Episodic fluid flow within continental rift basins: some insights from field data and mathematical models of the Rhinegraben

Vitrinite reflectance data from a petroleum exploration well in the northern Upper Rhinegraben show an unusual vertical maturity trend. Above and below a 500 m thick marl layer the vitrinite reflectance levels are consistent with modern, conductive, geothermal gradients. Between about 1000 and 1500 m depth, however, vitrinite reflectance levels are significantly elevated (about 0.6%Ro). This anomaly cannot be explained with one‐dimensional conductive or conductive–convective heat transfer models, and thermal effects of sedimentation or igneous intrusion seem implausible for this geological setting. The thermal anomaly that formed this maturation anomaly must have been hydrothermal in origin, two‐dimensional in nature, and persisted long enough to elevate the vitrinite reflectance values within this marl unit, yet it must have dissipated before the thermal perturbation would have altered the organic matter below and above the unit. In this study, we propose that the vitrinite reflectance anomalies were caused by a transient thermal inversion induced by episodic, lateral flow of hot (130–160°C) groundwater along conductive fractures and bedding planes. Heat flow constraints suggest that fluids must have moved rapidly up a vertical feeder fault from a depth of at least 3.6 km before migrating laterally. To test this hypothesis, we present a suite of simple, idealized mathematical models of groundwater flow, heat transfer, thermal degradation of kerogen and vitrinite systematics to explore the episodic flow that could have produced the observed thermal anomaly. In these simulations, a single, horizontal aquifer is sandwiched between two less permeable units: the total dimensions of the vertical section model are 4 km thick by 10 km long. The top of the aquifer coincides with the position of the observed thermal maturity anomaly in the Rhinegraben. Boundary conditions along the left edge of this aquifer were varied through time to allow for the migration of hot fluids out into the basin. Inflow temperature, horizontal velocity, duration and frequency of flow and thickness of the aquifer were varied. We found that a thermal maturity anomaly could only be produced by a rather restrictive set of hydrothermal conditions. It was possible to produce the observed vitrinite reflectance anomaly by a single hydrothermal flow event of 130°C fluid migrating laterally into the aquifer at a rate of 1 m a^?1 for about 10 000 years. The anomaly is spatially confined to near the left edge of the basin, near the feeder fault. If the flow event lasted longer than 100 000 years, then the maturation anomaly disappeared as the lower confining unit approached steady‐state thermal conditions. It is possible that such an event occurred about 5 million years ago in response to increases in fault permeability associated with far field Alpine tectonism.     

基于消费者行为分析的区域旅游市场规划方法研究

本文尝试从旅游者消费需求分析角度入手,探讨区域旅游市场规划的方法论创新。本论文引入了动机--行为投射模型,通过对建立在专题旅游市场调查基础上的消费行为数据库的深度剖析,论文揭示了旅游目标市场群体常见的三类消费心理趋向模式(补偿型需求、均衡型需求、超越型需求)以及相对应的旅游产品选择偏好规律。在此基础上提出了区域旅游目标市场群体的心理定位策略,以及相对应的从结构性旅游功能规划到旅游产品、旅游服务供给链连续递进的规划流程体系。即"行为分析(Behavior)-定位(Postioning)-旅游供给(Supple)",简称旅游市场规划的BPS方案。     

Hydraulic Analysis of a Self-Cleaning Drainage Outlet at the Hellenistic City of Priene

At the distal western end of a main drainage canal running along West Gate Street in the Hellenistic city of Priene, an unusual masonry outlet structure has been found. The housed structure contains a doubly curvilinear, contracting rectangular cross-section flow passageway that allows drainage water flow through the city perimeter wall. A computer model of the complexly-shaped drainage structure has been hydraulically analysed in order to determine the function of the curvilinear passageway. Results indicate that the internal shaping of the structure hydraulically conditions the flow to create multiple circulatory mixing flows that agitate and entrain debris in the outflow stream sufficient to “self-clean” the outlet, thus preventing clogging. Further calculations of flow free surface shapes at different flow rates indicate the maximum flow capacity that can be processed by the drainage system without overspillage to provide a lower-bound estimate of the steady-state water supply to the city. The design of a self-cleaning, continuous flow urban wastewater system implies that Greek city planners maintained a high level of awareness about the hygienic conditions needed to maintain the health of the populace. The Greek attitude toward technical refinements necessary to improve the quality of urban life could therefore not be better represented than in attention to detail related to water supply and waste water drainage systems as a key to improved life standards.     

