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91.
Constraints on the timing and conditions of high‐grade metamorphism,charnockite formation and fluid–rock interaction in the Trivandrum Block,southern India
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E. Blereau C. Clark R. J. M. Taylor T. E. Johnson I. C. W. Fitzsimons M. Santosh 《Journal of Metamorphic Geology》2016,34(6):527-549
Incipient charnockites have been widely used as evidence for the infiltration of CO2‐rich fluids driving dehydration of the lower crust. Rocks exposed at Kakkod quarry in the Trivandrum Block of southern India allow for a thorough investigation of the metamorphic evolution by preserving not only orthopyroxene‐bearing charnockite patches in a host garnet–biotite felsic gneiss, but also layers of garnet–sillimanite metapelite gneiss. Thermodynamic phase equilibria modelling of all three bulk compositions indicates consistent peak‐metamorphic conditions of 830–925 °C and 6–9 kbar with retrograde evolution involving suprasolidus decompression at high temperature. These models suggest that orthopyroxene was most likely stabilized close to the metamorphic peak as a result of small compositional heterogeneities in the host garnet–biotite gneiss. There is insufficient evidence to determine whether the heterogeneities were inherited from the protolith or introduced during syn‐metamorphic fluid flow. U–Pb geochronology of monazite and zircon from all three rock types constrains the peak of metamorphism and orthopyroxene growth to have occurred between the onset of high‐grade metamorphism at c. 590 Ma and the onset of melt crystallization at c. 540 Ma. The majority of metamorphic zircon growth occurred during protracted melt crystallization between c. 540 and 510 Ma. Melt crystallization was followed by the influx of aqueous, alkali‐rich fluids likely derived from melts crystallizing at depth. This late fluid flow led to retrogression of orthopyroxene, the observed outcrop pattern and to the textural and isotopic modification of monazite grains at c. 525–490 Ma. 相似文献
92.
Jeremy C. Ely Chris D. Clark Richard C. A. Hindmarsh Anna L. C. Hughes Sarah L. Greenwood Sarah L. Bradley Edward Gasson Lauren Gregoire Niall Gandy Chris R. Stokes David Small 《第四纪科学杂志》2021,36(5):946-960
Palaeo-ice sheets are important analogues for understanding contemporary ice sheets, offering a record of ice sheet behaviour that spans millennia. There are two main approaches to reconstructing palaeo-ice sheets. Empirical reconstructions use the available glacial geological and chronological evidence to estimate ice sheet extent and dynamics but lack direct consideration of ice physics. In contrast, numerically modelled simulations implement ice physics, but often lack direct quantitative comparison with empirical evidence. Despite being long identified as a fruitful scientific endeavour, few ice sheet reconstructions attempt to reconcile the empirical and model-based approaches. To achieve this goal, model-data comparison procedures are required. Here, we compare three numerically modelled simulations of the former British–Irish Ice Sheet with the following lines of evidence: (a) position and shape of former margin positions, recorded by moraines; (b) former ice-flow direction and flow-switching, recorded by flowsets of subglacial bedforms; and (c) the timing of ice-free conditions, recorded by geochronological data. These model–data comparisons provide a useful framework for quantifying the degree of fit between numerical model simulations and empirical constraints. Such tools are vital for reconciling numerical modelling and empirical evidence, the combination of which will lead to more robust palaeo-ice sheet reconstructions with greater explicative and ultimately predictive power. 相似文献
93.
Soil cracking effects on hydrological and erosive processes: a study case in Mediterranean cultivated vertisols
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Nesrine Inoubli Damien Raclot Roger Moussa Hamadi Habaieb Yves Le Bissonnais 《水文研究》2016,30(22):4154-4167
Shrink–swell soils, such as those in a Mediterranean climate regime, can cause changes in terms of hydrological and erosive responses because of the changing soil water storage conditions. Only a limited number of long‐term studies have focused on the impacts on both hydrological and erosive responses and their interactions in an agricultural environment. In this context, this study aims to document the dynamics of cracks, runoff and soil erosion within a small Mediterranean cultivated catchment and to quantify the influence of crack processes on the water and sediment supplied to a reservoir located at the catchment outlet using water and sediment measurements at a cultivated field outlet as baseline. Detailed monitoring of the presence of topsoil cracks was conducted within the Kamech catchment (ORE OMERE, Tunisia), and runoff and suspended sediment loads were continuously measured over a long period of time (2005–2012) at the outlets of a field (1.32 ha) and a catchment (263 ha). Analysis of the data showed that topsoil cracks were open approximately half of the year and that the rainfall regime and water table level conditions locally control the seasonal cracking dynamics. Topsoil cracks appeared to seriously affect the generation of runoff and sediment concentrations and, consequently, sediment yields, with similar dynamics observed at the field and catchment outlets. A similar time lag in the seasonality between water and sediment delivery was observed at these two scales: although the runoff rates were globally low during the presence of topsoil cracks, most sediment transport occurred during this period associated with very high sediment concentrations. This study underlines the importance of a good prediction of runoff during the presence of cracks for reservoir siltation considerations. In this context, the prediction of cracking effects on runoff and soil erosion is a key factor for the development of effective soil and water management strategies and downstream reservoir preservation. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
94.
