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1.
The Hikurangi Margin is a region of oblique subduction with northwest-dipping intermediate depth seismicity extending southwest from the Kermadec system to about 42°S. The current episode of subduction is at least 16–20 Ma old. The plate convergence rate varies along the margin from about 60 mm/a at the south end of the Kermadec Trench to about 45 mm/a at 42°S. The age of the Pacific lithosphere adjacent to the Hikurangi Trench is not known.The margin divides at about latitude 39°S into two quite dissimilar parts. The northern part has experienced andesitic volcanism for about 18 Ma, and back-arc extension in the last 4 Ma that has produced a back-arc basin onshore with high heaflow, thin crust and low upper-mantle seismic velocities. The extension appears to have arisen from a seawards migration of the Hikurangi Trench north of 39°S. Here the plate interface is thought to be currently uncoupled, as geodetic data indicate extension of the fore-arc basin, and historic earthquakes have not exceededM
s=7.South of 39°S there is no volcanism and a back-arc basin has been produced by downward flexure of the lithosphere due to strong coupling with the subducting plate. Heatflow in the basin is normal. Evidence for strong coupling comes from historic earthquakes of up to aboutM
s=8 and high rates of uplift on the southeast coast of the North Island.The reason for this division of the margin is not known but may be related to an inferred increase, from northeast to southwest, in the buoyancy of the Pacific lithosphere. 相似文献
2.
Daniel R. Block Euan Hague Winifred Curran Howard Rosing 《The Professional geographer》2018,70(2):284-290
Geographers have increasingly adopted community-based learning and research into their teaching and scholarly activities since Bunge and Harvey called for an applied public geography that is both useful and challenges societal inequalities. With few exceptions, however, there has been little discussion of methods for measuring this work. Many published assessments focus on the impacts of projects on students but overlook the impacts on community partners. Impacts on faculty and the larger university community are also often ignored. This article discusses literature on the evaluation of community–university research and service learning from a critical perspective. A discussion of service learning and community-based research (CBR) projects at two Chicago universities, DePaul and Chicago State, is presented. In both cases challenges were encountered to achieve full evaluation of projects, yet both included an evaluation of university and community partners that allowed for assessment of the projects’ value to all partners. 相似文献
3.
Summary On 29 October 1998, the RGS-IBG invited delegates to a conference entitled 'Social exclusion and the city', with Geoff Mulgan (founder of the think-tank Demos and a policy advisor to Prime Minister Blair) as the keynote speaker. The event was well attended, providing much useful insight into the government policies of the 'Social Exclusion Unit' and ongoing academic research into the processes and problems of 'social exclusion'. Here we provide some general observations on the conference and some suggestions regarding the possible directions of the debate on social exclusion and inclusion. 相似文献
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5.
Maciej J. Bojanowski Bogus?aw Bagiński Euan Clarkson Ray Macdonald Leszek Marynowski 《Contributions to Mineralogy and Petrology》2012,164(2):245-259
Zircon occurs in voids and cracks in phosphatic coprolites enclosed in siderite concretions in Mississippian shales near Edinburgh, Scotland. The zircon formed during hydrothermal alteration of early-diagenetic concretions and occurs as spherical aggregates of prismatic crystals, sometimes radiating. Vitrinite reflectance measurements indicate temperatures of ~270°C for the zircon-bearing concretions and the host shales. Molecular parameter values based on dibenzothiophene and phenanthrene distribution and occurrence of di- and tetra-hydro-products of polycyclic aromatic compounds suggest that the rocks experienced relatively high-temperature aqueous conditions related to hydrothermal fluids, perhaps associated with neighboring mafic intrusions. The zircon was dissolved from the concretions, transported in fluids, and reprecipitated in voids. This is the first record of the precipitation of authigenic zircon in sedimentary rock as a new phase, not as outgrowths. 相似文献
6.
