Preliminary 3–D geological model of the Kalgoorlie region,Yilgarn Craton,Western Australia,based on deep seismic‐reflection and potential‐field data

The granite‐greenstone terranes of the Eastern Goldfields Province, Yilgarn Craton, Western Australia, are a major Australian and world gold and nickel source. The Kalgoorlie region, in particular, hosts several world‐class gold deposits. To attempt to understand why these deposits occur where they do, it is important to understand the crustal architecture in the region and how the major mineral systems operate in this architecture. One way to understand these relationships is to develop a detailed 3–D geological model for the region. The best method to map the 3–D geometry of major geological structures is by acquisition and interpretation of seismic‐reflection profiles. To contribute to this aim, a grid of deep seismic‐reflection traverses was acquired in 1999 to examine the 3–D geometry of the region in an area including the Kalgoorlie mineral region and mineral fields to the north and west. This grid was tied to the 1991 regional deep seismic traverse and 1997 high‐resolution seismic profiles in the same region. The grid covers an area measuring approximately 50 km wide by 50 km long and extended to a depth of approximately 50 km (below the base of the crust in this region). The resulting 3–D geological model was further constrained by both surface geological data and geophysical interpretations, with the seismic interpretations themselves also constrained by gravity and magnetic modelling. The 3–D model was used to investigate the geometric relationships between the major faults and shear zones in the area, the relationship between the granite‐greenstone succession and the basement, and the spatial relationships between the greenstones and the granites. Interpretation of the grid of seismic lines and construction of the 3–D geological model confirmed the existence of the detachment surface and led to the recognition that the granite‐greenstone contact usually occurs at a much shallower level than the detachment. Also, west‐dipping faults in the vicinity of the Golden Mile, including the Abattoir Shear through to Boulder‐Lefroy Fault, appear to be more important than previously thought in controlling the structure of that area. An antiformal thrust stack occurs beneath a triangle zone centred on the Golden Mile. The Black Flag Group was deposited in a probable extensional setting, and late extension was also probably more important than previously thought. The granite‐gneiss domes were uplifted by the formation of antiformal thrust stacks at depth beneath them.     

Heat transfer and thermal boundary layers in high amplitude annulus waves

Abstract

The heat transfer by a rotating, differentially-heated annulus of fluid is measured throughout the high amplitude wave regime. Only Δr_w T was varied (although v(T₁₅ ).K(T₁₅ ) varied by 46%), and it is found that Nu = C₁(λ)Ra^? away from the symmetry and low amplitude to wave transition curves and this is independent of ω. (λ is the wavelength.) On the wave side of these transition curves a region exists in which Nu (symmetry) λ Nu λ C₁(λ)Ra^?. The local heat transfer rate also varies strongly with wave phase.

Using a selection of measured internal thermal fields in the steady, high amplitude wave regime, the side-wall thermal boundary layer structure is examined. It is found that Nu, = C₂·Gr₂ ^A2; both C ₂ and A ₂ are independent of ω and λ to first order. For the time mean profiles, A ₂ ≈ 0.25; in the high heat transfer portion of the wave A ₂ < ¼ and in the low heat transfer portion of the wave A ₂ > ?. These relations hold over most of the vertical extent of the side walls. The deviations of the boundary layers from the above behavior which occur on the remainder of the walls is illustrated. The average thicknesses of the wall boundary layers ∞ Ra^?¼ except in that phase of the wave in which the wall to mid-gap temperature difference is the largest.     

Searching for nanoflares

The evolution of hydrodynamic shocks generated by nanoflares in coronal loops is investigated in order to suggest requirements for future observations. The shocks produced by nanoflares of 10²⁴ ergs decay within a few seconds, which sets stringent requirements for Doppler or transverse velocity measurements. With feasible improvements, the Normal Incidence X-ray Telescope could observe the time-averaged emission from a nanoflare, however. We also consider collisions between shocks and show that these could also be observed.Operated by the Association of Universities for Research in Astronomy (AURA) under agreement with the Natinal Science Foundation.     

