Channelized and hillslope sediment transport and the geomorphology of mountain belts

This paper uses the results of landscape evolution models and morphometric data from the Andes of northern Peru and the eastern Swiss Alps to illustrate how the ratio between sediment transport on hillslopes and in channels influences landscape and channel network morphologies and dynamics. The headwaters of fluvial- and debris-flow-dominated systems (channelized processes) are characterized by rough, high-relief, highly incised surfaces which contain a dense and hence a closely spaced channel network. Also, these systems tend to respond rapidly to modifications in external forcing (e.g., rock uplift and/or precipitation). This is the case because the high channel density results in a high bulk diffusivity. In contrast, headwaters where landsliding is an important sediment source are characterized by a low channel density and by rather straight and unstable channels. In addition, the topographies are generally smooth. The low channel density then results in a relatively low bulk diffusivity. As a consequence, response times are greater in headwaters of landslide-dominated systems than in highly dissected drainages. The Peruvian and Swiss case studies show how regional differences in climate and the litho-tectonic architecture potentially exert contrasting controls on the relative importance of channelized versus hillslope processes and thus on the overall geomorphometry. Specifically, the Peruvian example illustrates to what extent the storminess of climate has influenced production and transport of sediment on hillslopes and in channels, and how these differences are seen in the morphometry of the landscape. The Swiss example shows how the bedding orientation of the bedrock drives channelized and hillslope processes to contrasting extents, and how these differences are mirrored in the landscape. An erratum to this article can be found at     

High-resolution seismic analysis of the coastal Mecklenburg Bay (North German Basin): the pre-Alpine evolution

The pre-Alpine structural and geological evolution in the northern part of the North German Basin have been revealed on the basis of a very dense reflection seismic profile grid. The study area is situated in the coastal Mecklenburg Bay (Germany), part of the southwestern Baltic Sea. From the central part of the North German Basin to the northern basin margin in the Grimmen High area a series of high-resolution maps show the evolution from the base Zechstein to the Lower Jurassic. We present a map of basement faults affecting the pre-Zechstein. The pre-Alpine structural evolution of the region has been determined from digital mapping of post-Permian key horizons traced on the processed seismic time sections. The geological evolution of the North German Basin can be separated into four distinct periods in the Rerik study area. During Late Permian and Early Triassic evaporites and clastics were deposited. Salt movement was initiated after the deposition of the Middle Triassic Muschelkalk. Salt pillows, which were previously unmapped in the study area, are responsible for the creation of smaller subsidence centers and angular unconformities in the Late Triassic Keuper, especially in the vicinity of the fault-bounded Grimmen High. In this area, partly Lower Jurassic sediments overlie the Keuper unconformably. The change from extension to compression in the regional stress field remobilized the salt, leading to a major unconformity marked at the base of the Late Cretaceous.     

Zircon M257 ‐ a Homogeneous Natural Reference Material for the Ion Microprobe U‐Pb Analysis of Zircon

We introduce and propose zircon M257 as a future reference material for the determination of zircon U‐Pb ages by means of secondary ion mass spectrometry. This light brownish, flawless, cut gemstone specimen from Sri Lanka weighed 5.14 g (25.7 carats). Zircon M257 has TIMS‐determined, mean isotopic ratios (2s uncertainties) of 0.09100 ± 0.00003 for ²⁰⁶pb/²³⁸U and 0.7392 ± 0.0003 for ²⁰⁷pb/²³⁵U. Its ²⁰⁶pb/²³⁸U age is 561.3 ± 0.3 Ma (unweighted mean, uncertainty quoted at the 95% confidence level); the U‐Pb system is concordant within uncertainty of decay constants. Zircon M257 contains ～ 840 μg g^?1 U (Th/U ～ 0.27). The material exhibits remarkably low heterogeneity, with a virtual absence of any internal textures even in cathodoluminescence images. The uniform, moderate degree of radiation damage (estimated from the expansion of unit‐cell parameters, broadening of Raman spectral parameters and density) corresponds well, within the “Sri Lankan trends”, with actinide concentrations, U‐Pb age, and the calculated alpha fluence of 1.66 × 10¹⁸ g^?1. This, and a (U+Th)/He age of 419 ± 9 Ma (2s), enables us to exclude any unusual thermal history or heat treatment, which could potentially have affected the retention of radiogenic Pb. The oxygen isotope ratio of this zircon is 13.9%o VSMOW suggesting a metamorphic genesis in a marble or calc‐silicate skarn.     

