Mechanisms of strain-rate- and reaction-dependent permeability in the mid-crust of the Southern Alps: insight from three-dimensional mechanical models

We used three-dimensional mechanical modelling to explore the interplay of rheology, modes of permeability creation and fluid flow in the mid-crust of an oblique orogen. We used the central Southern Alps and the Otago Schist of New Zealand to constrain our models. We also compared our models with the magnetotelluric survey along the Rangitata–Whataroa rivers that imaged a U-shaped zone of high conductivity, interpreted as interconnected fluids, beneath the central Southern Alps. Modelling was carried out using the numerical code FLAC^3D. We used a number of simple assumptions: an initially homogeneous starting material, deformation boundary conditions based on the tectonics of the South Island, the capability of the modelled material to develop an anisotropic permeability structure, strain rate and reaction-induced permeability increases, initial saturation and lithostatic pore pressures as a basis for our models. The initial isotropic permeability was 10⁻¹⁸ m². We modelled two possible mechanisms of permeability increase: (i) strain-rate dependent and (ii) reaction dependent. For a strong mid-crust, the models showed enhanced permeability and hence fluid interconnectivity in a symmetric region beneath the model main divide, both ends were turned up towards the brittle–ductile transition and fluid flow was the greatest in the across strike direction. For a weak mid-crust, the region of enhanced permeability was asymmetric and turned up towards the brittle–ductile regime close to the Alpine Fault. The strong mid-crust model reproduces the features that are common to all interpretations of the MT soundings and is our preferred model for the central Southern Alps.     

Modeling fluid flow in a heterogeneous, fault-controlled hydrothermal system

Previous studies have shown that most hydrothermal systems discharging at the land surface are associated with faulting, and that the location, temperature and rate of discharge of these systems are controlled by the geometry and style of the controlling fault(s). Unfortunately, the transport of heat and fluid in fault-controlled hydrothermal systems is difficult to model realistically; although heterogeneity and anisotropy are assumed to place important controls on flow in faults, few data or observations are available to constrain the distribution of hydraulic properties within active faults. Here, analytical and numerical models are combined with geostatistical models of spatially varying hydraulic properties to model the flow of heat and fluid in the Borax Lake fault of south-east Oregon, USA. A geometric mean permeability within the fault of 7 × 10⁻¹⁴ m² with 2× vertical/horizontal anisotropy in correlation length scale is shown to give the closest match to field observations. Furthermore, the simulations demonstrate that continuity of flow paths is an important factor in reproducing the observed behavior. In addition to providing some insight into possible spatial distributions of hydraulic properties at the Borax Lake site, the study highlights one potential avenue for integrating field observations with simulation results in order to gain greater understanding of fluid flow in faults and fault-controlled hydrothermal and petroleum reservoirs.     

Permeability of the continental crust: dynamic variations inferred from seismicity and metamorphism

The variation of permeability with depth can be probed indirectly by various means, including hydrologic models that use geothermal data as constraints and the progress of metamorphic reactions driven by fluid flow. Geothermal and metamorphic data combine to indicate that mean permeability ( k ) of tectonically active continental crust decreases with depth ( z ) according to log  k  ≈ −14–3.2 log  z , where k is in m² and z in km. Other independently derived, crustal-scale k – z relations are generally similar to this power-law curve. Yet there is also substantial evidence for local-to-regional-scale, transient, permeability-generation events that entail permeabilities much higher than these mean k – z relations would suggest. Compilation of such data yields a fit to these elevated, transient values of log  k  ≈ −11.5–3.2 log  z , suggesting a functional form similar to that of tectonically active crust, but shifted to higher permeability at a given depth. In addition, it seems possible that, in the absence of active prograde metamorphism, permeability in the deeper crust will decay toward values below the mean k – z curves. Several lines of evidence suggest geologically rapid (years to 10³ years) decay of high-permeability transients toward background values. Crustal-scale k – z curves may reflect a dynamic competition between permeability creation by processes such as fluid sourcing and rock failure, and permeability destruction by processes such as compaction, hydrothermal alteration, and retrograde metamorphism.     

