2013 Vol. 24, No. 5
In a number of geophysical or planetological settings, including Earth’s inner core, a silicate mantle crystallizing from a magma ocean, or an ice shell surrounding a deep water ocean—a situation possibly encountered in a number of Jupiter and Saturn’s icy satellites—a convecting crystalline layer is in contact with a layer of its melt. Allowing for melting/freezing at one or both of the boundaries of the solid layer is likely to affect the pattern of convection in the layer. We study here the onset of thermal convection in a viscous spherical shell with dynamically induced melting/freezing at either or both of its boundaries. It is shown that the behavior of each interface—permeable or impermeable— depends on the value of a dimensionless number P (one for each boundary), which is the ratio of a melting/freezing timescale over a viscous relaxation timescale. A small value of P corresponds to permeable boundary conditions, while a large value of P corresponds to impermeable boundary conditions. Linear stability analysis predicts a significant effect of semipermeable boundaries when the number P characterizing either of the boundary is small enough: allowing for melting/freezing at either of the boundary allows the emergence of larger scale convective modes. The effect is particularly drastic when the outer boundary is permeable, since the degree 1 mode remains the most unstable even in the case of thin spherical shells. In the case of a spherical shell with permeable inner and outer boundaries, the most unstable mode consists in a global translation of the solid shell, with no deformation. In the limit of a full sphere with permeable outer boundary, this corresponds to the “convective translation” mode recently proposed for Earth’s inner core. As another example of possible application, we discuss the case of thermal convection in Enceladus’ ice shell assuming the presence of a global subsurface ocean, and found that melting/freezing could have an important effect on the pattern of convection in the ice shell.
Seismic P velocity structure is determined for the upper 500 km of the inner core and lowermost 200 km of the outer core from differential travel times and amplitude ratios. Results confirm the existence of a globally uniform F region of reduced P velocity gradient in the lowermost outer core, consistent with iron enrichment near the boundary of a solidifying inner core. P velocity of the inner core between the longitudes 45oE and 180oE (quasi-Eastern Hemisphere) is greater than or equal to that of an AK135-F reference model whereas that between 180oW and 45oE (quasi-Western Hemisphere) is less than that of the reference model. Observation of this heterogeneity to a depth of 550 km below the inner core and the existence of transitions rather than sharp boundaries between quasi-hemispheres favor either no or very slow inner core super rotation or oscillations with respect to the mantle. Degree- one seismic heterogeneity may be best explained by active inner core freezing beneath the equatorial Indian Ocean dominating structure in the quasi-Eastern Hemisphere and inner core melting beneath equatorial Pacific dominating structure in the quasi-Western Hemisphere. Variations in waveforms also suggest the existence of smaller-scale (1 to 100 km) heterogeneity.
We conduct an experiment to investigate whether linearity in the observed velocity gradient in the volume of the inner core sampled by the PKP ray paths beneath Central America is a robust approximation. Instead of solving an optimization problem, we approach it within the Bayesian inference. This is an ensemble approach, where model specification is relaxed so that instead of only one solution, groups of reasonable models are acceptable. Furthermore, in transdimensional Bayesian inference used here, the number of basis functions needed to model observations is by itself an unknown. Our modeling reveals that in the ensemble of models, the most likely are those containing only 2 nodes (linear trend). Thus our result justifies the assumption used for the determination of inner core rotation with respect to the rest of the mantle that the observed gradient is constant in its nature (linear). Recent observations in seismology suggest that it is likely that the spatial variability in elastic parameters is a widespread phenomenon in the inner core. Future array observations will further constrain spatial extent and magnitude of velocity changes and show whether there is a significant difference between these observations in the two quasihemispheres of the inner core.
Attenuation of PKP(DF) in the Eastern Hemisphere is examined in terms of multiple scattering to simultaneously explain a puzzling relationship, a relatively fast velocity anomaly corresponding to strong attenuation. Reflectivity synthetics with one-dimensional random velocity fluctuations are compared with observations of PKP(DF)/PKP(Cdiff) amplitude ratios and differential travel times of PKP(Cdiff)-PKP(DF) for the equatorial paths. A Gaussian distribution of P-wave velocity fluctuations with the standard deviations of 5%, 6%, and 7% in the uppermost 200 km of the inner core is superimposed on the velocity structure that is slightly faster than the typical structure in the Eastern Hemisphere, which is likely to explain both the travel time and amplitude data as far as only the one-dimensional structure is considered. Further examinations of the statistic characteristic of scatterer distribution in two and three-dimensions are required to obtain a realistic conclusion.
