The loess of north-central China: Geotechnical properties and their relation to slope stability

Loess is an aeolian deposit consisting of predominantly silt-sized quartz particles, and containing variable amounts of clay-sized minerals. Loess is generally classified as a water-softening material, because upon wetting the loess fabric rapidly weakens or collapses. The strain hardening of Malan loess and the brittle failure of Lishi and Wucheng loess are in strong contrast to the failure behaviour of these loess deposits in a remoulded state. From tests carried out on samples with varying moisture contents, using a modified Bromhead ring shear apparatus, it was found that the effective apparent cohesion gradually increases and the effective internal friction angle decreases with an increase in moisture content. When the moisture content reaches a material-specific threshold, the effective cohesion decreases rapidly and the effective internal friction angle stabilises at a residual value.

The frequent failure of loess slopes in the western part of the Chinese Loess Plateau is closely related to progressive weathering along zones in these slopes, which causes a dramatic decrease in strength from the peak strength condition. Progressive weathering is common in the loess slopes in the western part of the Chinese Loess Plateau. During the process shear strength reduction along potential slip surfaces may be achieved by leaching of readily soluble salts, destruction of cementation bonds, and redistribution of particles. Localized collapse of the loess fabric causes internal deformation and consequently peak strength conditions are concentrated on a progressively smaller area of the failure plane. Therefore, the mode of failure of loess slopes is generally determined by brittle failure of the undisturbed, and unweathered, central parts of the slopes. It is important that both the weathered and unweathered strength of the loessmaterials in this area be established in order to analyze the stability of existing loessslopes, many of which are steepand lie directly above domestic and industrial urban areas.     

Fluids of anorogenic granites: A preliminary assessment

Summary Anorogenic granites are principally associated with alkaline ring complexes or anorthositic suites. Compositional relationships in the granites may be largely the result of variable compositions of fluid phases present during the initial melting and/or later evolution. For example, the negative K₂O-SiO₂ relationships in granites associated with alkaline rocks is consistent with: (1) evolution of the initial melts in a CO₂-rich environment, which causes the product of incongruent melting of biotite to be a prominent component of the derived magmas; and/or (2) crystallization of the granites in a CO₂-rich environment, which causes a residual liquid to be rich in biotite component. Limited evidence indicates that granites evolved in fluorine-rich environments are enriched in Na and may have negative relationships between Na₂O and SiO₂. Fluorine-rich environments may be more characteristic of the silicic rocks in rapakivi-granit e/anorthosite complexes than of granites in alkaline suites.

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Fluida von anorogenen Graniten: Eine erste, abschätzende Studie

Zusammenfassung Anorogene Granite sind prinzipiell mit alkalischen Ringkomplexen oder AnorthositSerien assoziiert. Die spezielle Zusammensetzung dieser Granite resultiert vermutlich hauptsächlich aus der variablen Zusammensetzung der während des initialen Schmelzprozesses und/oder späteren magmatischen Evolution vorhandenen fluiden Phase. Zum Beispiel geht die negative Korrelation zwischen K₂O und SiO₂ in Graniten, die mit alkalinen Gesteinen assoziiert sind, konform mit 1) der Entwicklung der Initialschmelze in einem CO₂-reichen Milieu, wodurch das Produkt des inkongruenten Schmelzens von Biotit als Komponente im Magma dominiert, und/oder 2) der Kristallisation der Granite in einem CO₂-reichen Milieu, das wiederum eine biotitreiche Residualschmelze hervorruft. Limitierte Hinweise deuten an, daß Granite, die in einem fluorreichen Milieuentstehen, an Na angereichert sind und eine negative Korrelation zwischen Na₂O und SiO₂ zeigen. Fluorreiches Milieu ist vermutlich für die sauren Gesteine innerhalb von Rapakivi-Granit/Anorthosit-Komplexen eher charakteristisch als für Granite in alkalischen Serien.

     

The politics of the British inner city

The reasons for the recent revival of the debate about the inner cities and its implications for geographical thinking are analysed. Changing government policies towards inner cities since the initial declaration of intent by the Labour government in 1968 are described, and the more recent Conservative government emphasis upon privatization, enterprise, deregulation and flexibility is discussed. Academic interest currently focuses on the way in which the terms of the debate are set politically with the concept of the ‘inner city’ being defined and redefined according to the political priorities of policy-makers. Running parallel with this process is the ‘imagineering’ of place, whereby places are sold on the basis of deliberately created images. Discussion of the London Docklands highlights the mixed record of redevelopment compared with the images purveyed by government and other agencies. A ‘hidden agenda’ (of race or dismantling of local government power) is shown to underlie political rhetoric about inner cities. The inner city thus emerges as a key ideological battleground between contending political groupings.     

Tidal and seasonal variations of sulfate ion in a New Jersey marsh system

Sulfate variations during a tidal cycle were investigated at three sites in a highly polluted tidal marsh. Sulfate- chlorinity relationships were determined in the light of fluctuations in temperature, dissolved oxygen and pH. The relationships were found to be seasonal in nature, being affected by temperature and rainfall effects on biologic productivity. Both reduction of sulfate ion to a sulfide species and oxidation of sulfide species to the sulfate ion were found to occur. Consideration of the sulfide oxidation process suggests the possibility that metals precipitated as sulfides may be mobilized and redistributed in the marsh system.     