Hydrothermal monitoring in a quiescent volcanic arc: Cascade Range,northwestern United States

Ongoing (1996–present) volcanic unrest near South Sister, Oregon, is accompanied by a striking set of hydrothermal anomalies, including elevated temperatures, elevated major ion concentrations, and ³He/⁴He ratios as large as 8.6 R_A in slightly thermal springs. These observations prompted the US Geological Survey to begin a systematic hydrothermal‐monitoring effort encompassing 25 sites and 10 of the highest‐risk volcanoes in the Cascade volcanic arc, from Mount Baker near the Canadian border to Lassen Peak in northern California. A concerted effort was made to develop hourly, multiyear records of temperature and/or hydrothermal solute flux, suitable for retrospective comparison with other continuous geophysical monitoring data. Targets included summit fumarole groups and springs/streams that show clear evidence of magmatic influence in the form of high ³He/⁴He ratios and/or anomalous fluxes of magmatic CO₂ or heat. As of 2009–2012, summit fumarole temperatures in the Cascade Range were generally near or below the local pure water boiling point; the maximum observed superheat was <2.5°C at Mount Baker. Variability in ground temperature records from the summit fumarole sites is temperature‐dependent, with the hottest sites tending to show less variability. Seasonal variability in the hydrothermal solute flux from magmatically influenced springs varied from essentially undetectable to a factor of 5–10. This range of observed behavior owes mainly to the local climate regime, with strongly snowmelt‐influenced springs and streams exhibiting more variability. As of the end of the 2012 field season, there had been 87 occurrences of local seismic energy densities approximately ≥ 0.001 J/m³ during periods of hourly record. Hydrothermal responses to these small seismic stimuli were generally undetectable or ambiguous. Evaluation of multiyear to multidecadal trends indicates that whereas the hydrothermal system at Mount St. Helens is still fast‐evolving in response to the 1980–present eruptive cycle, there is no clear evidence of ongoing long‐term trends in hydrothermal activity at other Cascade Range volcanoes that have been active or restless during the past century (Baker, South Sister, and Lassen). Experience gained during the Cascade Range hydrothermal‐monitoring experiment informs ongoing efforts to capture entire unrest cycles at more active but generally less accessible volcanoes such as those in the Aleutian arc.     

Changes in fluid pathways in a calcite vein mesh (Natih Formation,Oman Mountains): insights from stable isotopes

We present a structural, microstructural, and stable isotope study of a calcite vein mesh within the Cretaceous Natih Formation in the Oman Mountains to explore changes in fluid pathways during vein formation. Stage 1 veins form a mesh of steeply dipping crack‐seal extension veins confined to a 3.5‐m‐thick stratigraphic interval. Different strike orientations of Stage 1 veins show mutually crosscutting relationships. Stage 2 veins occur in the dilatant parts of a younger normal fault interpreted to penetrate the stratigraphy below. The δ¹⁸O composition of the host rock ranges from 21.8‰ to 23.7‰. The δ¹³C composition ranges from 1.5‰ to 2.3‰. This range is consistent with regionally developed diagenetic alteration at top of the Natih Formation. The δ¹⁸O composition of vein calcite varies from 22.5‰ to 26.2‰, whereas δ¹³C composition ranges from ?0.8‰ to 2.1‰. A first trend observed in Stage 1 veins involves a decrease of δ¹³C to compositions nearly 1.3‰ lower than the host rock, whereas δ¹⁸O remains constant. A second trend observed in Stage 2 calcite has δ¹⁸O values up to 3.3‰ higher than the host rock, whereas the δ¹³C composition is similar. Stable isotope data and microstructures indicate an episodic flow regime for both stages. During Stage 1, formation of a stratabound vein mesh involved bedding‐parallel flow, under near‐lithostatic fluid pressures. The ¹⁸O fluid composition was host rock‐buffered, whereas ¹³C composition was relatively depleted. This may reflect reaction of low ¹³C CO₂ derived by fluid interaction with organic matter in the limestones. Stage 2 vein formation is associated with fault‐controlled fluid flow accessing fluids in equilibrium with limestones about 50 m beneath. We highlight how evolution of effective stress states and the growth of faults influence the hydraulic connectivity in fracture networks and we demonstrate the value of stable isotopes in tracking changes in fluid pathways.     

Chronicles of vadose zone diagenesis: cone‐shaped iron oxide concretions,Triassic Trujillo Formation,Palo Duro Canyon,Texas

Unusual cone‐shaped iron oxide concretions occur in the Late Triassic, lower fluvial sandstone member of the Trujillo Formation at Palo Duro Canyon in the Texas panhandle. In situ concretions are significant because they record both historical information about past processes that occurred within the geologic unit and present‐day information about the ability of the unit to conduct fluids. The dominant orientation of the concretions is cone‐apex up, body radiating down and out, with long axis perpendicular to bedding. Concretion morphologies are associated with the sedimentary texture and primary bedding structure of the host rock and the corresponding hydrologic regime (i.e. advection versus dispersion for iron‐transport behavior). Three lithofacies in the lower Trujillo member exhibit different cone forms. Field observations of cone orientation and morphology suggest vadose conditions for diagenetic precipitation of iron oxide cements, with timing potentially represented by the major pre‐Miocene unconformity.     