Shear velocity and density contrast across the inner core boundary are essential for studying deep earth dynamics, geodynamo and geomagnetic evolution. In previous studies, amplitude ratio of PKiKP/PcP at short distances and PKiKP/P at larger distances are used to constrain the shear velocity and density contrast, and shear velocity in the top inner core is found to be substantially smaller than the PREM prediction. Here we present a large dataset of PKiKP/P amplitude ratio measured on 420 seismic records at ILAR array in Alaska for the distance range of 80o–90o, where the amplitude ratio is sensitive to shear velocity and density contrast. At high frequency (up to 6 Hz), mantle attenuation is found to have substantial effects on PKiKP/P. After the attenuation effects are taken into account, we find that the density contrast is about 0.2–1.0 g/cm3, and shear velocity of inner core is 3.2–4.0 km/s, close to the PREM (Preliminary Reference Earth Model) prediction (0.6 g/cm3 and 3.5 km/s, respectively). The relatively high shear velocity in inner core does not require large quantities of defects or melts as proposed in previous studies.
Based upon seven superconducting gravimeter (SG) records of 20 000 h length after the 2004 Sumatra earthquake, four methods, namely the ensemble empirical mode decomposition (EEMD), the multistation experiment (MSE) technique, the autoregressive (AR) method and the product spectrum analysis (PSA) method, are chosen jointly together to detect the inner core translational modes (1S1). After the conventional pretreatment, each of the seven simultaneous residual gravity series is divided into five segments with an 80% overlap, and then EEMD is applied to all the 35 residual SG series as a dyadic filter bank to get 35 filtered series. After then, according to different stations and different time windows, five new simultaneous gravity datasets are obtained. After using MSE for each of the five new datasets, the AR method is used to demodulate some known harmonic signals from the new sequences that obtained by using MSE, and three demodulated product spectra are obtained. Then, according to two criterions, two clear spectral peaks at periods of 4.548 9±2.3×10-5 and 3.802 3±3.2×10-5 h corresponding respectively to the singlets m=?1 and m=+1 are identified from various spectral peaks, and they are close to the predictions of the 1066A model given by Rieutord (2002), but no spectral peak corresponding to the singlet m=0 is found. We conclude that the selected two peaks might be the observed singlets of the Slichter triplet.
Hemispherical asymmetry in core dynamics induces degree-1 pressure variations at the core mantle boundary (CMB), which in turn deforms the overlaying elastic mantle, at the same time keeps center of mass of the whole Earth stationary in space. We develop a systematic procedure to deal with the degree-1 CMB pressure loading. We find by direct calculation a surprisingly negative load Love number h1=-1.425 for vertical displacement. Further analysis indicates that the negative h1 corresponds to thickening above the positive load that defies intuition that pressure inflation pushes overlaying material up and thins the enveloping shell. We also redefine the pressure load Love numbers in general to enable comparison between the surface mass load and the CMB pressure load for the whole spectrum of harmonic degrees. We find that the gravitational perturbations from the two kinds of loads at degrees n>1 are very similar in amplitude but opposite in sign. In particular, if the CMB pressure variation at degree 2 is at the level of ~1 hpa/yr (1 cm water height per year), it would perturb the variation of Earth’s oblateness, known as the J2, at the observed level.
An attempt has been made to search for the translational oscillations of the Earth’s solid inner core in the gravity measurements recorded with the superconducting gravimeters (SG) from the worldwide network of the Global Geodynamics Project (GGP). All the SG data were prepared and analyzed by the same method to remove accurately the signatures related to gravity tides, local barometric pressure, the Earth’s rotation, the longterm trend and so on. We obtained the estimations of the power spectral densities of each residual series and the estimations of the product spectral densities in the subtidal band (0.162–0.285 cph) were obtained by using a multistation stacking technique after further eliminating atmospheric effects. The inner core translation triplet was detected in the subtidal band. We find 6 groups of signal with high signal-to-noise ratio that are consistent with the characteristics of the triplet, and 4 groups of the results that are close to the previous studies and the differences are less than 0.92%. It implies that the groups of signatures all have the possibility to be related to the inner core translational oscillations.
