Evidence of a possible north-south shift of the western sector of the African plate—Discussion

A negative anomaly in the vertical magnetic field component at a site in Nigeria has recently been interpreted by Oni in terms of a possible north-south shift of the western sector of the African plate. In this paper, comments are made under the headings of magnetic field reversals, polar wander and secular variation, which cast doubt on Oni's interpretation.     

Production of dissolved organic carbon in forest soils along the north–south European transect

The aim of this study is to estimate the C loss from forest soils due to the production of dissolved organic C (DOC) along a north–south European transect. Dissolved organic matter (DOM) was extracted from the forest soils incubated at a controlled temperature and water content. Soils were sampled from forest plots from Sweden to Italy. The plots represent monocultures of spruce, pine and beech and three selected chronosequences of spruce and beech spanning a range of mean annual temperature from 2 to 14 °C. The DOM was characterized by its DOC/DON ratio and the C isotope composition δ¹³C. The DOC/DON ratio of DOM varied from 25 to 15 after 16 days of incubation and it decreased to between 16 and 10 after 126 days. At the beginning of incubation the δ¹³C values of DOC were 1‰ or 2‰ less negative than incubated soils. At the end of the experiment δ¹³C of DOC were the same as soil values. In addition to DOC production heterotrophic respiration and N mineralization were measured on the incubated soils. The DON production rates decreased from 30 to 5 μgN gC⁻¹ d⁻¹ after 16 days of incubation to constant values from 5 to 2 μgN gC⁻¹ d⁻¹ after 126 days at the end of experiment. The DIN production rates were nearly constant during the experiments with values ranging from 20 to 4 μgN gC⁻¹ d⁻¹. DOC production followed first-order reaction kinetics and heterotrophic respiration followed zero-order reaction kinetics. Kinetic analysis of the experimental data yielded mean annual DOC and respiration productions with respect to sites. Mean annual estimates of DOC flux varied from 3 to 29 g of C m⁻² (1–19 mg C g⁻¹ of available C), corresponding to mean DOC concentrations from 2 to 85 mg C L⁻¹.     

Measuring the north–south coseismic displacement component with high‐resolution multi‐aperture InSAR

Earthquake source inversions based on space‐borne Synthetic Aperture Radar interferometry (InSAR) are used extensively. Typically, however, only the line‐of‐sight (LoS) surface displacement component is measured, which is mainly sensitive to the vertical and E–W deformations, although well‐established methods also exist to estimate the flight‐path component, which is highly sensitive to the N–S displacement. With high‐resolution sensors, these techniques are particularly appealing, because accuracies in the order of 3 cm can be achieved, while retaining spatial resolutions between 45 m and a few km, depending on the required level of filtering. We discuss the application to COSMO‐SkyMed SAR imagery of the Spectral Diversity or Multi Aperture Interferometry technique, presenting the first SAR flight‐path displacement field associated with the Mw 6.3, 2009 L'Aquila event (central Apennines). Finally, we observe and characterize a previously unknown misregistration pattern.     

Seasonal patterns of pollen and the use of pollen as a chronometer in the north‐western Loess Plateau of China

The use of high‐resolution and highly precise age models is essential for quantitative environmental reconstructions. To assess the reliability of pollen as a chronometer in laminated lake sediments, pollen traps and lake cores, statistical methods were used to study seasonal pollen signals. The results showed that (a) the pollen assemblages from air traps and lake/reservoir surface sediment traps in the north‐western Loess Plateau of China displayed distinct seasonal patterns, which were confirmed by nonmetric multidimensional scaling (NMDS); and (b) The pollen assemblages of the dark and light layers in the Zhenhu Lake cores allowed detection of the seasonality of sediment deposition based on the results of the pollen traps. The NMDS method also showed statistically significant differences between the pollen assemblages of the dark and light layers. Furthermore, instrumental monitoring data and characteristic time markers of historical events (1958 and 2011) were used to cross‐check the chronologies obtained by the pollen assemblages, and the results strengthened the credibility and reliability of pollen as a chronometer.     

