate distribution constants and/or activity coefficients. In terms of $\text{Ge}\text{Ga}$ ratios and various other properties group IIF shows genetic links to the Eagle Station pallasites and $\text{CO}\text{CV}$ chondrites. Klamath Falls is a new high-Ni, low-Ir member of group IIIF that extends the concentration ranges in this group and makes these comparable to the ranges in large igneous groups such as IIIAB. Groups IAB and IIICD have been revised to extend the lower Ni boundary of group IIICD down to 62 mg/g. The iron having by far the highest known Ni concentration (585 mg/g), Oktibbeha County, is a member of group IAB and extends the concentration ranges of all elements in this nonmagmatic group. Morasko, a IAB iron associated with a crater field in Poland, is paired with the Seeläsgen iron discovered 100 km away. All explosion craters from which meteorites have been recovered were produced by IAB and IIIAB irons.     

Rb−Sr,K−Ar and fission track ages for granites from Penang Island,West Malaysia: an interpretation model for Rb−Sr whole-rock and for actual and experimental mica data

Penang Island represents the northwestern extension of the western magmatic belt of Peninsular Malaysia. Thirty-one samples of highly evolved biotite-and biotite-muscovite granites were used in an integrated study to unravel the complex magmatic, tectonic and cooling histories of these rocks. Highly distorted Rb–Sr whole-rock age patterns are evident. These are attributed to the partial post-magmatic Sr homogenization within the granite batholith which led to the rotation of isochrons towards younger ages and higher ^(87/86)Sr intercepts. The recognition of this mechanism allowed the establishment of a new Rb–Sr interpretation model. The intrusion ages of the granites can be extrapolated based on the evolutionary trend of the initial ^(87/86)Sr. Including the data of Bignell and Snelling, three episodes of granite emplacement at 307±8 Ma, 251±7 Ma and 211±2 Ma are suggested for Penang and the NW Main Range. The late-Triassic intrusive induced a hydrothermal conductive convection system which affected all the granites. It is considered to be responsible for the Rb–Sr whole-rock age distortion, the Rb–Sr and K–Ar biotite age resetting and the textural and mineralogical changes in the granites. The duration of the hydrothermal convections, deduced from the Rb–Sr whole rock ages, is about 6 Ma and 20 Ma in the northern and southern parts of Penang respectively. Fast regional cooling to 350±50°C within a time span of 1–3 Ma is recognized for the late-Triassic Feringgi intrusive from the mica ages, followed by a generally slow cooling rate of about 1°C/Ma. Fission track ages, in addition, indicate blockwise uplift along the N-S and NW-SE tending faults, thus resulting in the exposure of deeper crustal levels in southern and eastern Penang. A change in the tensional regime since Oligocene/Miocene, accompanied by a southwest tilting of the island, is indicated by the fission track apatite ages. Variable sometimes younger K–Ar, respectively Rb–Sr biotite ages mainly depend on the degree of hydrothermal overprint at different crustal levels. An increase of the reaction surface by grain size reduction influences Rb–Sr and K–Ar mica ages in similar ways, as has been demonstrated by experimental data.     

U-Pb zircon and Sm-Nd model ages of high-grade Moldanubian metasediments,Bohemian Massif,Czechoslovakia

We report single grain U-Pb ion-microprobe as well as conventional bulk size fraction ages for zircons from 3 metasediment samples of the Moldanubian Complex, Bohemian Massif, one of the largest crystalline complexes of the Hercynian foldbelt in Europe. These are complemented by whole-rock Sm-Nd model ages. The metasediments are of upper amphibolite to granulite grade and come from the Bory Massif in Moravia, NW of Brno (sample AA-1) and from the Varied Group (AA-2) and Monotonous Group (AA-3) in the Ceske Budejovice region of SW Bohemia.Ion-microprobe data for detrital zircons yielded ²⁰⁷Pb/ ²⁰⁶Pb ages between ca. 1750 Ma and 2680 Ma and reflect chronologically heterogeneous source terrains. One grain in sample AA-1 of the Bory granulite massif may be as old as 2684±14 Ma, and this constitutes the oldest reported zircon age for the Hercynian belt of central Europe. The single grain data are much less discordant than previously published conventional U-Pb analyses from bulk zircon samples and suggest a significant early Proterozoic crust-formation event between 2 and 2.2 Ga ago. The size fraction data are compatible with the single grain ages and give a fairly precise definition of the Hercynian event between 347 and 367 Ma ago while their upper Concordia intercept ages between 1700 and 2050 Ma represent the mean of the respective grain populations and are probably geohronologically meaningless. The Nd whole-rock model ages between 1.7 and 3.0 Ga confirm mid-Proterozoic to Archaean source terrains for the dated metasediments.     

Zur Metallogenese in Skandinavien

Ohne Zusammenfassung     

Natural hazard experiences and adaptations: A study of winter climate-induced road closures in Norway

The effects of global climate change include more extreme weather events that harm lifeline infrastructure such as road access. The questionnaire-based study takes a novel natural experiment approach to subjective personal experiences and perceptions of lifeline vulnerability in two seaside communities in Norway that have been sporadically isolated due to avalanches, heavy snowfall, and/or snowdrifts. The enquiry aims at filling a research gap on sudden winter climate-induced disconnections and road travel hazards in advanced societies. The results show that weather-induced road closures lead to worries about road travel and practical problems, but also that many people are able to adjust to reduce their vulnerability. The authors concluded that community characteristics such as available services and social and human capital are important for understanding people’s vulnerabilities, worries, and hazard preparedness.     

Clinoform growth in a Miocene,Para‐tethyan deep lake basin: thin topsets,irregular foresets and thick bottomsets

Late Miocene lacustrine clinoforms of up to 400 m high are mapped using a 1700 km² 3‐D seismic data set in the Dacian foreland basin, Romania. Eight Meotian clinoforms, constructed by sediment from the South Carpathians, prograded around 25 km towards southwest. The individual clinothems show thin (10–60 m thick), if any, topsets, disrupted foresets and highly aggradational bottomsets. Basin‐margin accretion occurred in three stages with changing of clinoform heights and foreset gradients. The deltaic system prograded into an early‐stage deep depocenter and contributed to high gradient clinoforms whose foresets were dominated by closely (100–200 m) spaced 1.5–2 km wide V‐shaped sub‐lacustrine canyons. During intermediate‐stage growth, 2–4 km wide canyons were dominant on the clinoform foresets. From the early to intermediate stages, the lacustrine shelf edges were consistently indented. The late‐stage outbuilding was characterised by smaller clinoforms with smoother foresets and less indentation along the shelf edge. Truncated and thin topsets persisted through all three stages of clinoform evolution. Nevertheless, the resulting long‐term flat trajectory shows alternating segments of forced and low‐amplitude normal regressions. The relatively flat trajectory implies a constant base level over time and was due to the presence of the Dacian–Black Sea barrier that limited water level rise by spilling to the Black Sea. Besides the characteristic shelf‐edge incision of the thin clinoform topsets and the resultant sediment bypass at the shelf edge, the prolonged regressions of the shelf margin promoted steady sediment supply to the basin. The high sediment supply at the shelf edges generated long‐lived slope sediment conduits that provided sustained sediment transport to the basin floor. Clinothem isochore maps show that large volumes of sediment were partitioned into the clinoform foresets, and especially the bottomsets. Sediment predominantly derived from frequent hyperpycnal flows contributed to very thick, ca. 300–400 m in total, bottomsets. Decreasing subsidence over time from the foredeep resulted in diminishing accommodation and clinoform height, reduced slope channelization and smoother slope morphology.