Located on the western Yangtze Block, the Sichuan (四川)-Yunnan (云南)-Guizhou (贵州) (SYG) Pb-Zn metallogenic province has been a major source of base metals for China. In the southeastern SYG province, structures are well developed and strictly control about 100 Pb-Zn deposits. Almost all the deposits are hosted in Devonian to Permian carbonate rocks. Leadzinc ores occur either as veinlets or disseminations in dolomitic rocks with massive and disseminated textures. Ore minerals are composed of pyrite, sphalerite and galena, and gangue minerals are calcite and dolomite. Sulfide minerals from four typical Pb-Zn deposits are analyzed for sulfur isotope compositions to trace the origin and evolution of hydrothermal fluids. The results show that δ34S values of sulfide minerals range from +3.50‰ to +20.26‰, with a broad peak in +10‰ to +16‰, unlike mantle-derived sulfur (0±3‰). However, the mean δ34Ssulfide and δ34S∑S-fluids values are similar to that of sulfatebearing evaporites in the host rocks (gypsum: ~+15‰ and barite: +22‰ to +28‰) and Cambrian to Permian seawater sulfate (+15‰ to +35‰). This suggests that reduced sulfur in hydrothermal fluids was likely derived from evaporates in the host rocks by thermochemical sulfate reduction (TSR). Calculated δ34S∑S-fluids values of the Shanshulin (杉树林), Qingshan (青山), Shaojiwan (筲箕湾) and Tianqiao (天桥) Pb-Zn deposits are +21.59‰, +18.33‰, +11.43‰ and +10.62‰, respectively, indicating sulfurbearing hydrothermal fluids may be evolved from the Shanshulin to Qingshan and then the Shaojiwan to Tianqiao deposition sites along the Yadu (垭都)-Ziyun (紫云) lithospheric fracture in the southeastern SYG province.
Daxing’anling (大兴安岭) area is one of the regions that Phanerozoic granites are extremely developed in NW China. At present, the Hercynian granitoid research lags behind the Mesozoic granitoid research. In this article, we have taken systematic petrochemistry and geochronology researches on the Hercynian granitoids in Daitongshan (代铜山) copper deposit and Lamahanshan (喇嘛罕山) silver polymetallic deposit, which were located at southern section of Daxing’anling metallogenic belt. Zircon SHRIMP U-Pb dating results show that, the granite aplites in Daitongshan and the gneissic granites in Lamahanshan were formed at (265±5)–(268±9) Ma and (252±2)–(252.6±3.4) Ma, respectively, which were both the products of late Herynian tectonic-magmatic events. Samples from Lama- hanshan are characterized by high SiO2 (69.72 wt.%–74.65 wt.%), high potassium (3.53 wt.%–4.55 wt.%) and low P2O5 (0.03 wt.%–0.12 wt.%), aluminum saturation index (A/CNK) range from 0.95 to 0.98, Rb, Nd and K are enriched, whereas the elements such as Nb, Ta, P and Ti are depleted, which belong to I-type granitoids. Characteristics of samples from Daitongshan are similar to H-type granitoids. The magma source may be mostly originated from the lithospheric mantle component which were transformated or affected by the subduction components, and its formation may be closely related with the subduction and orogenesis of the Paleo-Asian Ocean.
Petroleum exploration targets are extending gradually from the single conventional trap reservoirs to the large-scale unconventional continuous accumulations. Oil and gas reservoirs have been divided into two types based on the trapping mechanism and distribution of oil and gas: conventional single-trap reservoirs, such as the Daqing oil field in Songliao Basin and the Kela-2 gas field in Tarim Basin; and unconventional continuous petroleum accumulation, such as Upper Paleozoic tight gas and Mesozoic tight oil in Ordos Basin, and Upper Triassic tight gas in Sichuan Basin. Two typical geologic characteristics of continuous petroleum accumulation involve: (1) coexisting source and reservoir, petroleum pervasive throughout a large area tight reservoirs, and no obvious traps or well-defined water-oil and gas contracts; (2) nonbuoyancy accumulation, continuous petroleum charge, and no significant influence by buoyancy. Continuous petroleum accumulation generally have nmscale pore throats, and the diameters range of 10–500 nm. The geometry and connectivity of these pore throats has significant impact on the migration and distribution of oil and gas in continuous petroleum accumulation. China has numerous continuous petroleum accumulation containing various petroleum deposits, and the exploration of continuous resources is very promising. Unconventional petroleum geology will become an important new subject in petroleum geology in future, and the nano-technology will function greatly on research, exploration and development of the hydrocarbon accumulation in nano-pore-throats.
The shale of Yanchang Formation in Upper Triassic is the most important source rock for the Mesozoic petroleum reserviors in Ordos Basin. Chang 7 and Chang 4+5 members are the major source rock formation. Source rock samples, obtained from 22 cored wells in central Ordos Basin, were geochemically analyzed to determine the organic matter abundance, kerogen type and thermal maturity. Total organic carbon values ranged from 0.36% to 19.10%, 8.09% on average, indicating a good source rock potential. In this area, the shale is mature, as indicated by vitrinite reflectance values. Rock-Eval data revealed that the samples are dominated by type II kerogen. Compare the Chang 7 and Chang 4+5 members, which suggests that the Chang 7 shale has higher TOC, especially the highest lower Chang 7 members. The abundance of organic matter of Chang 4+5 and Chang 7 members is both richest in southeast basin. The kerogen type of Chang 4+5 and upper Chang 7 members is type II1, the counterpart of middle and lower Chang 7 member is type I. During the burial history, the total hydrocarbon-generating quantity of Chang 7 member is much more than that of Chang 4+5 members.
