Re-Os同位素在沉积地层精确定年及古环境反演中的应用进展

Re-Os同位素亲有机质的性质,使得富有机质沉积岩在沉积的过程中能够吸附富集海水中的Re、Os,沉积岩的沉积压实过程也是其中Re-Os同位素体系封闭计时的过程,沉积岩Re-Os同位素等时线年龄代表地层沉积时代,Os同位素初始比值187Os/188Os反映沉积时海水的Os同位素比值。Re-Os同位素体系在富有机质沉积岩中的成功应用,能够直接确定地层沉积时代,从而对地层界线进行直接厘定,并且能够对一些沉积矿床形成时代、冰川事件发生时代进行厘定和限制。通过沉积岩Re-Os同位素特征,可以对古环境进行反演,有助于了解全球大气海洋的演化,气候的变化,对研究生物灭绝等重大地质事件发生的时限和机制以及金属矿床的成矿物质来源具有重要意义。文章阐述了Re-Os同位素在富有机质沉积岩中应用的原理,并列举了Re-Os同位素在沉积地层精确定年及古环境反演中应用实例,说明了富有机质沉积岩Re-Os同位素分析精度及其影响因素,以使Re-Os同位素体系作为一种强有力的工具,解决更多的沉积地层时代问题以及更好地对古环境进行反演。     

基于解析信号振幅识别磁源体边界的分析解释方法

解析信号本质上是一种全通滤波器,由于其不受人为因素干扰,能够较好地反映原始信号包含的信息特征。该方法不仅是对非平稳信号进行分析的有效方法,也是检测信号局部特征的有力工具。本文通过对实信号的解析信号处理得到复信号,由该复信号的振幅,能清晰地刻画信号能量空间分布特征。模型试验表明,解析信号振幅方法减小了常规化极方法在勘探程度较低的地区引起假异常的问题,使地下磁性体与振幅中心位置对应。尤其在低纬度或地形切割较大的地区,该方法更具优势。将该方法应用于老挝爬立山铁矿区实测磁异常的分析,由此圈定了磁铁矿分布位置,并由区内56个钻孔证实该推断的可靠性。     

Seismic structure of the Helan–Liupan–Ordos western margin tectonic belt in North-Central China and its geodynamic implications

We study high-resolution three-dimensional P-wave velocity (Vp) tomography and anisotropic structure of the crust and uppermost mantle under the Helan–Liupan–Ordos western margin tectonic belt in North-Central China using 13,506 high-quality P-wave arrival times from 2666 local earthquakes recorded by 87 seismic stations during 1980–2008. Our results show that prominent low-velocity (low-V) anomalies exist widely in the lower crust beneath the study region and the low-V zones extend to the uppermost mantle in some local areas, suggesting that the lower crust contains higher-temperature materials and fluids. The major fault zones, especially the large boundary faults of major tectonic units, are located at the edge portion of the low-V anomalies or transition zones between the low-V and high-V anomalies in the upper crust, whereas low-V anomalies are revealed in the lower crust under most of the faults. Most of large historical earthquakes are located in the boundary zones where P-wave velocity changes drastically in a short distance. Beneath the source zones of most of the large historical earthquakes, prominent low-V anomalies are visible in the lower crust. Significant P-wave azimuthal anisotropy is revealed in the study region, and the pattern of anisotropy in the upper crust is consistent with the surface geologic features. In the lower crust and uppermost mantle, the predominant fast velocity direction (FVD) is NNE–SSW under the Yinchuan Graben and NWW–SEE or NW–SE beneath the Corridor transitional zone, Qilian Orogenic Belt and Western Qinling Orogenic Belt, and the FVD is NE–SW under the eastern Qilian Orogenic Belt. The anisotropy in the lower crust may be caused by the lattice-preferred orientation of minerals, which may reflect the lower-crustal ductile flow with varied directions. The present results shed new light on the seismotectonics and geodynamic processes of the Qinghai–Tibetan Plateau and its northeastern margin.     

Lithospheric structure beneath the East China Sea revealed by Rayleigh-wave phase velocities

We explore the variations of Rayleigh-wave phase-velocity beneath the East China Sea in a broad period range (5–200 s). Rayleigh-wave dispersion curves are measured by the two-station technique for a total of 373 interstation paths using vertical-component broad-band waveforms at 32 seismic stations around the East China Sea from 6891 global earthquakes.The resulting maps of Rayleigh-wave phase velocity and azimuthal anisotropy provide a high resolution model of the lithospheric mantle beneath the East China Sea. The model exhibits four regions with different isotropic and anisotropic patterns: the Bohai Sea, belonging to the North China Craton, displays a continental signature with fast velocities at short periods; the Yellow Sea, very stable unit associated with low deformation, exhibits fast velocities and limited anisotropy; the southern part of the East China Sea, with high deformation and many fractures and faults, is related to slow velocities and high anisotropic signature; and the Ryukyu Trench shows high-velocity perturbations and slab parallel anisotropy.     

