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The thermal structure of subduction zones exerts a major influence on deep-seated mechanical and chemical processes controlling arc magmatism, seismicity, and global element cycles. Accretionary complexes exposed inland may comprise tectonic blocks with contrasting pressure–temperature (P–T) histories, making it possible to investigate the dynamics and thermal evolution of former subduction interfaces. With this aim, we present new Lu–Hf geochronological results for mafic rocks of the Halilba?? Complex (Anatolia) that evolved along different thermal gradients. Samples include a lawsonite–epidote blueschist, a lawsonite–epidote eclogite, and an epidote eclogite (all with counter-clockwise P–T paths), a prograde lawsonite blueschist with a “hairpin”-type P–T path, and a garnet amphibolite from the overlying sub-ophiolitic metamorphic sole. Equilibrium phase diagrams suggest that the garnet amphibolite formed at ~0.6–0.7 GPa and 800–850 °C, whereas the prograde lawsonite blueschist records burial from 2.1 GPa and 420 °C to 2.6 GPa and 520 °C. Well-defined Lu–Hf isochrons were obtained for the epidote eclogite (92.38 ± 0.22 Ma) and the lawsonite–epidote blueschist (90.19 ± 0.54 Ma), suggesting rapid garnet growth. The lawsonite–epidote eclogite (87.30 ± 0.39 Ma) and the prograde lawsonite blueschist (ca. 86 Ma) are younger, whereas the garnet amphibolite (104.5 ± 3.5 Ma) is older. Our data reveal a consistent trend of progressively decreasing geothermal gradient from granulite-facies conditions at ~104 Ma to the epidote-eclogite facies around 92 Ma, and the lawsonite blueschist-facies between 90 Ma and 86 Ma. Three Lu–Hf garnet dates (between 92 Ma and 87 Ma) weighted toward the growth of post-peak rims (as indicated by Lu distribution in garnet) suggest that the HP/LT rocks were exhumed continuously and not episodically. We infer that HP/LT metamorphic rocks within the Halilba?? Complex were subjected to continuous return flow, with “warm” rocks being exhumed during the tectonic burial of “cold” ones. Our results, combined with regional geological constraints, allow us to speculate that subduction started at a transform fault near a mid-oceanic spreading centre. Following its formation, this ancient subduction interface evolved thermally over more than 15 Myr, most likely as a result of heat dissipation rather than crustal underplating.  相似文献   
53.
Analysis of water movement in paddy rice fields (I) experimental studies   总被引:2,自引:0,他引:2  
For the purpose of increasing the amount of ground water recharge, we investigated the hydraulic characteristics of water infiltration in a flooded paddy rice field in Ten-Chung, Chung-Hwa county, Taiwan. Experimental results based on mini-tensiometers and double ring infiltrometer measurements indicated that the least permeable layer occurred at the interface of the puddled topsoil and non-puddled subsoil. The average thickness of this layer was about 7.5 cm and saturated hydraulic conductivity ranged from 0.034 to 0.083 cm/day. Vertical infiltration flow was saturated within the plow sole layer and became unsaturated in the subsoil below the plow sole layer. The hydraulic conductivity of the subsoil, 20–30 times greater than that of the plow sole layer, revealed that the subsoil was more permeable than the plow sole layer. In situ measurements also demonstrated that breakage of the plow sole layer increased infiltration rate by a factor of 3.7. Increasing ponded water depth from 6 to 16 cm increased infiltration 1.5 fold. It is suggested that using the fallow paddy rice fields without puddling is a feasible way to enhance groundwater recharge, but for cultivated paddy rice fields, breaking the plow sole needs further study in terms of its recoverability and because of the potential contamination of the shallow aquifer by agrochemicals. The experimental data can be applied in numerical simulation models to quantify detailed water movement mechanisms and accurately estimate the amount of ground water recharge in paddy rice fields.  相似文献   
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The Güira de Jauco metamorphic sole, below the Moa-Baracoa ophiolite (eastern Cuba), contains strongly deformed amphibolites formed at peak metamorphic conditions of 650–660°C, approximately 8.6 kbar (~30 km depth). The geochemistry, based on immobile elements of the amphibolites, suggests oceanic lithosphere protholiths with a variable subduction component in a supra-subduction zone environment. The geochemical similarity and tectonic relations among the amphibolites and the basic rocks from the overlying ophiolite suggest a similar origin and protholith. New hornblende 40Ar/39Ar cooling ages of 77–81 Ma obtained for the amphibolites agree with this hypothesis, and indicate formation and cooling/exhumation of the sole in Late Cretaceous times. The cooling ages, geochemical evidence for a back-arc setting of formation of the mafic protoliths, and regional geology of the region allow proposal of the inception of a new SW-dipping subduction zone in the back-arc region of the northern Caribbean arc during the Late Cretaceous (ca. 90–85 Ma). Subduction inception was almost synchronous with the main plume pulse of the Caribbean–Colombian Oceanic Plateau (92–88 Ma) and occurred around 15 million years before arc-continent collision (75 Ma–Eocene) at the northern leading edge of the Caribbean plate. This chronological framework suggests a plate reorganization process in the region triggered by the Caribbean–Colombian mantle plume.  相似文献   
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The Common Fisheries Policy of the European Union adopted in 1983 is primarily focused on a science-based TAC management system with fixed national quota shares. The quality of the assessments of plaice and sole, the consistency of the scientific advice, the congruency of the management decisions, and the enforcement of the regulations are reviewed over a 15-year period. The conclusion is that there have been problems at each level, with the ultimate effect that the policy has not been able to restrict fishing mortality and that the level of overexploitation has only increased. It is argued that the present TAC system has created more problems than it has resolved. Attention is drawn to the autonomic economic developments within the fishery and to the potential for managing the fishery more effectively from the cost side rather than the profit side.  相似文献   
60.
mer Elitok  Kirsten Drüppel 《Lithos》2008,100(1-4):322-353
The Beyşehir–Hoyran ophiolite is situated in the western part of the Tauride belt (SW Turkey) and crops out at two localities north of the lake Beyşehir. It mainly comprises harzburgitic peridotites that were tectonically emplaced to their present position during the Late Eocene. The ophiolites themselves are tectonically overlain by either slope basin deposits with lava blocks (Eğirler formation) or massive Triassic limestone blocks (Deliktaş formation). High-grade sub-ophiolitic metamorphic rocks, i.e. epidote amphibolite, amphibolite, and pyroxene amphibolite, together with minor quartzite and calcschist, are observed at the base of the ophiolite sequence, where they occur as thin tectonic slices with an inverted metamorphic gradient. Average P–T conditions of 630–770 °C and c. 6 ± 1.5 kbar are calculated for the metamorphism of the amphibolites by conventional geothermobarometry, corresponding to a burial depth of 18–20 km. Both the sub-ophiolitic metamorphic rocks and the overlying mantle tectonites were intruded by isolated tholeiitic (Nb/Y = 0.041–0.108) diabase dikes, which do not transect the tectonic contact between the two units. Geochemical investigations of the amphibolites of the sub-ophiolitic rock suite show two different geochemical affinities, with the first group being alkaline in character (Nb/Y = 1–3.86) and the second one being tholeiitic (Nb/Y = 0.064–0.13). REE patterns, trace element plots and tectonomagmatic discrimination diagrams indicate that the most probable protoliths for alkaline amphibolites are within-plate type alkali basalts, whereas those of the tholeiitic group resemble tholeiitic island arc basalts. Similarities between the geochemical characteristics of the amphibolites and those of the volcanic rocks of the Eğirler formation strongly suggest that the latter are the protoliths of the amphibolites.  相似文献   
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