Rates of retrograde metamorphism and their implications for crustal rheology

The rate of reaction of a natural hornblende garnet granulite with water under a range of mid- to lower crustal conditions has been investigated experimentally. In runs of between 7 and 84 days small but measurable amounts of water were consumed, and sheet silicates (300°C, 300 MPa and 400°C, 400 MPa) and/or secondary actinolite (400°C, 400 MPa and 500°C, 500 MPa) were observed to have grown. When normalized to the surface area of the starting materials, hydration rates were in the range of 2–5 × 10⁻⁸ g m⁻² sec⁻¹. These reaction rates imply that a film of water that infiltrated a planar crack with a half width of 100 μm would be completely consumed within c. 100 years. These results imply that where water infiltrates the crust along faults or underlying shear zones in response to a deformation, it will remain as a free phase for only a finite period of time, which in some cases will be less than the repeat time for major earthquakes in the fault system. Thus, the rheology of fault zones and shear zones is likely to be cyclical, with the zone becoming stronger with time as water is consumed, and then weakened by infiltration of water after each rupture.     

Soil erosion and flooding on the eastern South Downs, southern England, 1976–2001

The South Downs in southern England have been farmed for 5000 years: the initial loess cover is now a thin, stony remnant as a result of erosion. During the 1980s, field monitoring of erosion events in an area of 36 km² showed that average rates of erosion are low (0.5–5.0 m³ ha⁻¹ yr⁻¹), but that occasional storms result in losses of over 200 m³ ha⁻¹ yr⁻¹ on individual fields. On soils that are only 15 cm thick, such rates pose a threat to future farming. Almost all recent erosion has been on fields of winter cereals, which are bare in the wet autumn period. Of greater significance in the short term are the effects of soil-laden runoff (or muddy floods) on urban areas. Flood damage to property has been a regular event since the conversion of this area to winter cereals in the 1970s. Soil conservation measures have been negligible and flood protection has consisted mainly of engineering works and limited land-use change at sites where property damage has occurred. Conversion to grass under the Set Aside, and an Environmentally Sensitive Area (ESA) scheme, could be an effective soil conservation and flood protection approach. In the longer term, increasing stoniness of soils, the economic incentives for arable farming in marginal areas, and the success of legal proceedings relating to flood damage, will determine the future of erosion on the South Downs. Current farming systems clearly are unsustainable and would become more so under future climates without substantial site-targeted land use change.     

Chemical modification of pyroclastic rock by hot water: an experimental investigation of mass transport at the fluid–solid interface

Hydrothermal water–(pyroclastic) rock interactions were examined using flow-through experiments to deduce the effect of mass transport phenomena on the reaction process. A series of experiments were conducted over the temperature range 75–250°C, with a constant temperature for each experiment, and at saturated vapour pressure, to estimate the apparent rate constants as a function of temperature.
Based on the chemistry of analysed solutions, the water–rock interaction in the experiments was controlled by diffusion from the reaction surface and by the existence of a surface layer at the rock–fluid interface, which regulated the chemical reaction rate. The reaction progress depended to a high degree on flow velocity and temperature conditions, with element abundances in the fluid significantly affected by these factors. Mass transport coefficients for diffusion from the rock surface to the bulk solution have been estimated. Ca is selectively depleted under lower temperature conditions ( T  < 150°C), whereas Na is greatly depleted under higher temperature conditions ( T  > 150°C), and K reaction rates are increased when flow velocity increases. Using these conditions, specific alkali and alkali earth cations were selectively leached from mineral surfaces. The 'surface layer' comprised a 0.5–1.8 mm boundary film on the solution side (the thickness of this layer has no dependence on chemical character) and a reaction layer. The reaction layer was composed of a Si, Al-rich cation-leached layer, whose thickness was dependent on temperature, flow velocity and reaction length. The reaction layer varied in thickness from about 10⁻⁴ to 10⁻⁷ mm under high temperature/low fluid velocity and low temperature/high fluid velocity conditions, respectively.     

