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

《Journal of Asian Earth Sciences》2014

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.  相似文献   

Causes of geothermal fields and characteristics of ground temperature fields in China     

MAO Xiao-ping  LI Ke-wen  WANG Xin-wei 《地下水科学与工程》2014,7(1):15-28

There are many arguments on energy sources and main controlling factors of geothermal fields, so a systematic study on the distribution of ground temperature fields shall be necessary. In this paper the thermal conduction forward method of geothermal field is used to simulate cooling rate of abnormal heat sources and heat transfer of the paleo-uplift model. Combined with a large number of geothermal field exploration cases and oil exploration well temperature curves of domestic and foreign, the following conclusions are drawn: (1) According to the magmatic activity time, the magmatism activities are divided into two categories: Magma active areas (activity time < 500 000 years) and weak/magma inactive areas (activity time > 500 000 years). The latter has a fast cooling rate (the cooling time of the magma pocket buried around 10 km is less than 200 000 years) after it has intruded into the shallow layer and it has no direct contribution to modern geothermal fields; (2) China belongs to a weak/magma inactive area such as Tengchong region and Qinghai-Tibet region because the chronological data of these regions show that its magma activity time is more than 500 000 years; (3) The temperature of most geothermal fields can be obviously divided into three segments in the vertical direction: A high geothermal gradient segment (Segment H) at the surface, then a low geothermal gradient segment (Segment L) at a secondary depth, and finally a lower temperature segment (Segment D) at a deeper depth. The temperature isoline presents a mirror reflection relation on the temperature profile, indicating that geothermal field is dominated by heat conduction, rather than having an abnormally high temperature “heat source” to provide heat; (4) Near-surface (0-5 km) materials’ lateral heterogeneity caused by tectonic movement shall probably be the main controlling factor of ground temperature fields.  相似文献   

Mud dispersal across a Cretaceous prodelta: Storm‐generated,wave‐enhanced sediment gravity flows inferred from mudstone microtexture and microfacies     

A. Guy Plint 《Sedimentology》2014,61(3):609-647

Determining sediment transport direction in ancient mudrocks is difficult. In order to determine both process and direction of mud transport, a portion of a well‐mapped Cretaceous delta system was studied. Oriented samples from outcrop represent prodelta environments from ca 10 to 120 km offshore. Oriented thin sections of mudstone, cut in three planes, allowed bed microstructure and palaeoflow directions to be determined. Clay mineral platelets are packaged in equant, face‐face aggregates 2 to 5 μm in diameter that have a random orientation; these aggregates may have formed through flocculation in fluid mud. Cohesive mud was eroded by storms to make intraclastic aggregates 5 to 20 μm in diameter. Mudstone beds are millimetre‐scale, and four microfacies are recognized: Well‐sorted siltstone forms millimetre‐scale combined‐flow ripples overlying scoured surfaces; deposition was from turbulent combined flow. Silt‐streaked claystone comprises parallel, sub‐millimetre laminae of siliceous silt and clay aggregates sorted by shear in the boundary layer beneath a wave‐supported gravity flow of fluid mud. Silty claystone comprises fine siliceous silt grains floating in a matrix of clay and was deposited by vertical settling as fluid mud gelled under minimal current shear. Homogeneous clay‐rich mudstone has little silt and may represent late‐stage settling of fluid mud, or settling from wave‐dissipated fluid mud. It is difficult or impossible to correlate millimetre‐scale beds between thin sections from the same sample, spaced only ca 20 mm apart, due to lateral facies change and localized scour and fill. Combined‐flow ripples in siltstone show strong preferred migration directly down the regional prodelta slope, estimated at ca 1 : 1000. Ripple migration was effected by drag exerted by an overlying layer of downslope‐flowing, wave‐supported fluid mud. In the upper part of the studied section, centimetre‐scale interbeds of very fine to fine‐grained sandstone show wave ripple crests trending shore normal, whereas combined‐flow ripples migrated obliquely alongshore and offshore. Storm winds blowing from the north‐east drove shore‐oblique geostrophic sand transport whereas simultaneously, wave‐supported flows of fluid mud travelled downslope under the influence of gravity. Effective wave base for sand, estimated at ca 40 m, intersected the prodelta surface ca 80 km offshore whereas wave base for mud was at ca 70 m and lay ca 120 km offshore. Small‐scale bioturbation of mud beds co‐occurs with interbedded sandstone but stratigraphically lower, sand‐free mudstone has few or no signs of benthic fauna. It is likely that a combination of soupground substrate, frequent storm emplacement of fluid mud, low nutrient availability and possibly reduced bottom‐water oxygen content collectively inhibited benthic fauna in the distal prodelta.  相似文献   

Matrix‐rich and associated matrix‐poor sandstones: Avulsion splays in slope and basin‐floor strata     

Viktor Terlaky  Robert William Charles Arnott 《Sedimentology》2014,61(5):1175-1197

