Wind erosion on the north‐eastern Tibetan Plateau: constraints from OSL and U‐Th dating of playa salt crust in the Qaidam Basin

The arid Qaidam Basin is the largest (~3.88 × 10⁴ km²) basin on the north‐eastern Tibetan Plateau. Wind erosion in the area has been regarded as an important trigger for intra‐basin tectonic balance upheaval, geomorphologic development and as a major supplier of dust to the Chinese Loess Plateau downwind. An initial estimate of the rate of wind erosion (Kapp et al., 2011) based on geological cross‐sections has suggested up to 3.2 × 10⁴ km³ of sediments has been deflated over the past 2.8 Ma, lowering the landscape by an average of 0.29 mm/yr. In this paper we re‐evaluate this estimate by dating surface crusts present on three playas within the basin. Understanding the development of these playas is crucial to assessing the overall role of the wind in shaping the regional landscape because they are typically capped with a thick salt crust which effectively protects them from wind erosion. Optically stimulated luminescence (OSL) and U‐series dating from a pit section and from the top of a deep drill core, together with results from magnetostratigraphy and a climate proxy record correlated to the marine oxygen isotope record, are used here to determine the age of the playa plains and suggest that the salt crusts have an age of c. 0.1 Ma. This young age and the wide distribution of resistant thick salt crusts of the playa plains indicate a much lower degree of wind erosion than previously suggested. The crusts protect the surface from significant surface erosion (including sediment exhumation and unloading) and whilst some wind erosion does occur, it is unlikely to be sufficient to trigger tectonic uplift of the basin or to be a major dust source for the Loess Plateau as previously suggested. Copyright © 2013 John Wiley & Sons, Ltd.     

Layer parallelisation: An unrecognised mechanism for palaeomagnetic rotations in fold belts

We report a new driving mechanism for vertical-axis rotation in fold belts: during directed folding, there always are layers which are tilted oblique to the regional shortening direction. These layers are geometrically incompatible with fold closure and must become accommodated towards parallelism with the other layers in the course of further shortening. Accommodation is achieved through vertical-axis rotations of the layers towards the shortening direction. A revision of palaeomagnetic data sets from fold belts shows that (1) incompatible layers occur in almost all data sets, reflecting a certain degree of non-cylindrical folding, and (2) a parallelisation of the layers by true vertical-axis rotations occurred when folding became intense. Layer parallelisation is a potential source of disturbance for palaeomagnetic and tectonic interpretation. It can be the explanation for some of the frequent outliers in palaeomagnetic data sets, and a basic model for the rotation pattern of palaeomagnetic directions.     

The Cold Carbonate Connection Between Mono Lake, Californiaand the Bransfield Strait, Antarctica

Ikaite is a rare form of carbonate – calciumcarbonate hexahydrate (CaCO3·6H2O) and isthe precursor to thinolites. Metastable 'ikaite'crystals, discovered in unconsolidated marinesediments in the King George Basin in the BransfieldStrait, Antarctica, are related to diageneticremineralization reactions of organic matter. StableC, O, and H-isotopes track the response of ikaitecrystals, during growth, to changing interstitialfluid conditions as a result of bacterial sulphatereduction and methanogenesis. Ikaites form inpreference over calcite or aragonite at the prevailingsurface sediment conditions of -1.6 °C and 200bar in the King George Basin.The calcareous tufa towers of the terrestrial,hypersaline Mono Lake of northern California areCaCO3-precipitates formed by the influx ofsubmerged springs of calcium-rich freshwaters enteringthe alkaline lake (CO2 = 0.5 m, pH =9.8). Under current climatic conditions the mineralcalcite precipitates, but during the colder Tiogaglacial period of Late Wisconsian age (12,000 to 9,000years BP), and possibly present day during the winterseason, the monoclinic calcium carbonate hexahydrate(ikaite) was the dominant phase formed. Thesepaleo-ikaites have since recrystallized to form thecalcitic pseudomorph 'thinolites'. They are foundelsewhere in recent and ancient sediments of polarregions, e.g., as 'glendonites'. The environmentaloccurrence of ikaites and their pseudomorphs deem themas potential paleoclimatic indicators of coldenvironments. The larger crystals are typicallyrestricted to colder, deeper organic-rich sediments orin moderately evaporitic basins. In these cases, theikaite formation and decomposition may be influencedby additives such as phosphate or amino acids.     

