Zur Mechanik der ostalpinen Deckenbildung

Zusammenfassung Die Mechanik der Deckenbildung ist eine aktuelle Streitfrage bei der Deutung alpiner Gebirgsbildung. Zwei Leitvorstellungen stehen zur Diskussion: 1. Tangentialdruck in der Erdkruste verursacht Faltungen und Deckenüberschiebungen in der mobilen geosynklinalen Zone; 2. Gravitationstektonik zufolge von endogen bedingten Hebungen und Senkungen verursacht die lateralen Massenverlagerungen der alpinen Orogenese.Im ersten Falle also Mobilismus (Wandertektonik), mit Einengung der Geosynklinale, begleitet von Verschluckung der Sialkruste oder crustal buckling. Im zweiten Falle Fixismus (Standtektonik), das heißt nur Durchbewegung der Tektonosphäre zufolge von Materialkreisläufen im geosynklinalen Bereiche, ohne daß notwendigerweise auch die Nebengebiete mitbeteiligt werden und sich nähern müssen.An der Hand von drei genetischen Profilen über Salzburg und die Hohen Tauern wird erläutert, daß es möglich ist, die ostalpine Orogenese mit Hilfe des zweiten Leitbildes zu erklären.Die Gravitationstektonik oder Sekundärtektogenese tritt in verschiedenen Stockwerken auf. Die Abgleitung der alpinen Sedimenthaut liefert Decken von helvetischem Typ. Wenn auch das kristalline Grundgebirge am lateralen Transport mitbeteiligt ist, entstehen Decken vom ostalpinen Typ. Die Verfließungen der tiefen, rheomorphen Teile der Kruste (Migma und z. T. palingenes Magma) liefern die Decken penninischen Charakters, welche im Tauernfenster aufgeschlossen sind.Die erste orogene Hauptphase war die tieforogene oder Gosauphase der mittleren Kreide, wobei die Proto-Austriden und Proto-Penniden in die Tauernvortiefe wanderten.Die zweite orogene Hauptphase war die hochorogene oder Molassephase des Mitteltertiärs, wobei die Tauernzone als Geantiklinale emporgewölbt wurde. In den nördlichen Kalkalpen fanden nordvergente, in den südlichen Kalkalpen südvergente Bewegungen statt.Zum Schlusse wurden die nördlichen Kalkalpen gegen die Molasse aufgeschoben, welche Bewegung im zentralen Tauernsektor wahrscheinlich mechanisch mit der Gailabschiebung in der Südflanke der Geantiklinale zusammenhängt.Die Deutung der Mechanik der ostalpinen Deckenbildung nach dem Leitbild der Schwerkrafttektonik führt also zum Ergebnis, daß Krustenverkürzung keine notwendige Begleiterscheinung alpiner Orogenese ist. Diese Schlußfolgerung steht in Übereinstimmung mit gravimetrischen und seismischen Beobachtungen.     

On Geodynamic Evolution of Simao Region (Southwestern Yunnan, China) during Late Paleozoic and Triassic

ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅＳｉｍａｏｒｅｇｉｏｎ (ＳＷＹｕｎｎａｎ ,Ｃｈｉｎａ) ,ｓｉｔｕａｔｅｄｂｅ ｔｗｅｅｎｔｈｅＡｉｌａｏｓｈａｎｂｅｌｔｉｎｔｈｅｅａｓｔａｎｄｔｈｅＬａｎｃａｎｇｊｉａｎｇｚｏｎｅｉｎｔｈｅｗｅｓｔ (Ｆｉｇ .1) ,ｉｓａｋｅｙａｒｅａｔｏｃｈｒｏｎｏｌｏｇｉｃａｌｌｙｃｏｎｓｔｒａｉｎｌａｔｅＰａｌｅｏｚｏｉｃｃｏｍｐｒｅｓｓｉｏｎａｌｄｅｆｏｒｍａｔｉｏｎｓａｎｄｔｈｕｓｏｆｐｒｉｍｅｉｎ ｔｅｒｅｓｔｗｈｅｎｄｉｓｃｕｓｓｉｎｇｔｈｅｐｌａｔｅ ｔｅｃｔｏｎｉｃｈｉ…     

La tectogenèse hercynienne dans la partie occidentale du bassin de l'Ahnet et la région de Bled El-Mass, Sahara Algérien: un continuum de déformation

The study area is located in the north of the Hoggar Basement. It is bounded to the east by the Idjerane-Azaz north-south-trending structural system, the Azzel-Matti Ridge to the south and to the north by the Timimoun Basin. Distribution of the structural elements and the determination of deviatoric stress tensors indicate two main directions for major stress (σ1), one striking northeast-southwest and the other west-northwest-east-southeast. Structural evolution during the Hercynian Orogeny can be considered as a continuous strain rather than as two contrasting tectonic events. The structural features can be attributed to the association of wrench faulting and thrusting, which is compatible with a strain regime of combined pure and simple shear. The basement fabric features exerted a very strong control on the structural evolution during the Hercynian deformation.     