Recommended Values of 239Pu, 240Pu and 239+240Pu Concentrations in Reference Material IAEA-315 (Marine Sediment) Estimated by Thermal Ionisation Mass Spectrometry, Inductively Coupled Plasma-Mass Spectrometry and Alpha Spectrometry

The recommended concentrations of ²³⁹Pu, ²⁴⁰Pu and ^239+240Pu in reference material IAEA‐315 (marine sediment) were estimated by three analytical methods: isotope dilution thermal ionisation mass spectrometry (TIMS), isotope dilution inductively coupled plasma‐mass spectrometry (ICP‐MS) and alpha spectrometry. The determination of ²³⁹Pu and ²⁴⁰Pu (^239+240Pu by alpha spectrometry) was carried out with samples from randomly selected bottles using each method. Plutonium‐238 was also measured by alpha spectrometry. A plutonium‐242 reference material was used as a spike for the quantitative analysis. The influence of ²⁴²Pu in the samples was therefore calculated; however, this contribution was less than the range of uncertainty and did not influence the final results. The obtained data were statistically analysed using variance component analysis and paired comparison. The combined standard uncertainties from “method/measurement”, “bottle” and “sub‐sample” were in the order of 3 to 6%. The main contributions to the uncertainty were from the material heterogeneity and from systematic differences between methods. Based on this study with twenty‐seven analyses using 10–14 g sample mass, concentrations of (38 ± 3) Bq kg^?1, (28 ± 3) Bq kg^?1 and (66 ± 4) Bq kg^?1 are proposed as recommended values for ²³⁹Pu, ²⁴⁰Pu and ^239+240Pu, respectively, and (9.5 ± 0.4) Bq kg^?1 for ²³⁸Pu as an information value in reference material IAEA‐315. In mass concentration units, these amount to (16.4 ± 1.2) ng kg^?1, (3.3 ± 0.4) ng kg^?1 and (0.015 ± 0.003) ng kg^?1 for ²³⁹Pu, ²⁴⁰Pu and ²³⁸Pu, respectively. The certified reference materials NIST 4350B and NIST 4354 were also analysed by TIMS for quality assurance of the method used in this study.     

Transition from eclogite to amphibolite-facies metamorphism in the Austroalpine Ulten Zone

Summary The Ulten Zone of the Austroalpine crystalline basement south-west of Meran (Italy) contains metapelitic schists and granoblastic paragneisses, leucocratic orthogneisses, migmatites (in both gneiss-lithologies), metabasites and ultramafic lenses. Metamorphic textures of the metapelitic schists and granoblastic paragneisses indicate two different metamorphic events, characterized by two mineral assemblages, which differ in mineral chemistry: (1) an eclogite facies mineral assemblage (M1) comprising Grt-Ky I-Bt. Ms-Kfs-PI-Qtz-Rt, and (2) an amphibolite facies mineral assemblage (M2) comprising Grt-KyII-Bt-Ms-PI-Qtz-Ilm±St. For the M1 event, pressures of at least 15kbar and temperatures of about 700°±50°C can be estimated. The later amphibolite facies overprint occurred at pressures of 6 to 8kbar and about 600°±50°C. The M1 and M2 assemblages belong to a continuous clockwise metamorphic evolution during the Variscan orogeny. Evidence for Alpine metamorphism can only be detected by sericite rims around kyanite and reset biotite ages. The migmatites, which contribute about 15–30vol.% of all rocks in the investigated area, were formed on the prograde path during the M1 event. Dissolution of H₂O in the melted part of the migmatites resulted in a CO₂dominated fluid, which was trapped in primary kyanite (M1) fluid inclusions. Secondary H₂O-rich fluid inclusions are found in quartz grains and may represent the fluid which enabled a pervasive equilibration during M2.

---

Übergang von eklogit-zu amphibolitfazieller Matamorphose in der austroalpinen Ultenzone

Zusammenfassung Die Ulten Zone, ein Teil des ostalpinen kristallinen basements, südwestlich von Meran, wird aus Metapeliten and granoblastischen Paragneisen, leukokraten Orthogneisen, Migmatiten (in beiden Lithologien), Metabasiten and ultramafischen Linsen aufgebaut. Metamorphe Texturen der Metapelite und granoblastischen Paragneise lassen auf zwei verschiedene metamorphe Ereignisse schließen, die durch unterschiedliche Mineral-chemismen und Paragenesen charakterisiert sind: (1) eine eklogitfazielle Paragenese (M1), bestehend aus Grt-KyI-Bt-Ms-Kfs-P1-Qtz-Rt und (2) eine amphibolitfazielle Paragenese (M2), bestehend aus Grt-KyII-Bt-Ms-P1-Qtz-Ilm±St. Für M1 konnten Minimaldrucke von 15kbar und Temperaturen von 700°±50°C abgeleitet werden. Die spätere amphibolitfazielle Überprägung fand bei 6 bis 8kbar und 600°±50°C statt. M1 und M2 gehören einer kontinuierlichen Metamorphoseentwicklung während der variszischen Orogenese an.Die Migmatite, ungefähr 15–30vol.% der Gesteine im untersuchten Gebiet, wurden am prograden Pfad während des M1 Ereignisses gebildet. Aufgrund der höheren Löslichkeit von H₂0 in der Schmelze, blieb ein CO₂, reiches Fluid zurück, das im primären Kyanit (M1) eingeschlossen wurde. Wässrige Flüssigkeitseinschlüsse können in Quarzkörnern gefunden werden. Dieses Fluid ist wahrscheinlich für die Reequilibrierung zu amphibolitfaziellen Bedingungen verantwortlich.

