Site-Specific Modeling of SH and P-SV Waves for Microzonation Study of Kolkata Metropolitan City, India

Kolkata, one of the oldest cities of India, is situated over the thick alluvium of the Bengal Basin, where it lies at the boundary of the zone III and zone IV of the seismic zonation map of India. An example of the study of site effects of the metropolitan Kolkata is presented based on theoretical modeling. Full synthetic strong motion waveforms have been computed using a hybrid method that combines the modal summation and finite difference techniques. The 1964 Calcutta earthquake, which was located at the southern part of Kolkata, is taken as the source region, with the focal mechanism parameters of dip?=?32°, strike?=?232° and rake?=?56°. Four profiles are considered for the computation of the synthetic seismograms from which the maximum ground acceleration (A _MAX) is obtained. Response spectra ratios (RSR) are then computed using a bedrock reference model to estimate local amplifications effects. The A _MAX varies from 0.05 to 0.17?g and the comparison of the A _MAX with the different intensity scales (MM, MSK, RF and MCS) shows that the expected intensity is in the range from VII to X (MCS) for an earthquake of magnitude 6.5 at an epicentral distance of about 100?km. This theoretical result matches with the empirical (historical and recent) intensity observations in Kolkata. The RSR, as a function of frequency, reaches the largest values (largest amplification) in the frequency range from 1.0 to 2.0?Hz. The largest site amplification is observed at the top of loose soil.     

Dendrogeomorphic dating of rockfalls on low‐latitude,high‐elevation slopes: Rodadero,Iztaccíhuatl volcano,Mexico

Dynamics and rates of rockfalls have been repeatedly studied in mountain environments with archival records as well as lichenometric, radiocarbon or dendrogeomorphic approaches. In this study, we test the potential of conifers growing at a low‐latitude, high‐elevation site as a dendrogeomorphic tool to reconstruct to calendar dates associated rockfall activity. Analysis is based on tree‐ring records of Mexican mountain pine (Pinus hartwegii Lindl.) growing at timberline [~4000 m above sea level (a.s.l.)] and at the runout fringe of a north–northeast (NNE)‐facing slope of the dormant Iztaccíhuatl volcano (Mexico), which is subject to frequent rockfalls. The potential and limitations of tree‐ring data are demonstrated based on 67 rockfall impacts dated in the increment‐ring series of 24 trees since ad 1836. While findings of this paper are site‐specific, the study clearly shows the potential of dendrogeomorphic approaches in extra‐Alpine, low‐latitude environments and for the understanding of rockfall processes in space and time. Copyright © 2011 John Wiley & Sons, Ltd.     

Review of geology, alteration and origin of iron oxide–apatite deposits in the Cretaceous Ningwu basin, Lower Yangtze River Valley, eastern China: Implications for ore genesis and geodynamic setting

In the Cretaceous Ningwu volcano-sedimentary basin in the Yangtze River Valley metallogenic belt, eastern China, there are three areas with a dense distribution of magnetite or hematite deposits: the Meishan deposit in the north; Washan, Nanshan and Taocun deposits in the center; and the Zhongjiu and Gushan deposits in the south. The mineralization in the Ningwu basin is associated mainly with subvolcanic intrusions, consisting of gabbro–diorite porphyry and/or gabbro–diorite. Alteration zoning of these deposits is pronounced, and includes: (1) an upper light colored zone of argillic, kaolinite, silica, carbonate and pyritic alteration (2) a middle dark colored zone of diopside, fluorapatite–magnetite, phlogopite, and garnet with fluorapatite–magnetite; (3) a lower light colored zone of extensive albitic alteration. However, at the Gushan iron deposit, the lower light colored zone and the middle dark colored zone are absent, whereas the principal alteration is represented by silicification, kaolinization, and carbonatization.The iron oxide–apatite deposits in the Ningwu basin are typically magmatic–metasomatic origin and are similar to the Kiruna-type deposits in Scandinavia, particularly with respect to mineral assemblages, fabric and structure of the iron ores, occurrence of the orebodies and wall rock alteration. The iron oxide–apatite deposits of the Ningwu basin contain magnetite and/or hematite, with diopside or actinolite and apatite gangue. They were formed in a rift or extensional environment and the mineralization is associated with alkaline magmatism. The time interval between magmatism and related mineralization is very short.     