Gum rosin (colophony): A suitable material for thermomechanical modelling of the lithosphere

We investigate the use of a ductile material with temperature-sensitive viscosity for thermomechanical modelling of the lithosphere. First, we consider the scaling of mechanical and thermal properties. For a normal field of gravity, the balance of stresses and body forces sets the stress scale, in proportion to the linear dimensions and the densities. The equation of thermal conduction sets the time scale. The activation enthalpy for creep sets the temperature scale; but the thermal expansivity provides an additional constraint on this temperature scale.

Gum rosin appears to be a suitable material for lithospheric modelling. We have measured its flow properties, at various temperatures, in a specially designed rotary viscometer with unusually low machine friction. The rosin is almost Newtonian. Strain rate depends upon stress to the power n, where 1.0 <n < 1.14. The viscosity varies over 5 orders of magnitude, from about 10² Pa s at 80°C, to about 10⁷ Pa s at 40°C. The activation enthalphy is thus about 250 kJ/mol. Measured with a needle probe, the thermal conductivity is 0.113 ± 0.001 W m⁻¹K⁻¹; the thermal diffusivity, (6±3) ×10⁻⁷ m² s⁻¹. Calculated from X-ray profiles, the thermal expansivity is about 3 × 10⁻⁴ K⁻¹. These thermal and mechanical properties make gum rosin suitable for thermomechanical models, where linear dimensions scale down by a factor of 10⁶; time, by 10¹¹; viscosity, by 10¹⁷; and temperature change, by 10¹.     

Social network interactions among mentally ill persons in community housing: research issues and agenda

This paper discusses social network interactions among individuals who have been hospitalized with mental illness and who currently reside in different types of community housing programmes. First, a conceptualisation of social networks that focuses on network interactions, both supportive and non-supportive, between network interactions may be more important influences on successful or unsuccessful adaptation of current and former psychiatric patients to community life than the structural dimensions of social networks more commonly analysed in the literature. The design of a current longitudinal study of social network interactions is presented. The results of two pilot studies that examine social network interactions is presented. The results of two pilot studies that examine social network interactions with data from several community housing programmes in southern Ontario, Canada are reviewed and an agenda is stated for further data collection and analysis.     

Characterization of hard‐to‐differentiate dune stratification types in the Permian Coconino Sandstone (Arizona,USA)

Dune stratification types, which include grainfall, grainflow and ripple lamination, provide a record of the fine‐scale processes that deposited sediment on palaeo‐dune foresets. While these facies are relatively easy to distinguish in some cross‐bedded sandstones, for others – like the Permian Coconino Sandstone of northern and central Arizona – discrete stratification styles are hard to recognize at the bedding scale. Furthermore, few attempts have been made to classify fine‐scale processes in this sandstone, despite its renown as a classic aeolian dune deposit and Grand Canyon formation. To interpret depositional processes in the Coconino Sandstone, cross‐bed facies were characterized using a suite of sedimentary textures and structures. Bedding parameters were described at multiple scales via a combination of field and laboratory methods, including annotated outcrop photomosaics, strike and dip measurements, sandstone disaggregation and laser‐diffraction particle analysis, high‐resolution scans of thin sections, and scanning electron microscopy. Cross‐beds were observed to be laterally extensive along‐strike, with most dip angles ranging from the mid‐teens to mid‐twenties. While some cross‐bed sets are statistically coarser near their bases, others exhibit no significant vertical sorting trends. Both massive and laminated textures are visible in high‐resolution scans of thin sections, but laminae contacts are commonly indistinct, making normal and reverse grading difficult to define. Diagenetic features, such as stylolite seams and large pores, are also present in some samples and might indicate alteration of original textures like detrital clay laminae and carbonate minerals. Observed textures and sedimentary structures suggest that the cross‐beds may consist of grainflow and grainfall deposits, but these remain difficult to differentiate at outcrop and thin‐section scales. This characterization of fine‐scale processes will play a critical part in the development of depositional models for the Coconino Sandstone and elucidate interpretations for similar cross‐bedded formations.     