Diatom-environmental relationships in 64 alkaline southeastern Ontario (Canada) lakes: a diatom-based model for water quality reconstructions 总被引：5，自引：0，他引：5
Lake eutrophication is a problem in many areas of Ontario, although the history of nutrient enrichment is poorly documented. The aim of this study was to construct a diatom-based transfer function to infer past phosphorus levels in Ontario lakes using paleolimnological analyses. The relationship between diatom assemblages and limnological conditions was explored from a survey of diatoms preserved in the surface sediments of 64 Southern Ontario lakes, spanning a total phosphorus gradient of 0.004 to 0.054 mg L-1. Over 420 diatom taxa were identified, 98 of which were sufficiently common to be considered in statistical analyses. Canonical correspondence analysis (CCA) determined that pH, ammonium, aluminum, spring total phosphorus (TP), strontium, total nitrogen (TN), maximum depth (MaxZ), chlorophyll a (Chla) and mean depth were significant variables in explaining the variance in the diatom species data. The environmental optima of common diatom taxa for the limnologically important variables (TP, pH, TN, MaxZ, Chla) were calculated using weighted averaging (WA) regression and calibration techniques, and transfer functions were generated. The diatom inference model for spring TP provided a robust reconstructive relationship (r2 = 0.637; RMSE = 0.007 mg L-1; r2
boot = 0.466; RMSEboot = 0.010 mg L-1). Other variables, including pH (r2 = 0.702; RMSE = 0.208; r2
boot = 0.485; RMSEboot = 0.234), TN (r2 = 0.574; RMSE = 0.0899 mg L-1; r2
boot = 0.380; RMSEboot = 0.127 mg L-1) and MaxZ (r2 = 0.554; RMSE = 1.05 m; r2
boot = 0.380; RMSEboot = 1.490 m), were also strong, indicating that they may also be reconstructed from fossil diatom communities. This study shows that it is possible to reliably infer lakewater TP and other limnological variables in alkaline Southern Ontario lakes using the WA technique. This method has the potential to aid rehabilitation programs, as it can provide water quality managers with the means to estimate pre-enrichment phosphorus concentrations and an indication of the onset and development of nutrient enrichment in a lake. 相似文献
7.
Eighteen lakes were added to a published training set of 46 British Columbia (BC) lakes in order to expand the original range of total phosphorus (TP) concentrations. Canonical correspondence analysis (CCA) was used to analyze the relationship between diatom assemblages and environmental variables. Specific conductivity and [TP] each explained significant (P0.05) directions of variance in the distribution of the diatoms. The relationship between diatom assemblages and [TP] was sufficiently strong to warrant the development of a weighted-averaging (WA) regression and calibration model that can be used to infer past trophic status from fossil diatom assemblages.The relationship between observed and inferred [TP] was not improved by the addition of more eutrophic lakes, however the [TP] range and the number of taxa used in the transfer function are now superior to the original model. Diatom species assemblages changed very little in lakes with TP concentrations greater than 85 µg 1–1, so we document the development of a model containing lakes with TP85 µg 1–1. The updated model uses 59 training lakes and covers a range of species optima from 6 to 41.9 µg 1–1 TP, and a total of 150 diatom taxa.The updated inference model provided a more realistic reconstruction of the anthropogenic history of a highly eutrophic BC lake. The model can now be used to infer past nutrient conditions in other BC lakes in order to assess changes in trophic status. 相似文献
8.