A common facies observed in deep‐water slope and especially basin‐floor rocks of the Neoproterozoic Windermere Supergroup (British Columbia, Canada) is structureless, coarse‐tail graded, medium‐grained to coarse‐grained sandstone with from 30% to >50% mud matrix content (i.e. matrix‐rich). Bed contacts are commonly sharp, flat and loaded. Matrix‐rich sandstone beds typically form laterally continuous units that are up to several metres thick and several tens to hundreds of metres wide, and commonly adjacent to units of comparatively matrix‐poor, scour‐based sandstone beds with large tabular mudstone and sandstone clasts. Matrix‐rich units are common in proximal basin‐floor (Upper Kaza Group) deposits, but occur also in more distal basin‐floor (Middle Kaza Group) and slope (Isaac Formation) deposits. Regardless of stratigraphic setting, matrix‐rich units typically are directly and abruptly overlain by architectural elements comprising matrix‐poor coarse sandstone (i.e. channels and splays). Despite a number of similarities with previously described matrix‐rich beds in the literature, for example slurry beds, linked debrites and co‐genetic turbidites, a number of important differences exist, including the stratal make‐up of individual beds (for example, the lack of a clean sandstone turbidite base) and their stratigraphic occurrence (present throughout base of slope and basin‐floor strata, but most common in proximal lobe deposits) and accordingly suggest a different mode of emplacement. The matrix‐rich, poorly sorted nature of the beds and the abundance and size of tabular clasts in laterally equivalent sandstones imply intense upstream scouring, most probably related to significant erosion by an energetic plane‐wall jet or within a submerged hydraulic jump. Rapid energy loss coupled with rapid charging of the flow with fine‐grained sediment probably changed the rheology of the flow and promoted deposition along the margins of the jet. Moreover, these distinctive matrix‐rich strata are interpreted to represent the energetic initiation of the local sedimentary system, most probably caused by a local upflow avulsion.  相似文献   

Influence of carbonate facies on fault zone architecture     

《Journal of Structural Geology》2014

Normal faults on Malta were studied to analyse fault propagation and evolution in different carbonate facies. Deformation of carbonate facies is controlled by strength, particle size and pore structure. Different deformation styles influence the damage characteristics surrounding faults, and therefore the fault zone architecture. The carbonates were divided into grain- and micrite-dominated carbonate lithofacies. Stronger grain-dominated carbonates show localised deformation, whereas weaker micrite-dominated carbonates show distributed deformation. The weaker micrite-dominated carbonates overlie stronger grain-dominated carbonates, creating a mechanical stratigraphy. A different architecture of damage, the ‘Fracture Splay Zone’ (FSZ), is produced within micrite-dominated carbonates due to this mechanical stratigraphy. Strain accumulates at the point of juxtaposition between the stronger grain-dominated carbonates in the footwall block and the weaker micrite-dominated carbonates in the hanging wall block. New slip surfaces nucleate and grow from these points, developing an asymmetric fault damage zone segment. The development of more slip surfaces within a single fault zone forms a zone of intense deformation, bound between two slip surfaces within the micrite-dominated carbonate lithofacies (i.e., the FSZ). Rather than localisation onto a single slip surface, allowing formation of a continuous fault core, the deformation will be dispersed along several slip surfaces. The dispersed deformation can create a highly permeable zone, rather than a baffle/seal, in the micrite-dominated carbonate lithofacies. The formation of a Fracture Splay Zone will therefore affect the sealing potential of the fault zone. The FSZ, by contrast, is not observed in the majority of the grain-dominated carbonates.  相似文献   

熔融制样波长色散X射线荧光光谱法测定白云石中钙镁硅铁铝   总被引：4，自引：3，他引：1  

褚宁  李卫刚  蒋晓光  张彦甫  毕孝瑞 《岩矿测试》2014,33(6):834-838

白云石属碳酸盐矿物,应用熔融制样X射线荧光光谱法测定其中主次量组分钙、镁、硅、铁、铝时,由于白云石灼烧减量大,在试料片制备过程中,如果以干基试料制备试料片,除灼烧减量外还有少量其他组分被分解出的大量CO₂带走,导致试料损失,测定结果偏低;如果以灼烧基试料制备试料片,由于灼烧后的试料极易吸收空气中的水分和CO₂,同样使测定结果偏低。基于质量守恒原理,本文直接以灼烧减量测量后的灼烧基试料质量作为试料量(即灼烧减量测定所称量的干基试料量扣除灼烧减量的量),以四硼酸锂为熔剂,5%碘化铵溶液为脱模剂,试料与熔剂的稀释比为1:10,于1050℃熔融15 min以上制备的试料片透彻、玻璃化程度高。以白云石标准物质和标准样品作为标准试料,制作各组分的标准曲线的相关系数在0.9940~0.9994之间;方法检出限为0.011%~0.48%;标准物质和标准样品的测定值与认定值基本一致,各组分的相对标准偏差(RSD,n=11)在0.5%~1.7%之间,方法具有较好的重现性。本方法以1050℃灼烧后的试样作为试料制备XRF分析样片,最大限度地降低了灼烧减量因素(空气中的水分和CO₂)对测定结果的影响,适用于白云石及其煅烧物中钙、镁、硅、铁、铝等组分的同时测定。  相似文献   