Indications of formation water flow and compartmentalization on the flank of a salt structure derived from salinity and seismic data

Vertical and lateral variations in lithology, salinity, temperature, and pressure determined from wireline LAS logs, produced water samples, and seismic data on the south flank of a salt structure on the continental shelf, offshore Louisiana indicate three hydrogeologic zones in the study area: a shallow region from 0 to 1.1 km depth with hydrostatically pressured, shale‐dominated Pleistocene age sediments containing pore waters with sea water (35 g l^?1) or slightly above sea water salinity; a middle region from 1.1 to 3.2 km depth with near hydrostatically pressured, sand‐dominated Pliocene age sediments that contain pore waters that range from seawater salinity to up to 5 times sea water salinity (180 g l^?1); and a deep section below 3.2 km depth with geopressured, shale‐dominated Miocene age sediments containing pore waters that range from sea water salinity to 125 g l^?1. Salt dissolution has generated dense, saline waters that appear to be migrating down dip preferentially through the thick Pliocene sandy section. Sand layers that come in contact with salt contain pore waters with high salinity. Isolated sands have near sea water salinity. Salinity information in conjunction with seismic data is used to infer fluid compartmentalization. Both vertical and lateral lithologic barriers to fluid flow at tens to hundreds of meters scale are observed. Fluid compartmentalization is also evident across a supradomal normal fault. Offset of salinity contours are consistent with the throw of the fault, which suggests that saline fluids migrated before fault formation.     

虚拟社区信息流导引现实社区人流的特征——以杭州市智能居住小区网站为例

虚拟社区的广泛应用已给现实社区带来了多方面的深刻影响。其中一类虚拟社区与现实社区已逐步融合,并形成了半虚拟半现实的社区结构,城市智能居住小区即属于这种情况,它为地理学研究虚拟社区问题提供了参考对象。本文的研究目的是为了揭示虚拟社区信息流对现实居住社区人流导引作用的特征。在对国内外研究进行回顾后,以杭州市智能小区为例,分析了其网站信息流导引人流的两点表现-存在性与同质性。研究发现虚拟社区对现实居住行为的导引是客观存在的,其中网站智能性在导引中发挥了巨大作用;而受影响人群的同质化是虚拟社区信息流对居住人群导引的突出特征。     

Evolution of porosity, permeability and fluid pressure in dilatant faults post-failure: implications for fluid flow and mineralization

Mineralization of brittle fault zones is associated with sudden dilation, and the corresponding changes in porosity, permeability and fluid pressure, that occur during fault slip events. The resulting fluid pressure gradients cause fluid to flow into and along the fault until it is sealed. The volume of fluid that can pass through the deforming region depends on the degree of dilation, the porosity and permeability of the fault and wall rocks, and the rate of fault sealing. A numerical model representing a steep fault cutting through a horizontal seal is used to investigate patterns of fluid flow following a dilatant fault slip event. The model is initialized with porosity, permeability and fluid pressure representing the static mechanical state of the system immediately after such an event. Fault sealing is represented by a specified evolution of porosity, coupled to changes in permeability and fluid pressure, with the rate of porosity reduction being constrained by independent estimates of the rate of fault sealing by pressure solution. The general pattern of fluid flow predicted by the model is of initial flow into the fault from all directions, followed by upward flow driven by overpressure beneath the seal. The integrated fluid flux through the fault after a single failure event is insufficient to account for observed mineralization in faults; mineralization would require multiple fault slip events. Downward flow is predicted if the wall rocks below the seal are less permeable than those above. This phenomenon could at least partially explain the occurrence of uranium deposits in reactivated basement faults that cross an unconformity between relatively impermeable basement and overlying sedimentary rocks.     

A vector of high‐temperature paleo‐fluid flow deduced from mass transfer across permeability barriers (quartz veins)

Quartz veins acted as impermeable barriers to regional fluid flow and not as fluid‐flow conduits in Mesoproterozoic rocks of the Mt Painter Block, South Australia. Systematically distributed asymmetric alteration selvedges consisting of a muscovite‐rich zone paired with a biotite‐rich zone are centered on quartz veins in quartz–muscovite–biotite schist. Geometric analysis of the orientation and facing of 126 veins at Nooldoonooldoona Waterhole reveals a single direction along which a maximum of all veins have a muscovite‐rich side, irrespective of their specific individual orientation. This direction represents a Mesoproterozoic fluid‐flow vector and the veins represent permeability barriers to the flow. The pale muscovite‐rich zones formed on the downstream side of the vein and the dark biotite‐rich zones mark the upstream side. The alteration couplets formed from mica schist at constant Zr, Ga, Sc, and involved increases in Si, Na, Al and decreases in K, Fe, Mg for pale alteration zones, and inverse alteration within dark zones. The asymmetry of the alteration couplets is best explained by the pressure dependence of mineral–fluid equilibria. These equilibria, in combination with a Darcian flow model for coupled advection and diffusion, and with permeability barriers imposed by the quartz veins, simulate the pattern of both fluid flow and differential, asymmetric metasomatism. The determined vector of fluid flow lies along the regional foliation and is consistent with the known distribution of regional alteration products. The presence of asymmetric alteration zones in rock containing abundant pre‐alteration veins suggests that vein‐rich material may have generally retarded regional fluid flow.