On the visualization of three-dimensional datasets

The effective visualization of three-dimensional (3D) datasets, both observationally and computationally sourced, is becoming common in solar physics. We present example plots of data from a 3D magnetohydrodynamical simulation, where depth perception is simulated using chromo-stereoscopy. The depth information is coded into the images using colours. When such images are viewed with double prism refraction ChromaDepthTM 3D glasses, a pronounced 3D effect is achieved. This visualization method is especially suited for working with and presenting computationally derived 3D datasets.     

Industrialization affects heavy metal and carbon isotope concentrations in recent Baltic Sea sediments

Recent sediment cores of the western Baltic Sea were analyzed for heavy metal and carbon isotope contents. The sedimentation rate was determined from radiocarbon dates to be 1.4 mm/yr. The ‘recent age’ of the sediment was about 850 yr. Within the upper 20 cm of sediment, certain heavy metals became increasingly enriched towards the surface; Cd, Pb, Zn and Cu increased 7-, 4-, 3- and 2-fold, respectively, whereas Fe, Mn, Ni and Co remained unchanged. Simultaneously, the radiocarbon content decreased by about 14 per cent. The enrichment in heavy metals as well as the decrease in the ¹⁴C-concentration during the last 130 ± 30yr parallels industrial growth as reflected in European fossil fuel consumption within that same period of time. The near-surface sediments are affected by residues released from fossil fuels at the rate of about 30 g/m² yr for the past two decades. The residues have a pronounced effect on the heavy metal and carbon isotope composition of the most Recent sediments allowing estimates to be made for sedimentation, erosion and heavy metal pollution.     

A review of: “Proceedings of the second asian congress of fluid mechanics”

Abstract

Edited by T. C. Lin. Science Press, Beijing and VNU Science Press, Utrecht, 1984. xvi+ 1154pp. $143 (ISBN 90 6764 005 0).     

Arbeit deutscher Geologen im Ausland seit Kriegsende

Ohne Zusammenfassung     

High-pressure granulite facies metamorphism in the Pan-African belt of eastern Tanzania: P–T–t evidence against granulite formation by continent collision

To constrain the tectonic history of the Pan-African belt in Tanzania, we have studied the P–T evolution of granulites from northern and eastern Tanzania representative for a large part of the southern Pan-African belt of East Africa (e.g. Pare, Usambara, Ukaguru and Uluguru Mountains). Thermobarometry (conventional and multireaction equilibria) on enderbites and metapelites gives 9.5–11 kbar and 810±40 °C during peak metamorphism at 650–620 Ma. This is consistent with the occurrence of both sillimanite and kyanite in metapelites and of the high-P granulite facies assemblage garnet–clinopyroxene–quartz in mafic rocks. Peak metamorphic conditions are surprisingly similar over a very large area with N-S and E-W extents of about 700 and 200 km respectively. The prograde metamorphic evolution in the entire area started in the kyanite field but evolved mainly within the sillimanite stability field. The retrograde P–T evolution is characterized by late-stage kyanite in metapelites and garnet–clinopyroxene coronas around orthopyroxene in meta-igneous rocks. This is in agreement with thermobarometric results and isotopic dating, indicating a period of nearly isobaric and slow cooling prior to tectonic uplift. The anticlockwise P–T path could have resulted from magmatic underplating and loading of the lower continental crust which caused heating and thickening of the crust. Substantial postmetamorphic crustal thickening of yet unknown age (presumably after 550 Ma) led subsequently to the exhumation of high-P granulites over a large area. The results are consistent with formation of the Pan-African granulites at an active continental margin where tonalitic intrusions caused crustal growth and heating 70–100 Ma prior to continental collision. The P–T–t path contradicts recent geodynamic models which proposed tectonic crustal thickening due to continental collision between East and West Gondwana as the cause of granulite formation in the southern part of the Pan-African belt.     