The Re-Os isotopic systematics of two ca. 2.7-Ga komatiite flows from Belingwe, Zimbabwe are examined. Rhenium and Os concentrations in these rocks are similar to concentrations in other Archean, Proterozoic, and Phanerozoic komatiites. Despite the excellent preservation of primary magmatic minerals, the Re-Os systematics of whole-rock samples of the komatiites show open-system behavior. Consistent model ages for several whole-rock samples suggest a disturbance to the system during the Proterozoic. Despite the open-system behavior in the whole rocks, Re-Os systematics for concentrates of primary magmatic olivine and spinel indicate generally closed-system behavior since the magmatic event that produced the rocks. Regression of the data for the mineral concentrates yields an age of 2721 ± 21 Ga, which is consistent with Pb-Pb and Sm-Nd ages that have been previously reported for the komatiites (Chauvel et al., 1993), and an initial 187Os/188Os ratio of 0.11140 ± 84 (γOs = +2.8 ± 0.8).The 2 to 3% enrichment in 187Os/188Os ratio of the mantle source of the komatiites, relative to the chondritic composition of the contemporaneous convecting upper mantle, most likely reflects either the incorporation of substantially older (≥ 4.2 Ga), Re-rich recycled mafic crust into the mantle source of the komatiites or the contribution of suprachondritic Os to the source from the putative 187Os-enriched outer core. The former interpretation would indicate the Hadean formation and recycling of mafic crust. The latter interpretation would require early formation of a substantial inner core followed by upwelling of a mantle plume from the core-mantle boundary, at least as far back as the Late Archean. Either interpretation requires large-scale mantle convection during the first half of Earth history. 相似文献
9.
Elizabeth E. Alexson Euan D. Reavie Richard P. Axler Sergiy V. Yemets Pavel A. Krasutsky Mark B. Edlund Robert W. Pillsbury Diane Desotelle 《Journal of Paleolimnology》2018,59(3):373-395
The St. Louis River Estuary (SLRE), a freshwater estuary bordering Duluth, Minnesota, Superior, Wisconsin, and the most western point of Lake Superior (46.74°, ? 92.13°), has a long history of human development since Euro-American settlement ~ 200 years ago. Due to degradation from logging, hydrologic modification, industrial practices, and untreated sewage, the SLRE was designated an Area of Concern in 1987. Action has been taken to restore water quality including the installation of the Western Lake Superior Sanitary District in 1978 to help remove beneficial use impairments. A better understanding of historical impacts and remediation is necessary to help document progress and knowledge gaps related to water quality, so a paleolimnological study of the SLRE was initiated. Various paleolimnological indicators (pigments, diatom communities, and diatom-inferred phosphorus) were analyzed from six cores taken throughout the SLRE and another from western Lake Superior. Reductions in eutrophic diatom taxa such as Cyclotella meneghiniana and Stephanodiscus after 1970 in certain cores suggest an improvement in water quality over the last 40 years. However, in cores taken from estuarine bay environments, persistence of eutrophic taxa such as Cyclostephanos dubius and Stephanodiscus binderanus indicate ongoing nutrient problems. Sedimentary pigments also indicate cyanobacteria increases in bays over the last two decades. Diatom model-inferred phosphorus and contemporary monitoring data suggest some of the problems associated with excess nutrient loads have been remediated, but modern conditions (internal phosphorus loading, changing climate) may be contributing to ongoing water quality impairments in some locations. The integrated record of biological, chemical, and physical indicators preserved in the sediments will aid state and federal agencies in determining where to target their resources. 相似文献
10.
Evidence for different scaling of earthquake source parameters for large earthquakes depending on faulting mechanism 总被引：2，自引：0，他引：2
Scaling relationships between seismic moment, rupture length, and rupture width have been examined. For this purpose, the data from several previous studies have been merged into a database containing more than 550 events. For large earthquakes, a dependence of scaling on faulting mechanism has been found. Whereas small and large dip-slip earthquakes scale in the same way, the self-similarity of earthquakes breaks down for large strike-slip events. Furthermore, no significant differences in scaling could be found between normal and reverse earthquakes and between earthquakes from different regions. Since the thickness of the seismogenic layer limits fault widths, most strike-slip earthquakes are limited to rupture widths of between 15 and 30 km while the rupture length is not limited. The aspect ratio of dip-slip earthquakes is similar for all earthquake sizes. Hence, the limitation in rupture width seems to control the maximum possible rupture length for these events. The different behaviour of strike-slip and dip-slip earthquakes can be explained by rupture dynamics and geological fault growth. If faults are segmented, with the thickness of the seismogenic layer controlling the length of each segment, strike-slip earthquakes might rupture connected segments more easily than dip-slip events, and thus could produce longer ruptures than dip-slip events of the same width 相似文献