This study evaluated the hydrocarbon-bearing potential of Upper Jurassic marine source rocks in the Qiangtang (羌塘) Basin through a comprehensive organic geochemical analysis of the samples from a large number of outcrops in different structural units to predict the location of favorable hydrocarbon kitchens, based on the evaluation standards of Mesozoic marine source rocks in the Qiangtang Basin. Rocks’ depositional environment, thickness and organic geochemistry feature were analyzed in this study. The principal controlling factors of the occurrences of favorable source rocks were analyzed. Upper Jurassic Suowa (索瓦) Formation source rocks are mainly platform limestone in the Dongcuo (洞错)-Hulu (葫芦) Lake deep sag and Tupocuo (吐坡错)-Baitan (白滩) Lake deep sag. Lithologically, the Suowa Fromation is made up of a suite of marls in intra-platform sags, micrites and black shales, which were all deposited in the closed, deep and static water depositional environment. Marl could form hydrocarbon-rich source rocks and its organic matter type is mainly II type in mature to highly-mature stage, the limestone forms a medium-level source rock. In addition, the favorable source kitchen of limestone is larger than that of mudstone. This study provides an important reference for the evaluation of Jurassic marine source rocks and for prediction of petroleum resources in the Qiangtang Basin.
We use multi-frequency ground penetrating radar (GPR) to detect and map debris thickness and shallow layer structure of buried ice-body under debris-covered. The basis analysis is depend on mainly (1) the stacking wave velocity in a common mid-point (CMP) survey, (2) the ratio of attenuation of an electromagnetic wave passing through different media, and (3) the vertical resolution. Through a series of analysis, it was found that the optimal average velocity in the shallow layer (0–4 m) is 0.06 m/ns. Images obtained with antennas having different frequencies have different characteristics; i.e., the 50 MHz antenna provides much worse vertical resolution than the 200 and 100 MHz antennas (90 and 160 ns, respectively) and the performance of the 200 MHz antenna for a debris-covered glacier is best. This study provides the basis for accurate analysis of the structure and thickness of the debris layer in the zone of ablation of a debris-covered glacier, this study also provides a reference for research into the formation mechanism and estimation of the ice volume of glacier covered by debris.
Investigation of infiltration through unsaturated zone which consists of both porous and fractured media is important for comprehensively understanding water circulation and effectively managing groundwater resources and contamination control. Infiltration experiments for three kinds of porous-fractured layered structures were conducted with application of a rainfall simulator in this in-vestigation. During experiments, the volumetric water contents of porous media and on the interface of porousfractured media were monitored by moisture sensors (TDT). The infiltration rate, water amount in the profile and on the interface between the soil and the fractured bedrock, and outflow from the layered structures were analyzed to identify the effects of porousfractured interface on water movement in the upper porous media and the effects of various kinds of porous media on infiltration in fractured rocks. It has been observed from the experiment results that the porous media and the fractured rock bear considerable reciprocal impact each other on infiltration processes and water content distribution. The results showed fractured rock prevented vertical water movement in the layered structure, and it decreases infiltration rate of layered structure and slows the process for upper porous media saturation.
Debris flows are the one type of natural disaster that is most closely associated with human activities. Debris flows are characterized as being widely distributed and frequently activated. Rainfall is an important component of debris flows and is the most active factor when debris flows occur. Rainfall also determines the temporal and spatial distribution characteristics of the hazards. A reasonable rainfall threshold target is essential to ensuring the accuracy of debris flow pre-warning. Such a threshold is important for the study of the mechanisms of debris flow formation, predicting the characteristics of future activities and the design of prevention and engineering control measures. Most mountainous areas have little data regarding rainfall and hazards, especially in debris flow forming regions. Therefore, both the traditional demonstration method and frequency calculated method cannot satisfy the debris flow prewarning requirements. This study presents the characteristics of pre-warning regions, included the rainfall, hydrologic and topographic conditions. An analogous area with abundant data and the same conditions as the pre-warning region was selected, and the rainfall threshold was calculated by proxy. This method resolved the problem of debris flow pre-warning in areas lacking data and provided a new approach for debris flow pre-warning in mountainous areas.