Magnetostratigraphy of deep drilling core SG-1 in the western Qaidam Basin (NE Tibetan Plateau) and its tectonic implications

The Qaidam Basin is the largest intermontane basin of the northeastern Tibetan Plateau and contains a continuous Cenozoic sequence of lacustrine sediments. A ~ 1000-m-deep drilling (SG-1) with an average core recovery of ~ 95% was carried out in the depocenter of the Chahansilatu playa (sub-depression) in the western Qaidam Basin, aimed to obtain a high-resolution record of the paleoenvironmental evolution and the erosion history. Stepwise alternating field and thermal demagnetization, together with rock magnetic results, revealed a stable remanent magnetization for most samples, carried by magnetite. The polarity sequence consisted of 16 normal and 15 reverse zones which can be correlated with chrons 1n to 2An of the global geomagnetic polarity time scale. Magnetostratigraphic results date the entire core SG-1 at ~ 2.77 Ma to ~ 0.1 Ma and yielded sediment accumulation rate (SAR) ranging from 26.1 cm/ka to 51.5 cm/ka. Maximum SARs occurred within the intervals of ~ 2.6–2.2 Ma and after ~ 0.8 Ma, indicating two episodes of erosion, which we relate to pulse tectonic uplift of the NE Tibetan Plateau with subsequent global cooling.     

Three generations of monazite in Austroalpine basement rocks to the south of the Tauern Window: evidence for Variscan, Permian and Eo-Alpine metamorphic events

Three monazite generations were observed in garnet-bearing micaschists from the Schobergruppe in the basement to the south of the Tauern Window, Eastern Alps. Low-Y monazite of Variscan age (321?±?14?Ma) and high-Y monazite of Permian age (261?±?18?Ma) are abundant in the mica-rich rock matrix and in the outer domains of large garnet crystals. Pre-Alpine monazite commonly occurs as polyphase grains with low-Y Variscan cores and high-Y Permian rims. Monazite of Eo-Alpine age (112?±?22?Ma) is rarer and was observed as small, partly Y-enriched grains (3?wt. %?Y₂O₃) in the rock matrix and within garnet. Based on monazite-xenotime thermometry, Y?+?HREE values in monazite indicate minimum crystallization conditions of 500?°C during the Variscan and 650?°C for the Permian and Alpine events, respectively. Garnet zoning and thermobarometric calculations with THERMOCALC 3.21 record an amphibolite facies, high-pressure stage of ~600?°C/13?C16?kbar, followed by a thermal maximum at 650?C700?°C and 6?C9?kbar. The Eo-Alpine age for these two events is supported by inclusions of Cretaceous monazite in the garnet domains used for thermobarometric constraints and through the high growth temperatures of Eo-Alpine monazite, which is consistent with that of the thermal maximum (~700?°C). The age and growth conditions of a few Mn-rich garnet cores, sporadically present within Eo-Alpine garnet, are unclear because inclusions of monazite, plagioclase and biotite necessary for thermobarometric- and age constraints are absent. However, based on monazite thermometry, Permian and Variscan metamorphic conditions were high enough for the growth of pre-Alpine garnet. The formation of Variscan garnet and its later resorption, plus Y-release, would also explain the high Y in Permian monazite, which cannot originate from preexisting Variscan monazite only. Monazite of Variscan, Permian and/or Eo-Alpine ages were also observed in other garnet-bearing micaschists from the Schobergruppe. This suggests that the basement of the Schobergruppe was overprinted by three discrete metamorphic events at conditions of at least lower amphibolite facies. While the Variscan event affected all parts of this basement, the younger events are more pronounced in its structurally lower units.