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951.
Grant M. Cox Christopher J. Lewis Alan S. Collins Galen P. Halverson Fred Jourdan John Foden David Nettle Fayek Kattan 《Gondwana Research》2012,21(2-3):341-352
The Ad Dawadimi Terrane is an Ediacaran basin of the Arabian Nubian Shield (ANS), Saudi Arabia. This basin terrane is situated in the far eastern part of the ANS and represents the youngest accretion event of the exposed ANS. Therefore, the timing of events within the basin is key to understanding both the closure of the Mozambique Ocean and the amalgamation of Gondwana along the northern East African Orogen. Here we present U/Pb detrital zircon data for the Abt Formation, the principle basin sediments of the Ad Dawadimi Terrane, along with 40Ar/39Ar ages on muscovite and whole rock Sm/Nd data. These data indicate that deep-water deposition in the Abt Basin did not end until after ca. 620 Ma and that deformation and greenschist-facies metamorphism of the Abt Formation occurred at 620 ± 3 (2σ) Ma along an active margin. This is the youngest terrane amalgamation event reported so far in the Arabian–Nubian Shield, but we suggest even younger sutures lie further east beneath the Phanerozoic cover of eastern Saudi Arabia. Our results suggest that the Ediacaran basins of the eastern ANS were not part of the Huqf basin in Oman, which was instead part of a passive margin of Neoproterozoic India, separated from the active margin of Africa by the Mozambique Ocean that probably did not close until the late Ediacaran or early Cambrian. 相似文献
952.
953.
Assessing spatial variability in soil characteristics with geographically weighted principal components analysis 总被引:1,自引:0,他引:1
Sandeep?KumarEmail author Rattan?Lal Christopher?D.?Lloyd 《Computational Geosciences》2012,16(3):827-835
Dimensionality reduction methods such as principal components analysis (PCA) provide a means of identifying trends in soil
characteristics which may be represented by a wide range of variables. However, these characteristics may be highly spatially
variable and so the results from PCA represent, in some sense, an “average” of locally distinct characteristics. One approach
to account for these local differences is to introduce a geographical weighting scheme into the PCA process. In this paper,
such an approach is assessed in the exploration of soil characteristics in the state of Pennsylvania, USA. Data from 878 georeferenced
soil profiles which include different soil parameters (n = 12) were extracted from the National Soil Survey Center database. Where data are parts of compositions (e.g., percentages
of sand, silt, and clay), analysis using raw data is not appropriate and such data were transformed using log ratios (specifically,
balances). Single variables (i.e., those which are not parts of compositions) were logged. The first two principal components
explain over 50% of the variance. The mapped values suggest marked spatial variation in soil characteristics, but it is not
possible to assess which of these variables explain most variation in particular regions from the simple maps of raw variables.
Geographically weighted PCA (GWPCA) provides additional information which is obscured by PCA, and it also provides a set of
component scores and loadings at all data locations. The soil variable with the largest loading at most locations of Pennsylvania
is the logged base saturation (BSln), and this supports the findings of the conventional PCA analysis. While BSln loads most
highly in most of the eastern third, the middle and the south west of the state, the northwest is less spatially consistent
in terms of the variables which explain most variation. For GWPC 1, the variable with the second largest loading at most locations
(i.e., primarily the south and west) is CEC.B1 (the log ratio of Ca, Mg, and Na to K and EXACID), while CEC.B2 (the log ratio
of Ca and Mg to Na), pHln (logged pH) and BSln dominate in other areas. The GWPCA results suggest that there is marked spatial
variation in multivariate soil characteristics across Pennsylvania state and that results from standard PCA obscure this considerable
variation. 相似文献
954.