---

---

With 5 Figures     

Exclusive rights to fish: Towards a rational fisheries policy

Discusses the tragedy of the commons with regard to fish resources in the North Atlantic, and points to the fact that the Atlantic cod has now been so heavily exploited that a strong regulation of fisheries is needed to preserve an exploitable stock. The author argues for exclusive economic zones dividing up the remaining loopholes among coastal nations and individually transferable quotas.     

A nonlinear tensor field and the second metric tensor

In our preceding paper {see [L. Sh. Grigorian and S. Gottlöber, Astrofizika (in press)]} we investigated a self-gravitating system consisting of a scalar field and a linear tensor field _ik= _ki with minimal coupling and with allowance for the action of vacuum polarization effects. In the present paper we investigate the case of a nonlinear tensor field _ik. The action S ⁽⁾ of the field _ik is determined by the difference R_ik — _ik, where R_ik is the space-time Ricci tensor and Rik is the analogous quantity constructed using the metric _ik=g_ik+_ik induced by _ik ( is a free parameter). Here S ⁽⁾ coincides with the previously known expression for the action of a linear field _ik. Equations of motion are derived for _ik in curved space-time. The energy-momentum metric tensor, determining the contribution of _ik to the gravitational field equations, is calculated.Translated from Astrofizika, Vol. 39, No. 1, pp. 135–144, January-March, 1996.     

Effects of water on strength and failure mode of coarse-grained granites at 300°C

Summary Deformation experiments have been performed in a triaxial compression cell at a temperature of 300°C and confining pressures up to 65 MPa using samples of homogeneous, fresh two-mica-granite (RM) and monzogranite (CM). The cylindrical specimens (d=70 mm, h=140 mm, V=540 cm³) were tested undrained under dry (105°C), as received, and water saturated conditions at deformation rates between and . The mechanical behaviour of the two types of coarse-grained, crystalloblastic granites is critically influenced by mineralogical composition, porosity, and the amount of intergranular water present in the samples. The failure stress of the CM granite is at about 65% of that of the RM granite; in both rocks strength decreases with increasing porosity and water content.The presence of interstitial water causes a failure mode of non-localized, homogeneously distributed microcracking in the central parts of the samples, whereas, in runs with dry granites, strain localization along a single shear fracture was observed. When aqueous fluids are present, the macroscopic style of deformation of granites appears to be ductile even at lowP andT conditions. Strength and angle of internal friction are reduced to very low values. The style of deformation, as well as the reduction of strength of the water-saturated rock samples, is due to mechanical and chemical effects of intergranular water at elevated temperatures.The maximum differential stresses measured for these coarse-grained granites are much lower than the strength commonly reported for other granites, e. g. Westerly and Charcoal granites. Our data suggest that the strength of the granitic crust under differential stress is lower than currently deduced from laboratory experiments.     

Early Cretaceous volcaniclastic and quartzose sandstones from north central Nepal: composition,sedimentology and geotectonic significance

Early Cretaceous sandstones and shales from the Thakkhola region (Chukh and Tangbe Formations) were deposited on the subsiding northern (Tethyan) margin of Gondwana; current directions indicate a source area to the south. Whereas the Berriasian Chukh Formation is dominated by quartz arenites, volcaniclastic sandstones and minor conglomerates dominate the Tangbe Formation of Valanginian to Aptian age. Upsection, the composition of the volcaniclastic components changes from mainly basaltic to dominantly dacitic/rhyolitic. Magmatism was preceded and accompanied by strong uplift in the source area of the Chukh and Tangbe clastics, resulting in the erosion of probably more than several hundred meters of pre-Cretaceous sedimentary, metamorphic and plutonic rocks. A geochemical whole rock analysis of basaltic pebble fragments shows their alkaline character and suggests a within-plate geotectonic setting for the volcanic source. This volcanotectonic event is probably related to rifting between Australia and Greater India, where seafloor spreading began in late Valanginian/Hauterivian times.     

Cu 2p absorption spectroscopy as a probe for the site occupancy of (ZnxCu1−x)WO4 solid solution

The solid solution sanmartinite (ZnWO₄)—cuproscheelite (CuWO₄) has been studied using Cu 2p X-ray absorption spectroscopy. While a single L₃ absorption peak is observed for CuWO₄, two distinct L₃ absorption peaks with a separation of ～0.8 eV are observed for the intermediate samples in the solid solution. The two peaks represent distinct Cu sites: one with all CuO₆ next nearest neighbours in the (Cu,Zn)O₆ chains, another having at least one ZnO₆ next nearest neighbour. Both sites show a linear increase in covalency as a function of increasing Cu-content. The relative intensities of the two absorption peaks is dependent upon the Cu-content and has been used to model the site occupancies. The results reveal that the local structural effects can be associated with a composition-dependent structural phase transition from P2/c (ZnWO₄) to P $\bar 1$ (CuWO₄). Deviations from a single-site model are explained in terms of the local environments, and evidence for site preferences and local clustering are explored.