Finite element modeling of slug tests in an aquifer with stratigraphical and structural heterogeneities

Slug interference tests using an array of multilevel active and monitoring wells permit enhanced aquifer characterization. Analyses of these field test data rely on numerical inverse models. In order to provide synthetic data sets and to have a better understanding of the flow mechanisms, we used a three-dimensional finite element analysis (FEHM) to explore the effects of idealized, stratigraphical (strata) and structural (faults) heterogeneities with low permeability values on the transient head field that is associated with slug tests in an aquifer. Firstly, we tested our model on homogeneous aquifers and the effectiveness of our modeling strategies have been validated via the excellent agreement of our modeling results with those of the semi-analytical model of Liu & Butler (1995) . In our heterogeneity investigations, we embedded vertical and horizontal zones of lower permeability into a homogenous, isotropic, and confined aquifer to represent low-permeability faults and strata respectively. A slugged interval is located at the center of the aquifer. Effects of strata thickness and permeability contrast as well as other effects associated with the offset of low-permeability strata were explored. In particular, modeling results are represented by contour maps of peak travel time and maximum head perturbation of generated hydraulic pulses. Furthermore, various phenomena, such as real-time matrix diffusion, intermittent matrix–fracture interactions, and faster pulse arrival through a longer flow trajectory, are concretely presented in the snapshots of head perturbations in the aquifer. Our finite element modeling provides useful information for understanding the behaviors of diffusive pressure propagation in an aquifer with stratigraphical and/or structural heterogeneities, and for designing hydraulic tomography to enhance aquifer characterization.     

The seventh and eighth Belgian archaeological expeditions to ed-Dur (Umm al-Qaiwain, UAE)

The results of the 1993 and 1994 seasons by a Belgian team from Gent University at ed-Dur in the Emirate of Umm al-Qaiwain are presented. The excavations concentrated on area BS, where a total of 2650 m² were excavated. This large exposure provided a vast amount of material of the late 1st century BC and the 1st century AD. Only some rare objects (from different areas) are presented.     

The interplay of permeability and fluid properties as a first order control of heat transport, venting temperatures and venting salinities at mid-ocean ridge hydrothermal systems

While the fundamental influence of fluid properties on venting temperatures in mid-ocean ridge (MOR) hydrothermal systems is now well established, the potential interplay of fluid properties with permeability in controlling heat transfer, venting temperatures, and venting salinities has so far received little attention. A series of numerical simulations of fully transient fluid flow in a generic, across-axis model of a MOR with a heat input equivalent to magmatic supply at a spreading rate of 10 cm year⁻¹ shows a strong dependence of venting temperature and salinity on the system's permeability. At high permeability, venting temperatures are low because fluid fluxes are so high that the basal conductive heating cannot warm the large fluid masses rapidly enough. The highest venting temperature around 400°C as well as sub-seafloor fluid phase separation occur when the permeability is just high enough that the fluid flux can still accommodate all heat input for advection, or for lower permeabilities where advection is no longer capable to transfer all incoming magmatic heat. In the latter case, additional mechanisms such as eruptions of basaltic magma may become relevant in balancing heat flow in MOR settings. The results can quantitatively be explained by the 'fluxibility' hypothesis of Jupp & Schultz (Nature, 403 , 2000, 880), which is used to derive diagrams for the relations between basal heat input, permeability and venting temperatures. Its predictive capabilities are tested against additional simulations. Potential implications of this work are that permeability in high-temperature MOR hydrothermal systems may be lower than previously thought and that low-temperature systems at high permeability may be an efficient way of removing heat at MOR.     

Excavations at ed-Dur (Umm al-Qaiwain, U.A.E.) — Preliminary report on the sixth Belgian season (1992)

The results of six weeks'research in 1992 by a Belgian team from Gent University at ed-Dur in the Emirate of Umm al-Qaiwain are presented. The excavations concentrated on the areas BQ and BR, where a total of almost 2700 m² were excavated. Both areas contained mostly graves, all plundered, however, except for some plain earth-burials. These large exposures provided a vast amount of various objects and pottery and gave further information on burial practices of the 1st cent. A.D. in southeastern Arabia.     

Managing the maritime heritage: the National Maritime Archaeological Museum and National Centre for Underwater Research, Cartagena, Spain

The rapid developmental work of the Museum-Centre and its policies within a limited budget are set in the context of world-wide problems of management and preservation of the underwater heritage in the face of accelerating threats to its survival. The impact of engineering projects and lack of official control or supervision of developer-funded maritime archaeological interventions is something of which governments are insufficiently aware. The construction of a marina at Mazarron has resulted in progressive denudation of the bay, exposure of two Phoenician wrecks and thousands of Phoenician potsherds, necessitating the annual resurvey of 70,000 m² of seabed. Among protective measures an iron 'strongbox' has been designed and successfully used on small wreck-sites. © 2000 The Nautical Archaeology Society     