Igneous series in island arcs: The northeastern Caribbean compared with worldwide island-arc assemblages

Two major magma series are recognised in the northeastern Carribean island arcs and correlated with numerous examples elsewhere, mainly in the western Pacific. The Primitive Island Arc (PIA) series is apparently the oldest in each of its island-arc occurrences and is followed by the calcalkaline (CA) series, which locally co-occurs with a high-potassium (HK) variety, especially in topographically larger island areas. Also occurring in the northeastern Caribbean are tectonically emplaced examples of metamorphosed mid-ocean ridge basalt (MORB) and a diabase dike swarm possibly related to a rifting event. PIA series is distinguished from the CA by its lower content of LIL elements, including REE. Th, U, Zr, K, and Ba. The difference is most striking in the abundant siliceous differentiates of this series, in which these elements are scarcely higher in abundance than in mafic examples. The HK series is considered to be a subseries of the CA which is higher in K, Ba, Sr, Mg, and Ni, but has REE, Th, and other element levels very similar to the CA. The PIA series has been confused with MORB but has lower REE (with flatter patterns generally), much lower Ni, lower Ti, Zr, and Mg, and appears to be intrinsically hydrated. The abundance of siliceous differentiates also separates this series from MORB. PIA magma is believed to be generated by massive melting of hydrated mantle which rises in front of the descending snout of a newly subducting slab at the initial stage of island-arc genesis; i.e., before the island-arc platform has formed. CA magma has a different source and is probably the partially fused material of the slab itself, more or less modified by volcanic-sedimentary materials dragged to depth during subduction.     

Geochemistry of Cenezoic volcanic rocks, Baja California, Mexico: Implications for the petrogenesis of post-subduction magmas

Late Cenozoic volcanism in Baja California records the effects of cessation of subduction at a previously convergent, plate margin. Prior to 12.5 m.y., when subduction along the margin of Baja ceased, the predominant volcanic activity had a calc-alkaline signature, ranging in composition from basalt to rhyolite. Acidic pyroclastic activity was common, and possibly represented the westermost, distal edge of the Sierra Madre Occidental province. After 12.5 m.y., however, the style and composition of the magmatic products changed dramatically. The dominant rock type within the Jaraguay and San Borja volcanic fields is a magnesian andesite, with up to 8% MgO at 57% SiO₂, low Fe/Mg ratios, and high Na/K ratios. These rocks have unusual trace-element characteristics, with high abundances of Sr (up to 3000 ppm), low contents of Rb; K/Rb ratios are very high (usually over 1000, and up to 2500), and Rb/Sr ratios are low (less than 0.01). Furthermore, La_n/Yb_n ratios are high, consistent with derivation from a mantle source with fractionated REE patterns. ⁸⁷Sr/⁸⁶Sr ratios are less than 0.7048, and usually less than 0.7040, whereas the pre-12.5 m.y. lavas have ⁸⁷Sr/⁸⁶Sr ratios between 0.7038 and 0.7063. We have previously termed these rocks bajaites, in order to distinguish them from other magnesian andesites. Bajaites also occur in southernmost Chile and the Aleutian Islands, areas which also have histories of attempted or successful ridge subduction.It is proposed that the bajaite series is produced during the unusual physico-chemical conditions operating during the subduction of young oceanic lithosphere, or subduction of a spreading centre. During normal subduction, the oceanic crust dehydrates, releasing volatiles (water, Rb and other large-ion lithophile elements) into the overlying wedge. Subduction of younger crust will result in a progressive decrease, and eventual cessation of the transfer of volatiles when subduction stops. Thermal rebound of the mantle may cause the slab to melt, perhaps under eclogitestable conditions. The resulting melt will be heavy-REE-depleted, perhaps dacitic, but will otherwise inherit MORB-like Rb/Sr and K/Rb ratios. The ascending melt will react with the mantle to form the source of the bajaitic rocks. Furthermore, any amphibole in the mantle, stabilised during the higher PH₂O conditions of earlier subduction, will break down and contribute a high-K/Rb ratio component.The implications of this study are that firstly, the subducted slab does not contribute a highly fractionated REE component in most modern arcs (i.e. the slab does not melt); secondly, Rb has a very short residence time in the mantle, and its abundance in arc rocks is a direct reflection of the input from the dehydrating slab; and thirdly, bajaitelike rocks may provide recognition of attempted or successful ridge subduction in the geologic past.     

A seismic hazard map of India and adjacent areas

We have produced a probabilistic seismic hazard map showing peak ground accelerations in rock for India and neighboring areas having a 10% probability of being exceeded in 50 years. Seismogenic zones were identified on the basis of historical seismicity, seismotectonics and geology of the region. Procedures for reducing the incompleteness of earthquake catalogs were followed before estimating recurrence parameters. An eastern United States acceleration attenuation relationship was employed after it was found that intensity attenuation for the Indian region and the eastern United States was similar. The largest probabilistic accelerations are obtained in the seismotectonic belts of Kirthar, Hindukush, Himalaya, Arakan-Yoma, and the Shillong massif where values of over 70% g have been calculated.