Variations of b‐values at the western edge of the Ryukyu Subduction Zone,north‐east Taiwan

Using earthquakes relocated in north‐east Taiwan, we estimated b‐value distribution along a cross‐section located near the Ryukyu slab edge, and four b‐value anomalous areas are evidenced: (1) a high b‐value body lying on top of a low Vp, low Vs and high Vp/Vs sausage‐like body was considered as a region of enhanced partial melt or water supply above which seismicity occurs; (2) beneath the Ilan Plain, an anomalous area characterized by b‐values slightly higher than 1.1 might give evidence to the magma conduits to the Kueishantao Island; (3) above the Ryukyu Wadati‐Benioff zone, at depths ranging from 90 to 110 km, a high b‐value anomaly might correspond to the depth where dehydration occurs in the subducting oceanic plate; and (4) a low b‐value area located within the Ryukyu slab, at depths ranging from 70 to 90 km, might be linked to the compressive mechanisms shown by focal mechanisms and the bending of the subducting plate.     

Proterozoic–Early Paleozoic evolution in western South America—a discussion

The hypothesis of exotic terranes in Perú, Bolivia, Argentina and Chile generated discussions on the mode of transfer and extent of accretional events that may have occurred in the southern Andes during the Late Proterozoic–Early Paleozoic. Initially, a tectogenesis based on autochthonous mobile fold belts was discussed. Following ideas emphasised the fragmentation of the supercontinent Rodinia, Laurentia moving along the West Gondwana border and colliding with the Gondwana western margin. The most important effect of this Laurentia/Gondwana relationship was attributed to the Argentine Precordillera (or Cuyania) terrane splitting off from Laurentia and docking to Gondwana in the Early Paleozoic. In this study, the most cited arguments for this Laurentia/Precordillera relationship are discussed, emphasising paleontological considerations. It is shown that these arguments do not exclude a close original vicinity of the Precordillera terrane to Gondwana.The Precordillera terrane is suggested to be part of a hypothetical platform, which developed between South America, Africa and Antarctica (SAFRAN platform), and which was displaced to its actual position by transcurrent faults. The collisional events in the Sierras Pampeanas ensued from strike–slip movements and were responsible for the S and I type transpressional magmatism along the Pampean and Famatinian terranes. The final result of this continent-parallel movement of terrane slices is similar to that of a terrane split off from Laurentia, but the first-named way of formation easier explains the general continuity of plate convergence at the western border of Gondwana than the Laurentia/Precordillera connection does.     

Evidence for a mid‐Pleistocene shift of ice‐drift pattern in the Nordic seas

Sediment proxy records from a continuous, 1.5 million year long deep‐sea sediment core from a site in the western Norwegian Sea were used to obtain new insights into the nature of palaeoceanographic change in the northern North Atlantic (Nordic seas) during the climatic shift of the Mid‐Pleistocene Revolution (MPR). Red‐green sediment colour and magnetic susceptibility records both reveal significant differences in their mean values when comparing the intervals older than 700 000 yr (700 ka) with those from the past 500 kyr. The timing and duration of these changes indicates that the MPR in the Nordic seas is characterised by a gradual transition lasting about 200 kyr. Together with further sedimentological evidence this suggests that the mid‐Pleistocene climate shift was accompanied by a general change in ice‐drift pattern. It is further proposed that prior to the onset of the major late Pleistocene glaciations in the Northern Hemisphere a significant proportion of the ice in the eastern Nordic seas originated from a southern provenance, whereas later it dominantly came from the surrounding landmasses. Copyright © 2003 John Wiley & Sons, Ltd.     

Active faulting south of the Himalayan Front: Establishing a new plate boundary

New tectonic uplifts south of the Salt Range Thrust and Himalayan Front Thrust (HFT) represent an outward step of the plate boundary from the principal tectonic displacement zone into the Indo-Gangetic Plain. In Pakistan, the Lilla Anticline deforms fine-grained overbank deposits of the Jhelum River floodplain 15 km south of the Salt Range. The anticline is overpressured in Eocambrian non-marine strata. In northwest India south of Dehra Dun, the Piedmont Fault (PF) lies 15 km south of the HFT. Coalescing fans derived from the Himalaya form a piedmont (Old Piedmont Zone) 15–20 km wide east of the Yamuna River. This zone is uplifted as much as 15–20 m near the PF, and bedding is tilted 5–7° northeast. Holocene thermoluminescence-optically-stimulated luminescence dates for sediments in the Old Piedmont Zone suggest that the uplift rate might be as high as several mm/a. The Old Piedmont Zone is traced northwest 200 km and southeast another 200 km to the Nepal border. These structures, analogous to protothrusts in subduction zones, indicate that the Himalayan plate boundary is not a single structure but a series of structures across strike, including reactivated parts of the Main Boundary Thrust north of the range front, the HFT sensu stricto, and stepout structures on the Indo-Gangetic Plain. Displacement rates on all these structures must be added to determine the local India-Himalaya convergence rate.     