Measurements of gas permeability and diffusivity of tight reservoir rocks: different approaches and their applications

Permeability and diffusivity are critical parameters of tight reservoir rocks that determine their viability for commercial development. Current methods for measuring permeability and/or diffusivity may lead to erroneous results when applied to very tight rocks including gas shales, coal, and tight gas sands, as well as rocks considered as seals for nuclear waste repositories and strata for geological sequestration of CO₂. The use of He as routinely applied to measure porosity, permeability, and diffusivity may result in non-systematic errors because of the molecular sieving effect of the fine pore structure to larger molecules such as reservoir gases. Utilizing gases with larger adsorption potentials than He, such as N₂, and including all reservoir gases to measure porosity or permeability of rocks with high surface area is a viable alternative, but requires correcting for adsorption in the analyses. This study expands several approaches to measure permeability and diffusivity with considerations for gas adsorption, which has not been explicitly considered in previous studies. We present new models that explicitly correct for adsorption during pulse-decay measurements of core under reservoir conditions, as well as on crushed samples used to approximate permeability or diffusivity. We also present a method to determine permeability or diffusivity from on-site drill-core desorption test data as carried out to determine gas in place in coals or gas shales. Our new approach utilizes late-time data from experimental pressure-decay tests, which we show to be more reliable and theoretically (and practically) accurate than the early-time approach commonly used to estimate gas-transport properties.     

Property Rights versus "Public Necessity": A Perspective on Emergency Powers and the Supreme Court

Chief Justice Earl Warren once wrote that a free government is continuously "on trial for its life." ¹ And never are the foundations of constitutional liberties more fragile than in periods of emergency, when government invokes extraordinary powers. Invariably, emergency powers involve the immediate curtailment of some rights; at their extreme in martial law, they can warrant an entire suspension of normal civilian governmental functions, as well as full suspension of due-process guarantees. ² Once the constitutional fabric has been stretched to accommodate urgent public necessity in such situations, moreover, restoration to its earlier condition is not automatic or inevitable. On the contrary, as Justice Robert Jackson presciently warned, once the Supreme Court validates as constitutional the abridgement of essential rights during an emergency—and especially when the Court does so in relation to "the vague, undefined and undefinable 'war power'"—any principle that is thus articulated to justify such emergency action "then lies about like a loaded weapon ready for the hand of any authority that can bring forward a plausible claim of an urgent need." ³     

Hydrothermal convection in and around mineralized fault zones: insights from two- and three-dimensional numerical modeling applied to the Ashanti belt, Ghana

Among hydrogeological processes, free convection in faults has been cited as a possible cause of gold mineralization along major fault zones. Here, we investigate the effects of free convection to determine whether it can cause giant orogenic gold deposits and their regular spatial distribution along major fault/shear zones. The approach comprises: (i) coupled two- and three-dimensional numerical heat- and fluid-flow simulations of simplified geological models; and (ii) calculation of the rock alteration index (RAI) to delineate regions where precipitation/dissolution can occur. Then, comparing the deduced alteration patterns with temperature distribution, potential areas of gold mineralization, defined by T  > 200°C and RAI < 0, are predicted. The models are based on the orogenic Paleoproterozoic ore deposits of the Ashanti belt in western Africa. These deposits occur in the most permeable parts of the fault zone, where the lateral permeability contrast is the highest. For a simple geometry, with a fault zone adjacent to a sedimentary basin half as permeable, we note a transition from three-dimensional circulation within the fault to a two-dimensional convective pattern in the basin far from the fault. Moreover, whereas two-dimensional undulated isotherms dominate in the basin, three-dimensional corrugated isotherms result from the preferred convective pattern within the fault, thus enhancing a periodic distribution of thermal highs and lows. In our most elaborate three-dimensional model with an imposed lateral permeability gradient, the RAI distribution indicates that fluid circulation in fault zones gives rise to a spatial periodicity of alteration patterns consistent with field data.     