Clinton Rissmann Bruce Christenson Cynthia Werner Matthew Leybourne Jim Cole Darren Gravley 《Applied Geochemistry》2012
Carbon dioxide emissions and heat flow have been determined from the Ohaaki hydrothermal field, Taupo Volcanic Zone (TVZ), New Zealand following 20 a of production (116 MWe). Soil CO2 degassing was quantified with 2663 CO2 flux measurements using the accumulation chamber method, and 2563 soil temperatures were measured and converted to equivalent heat flow (W m−2) using published soil temperature heat flow functions. Both CO2 flux and heat flow were analysed statistically and then modelled using 500 sequential Gaussian simulations. Forty subsoil CO2 gas samples were also analysed for stable C isotopes. Following 20 a of production, current CO2 emissions equated to 111 ± 6.7 T/d. Observed heat flow was 70 ± 6.4 MW, compared with a pre-production value of 122 MW. This 52 MW reduction in surface heat flow is due to production-induced drying up of all alkali–Cl outflows (61.5 MW) and steam-heated pools (8.6 MW) within the Ohaaki West thermal area (OHW). The drying up of all alkali–Cl outflows at Ohaaki means that the soil zone is now the major natural pathway of heat release from the high-temperature reservoir. On the other hand, a net gain in thermal ground heat flow of 18 MW (from 25 MW to 43.3 ± 5 MW) at OHW is associated with permeability increases resulting from surface unit fracturing by production-induced ground subsidence. The Ohaaki East (OHE) thermal area showed no change in distribution of shallow and deep soil temperature contours despite 20 a of production, with an observed heat flow of 26.7 ± 3 MW and a CO2 emission rate of 39 ± 3 T/d. The negligible change in the thermal status of the OHE thermal area is attributed to the low permeability of the reservoir beneath this area, which has limited production (mass extraction) and sheltered the area from the pressure decline within the main reservoir. Chemistry suggests that although alkali–Cl outflows once contributed significantly to the natural surface heat flow (∼50%) they contributed little (<1%) to pre-production CO2 emissions due to the loss of >99% of the original CO2 content due to depressurisation and boiling as the fluids ascended to the surface. Consequently, the soil has persisted as the major (99%) pathway of CO2 release to the atmosphere from the high temperature reservoir at Ohaaki. The CO2 flux and heat flow surveys indicate that despite 20 a of production the variability in location, spatial extent and magnitude of CO2 flux remains consistent with established geochemical and geophysical models of the Ohaaki Field. At both OHW and OHE carbon isotopic analyses of soil gas indicate a two-stage fractionation process for moderate-flux (>60 g m−2 d−1) sites; boiling during fluid ascent within the underlying reservoir and isotopic enrichment as CO2 diffuses through porous media of the soil zone. For high-flux sites (>300 g m−2 d−1), the δ13CO2 signature (−7.4 ± 0.3‰ OHW and −6.5 ± 0.6‰ OHE) is unaffected by near-surface (soil zone) fractionation processes and reflects the composition of the boiled magmatic CO2 source for each respective upflow. Flux thresholds of <30 g m−2 d−1 for purely diffusive gas transport, between 30 and 300 g m−2 d−1 for combined diffusive–advective transport, and ?300 g m−2 d−1 for purely advective gas transport at Ohaaki were assigned. δ13CO2 values and cumulative probability plots of CO2 flux data both identified a threshold of ∼15 g m−2 d−1 by which background (atmospheric and soil respired) CO2 may be differentiated from hydrothermal CO2. 相似文献
955.
Evaluating roadside rockmasses for rockfall hazards using LiDAR data: optimizing data collection and processing protocols 总被引:3,自引:0,他引:3
Matthew J. Lato Mark S. Diederichs D. Jean Hutchinson Rob Harrap 《Natural Hazards》2012,60(3):831-864
Highways and railroads situated within rugged terrain are often subjected to the hazard of rockfalls. The task of assessing
roadside rockmasses for potential hazards typically involves an on-site visual investigation of the rockmass by an engineer
or geologist. At that time, numerous parameters associated with discontinuity orientations and spacing, block size (volume)
and shape distributions, slope geometry, and ditch profile are either measured or estimated. Measurements are typically tallied
according to a formal hazard rating system, and a hazard level is determined for the site. This methodology often involves
direct exposure of the evaluating engineer to the hazard and can also create a potentially non-unique record of the assessed
slope based on the skill, knowledge and background of the evaluating engineer. Light Detection and Ranging (LiDAR)–based technologies
have the capability to produce spatially accurate, high-resolution digital models of physical objects, known as point clouds.
Mobile terrestrial LiDAR equipment can collect, at traffic speed, roadside data along highways and rail lines, scanning continual
distances of hundreds of kilometres per day. Through the use of mobile terrestrial LiDAR, in conjunction with airborne and
static systems for problem areas, rockfall hazard analysis workflows can be modified and optimized to produce minimally biased,
repeatable results. Traditional rockfall hazard analysis inputs include two distinct, but related sets of variables related
to geological or geometric control. Geologically controlled inputs to hazard rating systems include kinematic stability (joint
identification/orientation) and rock block shape and size distributions. Geometrically controlled inputs include outcrop shape
and size, road, ditch and outcrop profile, road curvature and vehicle line of sight. Inputs from both categories can be extracted
or calculated from LiDAR data, although there are some limitations and special sampling and processing considerations related
to structural character of the rockmass, as detailed in this paper. 相似文献
956.