The role of hydrothermal fluids in sedimentation in saline alkaline lakes: Evidence from Nasikie Engida,Kenya Rift Valley

Saline alkaline lakes that precipitate sodium carbonate evaporites are most common in volcanic terrains in semi‐arid environments. Processes that lead to trona precipitation are poorly understood compared to those in sulphate‐dominated and chloride‐dominated lake brines. Nasikie Engida (Little Magadi) in the southern Kenya Rift shows the initial stages of soda evaporite formation. This small shallow (<2 m deep; 7 km long) lake is recharged by alkaline hot springs and seasonal runoff but unlike neighbouring Lake Magadi is perennial. This study aims to understand modern sedimentary and geochemical processes in Nasikie Engida and to assess the importance of geothermal fluids in evaporite formation. Perennial hot‐spring inflow waters along the northern shoreline evaporate and become saturated with respect to nahcolite and trona, which precipitate in the southern part of the lake, up to 6 km from the hot springs. Nahcolite (NaHCO₃) forms bladed crystals that nucleate on the lake floor. Trona (Na₂CO₃·NaHCO₃·2H₂O) precipitates from more concentrated brines as rafts and as bottom‐nucleated shrubs of acicular crystals that coalesce laterally to form bedded trona. Many processes modify the fluid composition as it evolves. Silica is removed as gels and by early diagenetic reactions and diatoms. Sulphate is depleted by bacterial reduction. Potassium and chloride, of moderate concentration, remain conservative in the brine. Clastic sedimentation is relatively minor because of the predominant hydrothermal inflow. Nahcolite precipitates when and where pCO₂ is high, notably near sublacustrine spring discharge. Results from Nasikie Engida show that hot spring discharge has maintained the lake for at least 2 kyr, and that the evaporite formation is strongly influenced by local discharge of carbon dioxide. Brine evolution and evaporite deposition at Nasikie Engida help to explain conditions under which ancient sodium carbonate evaporites formed, including those in other East African rift basins, the Eocene Green River Formation (western USA), and elsewhere.     

Concentration, isotopic composition, and sources of lead in Southern Ocean air during 1999/2000, measured at the Cape Grim Baseline Air Pollution Station, Tasmania

Southern Ocean aerosols were collected at the Cape Grim Baseline Air Pollution Station from onshore air under baseline conditions between February 1999 and April 2000. Thermal ionization techniques (TIMS) and isotope dilution mass spectrometry (IDMS) were used to measure the isotopic composition and concentration of lead in the air giving concentrations as low as 0.6 ± 0.1 pg · m⁻³. Air collected under baseline conditions for 12 months (May 1999-April 2000) yielded an overall lead concentration of 11.0 ± 0.2 pg · m⁻³ and isotopic composition of ²⁰⁶Pb/²⁰⁷Pb = 1.154, ²⁰⁸Pb/²⁰⁷Pb = 2.387 and ²⁰⁶Pb/²⁰⁴Pb = 17.93. The range in isotopic ratios was consistent with the mixing of lead from major population centers in the Southern Hemisphere in the mid to high latitudes, except for the presence of highly radiogenic lead in some samples. Contributions from radiogenic lead of up to ∼0.8% were observed. Three periods with the highest percentage contribution of radiogenic lead (>0.5%) were investigated in more detail, and 4-d back-trajectories and radon concentrations were used to help identify the sources. The sources are probably associated with the mining and processing of uranium rich ores in southern Africa and possibly South Australia.     

Detecting trends in seagrass abundance using aerial photograph interpretation: Problems arising with the evolution of mapping methods

In the late 1970s and early 1980s an inventory of seagrass distribution in southeast Australia was made from multiscale aerial photographs using a simple analog mapping method, referred to here as theCamera lucida technique. Four priority estuaries were remapped in 2003, also using multi-scale aerial photographs, but using a geographic information system (GIS)-based analog mapping technique. We quantified the degree of error that arose when estimates of seagrass distribution were compared for the two techniques be ressessing. with GIS, the same aerial photographs used earlier for four estuaries: Port Hacking, Bermagui River, Merimbula Lake, and St Georges Basin.Camera lucida produced a larger estimate of total seagrass area than GIS in all four locations: 8% of total seagrass area or 12.9 ha in Port Hacking, 15% or 5 ha in Bermagui River, 20% or 38.0 ha in Merimbula Lake, and 243% or 502.2 ha in St Georges Basin. The consistent and moderate discrepancy in Port Hacking, Bermagui River, and Merimbula Lake was attributed mainly to differences betweenCamera lucida and GIS, such as the tendency ofCamera lucida to overestimate seagrass area by amalgamating disjunct seagrass patches into continuous meadows. The large discrepancy at St Georges Basin was attributed to interoperator error; i.e., the operators who mapped photographs taken in 1979 had widely differing interpretations of the aerial photographs. A 20-yr trend was calculated for each estuary using either theCamera lucida or revised GIS value as the start point. Perceived management status of the seagrass depended greatly on the initial data used. In St Georges Basin, a catastrophic decline (65%, 553.9 ha) occurred based on theCamera lucida-GIS comparison but only a small decline (14.7%, 51.8 ha) when all photos were GIS assessed, In view of the technical developments in habitat mapping, evaluation of change in seagrass abundance using remote sensing needs an assessment of intra-operator error, inter-operator error, and systemic error before maps produced by superseded technology are considered useful baselines.