Effect of macropore tortuosity and morphology on preferential flow through saturated soil: A Lattice Boltzmann study     

《Computers and Geotechnics》2014

Unlike micropores where water moves upward or downward based on hydraulic gradient, in macropores, water flows predominantly downward due to the gravity. Therefore, models based on capillary flow are not capable of simulating macropore flow. There are attempts to model the macropore flow using two domains, one for capillary flow and another one for macropores. These models use Richard’s equation for capillary flow and Poiseuille’s law for macropores in which the macropore is approximated to be cylindrical or planar. This study quantifies the magnitudes of the errors induced by this assumption. Influence of macropore shapes and tortuosity was quantified by using a 3D Lattice Boltzmann model, which is capable of simulating fluid flow in micropores as well as macropores of cracked clays. Artificial macropores of constant sectional area and volume, but different shapes were generated in 3D and the influence of macropore shapes, shape related parameters, and tortuosity were systematically investigated. Macropore flow rate decreases with different shapes compared to cylindrical macropores and increase in aspect ratio of sectional shape leads to decrease in macropore flow rate. The maximum effect of bends/turnings along the tortuous macropore was about 25% on overall decrease of flow rate due to tortuosity. However, more detailed study is required on the influence of bends on macropore flow rate. The macropore flow rate reduces by about 70% for tortuosity of 1.41. A prediction equation is verified to predict the flow rate of different shapes and tortuous macropores based on straight cylindrical macropore using aspect ratio and tortuosity factor.  相似文献   

A constitutive model for unsaturated soils with consideration of inter-particle bonding     

《Computers and Geotechnics》2014

The paper presents a physically-based constitutive model for unsaturated soils that considers the bonding effect of water menisci at inter-particle contacts. A bonding factor has been used to represent the magnitude of the equivalent bonding stress, defined as the bonding force per unit cross-sectional area. The average skeleton stress is employed to represent the effect of average fluid pressures within soil pores. Based on an empirical relationship between the bonding factor ζ and the ratio e/e_s (where e and e_s are void ratios at unsaturated and saturated states, respectively, at the same average skeleton stress), we propose an elasto-plastic constitutive model for isotropic stress states, and then extend this model to triaxial stress states within the framework of critical state soil mechanics. Because only one yield surface is needed in the proposed model, a relatively small number of parameters are required. Comparisons between experimental data and model results show that, in most cases, the proposed model can reasonably capture the important features of unsaturated soil behavior.  相似文献   

Numerical studies on the effect of temperature on the unsaturated hydraulic response of geotextiles     

《Computers and Geotechnics》2014

A series of coupled thermo-hydraulic simulations were performed on a soil–geotextile column to understand the effect of temperature on suction distribution throughout the soil column and on the hydraulic performance of the geotextile as a drainage/capillary barrier layer. Two different constant temperatures of 0 °C and 38 °C and a temperature gradient of 4 °C along the column were modeled. Changing the temperature from 0 °C to 38 °C did not have a significant effect on the suction head distribution in the soil–geotextile column. The temperature gradient resulted in appreciable thermal vapor flow and changes in suction head and hydraulic conductivity of the geotextile. During drainage, the temperature gradient and lower temperature at the top of the column increased suction in the geotextile and its ability to function as a capillary barrier. During capillary rise, the temperature gradient and lower temperature at the top of the column decreased the suction in the geotextile and its ability to function as a capillary barrier. Changing the direction of the thermal gradient reversed the water vapor flow direction and its effect on the suction in the geotextile. A temperature gradient did not have a noticeable effect on the suction head of the geotextile when positive pore pressure was developed in the geotextile and adjacent soil during drainage.  相似文献   

Design of drainage blankets for leachate recirculation in bioreactor landfills using two-phase flow modeling     

《Computers and Geotechnics》2014

Drainage blankets (DB) are used for leachate recirculation in bioreactor landfills and consist of highly permeable material placed over a large area of the landfill with the leachate injection pipe embedded in the material at specified locations. DBs are generally installed at different depth levels during the waste filling operations. Very limited information is reported on performance of DBs, and that which exists is based on a small number of field monitoring and modeling studies. A rational method for the design of landfills using DBs has not been developed. This study performs a parametric analysis based on a validated two-phase flow model and presents design charts to guide the design of DBs for given hydraulic properties of MSW, the leachate injection rate and the dimensions and locations of the DB as measured from the leachate collection and recirculation system (LCRS) located at the bottom of the landfill cell. Numerical simulations were performed for the two established MSW conditions: homogeneous–isotropic and heterogeneous–anisotropic waste. The optimal levels of leachate saturation, wetted width, wetted area and developed pore water and pore gas pressures were determined, and design charts using the normalized parameters were developed. An example is presented on the use of design charts for typical field application.  相似文献