Matthew S. HUBER Ludovic FERRIÈRE Anna LOSIAK Christian KOEBERL 《Meteoritics & planetary science》2011,46(9):1418-1424
Abstract– Planar deformation features (PDFs) in quartz, one of the most commonly used diagnostic indicators of shock metamorphism, are planes of amorphous material that follow crystallographic orientations, and can thus be distinguished from non‐shock‐induced fractures in quartz. The process of indexing data for PDFs from universal‐stage measurements has traditionally been performed using a manual graphical method, a time‐consuming process in which errors can easily be introduced. A mathematical method and computer algorithm, which we call the Automated Numerical Index Executor (ANIE) program for indexing PDFs, was produced, and is presented here. The ANIE program is more accurate and faster than the manual graphical determination of Miller–Bravais indices, as it allows control of the exact error used in the calculation and removal of human error from the process. 相似文献
957.
Matthew R. M. IZAWA Roberta L. FLEMMING Neil R. BANERJEE Philip J. A. McCAUSLAND 《Meteoritics & planetary science》2011,46(5):638-651
Abstract– A new method for assessing the shock stage of enstatite chondrites has been developed, using in situ micro‐X‐ray diffraction (μXRD) to measure the full width at half maximum (FWHMχ) of peak intensity distributed along the direction of the Debye rings, or chi angle (χ), corresponding to individual lattice reflections in two‐dimensional XRD patterns. This μXRD technique differs from previous XRD shock characterization methods: it does not require single crystals or powders. In situ μXRD has been applied to polished thin sections and whole‐rock meteorite samples. Three frequently observed orthoenstatite reflections were measured: (020), (610), and (131); these were selected as they did not overlap with diffraction lines from other phases. Enstatite chondrites are commonly fine grained, stained or darkened by weathering, shock‐induced oxidation, and metal/sulfide inclusions; furthermore, most E chondrites have little olivine or plagioclase. These characteristics inhibit transmitted‐light petrography, nevertheless, shock stages have been assigned MacAlpine Hills (MAC) 02837 (EL3) S3, Pecora Escarpment (PCA) 91020 (EL3) S5, MAC 02747 (EL4) S4, Thiel Mountains (TIL) 91714 (EL5) S2, Allan Hills (ALHA) 81021 (EL6) S2, Elephant Moraine (EET) 87746 (EH3) S3, Meteorite Hills (MET) 00783 (EH4) S4, EET 96135 (EH4–5) S2, Lewis Cliff (LEW) 88180 (EH5) S2, Queen Alexandra Range (QUE) 94204 (EH7) S2, LaPaz Icefield (LAP) 02225 (EH impact melt) S1; for the six with published shock stages, there is agreement with the published classification. FWHMχ plotted against petrographic shock stage demonstrates positive linear correlation. FWHMχ ranges corresponding to shock stages were assigned as follows: S1 < 0.7°, S2 = 0.7–1.2°, S3 = 1.2–2.3°, S4 = 2.3–3.5°, S5 > 3.5°, S6—not measured. Slabs of Abee (EH impact‐melt breccia), and Northwest Africa (NWA) 2212 (EL6) were examined using μXRD alone; FWHMχ values place both in the S2 range, consistent with literature values. Micro‐XRD analysis may be applicable to other shocked orthopyroxene‐bearing rocks. 相似文献
958.
