黄海及其沿岸历史地震初步研究

黄海及其边缘，经历史地震资料整理，地震参数估定与编目，截至1949年8月，共有M≥3级地震2187次，其中M≥43/4级地震259次，资料显示大致有三个相对集中活跃区，16世纪曾有过历史地震活动高潮。     

南海西部围区中特提斯东延通道问题

通过对南海西部围区中生代岩相古地理资料的收集整理和分析,编制了该区T_2,T_3~1,T_3~2,T_3~3,J_1-J_2,J_3-K_1共6个时段的岩相古地理简图,根据其中生代海相地层的时空分布和岩相特征,讨论了尚存争议的中特提斯进入南海的通道问题。实际资料表明,黑水河盆地在三叠纪受印支运动影响完成了从海到陆的过程,之后不再出现海相沉积,中特提斯不可能从红河裂谷带进入南海。在新加坡所见的晚三叠世至早侏罗世浅海至陆相沉积代表古特提斯的残余海,到中侏罗世完全消失。早侏罗世时期在印支半岛南部出现的近南北走向的海湾可能经过泰国湾与当时的滇缅海相通;但是这个海湾浅而短暂,滇缅海能否从这里进入南海值得怀疑,更不可能是中特提斯的通道。在南海西部围区,迄今已证实的中生代洋壳碎片(蛇绿岩套)和深海沉积仅见于南部Woyla—Maratus—Lupar一线及其附近。这套延伸2000余km,从洋壳、深海到浅海岩相齐全的岩石所代表的晚侏罗世至早白垩纪世大洋应是中特提斯洋的一部分。中特提斯东延而最可能是走南路,即从班公一怒江带南下之后,经Woyla线穿过苏门答腊岛,绕加里曼丹岛南缘到Maratus线,向北再经沙巴到Lupar线,在沙捞越北部或纳土纳岛附近进入南海。     

Smoothing and Change Point Detection for Gamma Ray Count Data

Gamma ray detectors are used to measure the natural radioactivity of rocks. For a number of boreholes drilled at a site the gamma ray detector is lowered into each borehole and counts of gamma ray emissions at different depths are recorded as the instrument is gradually raised to ground level. The profile of gamma counts can be informative about the geology at each location. The raw count data are highly variable, and in this paper we describe the use of adaptive smoothing techniques and change point models in order to identify changes in the geology based on the gamma logs. We formulate all our models for the data in the framework of the class of generalized linear models, and describe computational methods for Bayesian inference and model selection for generalized linear models that improve on existing techniques. Application is made to gamma ray data from the Castelreagh Waste Management Centre which served as a hazardous waste disposal facility for the Sydney region between March 1974 and August 1998. Understanding the geological structure of this site is important for further modelling the transport of pollutants beneath the waste disposal area.     

Zircon formation during fluid circulation in eclogites (Monviso, Western Alps): implications for Zr and Hf budget in subduction zones

The zircons from an eclogite and an enclosed eclogite-facies vein from the Monviso ophiolite (Western Alps) display contrasting chemical and morphologic features and document different stages of the evolution of the ophiolite. The zircons from the eclogite show a typical magmatic zoning and are enriched in heavy rare earth elements (HREEs) over middle rare earth elements (MREEs) and have an accentuated negative Eu anomaly, which indicates that the grains co-crystallised with plagioclase. These magmatic zircons document the formation of oceanic crust at 163 ± 2 Ma. In contrast, zircons from the vein contain inclusions of garnet, omphacite, and rutile, which indicate that they crystallised under eclogite-facies conditions. The vein zircons have Th/U ratios < 0.09, lack Eu anomalies, and are only weakly enriched in HREE with respect to MREE. These features are consistent with a garnet-bearing, plagioclase-free, i.e., eclogite-facies paragenesis. Vein zircons yield an age of 45 ± 1 Ma, which is evidence for Eocene subduction-zone metamorphism of the Monviso ophiolite.In the vein, the apparent coexistence of zircon, omphacite, and garnet permits the determination of a set of trace element distribution coefficients among these minerals at high pressure. This set of partitioning can demonstrate chemical equilibrium among these phases in rocks that show less clear evidence of textural equilibrium. In addition, zircon age can now be linked to sensors of metamorphic pressure-temperature conditions. The presence of zircon and rutile in the vein is another example of high field strength element (HFSE) mobility over short distances in aqueous fluids at eclogite-facies conditions. However, the concentrations of Zr and Hf in the aqueous fluid are estimated to be at least a factor of 10 less than primitive mantle values.Mass balance calculations demonstrate that zircon hosts > 95% of the bulk Zr, 90% of Hf, and ∼25% of U in the vein. Zircon is a residual phase in subducted basalts and sediments up to temperatures of at least 800 to 900 °C. Therefore, residual zircon in subducted crust, together with rutile, control the HFSE in liberated subduction zone fluids/melts and might be partly responsible for negative Zr and Hf anomalies in subduction zone magmas.     