Supreme Court Activism in Economic Policy in the Waning Days of the New Deal: Interpreting the Fair Labor Standards Act, 1941–1946

Students of the Supreme Court universally agree that it made a dramatic shift in 1937. First, in West Coast Hotel Company v. Parrish, ¹ it retreated from the unbridled use of the Fourteenth Amendment's Due Process Clause to invalidate state economic regulatory legislation. Then, in National Labor Relations Board v. Jones and Laughlin Steel Corporation , ² the Justices widened the reach of congressional power under the Commerce Clause. This looser reading of the Commerce Clause was solidified in 1941 with United States v. Darby Lumber Company ³ and Wickard v. Filburn. ⁴ So decisive were these cases in dividing what went before from what came afterward that Bernard Schwartz has said, "The 1937 reversal marked the accession of what may be considered the second Hughes Court—so different was its jurisprudence from that of the Hughes Court that had preceded it." ⁵ Whereas the defining jurisprudence of the former had been close supervision of economic policy, the latter refused to second guess the economic wisdom of congressional (and state) regulatory initiatives. Alpheus T. Mason summarized Justice Harlan Fisk Stone's approach, which was indicative of the entire Court of this era, as one that would not say that "no economic legislation would ever violate constitutional restraints, [but that] … in this area the court's role would be strictly confined." ⁶ Confirming this approach, between 1937 and 1957 the Supreme Court struck down only four federal statutes as unconstitutional, none of which were economic in nature. ⁷     

The distribution and morphometry of lakes and reservoirs in British Columbia: a provincial inventory

An inventory of provincial lakes and reservoirs has been developed to characterise and assess the distribution and morphometry of standing water bodies in British Columbia. In the province, there are over 241,500 lakes and reservoirs greater than 1,000 m ² in size. These water bodies cover 2.37 percent of the province area and contain an estimated 521 km ³ of water (312 km ³ in natural lakes and 209 km ³ in reservoirs). A hypsometric relation suitable for order-of-magnitude estimates of lake volume from lake area is presented. Based on the distribution and morphometric attributes of lakes, several distinctive limnologic regions were identified, including the northeastern Alberta Plateau (highest proportion of circular lakes), the southwestern Alberta Plateau (lowest lake density/coverage and highest proportion of irregularly shaped lakes) and the Milbanke Strandflat (highest density of lakes). Observed regional and scale-related patterns in lake distribution/morphometry appear to be largely related to geomorphic controls, particularly tectonic and glacial history. Large-scale hydrologic implications of these standing water bodies and potential ecosystem/water resource management applications of the provincial inventory are also discussed.     

Get Your Tongue out of My Mouth 'Cause I'm Kissin' You Goodbye: The Politics of Ideas

The seminal work of Derthick and Quirk (1985 ) argues that public policy changes when the intellectual debate of ideas establishes that policy options are legitimate and current policies are ineffective. ¹ Overlooked in their otherwise exceptional analysis is the role that country music plays in the politics of ideas. ² The thesis of this article is that country music is a crucial part of the politics of ideas, and, in fact, many policy debates are resolved in country music well before the intellectual community of policy analysts reaches a consensus. This article will recap some of the key policy debates in which country music set the agenda, ³ established the key policy alternatives, or resolved the policy debate and permitted the adoption of public policy ( Anderson, 1994 ). ⁴     

PURPLE-DYE PRODUCTION IN LYCIA – RESULTS OF AN ARCHAEOZOOLOGICAL FIELD SURVEY IN ANDRIAKE (SOUTH-WEST TURKEY)

Summary.   In 2003 a substantial deposit of heavily fragmented shells of purple snails was by chance located at the site of Andriake, the ancient port of the Lycian city of Myra near Demre, in southern Turkey. The archaeozoological and archaeological record of a survey project in 2004 proved the identification of a large installation for the production of purple dye. Adjacent to a well preserved Hadrianic granary and partly superimposing the remains of a U-shaped building, the deposit is wrapped around three sides of a subterranean cistern. Apparently dating to the sixth century AD, the deposit contains approximately 300 m³ of shell debris. The unique chance of examining an undisturbed and well preserved manufacturing place for purple dye provides an excellent opportunity to investigate the processual particularities as well as the ecological impact of purple-dye production in antiquity.     

Justice Sandra Day O'Connor: The Framers' "First Woman"

Sandra Day O'Connor's appointment to the Supreme Court was a historic stride in American women's slow but determined march towards full equality. At our nation's birth, Abigail Adams urged her husband and other members of the Continental Congress to "Remember the Ladies" in their new government. ¹ "We know better than to repeal our Masculine systems," John Adams replied only half jokingly. ² More than two centuries would pass before a woman donned Supreme Court robes to help interpret the United States Constitution.