Christina?J.?ManningEmail author Matthew?F.?Thirlwall 《Contributions to Mineralogy and Petrology》2014,167(1):959
The off-rift central volcano of Öræfajökull has very distinctive EM1-like isotopic compositions compared with other Icelandic lavas. New Pb–Nd–Sr isotopic data from Öræfajökull show strong correlations interpreted as a result of mixing. End-members are a depleted mantle source incorporating 0.5 % subduction-processed sediment and a mantle source with an isotopic signature similar to lavas of the Reykjanes Peninsula. Sr–Nd–Pb isotopic correlations of Icelandic Eastern Rift Zone (ERZ) lavas are almost completely distinct from those of the Reykjanes Peninsula and the Western Rift Zone (WRZ) and require a high-207Pb/204Pb, low-143Nd/144Nd end-member that resembles Öræfajökull compositions, which is very distinct from the enriched end-members suggested for the Reykjanes Peninsula and the WRZ. Given the similar depth and degree of melting at rift zones, variation in the observed enriched end-members between rift zones must indicate spatial variations in enriched mantle sources within the shallow mantle under Iceland rather than purely mixing of melts from a bi-lithological mantle. This is consistent with observations that the ERZ lavas erupted closest to Öræfajökull exhibit the most Öræfajökull-like isotopic compositions, implying that a homogenised Öræfajökull source with positive ?207Pb is focused under the Öræfajökull centre and its associated flank zone. This then mixes laterally with the dominant negative-?207Pb ERZ mantle source. Like Reykjanes Peninsula and WRZ lavas, the ERZ mantle source has strongly negative Δ207Pb and low K/Nb (<170), and these provide evidence for a recycled oceanic crust contribution. The range in 206Pb/204Pb in mantle sources with negative Δ207Pb was probably generated by heterogeneity in 206Pb/204Pb and μ in the recycled oceanic crust, which is the dominant source of incompatible elements in Icelandic lavas. 相似文献
959.
Accurate sea-level reconstruction is critical in understanding the drivers of coastal evolution. Inliers of shallow marine limestone and aeolianite are exposed as zeugen (carbonate-capped erosional remnants) on the southern coast of the Arabian/Persian Gulf. These have generally been accepted as evidence of a eustatically driven, last-interglacial relative sea-level highstand preceded by a penultimate glacial-age lowstand. Instead, recent optically stimulated luminescence (OSL) dating suggests a last glacial age for these deposits, requiring > 100 m of uplift since the last glacial maximum in order to keep pace with eustatic sea-level rise and implying the need for a wholesale revision of tectonic, stratigraphic and sea-level histories of the Gulf. These two hypotheses have radically different implications for regional neotectonics and land–sea distribution histories. Here we test these hypotheses using OSL dating of the zeugen formations. These new ages are remarkably consistent with earlier interpretations of the formations being last interglacial or older in age, showing that tectonic movements are negligible and eustatic sea-level variations are responsible for local sea-level changes in the Gulf. The cause of the large age differences between recent studies is unclear, although it appears related to large differences in the measured accumulated dose in different OSL samples. 相似文献
960.
The Bolla Bollana Formation is an exceptionally thick (ca 1500 m), rift‐related sedimentary succession cropping out in the northern Flinders Ranges, South Australia, which was deposited during the Sturtian (mid Cryogenian) glaciation. Lithofacies analysis reveals three distinct facies associations which chart changing depositional styles on an ice‐sourced subaqueous fan system. The diamictite facies association is dominant, and comprises both massive and stratified varieties with a range of clast compositions and textures, arranged into thick beds (1 to 20 m), representing stacked, ice‐proximal glaciogenic debris‐flow deposits. A channel belt facies association, most commonly consisting of normally graded conglomerates and sandstones, displays scour and fill structure of ca 10 m width and 1 to 3 m depth: these strata are interpreted as channelized turbidites. Rare mud‐filled channels in this facies association bear glacially striated lonestones. Finally, a sheet heterolithics facies association contains a range of conglomerates through sandstones to silty shales arranged into clear, normally graded cycles from the lamina to bed scale. These record a variety of non‐channelized turbidites, probably occupying distal and/or inter‐channel locations on the subaqueous fan. Coarsening and thickening‐up cycles, capped by dolomicrites or mudstones, are indicative of lobe build out and abandonment, potentially as a result of ice lobe advance and stagnation. Dropstones, recognized by downwarped and punctured laminae beneath pebbles to boulders in shale, or in delicate climbing ripple cross‐laminated siltstones, are clearly indicative of ice rafting. The co‐occurrence of ice‐rafted debris and striated lonestones strongly supports a glaciogenic sediment source for the diamictites. Comparison to Pleistocene analogues enables an interpretation as a trough mouth fan, most probably deposited leeward of a palaeo‐ice stream. Beyond emphasizing the highly dynamic nature of Sturtian ice sheets, these interpretations testify to the oldest trough mouth fan recorded to date. 相似文献