华北强震断层面解和震源深度特征

本文利用远震P波及SH波和区域地震P_(nl)波波形反演方法,测定了华北9次强震的断层面解、震源深度及地震矩.结合其他作者测定的7次地震的结果,分析了华北16次强震断层面解及华北地震震源深度特征.结果表明,华北多数强震为走滑型地震,也有个别正断层型及逆断层型地震;16次地震多数发生在地壳中部10-25km深度范围内.     

A simulation model of alluvial stratigraphy

The quantitative model presented simulates the development of a two-dimensional alluvial sedimentary succession beneath a floodplain traversed by a single major river. Several inter-related effects which influence the distribution of channel-belt sand and gravel bodies within overbank fines are accounted for. These are (a) laterally variable aggradation, (b) compaction of fine sediment, (c) tectonic movement at floodplain margins, and (d) channel avulsion. Selected experiments with the model show how the interconnectedness and areal density of channel-belt deposits decrease with increasing floodplain width/channel-belt size, mean avulsion period, and channel-belt aggradation rate. Separation of stream patterns based on interconnectedness and channel deposit density is difficult. Tectonic movements do not have a significant influence upon the successions unless a preferred direction of tilting is maintained (half-graben). Then channel-belt deposits showing offlap tendencies tend to cluster adjacent to the active floodplain margin, leaving dominantly fine-grained alluvium to accumulate on the inactive side. Individual channel-belt deposits thicken during aggradation, although a self-regulating limit to such thickening is likely to operate. ‘Multistorey’features resulting from aggradation may be difficult to tell apart from those arising through superposition of distinct channel-belt deposits of avulsive origin.     

Mafic Pegmatites Intruding Oceanic Plateau Gabbros and Ultramafic Cumulates from Bolivar, Colombia: Evidence for a 'Wet' Mantle Plume?

The fault-bounded Bolívar Ultramafic Complex (BUC) onthe eastern fringes of the Western Cordillera of Colombia wastectonically accreted onto the western coast of South Americain the late Cretaceous–early Tertiary, along with pillowbasalts of the Caribbean–Colombian Oceanic Plateau (CCOP).The complex consists of a lower sequence of ultramafic cumulates,successively overlain by layered and isotropic gabbroic rocks.The gabbros grade into, and are intruded by, mafic pegmatitesthat consist of large magnesiohornblende and plagioclase crystals.These pegmatites yield a weighted mean ⁴⁰Ar–³⁹Ar step-heatingage of 90·5 ± 0·9 Ma and thus coincidewith the timing of peak CCOP volcanism. The chemistry of theBUC is not consistent with a subduction-related origin. However,the similarity in Sr–Nd–Pb–Hf isotopes betweenthe CCOP and the BUC, in conjunction with their indistinguishableages, suggests that the BUC is an integral part of the plume-derivedCCOP. The parental magmas of the Bolívar complex wereprobably hydrous picrites that underwent 20–30% crystallization.The residual magmas from this fractionation contained     

AFRICA IS COMING TO THE CAPE*

ABSTRACT. At the southern tip of Africa, culmination of one of the world'sgreat population shifts is there for the witnessing: a southward movement of Bantu‐speaking Black Africans, previously stalled in South Africa'sEastern Cape first by climatic factors and later by European‐imposed racial segregation. Cape Town, throughout its demographic history a “Coloured” and “White” city, is now Africanizing. Liberation from apartheid has brought rapid African influx, and the metropolitan population has more than doubled in less than twenty years. Sociocultural tensions and sociocultural possibilities ensue.     

Petrogenesis of the Kinsman Intrusive Suite: Peraluminous Granitoids of Western New Hampshire