Intraplate tectonics of the western North American plate

Fault-plane solutions, Cenozoic geology, and in-situ stress measurements are used to infer contemporary extension between subplates of the western North American plate. Intraplate, northwest extension is accommodated by strike-slip and oblique normal faulting along present-day seismic zones. Cenozoic volcanism, facilitated by regional extension above a subducting plate, may have spread radially outward from the northern Great Basin to form a continental triple junction with the principal arms: the central Great Basin, the Snake River Plain and a southeast-trending zone of rhyolite domes in the southern Columbia Plateau.     

Genesis of Podiform Chromite Deposits——Evidence from the Luobosa Chromite Deposits, Tibet

Taking for example the Luobosa chromite deposit in Tibet combined with other deposits of the same type in the world, the paper discusses the genetic mechanism of podiform chromite deposits. The study indicates that chromite and dunite-harzburgite are both the products of different degrees of partial melting of the same primary pyrolite(spinel lherzolite) and that chromite and dunite are the end product of higher degree of partial melting.The melting mechanism lies in the convcrsion of the two subgroups of pyroxenes(Cpx and Opx) into olivine and spinel respectively as a result of their incongruent melting, accompanied by regulation and reformation of accessory chrome spinel and such rock-forming minerals as Ol, Opx and Cpx, thus resulting in the close association of chromite deposits and dunite. The results of this study are of general significance for understanding the genesis of chromite deposits of the same type in the world.     

PGE mineralization in the western sector of the eastern Bushveld Complex

Summary Unusual facies of the Merensky Reef, the UG-2 and the UG-1 chromitite layers are developed in the western sector of the eastern Bushveld Complex. Within the basal pyroxenite of the Merensky unit, mineralization can be developed at up to four levels. Some of these contain significant mineralization with an increase in the Pt/Pd ratio upward in the succession.The UG-2 chromitite layer consists of a lower, sulphide-rich layer and an upper, sulphide-poor layer. Although these two layers are separated by a pyroxenite parting in places, both contain high platinum-group element (PGE) values. Textural features such as inclusions of base metal sulphides in chromite grains, and the moulding of sintered chromite grains around sulphides, indicates that immiscible sulphide liquid separated prior to or simultaneously with chromite crystallization. The presence of platinum minerals within the sulphides of the inclusions and enclosed in all the base metal sulphides interstitial to chromite, indicates that the PGE were extracted from the magma by the sulphide liquid.Textural and compositional evidence suggests that the sulphide enrichment in the UG-1 chromitite layer is also of magmatic origin, but that these sulphides underwent remobilization at high temperatures.Magma mixing processes are considered to have produced the chromitite layers. The high sulphide content associated with the chromitite layers in the upper critical zone in this sector is ascribed to favourable compositions and proportions of the magmas involved in the mixing process.

---

PGE-Vererzung im westlichen Sektor des östlichen Bushveld-Komplexes

Zusammenfassung Ungewöhnliche Fazies des Merensky-Reefes sowie der UG-2 und der UG-1 Chromitite kommen im westlichen Sektor des östlichen Bushveld Komplexes vor. In den basalen Pyroxeniten der Merensky-Einheit liegt Vererzung in bis zu vier verschiedenen Niveaus vor. Einige von diesen enthalten signifikante Metallgehalte, wobei das Pt/Pd Verhältnis gegen das Hangende hin zunimmt.Der UG-2 Chromitit besteht aus einer unteren, Sulfid-reichen, und einer oberen, Sulfid-armen Lage. Obwohl diese beiden Lagen stellenweise durch eine pyroxenitische Zwischenschicht getrennt sind, enthalten beide hohe Platin-Gruppen-Elementgehalte (PGE). Texturen wie z.B. Einschlüsse von Buntmetallsulfiden in Chromitkörnern, und die Anordnung von gesinterten Chromitkörnern um Sulfide herum weisen darauf hin, daß eine unmischbare Sulfidschmelze vor oder gleichzeitig mit der Chromitkristallisation abgetrennt wurde. Das Vorkommen von Platin-Mineralen in den Sulfiden der Einschlüsse, und in allen Buntmetallsulfiden die zwischen Chromitkörnern vorkommen, zeigen, daß die PGE durch eine Sulfidschmelze aus dem Magma entfernt worden sind.Texturelle und chemische Parameter zeigen, daß die Sulfidanreicherung in den UG-1 Chromititen auch einen magmatischen Ursprung hat, jedoch waren diese Sulfide später von einer Hochtemperatur-Mobilisation betroffen.Die Chromitit-Lagen werden durch Magmen-Mischung, der hohe Sulfid-Gehalt in den Chromitit-Lagen der oberen Kritischen Zone in diesem Sektor durch günstige Zusammensetzungen und Verhältnisse der Magmen, die an diesem Mischungsprozess teilgenommen haben erklärt.