In search of the Australasian tektite source crater: The Tonle Sap hypothesis

Abstract— The source crater for Australasian tektites remains to be positively identified We suggest that Tonle Sap, a roughly oval lake in south-central Cambodia, may represent the remnant of that crater. The size of the lake (about 100 km × 35 km), location (Indochina), inferred geologic age (recent), and orientation of the lake, as well as the geographical distribution of tektites, are consistent with this suggestion. The elongated shape of the lake with its long axis pointing toward Australia may be the result of an oblique impact of a NW to SE-moving object a few km in diameter. The absence of a raised rim and a central peak may be related to a low impact angle, soft target rocks, or high post-impact erosion and sedimentation rates. The scarcity of Muong Nong-type (layered) tektites near Tonle Sap may be due to extensive post-impact alluvial deposition, which buried the tektites. The chemical composition of Upper Indosinias formation sandstones from Phnom Batheay was determined. There are significant differences between the composition of indochinite tektites and these rocks, which are thus unlikely to represent tektite source rocks.     

Hydraulic Performance of the Horizontal Reactive Media Treatment Well: Pilot and Numerical Study

This study is focused on a passive treatment system known as the horizontal reactive treatment well (HRX Well®) installed parallel to groundwater flow, which operates on the principle of flow focusing that results from the hydraulic conductivity (K) ratio of the well and aquifer media. Passive flow and capture in the HRX Well are described by simplified equations adapted from Darcy's Law. A field pilot-scale study (PSS) and numerical simulations using a finite element method (FEM) were conducted to verify the HRX Well concept and test the validity of the HRX Well-simplified equations. The hydraulic performance results from both studies were observed to be within a close agreement to the simplified equations and their hydraulic capture width approximately five times greater than the well diameter (0.20 m). Key parameters affecting capture included the aquifer thickness, well diameter, and permeability ratio of the HRX Well treatment media and aquifer material. During pilot testing, the HRX Well captured 39% of flow while representing 0.5% of the test pit cross-sectional volume, indicating that the well captures a substantial amount of surrounding groundwater. While uncertainty in the aquifer and well properties (porosity, K, well losses), including the effects of boundary conditions, may have caused minor differences in the results, data from this study indicate that the simplified equations are valid for the conceptual design of a field study. A full-scale HRX Well was installed at Site SS003 at Vanderberg Air Force Base, California, in July/August 2018 based on outcomes from this study.     

Efficient sampling and data reduction techniques for probabilistic seismic lifeline risk assessment

Probabilistic seismic risk assessment for spatially distributed lifelines is less straightforward than for individual structures. While procedures such as the ‘PEER framework’ have been developed for risk assessment of individual structures, these are not easily applicable to distributed lifeline systems, due to difficulties in describing ground‐motion intensity (e.g. spectral acceleration) over a region (in contrast to ground‐motion intensity at a single site, which is easily quantified using Probabilistic Seismic Hazard Analysis), and since the link between the ground‐motion intensities and lifeline performance is usually not available in closed form. As a result, Monte Carlo simulation (MCS) and its variants are well suited for characterizing ground motions and computing resulting losses to lifelines. This paper proposes a simulation‐based framework for developing a small but stochastically representative catalog of earthquake ground‐motion intensity maps that can be used for lifeline risk assessment. In this framework, Importance Sampling is used to preferentially sample ‘important’ ground‐motion intensity maps, and K‐Means Clustering is used to identify and combine redundant maps in order to obtain a small catalog. The effects of sampling and clustering are accounted for through a weighting on each remaining map, so that the resulting catalog is still a probabilistically correct representation. The feasibility of the proposed simulation framework is illustrated by using it to assess the seismic risk of a simplified model of the San Francisco Bay Area transportation network. A catalog of just 150 intensity maps is generated to represent hazard at 1038 sites from 10 regional fault segments causing earthquakes with magnitudes between five and eight. The risk estimates obtained using these maps are consistent with those obtained using conventional MCS utilizing many orders of magnitudes more ground‐motion intensity maps. Therefore, the proposed technique can be used to drastically reduce the computational expense of a simulation‐based risk assessment, without compromising the accuracy of the risk estimates. This will facilitate computationally intensive risk analysis of systems such as transportation networks. Finally, the study shows that the uncertainties in the ground‐motion intensities and the spatial correlations between ground‐motion intensities at various sites must be modeled in order to obtain unbiased estimates of lifeline risk. Copyright © 2010 John Wiley & Sons, Ltd.     