The Kinsman Intrusive Suite occurs in six major plutons of westernNew Hampshire, covering a total area of 2240 km². It is an Acadian-agesyntectonic gneissic S-type peraluminous granitoid, rangingin composition from quartz diorite to granite. Much of the Kinsmanis characterized by very large (up to 120 mm in maximum dimension)megacrysts of alkali feldspar, but the bulk chemistry of therocks indicates that these cannot be phenocrysts crystallizedfrom initially homogeneous melts. Locally, there is abundant(20 per cent) almandine-rich garnet, and graphite is a commonaccessory.In contrast to the unannealed orthoclase in surroundingmetapelites, the alkali feldspar of the Kinsman has, for themost part, inverted to maximum microcline. The garnets havecore temperatures in the range of 800 to 900 ?C, and are pseudomorphedby, or show reaction rims to, biotite. Plagioclase commonlyshows zoning, some of it oscillatory. These features are magmaticin nature, and argue against the conclusions of previous investigatorsthat the mineralogy and textures of the rock are due to regionalmetamorphism of a previously-crystallized two-mica granitoidwhich has undergone prograde reactions such as:muse + bio +3 qtz 2 Kfs + gar + 2H₂O.The intrusives have also producedrecognizable contact-metamorphic features in the wallrocks andare probably coeval with the dominant M2 Acadian metamorphism.Majorelement analytical data for the Kinsman suite has been examinedby least squares mixing-model and extended Q-mode factor analysis.These calculations, supported by consideration of REE data,suggest that the most likely origin for the Kinsman magmas isby deep-crustal anatexis of slightly calcareous metapelites,and involves a reaction such as:bio + Al₂SiO₅ + qtz + feldspars gar + cord + Kfs + plag + melt.In non-calcareous pelites thisreaction produces a water-undersaturated peraluminous melt attemperatures above 700 ?C, and allows for the early crystallizationor recrystallization of K-feldspar, plagioclase, and garnetin a crystal-liquid mush or migma. Geochemically, garnet + plagioclaseare treated as restite, and a minimum-melt granite as the magmain the Q-mode and mixing-model calculations. The variabilityin chemistry of the Kinsman Intrusive Suite is best explainedon the basis of mixing of leucogranitic anatectic melts withgarnet-plagioclase restitic material and a quartz-feldspar-sillimanite-biotiterock, but only very slightly affected by crystal settling.     

Effect of Carbon Dioxide on Dehydration Melting Reactions and Melt Compositions in the Lower Crust and the Origin of Alkaline Rocks

Dehydration melting experiments of alkali basalt associatedwith the Kenya Rift were performed at 0·7 and 1·0GPa, 850–1100°C, 3–5 wt % H₂O, and f_O2 nearnickel–nickel oxide. Carbon dioxide [X_CO2 = molar CO₂/(H₂O+ CO₂) = 0·2–0·9] was added to experimentsat 1025 and 1050°C. Dehydration melting in the system alkalibasalt–H₂O produces quartz- and corundum-normative trachyandesite(6–7·5 wt % total alkalis) at 1000 and 1025°Cby the incongruent melting of amphibole (pargasite–magnesiohastingsite).Dehydration melting in the system alkali basalt–H₂O–CO₂produces nepheline-normative tephriphonolite, trachyandesite,and trachyte (10·5–12 wt % total alkalis). In thelatter case, the solidus is raised relative to the hydrous system,less melt is produced, and the incongruent melting reactioninvolves kaersutite. The role of carbon dioxide in alkalinemagma genesis is well documented for mantle systems. This studyshows that carbon dioxide is also important to the petrogenesisof alkaline magmas at the lower pressures of crustal systems.Select suites of continental alkaline rocks, including thosecontaining phonolite, may be derived by low-pressure dehydrationmelting of an alkali basalt–carbon dioxide crustal system. KEY WORDS: alkali basalt; alkaline rocks; carbon dioxide; dehydration melting; phonolite     

Thermoluminescence dating of Dimlington Stadial deposits in eastern England

The loess component of a solifluction deposit of the Dimlington Stadial exposed at the inland site of Eppleworth in eastern England gave a thermoluminescence date of 17.5 ± 1.6 × 10³ years. The solifluction deposit is overlain by a slightly weathered till correlated with the Skipsea Till of coastal exposures. which lies between organic horizons with radiocarbon dates of 18,500–18,240 B.P. and 13,045 B.P. Although the till must have been deposited during the Dimlington Stadial (after 18,240 B.P. at Dimlington and after 17,500 B.P. at Epplcworth), it gave apparent TL dates of 42.1 ± 3.6 × 10³ years at Eppleworth and 102 ± 9 × 10³ years at Dimlington, indicating that the components of the till were not exposed to light immediately before deposition.