---

---

With 7 Figures     

Quantifying the P–T–t conditions of north–south Lhasa terrane accretion: new insight into the pre‐Himalayan architecture of the Tibetan plateau

An integrated field, petrological and geochronological study of the Basong Tso region of south‐eastern Tibet has constrained the timing and P–T conditions of north–south Lhasa terrane accretion and provides new insight into the tectonothermal evolution of the Tibetan plateau. Two distinct high‐grade metamorphic belts are recognized in the region: a southern belt (the Basong Tso complex) that consists of sheared schist and orthogneiss; and a northern belt (the Zhala complex) that comprises paragneiss and granite. Combined pseudosection modelling and U–Pb geochronology of monazite and zircon indicates that the Basong Tso complex records peak metamorphic conditions of 9 ± 0.5 kbar and 690 ± 25 °C at c. 204–201 Ma, whereas the Zhala complex experienced peak metamorphic conditions of 5.0 ± 1.0 kbar and 740 ± 40 °C at c. 198–192 Ma. Microstructural analysis suggests that the two belts share a common early prograde history, after which the Basong Tso complex attained peak conditions following rapid burial, and the Zhala complex approached peak conditions along an isobaric path. Overall it is inferred that the Basong Tso and Zhala complexes represent the lower and upper structural levels of an evolving orogen that underwent Barrovian‐type metamorphism following collision (M1), followed by Buchan‐style overprinting at higher structural levels due to heat advection by syn‐tectonic granites (M2). Mylonitization (sensu lato) of the Basong Tso complex and juxtaposition of the two units occurred after attainment of peak conditions. The dominance of Mesozoic regional metamorphism across most of the Tibetan plateau indicates that Cenozoic crustal thickening processes, where present, are only manifested at depth.     

Transforming the Miocene Altyn Tagh fault slip into shortening of the north‐western Qilian Shan: insights from the drainage basin geometry

Although it has long been proposed that active crustal shortening within the Qilian Shan accommodates slip along the Altyn Tagh fault, questions regarding the temporal and spatial amount of slip transfer remain debated. Traditional shortening calculations cannot yet be made accurately, because the geology is not known well enough. Following the view that long, linear drainage basins in the north‐western Qilian Shan developed in response to internal shortening associated with slip transfer along the Altyn Tagh fault, we targeted this special irregular drainage basin geometry to develop an alternative way to estimate shortening. We infer a total of ~66 km of shortening in the western Qilian Shan with a north‐eastward decrease in the magnitude of shortening. This ~39% shortening strain is comparable to previous estimates. Estimates of dates of initiation support the view that significant shortening and topographical growth within the north‐western Qilian Shan occurred in post‐Middle Miocene time.     

Kilometre-scale sheath folds in the western Betics (south of Spain)

The three-dimensional exposures of the Sierra Blanca marbles and neighbouring rocks in the western Betic cordillera of the south of Spain show tight to isoclinal F₃ folds re-folded by a larger sheath-like structure. The relationships between both structures are as follows: (1) The mean axial surface for Sierra Blanca sheath fold can be defined as the great circle of best fit through the ß-axes of sub-cylindrical F₃ folding domains. (2) The asymmetric F₃-folds indicate a single mainly eastward normal sense of shear. (3) The mean attitude of the axial-plane crenulation foliation, Sc, is also sub-parallel to the mean axial surface for Sierra Blanca sheath fold. The regional context of the Sierra Blanca sheath fold is finally discussed, and a model of a heterogeneous high-strain extensional zone proposed.     

The rural sector in the development prospects and problems of a typical African country — Nigeria

This paper begins by theoretically presenting the rural sector of developing countries in a national development context. The critical role of the sector is highlighted and the system effects of inadequate equipment of the sector for such role are considered. The Nigerian situation is used to illustrate the prospects and the problems of the rural sector in most African countries. Remedial policy measures are shown to lie mainly in the provision of infrastructural facilities, in forging effective linkage relationships with the urban sector and in rural economy diversification towards a nationally beneficial integrated development.