Porphyry Cu–Au–Mo–epithermal Ag–Pb–Zn–distal hydrothermal Au deposits in the Dexing area, Jiangxi province, East China—A linked ore system

Based on previous studies and detailed field investigations of the Dexing porphyry copper deposit, the Yinshan Ag-Pb-Zn deposit and the Jinshan shear zone – hosted gold deposit in the Dele Jurassic volcanic basin, in the northeastern Jiangxi province, East China, we propose that the three deposits share spatial, temporal and genetic relationships and belong to the same metallogenic system. Dexing is a typical porphyry Cu–Au–Mo deposit in which both ore-forming fluid and metals are derived from the granite porphyry. The Yinshan deposit consists of a porphyry copper ore located in the cupola of a quartz porphyry stock, in the lower part, and Ag–Pb–Zn ore veins in the upper part. The hydrothermal fluids were mainly derived from the magma in the early stages of the mineralizing event and became mixed with meteoric waters in the late stages. Its ore metals are magma-derived. Both the Jinshan base metal veins and the Hamashi, Dongjie and Naikeng quartz vein-type gold deposit are hosted by brittle–ductile structures, which are distal in relation to the porphyry intrusions and were formed by mixed magmatic fluids and meteoric water, whereas the gold was mainly leached from the country rocks (Mesoproterozoic Shuangqiaoshan Group phyllite and schist). The deposits show a distinct spatial arrangement from porphyry Cu, to epithermal Ag–Pb–Zn and distal Au. We suggest a porphyry–epithermal–distal vein ore system model for this group of genetically related mineral deposits. They were formed in a back-arc setting in a Middle Jurassic active continental margin, with magmas derived from the subducted slab.     

Mesozoic metallogeny in East China and corresponding geodynamic settings — An introduction to the special issue

The giant East China Mesozoic metallogenic province hosts some of the World’s largest resources of tungsten, tin, molybdenum, antimony and bismuth. Ores of gold, silver, mercury, lead, zinc, copper, uranium and iron are also of major importance. The province and its constituent metallogenic belts or regions (South China; Middle–Lower Yangtze River Valley; East Qinling–Dabie; Interior of North China Craton; Yan-Liao and North-east China) are the products of several pulses of igneous activity and mineralisation between ~240 and ~80 Ma. Each successive stage has produced a distinctive suite of deposits that can be readily related to the geodynamic evolution of the region during the Mesozoic. This geodynamic evolution is linked to a complex series of tectonic events, involving far-field-subduction, plate collisions, crustal thickening, post-collision collapse and rifting.     

A model of boiling for fluid inclusion studies: Application to the Bolaños Ag–Au–Pb–Zn epithermal deposit,Western Mexico

Boiling can be inferred from fluid inclusion microthermometry studies when a progressive increase in apparent salinity is observed along with a decrease of homogenization temperature (T_H) and depth, thus reflecting the partitioning of non-volatile solutes into the liquid phase during steam loss. We propose a model for fluid evolution during boiling based on mass and heat balance equations, which establishes paths in the T_H-salinity space that can be compared with fluid inclusion data to confirm or discard boiling. Additionally, the model allows calculating paleo-depths, for which the effect of steam bubbles lowering the hydrostatic pressure is taken into account.     

Tectonics of the Lepontine Alps: ductile thrusting and folding in the deepest tectonic levels of the Central Alps

The Lepontine dome represents a unique region in the arc of the Central and Western Alps, where complex fold structures of upper amphibolite facies grade of the deepest stage of the orogenic belt are exposed in a tectonic half-window. The NW-verging Mont Blanc, Aar und Gotthard basement folds and the Lower Penninic gneiss nappes of the Central Alps were formed by ductile detachment of the upper European crust during its Late Eocene–Early Oligocene SE-directed underthrust below the upper Penninic and Austroalpine thrusts and the Adriatic plate. Four underthrust zones are distinguished in the NW-verging stack of Alpine fold nappes and thrusts: the Canavese, Piemont, Valais and Adula zones. Up to three schistosities S1–S3, folds F1–F3 and a stretching lineation XI with top-to-NW shear indicators were developed in the F1–F3 fold nappes. Spectacular F4 transverse folds, the SW-verging Verzasca, Maggia, Ziccher, Alpe Bosa and Wandfluhhorn anticlines and synclines overprint the Alpine nappe stack. Their formation under amphibolite facies grade was related to late ductile folding of the southern nappe roots during dextral displacement of the Adriatic indenter. The transverse folding F4 was followed since 30 Ma by the pull-apart exhumation and erosion of the Lepontine dome. This occurred coevally with the formation of the dextral ductile Simplon shear zone, the S-verging backfolding F5 and the formation of the southern steep belt. Exhumation continued after 18 Ma with movement on the brittle Rhone-Simplon detachment, accompanied by the N-, NW- and W-directed Helvetic and Dauphiné thrusts. The dextral shear is dated by the 29–25 Ma crustal-derived aplite and pegmatite intrusions in the southern steep belt. The cooling by uplift and erosion of the Tertiary migmatites of the Bellinzona region occurred between 22 and 18 Ma followed by the exhumation of the Toce dome on the brittle Rhone–Simplon fault since 18 Ma.     