Phase equilibria modelling and LASS monazite petrochronology: P–T–t constraints on the evolution of the Priest River core complex,northern Idaho
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L. M. Stevens J. A. Baldwin J. M. Cottle A. R. C. Kylander‐Clark 《Journal of Metamorphic Geology》2015,33(4):385-411
Phase equilibria modelling, laser‐ablation split‐stream (LASS)‐ICP‐MS petrochronology and garnet trace‐element geochemistry are integrated to constrain the P–T–t history of the footwall of the Priest River metamorphic core complex, northern Idaho. Metapelitic, migmatitic gneisses of the Hauser Lake Gneiss contain the peak assemblage garnet + sillimanite + biotite ± muscovite + plagioclase + K‐feldspar ± rutile ± ilmenite + quartz. Interpreted P–T paths predict maximum pressures and peak metamorphic temperatures of ~9.6–10.3 kbar and ~785–790 °C. Monazite and xenotime 208Pb/232Th dates from porphyroblast inclusions indicate that metamorphism occurred at c. 74–54 Ma. Dates from HREE‐depleted monazite formed during prograde growth constrain peak metamorphism at c. 64 Ma near the centre of the complex, while dates from HREE‐enriched monazite constrain the timing of garnet breakdown during near‐isothermal decompression at c. 60–57 Ma. Near‐isothermal decompression to ~5.0–4.4 kbar was followed by cooling and further decompression. The youngest, HREE‐enriched monazite records leucosome crystallization at mid‐crustal levels c. 54–44 Ma. The northernmost sample records regional metamorphism during the emplacement of the Selkirk igneous complex (c. 94–81 Ma), Cretaceous–Tertiary metamorphism and limited Eocene exhumation. Similarities between the Priest River complex and other complexes of the northern North American Cordillera suggest shared regional metamorphic and exhumation histories; however, in contrast to complexes to the north, the Priest River contains less partial melt and no evidence for diapiric exhumation. Improved constraints on metamorphism, deformation, anatexis and exhumation provide greater insight into the initiation and evolution of metamorphic core complexes in the northern Cordillera, and in similar tectonic settings elsewhere. 相似文献
95.
Many studies have investigated the exchange processes that occur between rivers and groundwater systems and have successfully quantified the water fluxes involved. Specifically, these exchange processes include hyporheic exchange, river–aquifer exchange (groundwater discharge and river loss) and bank storage exchange. Remarkably, there are relatively few examples of field studies where more than one exchange process is quantified, and as a consequence, the relationships between them are not well understood. To compare the relative magnitudes of these common exchange processes, we have collected data from 54 studies that have quantified one or more of these exchange flux types. Each flux value is plotted against river discharge at the time of measurement to allow the different exchange flux types to be compared. We show that there are positive relationships between the magnitude of each exchange flux type and increasing river discharge across the different studies. For every one order of magnitude increase in river discharge, the hyporheic, river–aquifer and bank storage exchange fluxes increase by factors of 2.7, 2.9 and 2.5, respectively. On average, hyporheic exchange fluxes are almost an order of magnitude greater than river–aquifer exchange fluxes, which are, in turn, approximately four times greater than bank storage exchange fluxes for the same river discharge. Unless measurement approaches that can distinguish between different types of exchange flux are used, there is potential for hyporheic exchange fluxes to be misinterpreted as river–aquifer exchange fluxes, with possible implications for water resource management decisions. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
96.