Successional changes in soil and hyporheic nitrogen fertility on an alluvial flood plain: implications for riparian vegetation

In floodplain primary succession, vegetation colonizes nitrogen-poor alluvial deposits and fertility improves as soil nitrogen accumulates over time. It is generally assumed that vegetation assimilates the vast majority of its nitrogen from the soil; however, recent studies have suggested that the hyporheic zone also may be an important nitrogen source. We investigated the potential relative importance of hyporheic nitrogen by comparing fertility indices, specifically total (TN), dissolved inorganic (DIN), potentially mineralizable (PMN) and ion exchange resin nitrogen (IERN) in both soils and the hyporheic zone at early, mid and late succession stands on an expansive river flood plain. We also constructed mesocosms to assess growth of cottonwood cuttings with access to soil and/or hyporheic water. We found TN and PMN increased from early to mid succession in both the soil (to 10 cm) and hyporheic zone (in a 10 cm layer). While TN, DIN and PMN were an order of magnitude higher in the soil than in the hyporheic zone, IERN was higher in the hyporheic zone, indicating that subsurface flow through the flood plain may be important in delivering nitrogen to the root zone. However, even when flux was added to the hyporheic PMN pool, nitrogen availability in the hyporheic zone (in a 10 cm layer) was vastly lower than soil PMN (to 10 cm). Further, the instantaneous standing stock of DIN in the surface soil alone was about equal to the sum of the DIN pool, the mean subsurface flux and the PMN pool in a 10 cm layer of hyporheic zone. In the mesocosm experiment, cottonwood cuttings with access to both soil and hyporheic water grew fastest; however, they also had the lowest foliar nitrogen concentrations, indicating that this was not due to greater nitrogen availability. In the field, nitrogen content of cottonwood foliage increased along with soil (but potentially hyporheic as well) nitrogen accumulation during succession, suggesting the vegetation responded to increasing nitrogen fertility. We conclude that at least on a per unit-volume basis, the hyporheic zone probably provides little nitrogen relative to the surface soil, except on new alluvial bars that characteristically are nitrogen poor. Therefore, the hyporheic zone is probably a much smaller nitrogen source for mature forests relative to the surface soil unless the vegetation exploits a much larger volume of the hyporheic zone than surface soil.     

The lower symmetric regime and its transition to waves in rotating annulus convection

Measurements of the temperature and zonal velocity fields which develop in a rotating annulus of fluid with an upper surface, differentially heated from the inner to outer cylinder, are described for the lower symmetric regime (small radial temperature differences). The temperature field is essentially conductive for moderate to large rotation rates, Ω (>1.0 sec⁻¹). The zonal velocity field is poorly approximated by the thermal wind equation.Measurements of the transition to waves from the lower symmetric regime at very large rotation rates are presented for positive and negative radial temperature differences. They suggest that the centrifugal buoyancy force and the free surface curvature may be important factors for the lower symmetric-wave transition at large Ω. By varying the stratification of the fluid over a range of 10³ independently of the radial temperature difference, Δr_wT, it is conclusively shown that several theories are correct in predicting that the lower symmetric transition is independent of the stratification at small Δr_wT > 0 for large enough Ω.     

Fish Remains from Homestead Cave and Lake Levels of the Past 13,000 Years in the Bonneville Basin

A late Quaternary ichthyofauna from Homestead Cave, Utah, provides a new source of information on lake history in the Bonneville basin. The fish, represented by 11 freshwater species, were accumulated between 11,200 and 1000 ¹⁴C yr B.P. by scavenging owls. The ⁸⁷Sr/⁸⁶Sr ratio of Lake Bonneville varied with its elevation; ⁸⁷Sr/⁸⁶Sr values of fish from the lowest stratum of the cave suggest they grew in a lake near the terminal Pleistocene Gilbert shoreline. In the lowest deposits, a decrease in fish size and an increase in species tolerant of higher salinities or temperatures suggest multiple die-offs associated with declining lake levels. An initial, catastrophic, post-Provo die-off occurred at 11,300–11,200 ¹⁴C yr B.P. and was followed by at least one rebound or recolonization of fish populations, but fish were gone from Lake Bonneville sometime before 10,400 ¹⁴C yr B.P. This evidence is inconsistent with previous inferences of a near desiccation of Lake Bonneville between 13,000 and 12,000 ¹⁴C yr B.P. Peaks in Gila atraria frequencies in the upper strata suggest the Great Salt Lake had highstands at 3400 and 1000 ¹⁴C yr B.P.