四川自然重砂异常特征及其成矿潜力响应

在自然重砂数据库系统(ZSAPS2.0)平台下,通过对四川省1∶20万自然重砂测量数据中的全省自然重砂矿物进行整理、统计和分析,开展了四川省自然重砂异常圈定及异常区带划分。结合四川省矿产资源潜力评价工作相关信息认为,自然重砂异常在矿产预测中具有一定的指示作用,部分具有较好的成矿潜力,所划分的自然重砂异常可作为全省开展相应矿种找矿工作部署的综合信息之一,可为省级找矿预测提供指导性信息。     

利用地震模拟技术研究逆断层的断点位置

在地震资料解释阶段,可根据解释结果进行正演模拟,以验证解释结果的可靠性,并通过模型正演方式了解地下波场的细微变化,提高解释精度。依据内蒙古鄂托克前旗沙章图矿区DF2逆断层的赋存形态,建立的地质模型,并围绕其逆断层的断点位置、埋深、地层速度等,对各种接收排列进行射线追踪实验。通过改变开动的道数、道距及偏移距,观察各种参数下射线追踪的分布情况及模拟时间剖面的面貌。正演结果表明利用射线模型追踪的模拟地震反射剖面与实际时间剖面完全吻合,其确定的逆断层断点位置及断面倾角合理。     

Assessment of soil–pile–structure interaction influencing seismic response of mid-rise buildings sitting on floating pile foundations

The role of the seismic soil–pile–structure interaction (SSPSI) is usually considered beneficial to the structural system under seismic loading since it lengthens the lateral fundamental period and leads to higher damping of the system in comparison with the fixed-base assumption. Lessons learned from recent earthquakes show that fixed-base assumption could be misleading, and neglecting the influence of SSPSI could lead to unsafe design particularly for structures founded on soft soils. In this study, in order to better understand the SSPSI phenomena, a series of shaking table tests have been conducted for three different cases, namely: (i) fixed-base structure representing the situation excluding the soil–structure interaction; (ii) structure supported by shallow foundation on soft soil; and (iii) structure supported by floating (frictional) pile foundation in soft soil. A laminar soil container has been designed and constructed to simulate the free field soil response by minimising boundary effects during shaking table tests. In addition, a fully nonlinear three dimensional numerical model employing FLAC3D has been adopted to perform time-history analysis on the mentioned three cases. The numerical model adopts hysteretic damping algorithm representing the variation of the shear modulus and damping ratio of the soil with the cyclic shear strain capturing the energy absorbing characteristics of the soil. Results are presented in terms of the structural response parameters most significant for the damage such as foundation rocking, base shear, floor deformation, and inter-storey drifts. Comparison of the numerical predictions and the experimental data shows a good agreement confirming the reliability of the numerical model. Both experimental and numerical results indicate that soil–structure interaction amplifies the lateral deflections and inter-storey drifts of the structures supported by floating pile foundations in comparison to the fixed base structures. However, the floating pile foundations contribute to the reduction in the lateral displacements in comparison to the shallow foundation case, due to the reduced rocking components.     

Andean earthquakes triggered by the 2010 Maule,Chile (Mw 8.8) earthquake: Comparisons of geodetic,seismic and geologic constraints

The Maule, Chile, (M_w 8.8) earthquake on 27 February 2010 triggered deformation events over a broad area, allowing investigation of stress redistribution within the upper crust following a mega-thrust subduction event. We explore the role that the Maule earthquake may have played in triggering shallow earthquakes in northwestern Argentina and Chile. We investigate observed ground deformation associated with the M_w 6.2 (GCMT) Salta (1450 km from the Maule hypocenter, 9 h after the Maule earthquake), M_w 5.8 Catamarca (1400 km; nine days), M_w 5.1 Mendoza (350 km; between one to five days) earthquakes, as well as eight additional earthquakes without an observed geodetic signal. We use seismic and Interferometric Synthetic Aperture Radar (InSAR) observations to characterize earthquake location, magnitude and focal mechanism, and characterize how the non-stationary, spatially correlated noise present in the geodetic imagery affects the accuracy of our parameter estimates. The focal mechanisms for the far-field Salta and Catamarca earthquakes are broadly consistent with regional late Cenozoic fault kinematics. We infer that dynamic stresses due to the passage of seismic waves associated with the Maule earthquake likely brought the Salta and Catamarca regions closer to failure but that the involved faults may have already been at a relatively advanced stage of their seismic cycle. The near-field Mendoza earthquake geometry is consistent with triggering related to positive static Coulomb stress changes due to the Maule earthquake but is also aligned with the South America-Nazca shortening direction. None of the earthquakes considered in this study require that the Maule earthquake reactivated faults in a sense that is inconsistent with their long-term behavior.     