Significance of post‐peak metamorphic reaction microstructures in the ultrahigh temperature Eastern Ghats Province,India
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David E. Kelsey Laura J. Morrissey Martin Hand Chris Clark Renee Tamblyn Adrian A. Gaehl Sarah Marshall 《Journal of Metamorphic Geology》2017,35(9):1081-1109
Ultrahigh temperature (UHT) granulites in the Eastern Ghats Province (EGP) have a complex P–T–t history. We review the P–T histories of UHT metamorphism in the EGP and use that as a framework for investigating the P–T–t history of Mg–Al‐rich granulites from Anakapalle, with the express purpose of trying to reconcile the down‐pressure‐dominated P–T path with other UHT localities in the EGP. Mafic granulite that is host to Mg–Al‐rich metasedimentary granulites at Anakapalle has a protolith age of c. 1,580 Ma. Mg–Al‐rich metasedimentary granulites within the mafic granulite at Anakapalle were metamorphosed at UHT conditions during tectonism at 960–875 Ma, meaning that the UHT metamorphism was not the result of contact metamorphism from emplacement of the host mafic rock. Reworking occurred during the Pan‐African (c. 600–500 Ma) event, and is interpreted to have produced hydrous assemblages that overprint the post‐peak high‐T retrograde assemblages. In contrast to rocks elsewhere in the EGP that developed post‐peak cordierite, the metasedimentary granulites at Anakapalle developed post‐peak, generation ‘2’ reaction products that are cordierite‐absent and nominally anhydrous. Therefore, rocks at Anakapalle offer the unique opportunity to quantify the pressure drop that occurred during so‐called M2 that affected the EGP. We argue that M2 is either a continuation of M1 and that the overall P–T path shape is a complex counter‐clockwise loop, or that M1 is an up‐temperature counter‐clockwise deviation superimposed on the M2 path. Therefore, rather than the rocks at Anakapalle having a metamorphic history that is apparently anomalous from the rest of the EGP, we interpret that other previously studied localities in the EGP record a different part of the same P–T path history as Anakapalle, but do not preserve a significant record of pressure decrease. This is due either to the inability of refractory rocks to extensively react to produce a rich mineralogical record of pressure decrease, or because the earlier high‐P part of the rocks history was erased by the M1 loop. Irrespective of the specific scenario, models for the tectonic evolution of the EGP must take the substantial pressure decrease during M2 into account, as it is probable the P–T record at Anakapalle is a reflection of tectonics affecting the entire province. 相似文献
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100.
Limited differences among habitats in deep‐sea macro‐infaunal communities off New Zealand: implications for their vulnerability to anthropogenic disturbance
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Daniel Leduc Ashley A. Rowden Malcolm R. Clark David A. Bowden Andrew R. Thurber 《Marine Ecology》2016,37(4):845-866
The spread of human activities into the deep sea may pose a high risk to benthic communities and affect ecosystem integrity. The deep sea is characterized by physical and biological heterogeneity and different habitat types are likely to differ in their vulnerability to anthropogenic impacts. However, across‐habitat comparisons are rare, and no comprehensive ecological risk assessment has yet been developed. To address this gap in our knowledge, we compared macro‐infaunal community structure in four habitats (slope, canyons, seamounts and methane seeps) at depths between 700 and 1500 m in the Hikurangi Margin and Bay of Plenty regions off New Zealand. The most striking contrast in community structure was between the two study regions, due to an order of magnitude difference in macro‐infaunal abundance that we believe was caused by differences in surface productivity and food availability at the sea bed. We found differences in structural and functional attributes of macro‐infaunal communities among some habitats in the Hikurangi Margin (slope, canyon and seep), but not in the Bay of Plenty. We posit that differences between canyon and slope communities on the Hikurangi Margin are due to enhanced food availability inside canyons compared with adjacent slope habitats. Seep communities were characterized by elevated abundance of both symbiont‐bearing and heterotrophic taxa, and were the most distinct, and variable, among the habitats that we considered on the Hikurangi Margin. Communities of seamounts were not distinct from slope or canyon communities on the Hikurangi Margin, probably reflecting similar environmental conditions in these habitats. The communities of deep‐sea canyon and seep habitats on the Hikurangi Margin were sufficiently dissimilar from each other and from slope habitats to warrant separate management consideration. By contrast, the low dissimilarity between communities of canyon and slope habitats in the Bay of Plenty suggests that habitat‐based management is not required in this region, for macro‐infauna at least. Although the two study regions share similar species pools, populations of the Hikurangi Margin region may be less vulnerable than the sparser populations of the Bay of Plenty due to the higher availability of potential colonizers and faster population growth. Thus regions, and habitats in some regions, should be subject to separate ecological risk assessment to help identify the key risks and consequences of human activities, and to inform options for reducing or mitigating impacts. 相似文献