The Italian strong motion network

The Italian Strong Motion Network is a permanent monitoring system run by the Italian national emergency management department (Dipartimento della Protezione Civile, DPC). The network is known as RAN (Rete Accelerometrica Nazionale). An extensive project for updating and improving the technology of RAN instruments as well as the number of recording points was performed in the last 10 years. A wide site selection survey was carried out from eastern Sicily along the Italian peninsula, covering high seismic risk areas. The recording station density and the choice of high-quality digital strong motion instruments ensure reliability of the RAN network in the long-term. At the end of 2008, the free field sites selection and instruments installation, planned in the project, were quite completed. In planning and drawing the new RAN, special attention has been devoted to the robustness of the transmission systems, and to the distribution of new stations in order to ensure plenty of data during a seismic emergency. We spent special care both in the estimation of the RAN site responses and in the diffusion of the strong motion data. In order to better identify damaged earthquake areas, improved ground motion parameters need to be set. Such parameters will also assist future progress for engineering seismic design techniques as well as disaster mitigation.     

西秦岭地区卡林-类卡林型金矿床及其成矿时间、构造背景和模式

秦岭造山带是世界第二大卡林-类卡林型金矿省,其地质背景与美国西部卡林-类卡林型金矿省明显不同,位于大陆内部的碰撞造山带,而非活动大陆边缘的盆岭省。秦岭卡林-类卡林型金矿主要赋存于板块缝合带及其前陆冲断带的海西-印支构造层中,以泥盆系和三叠系为主,主要岩性为瀉湖-浅海相的含碳细碎屑岩-碳酸盐岩建造,即碳硅泥岩系;成矿元素为Au-As-Sb-Hg-Ag,缺乏T1,共生矿种为Sb,Hg,Ag,U和重晶石等;成矿温度为160～300℃,流体盐度为4％～10％(NaClcq),压力为5～50MPa,深度约0．5～5km,属于中低温中浅成热液矿床,与内华达卡林-类卡林型金矿一致。成矿流体具有建造水特点,C2H6含量较高,显示了有机流体参与成矿。成矿同位素年龄介于220～100Ma之间,以170Ma为高峰,地球动力学背景属碰撞造山过程挤压-伸展转变期的减压增温体制,成矿构造模式为碰撞造山成岩成矿和流体作用模式。陆陆碰撞过程中,沿龙门山、阿坝-黑水-平青、若尔盖-文县、玛曲-勉县-略阳、白龙江、双河-公馆、凤县-镇安、安康等大型断裂带的陆内俯冲作用不仅导致了西秦岭地壳缩短增厚隆升,而且使俯冲板片变质脱水、熔融,派生成矿流体和相关熔体,流体和熔体的上升到仰冲板片次级构造带,形成了卡林-类卡林型和造山型金矿成矿系统。据此提出,西秦岭深部可能存在潜力巨大的造山型金矿省。     

Velocity gradients perpendicular to the galactic plane in the solar vicinity

One of the most important topics in astrophysics concern to the study of how the stars are born. Observational facts show that nearly all stars were born in groups within large massive complexes of gas and dust. An idea almost universally accepted, comes from the fact that gas associated with star formation is very inhomogeneous over a large range of size scales. The stars need to be formed by the fragmentation of large gas complexes. Studies of the interstellar medium show the existence of a hierarchical structure covering at least four orders of magnitude in density and length scale.Recently various studies have been made in an attempt to understand the process, or processes, responsible for the decay of large scales to small ones. Several processes have been suggested, however, some of them seem to be responsible only for generating the hierarchical structure on small scales.A study of the velocity distribution of molecular clouds in the solar vicinity, shows the existence of large velocity gradients perpendicular to the galactic plane. As can be shown, these gradients may be one of the most important sources of the energy responsible for the production of the interstellar turbulence on the largest size scales.On leave from Observatorio Astronomico da Serra da Piedade, Depto. de Fisica, ICEx, UFMG, C.P. 702, 30.161, Belo Horizonte-MG, Brasil.