Review of the chronostratigraphic charts in the Sinú-San Jacinto basin based on new seismic stratigraphic interpretations

Disperse and punctual studies; absence of integration of data ranging from local to regional focus; interpretations based only on lithostratigraphic features; and interpretation of data premised on an allochthonous origin of the Caribbean plate, are some of factors that increase the confusion and uncertainty in understanding the Sinú-San Jacinto Basin. The sedimentary record of Upper Cretaceous to Eocene has been traditionally interpreted as the record of deep-water settings. However, recently these sediments have been related to shallow marine and deltaic settings. Second problematic point is about the deposition environment of the Oligocene to Late Miocene succession. Some studies suggest canyons, turbidites and sediments deposited in deep-water settings. However, recent studies propose deltaic and shallow marine settings. The last stratigraphic problem is related to the controversial fluvial vs. shallow marine interpretations of the Pliocene sediments. Based upon seismic stratigraphic analysis in recent and reprocessed 2D seismic data, integrated with well data, we propose chronostratigraphic charts for the northern, central and southern zones of the Sinú-San Jacinto Basin. Twenty seismic facies based on amplitude, continuity, frequency and geometry of seismic reflectors and twelve seismic sequences were recognized. The seismic stratigraphic analysis in this study suggests that the sediments of Upper Cretaceous to Paleocene/Eocene were associated to continental to shallow marine settings. Lagoons, coastal plain and carbonate platform dominated during this period. The Oligocene to Middle Miocene record was characterized by deep-water deposition, whereas the Late Miocene to recent sedimentation was characterized by falling base level, characterized by deltaic and fluvial deposits. Five syn-rift sequences with wedge-shaped geometry were identified in this study. Three Triassic to Jurassic syn-rift sequences were characterized by seismic facies typical of fluvial to lacustrine and flood plain sedimentation. Two Cretaceous to Paleocene syn-rift sequences were characterized by seismic facies related to lagoons to coastal plain settings. Normal high-angle faults with a northeast-southwest direction related to rifting processes controlled the development of these sequences. The sheet-drape post-rift section was characterized by passive margin settings in the northern part of the Sinú-San Jacinto Basin and by diachronic tectonic inversion of older normal faults during Cenozoic, predominantly in the central and southern zones. The stratigraphic record related to the Mesozoic to Early Cenozoic rifting; the shallow marine sedimentation during Eocene and the tectono-stratigraphic continuity across the northern Colombia and northwestern Venezuela is coherent and well explained by the in situ origin of the Caribbean plate and is not explained by the “allochthonous” model.     

祁连东部西格拉高Sr/Y型花岗岩LA-ICP-MS锆石U-Pb定年及其地球化学特征

西格拉花岗岩体位于祁连东部的米屈花岗岩带上,空间上呈NW-NWW向展布,岩体由花岗闪长岩-花岗岩-正长岩组成,主要岩性为花岗闪长岩。LA-ICP-MS锆石U-Pb定年结果表明,西格拉花岗闪长岩存在两期岩浆作用,年龄分别为465.6±6.5Ma、443.2±4.8Ma,其中,前者与祁连洋俯冲极性发生转变的时限(~463Ma)相近,而后者与祁连洋闭合的时间(~445Ma)相似。早期淡色花岗闪长岩具正铕异常(δEu=2.44),晚期暗色花岗闪长岩则为弱负/正铕异常(δEu=0.94~1.32),两者均显示出高Si、Al,富Na贫K,富集Ba、Sr、LREE,高Sr/Y值,低Y、HREE含量,亏损HFSE(Nb、Ta、Ti、P)的特征,与Martin et al.(2005)划分的高硅埃达克岩(HSA)相似。其源区可能均残留石榴子石、普通角闪石及少量金红石,而斜长石则大量参与部分熔融过程,推测早期花岗闪长岩的源区深度可能超过50km;整体上,由早期→晚期反映出加厚地壳减薄、地幔组分贡献增强的趋势;两者可能均为加厚下地壳物质部分熔融的产物。