An estimate of hypocentre location accuracy in a large network: possible implications for tectonic studies in Italy

Data recorded by the Italian Telemetered Seismic Network (ITSN) of the Istituto Nazionale di Geofisica (ING) have been widely used in recent years to image slab structures and to find evidence for active processes along the Italian Peninsula. However, the use of seismic data for geostructural purposes may be affected by the well-known trade-off between earthquake location and seismic-velocity parameters. Furthermore, the confidence ellipse predicted by standard procedures may be inadequate for the representation of the probable error of a computed localization. This paper evaluates the probable errors on the hypocentre determinations of the seismic events recorded by the ITSN, using a Monte Carlo method.
We compute synthetic arrival times using a 1-D velocity model appropriate as an average for the Italian area. The hypocentres used are all those recorded by the ITSN during the period January 1992 to March 1994 (1972 events). Station locations are those of the current ITSN configuration. The synthetic arrival times are perturbed with a Gaussian distribution of errors and input to ING's standard hypocentral location procedure, but using crustal velocities differing by 10 per cent from those used to generate them. Each simulation is repeated at least 30 times. Average absolute shifts of hypocentres are assessed in grid cells of linear dimension 33 km covering the whole Italian region.
For regions within the ITSN, shifts are typically 5–10 km in location and up to 20 km in depth. However, for offshore and coastal regions, they are much greater: 50 km or more in both location and depth (far exceeding the equivalent uncertainties quoted by ING bulletins). Possible consequences of this are highlighted by producing a cross-section of subcrustal hypocentres from the Adriatic to the Tyrrhenian Sea, where the large uncertainty in depth precludes any confident interpretation of dipping tectonic features.     

Periodicity of magnetic intensities in magnetic anomaly profiles: the Cenozoic of the South Atlantic

Spectral analyses of several published magnetic anomaly profiles from Candé & Kent (1992a) were undertaken prior to analysing, in the same way, raw magnetic anomaly data from similar parts of the South Atlantic. It was found that similar and distinct medium and short wavelengths were present in both the published and raw data. When these are converted into the time domain using the average rate of spreading for each profile, these periodicities appear similar, possibly identical, to those expected from the long-term eccentricity orbital parameters (Fischer, DeBoer & Premoli Silva 1990). While such correlations are not necessarily causative, they suggest that magnetohydro-dynamical processes near the core-mantle boundary may be affected by gravitational changes due to planetary orbital perturbations.     

Paleogeography and structure of the central Mediterranean: Sicily and its offshore area

The geology of the mainland and offshore of Sicily is illustrated by a few geologic sections and seismic profiles across the late Cenozoic orogenic belt of central and western Sicily and across the Sardinia Channel and Sicily Straits. This belt is the result of several tectonic events. Deformation involved mainly the sedimentary cover of the old African continental margin characterized by a broad basinal domain, flanked along its external (southern) margin by a shallow-water carbonate platform attached to Africa in the Triassic. Compressional deformation started in the more internal basinal rock assemblages overlying a thinned crust. The most important structural characteristic of the early phase of thrusting is the duplex pile forming the bulk of the chain in Central Western Sicily. The structure consists of a basal allochthon, made up of Permian to Middle Triassic layers, an intermediate duplex wedge, composed of competent Mesozoic carbonates, and a roof complex, including Upper Mesozoic-Lower Tertiary less competent rocks. Large-scale clockwise rotation of the thrusts accompanied transpressional movements in the hinterland during the Pliocene. Right oblique reverse faults modified the previous tectonic contacts between the allochthons in the hinterland zones. Contemporaneous south-directed imbrications affected the southern external areas, progressively incorporating foreland and piggyback basins. Development of the Gela Thrust System appears to be linked to the transpressional event; its accretion is also related to contemporaneous underthrusting at deeper levels of Mesozoic carbonate substratum. The older buried thrust sheets were pushed up to the surface breaching the deformed Tertiary cover of the Gela TS. Northwards in the belt post-Messinian normal growth faults opened half graben whose sedimentary fill underwent structural inversion. Alternation of extension and compression tectonics characterizes the Sicilian continental margin in the last million years.