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91.
Finely cellular plagioclase intergrowths have been studied in xenocrystic andesine (An32) and andesine mantled K-feldspars within mafic magmatic enclaves in a quartz-feldspar porphyry from the Proterozoic subvolcanic Hammarudda complex, Åland rapakivi batholith, SW Finland. The cellular intergrowths usually occur as 0.2–2.0 mm mantles around xenocrysts but also as entirely cellular grains, and are built up of a network of two distinct phases: one relatively Na-rich (An31) and one relatively Ca-rich (An50). The grains are also covered by a thin (0.08–0.12 mm), continuous, normally zoned rim outside the cellular mantle. Small inclusions (0.01–0.05 mm) of Fe–Mg minerals are concentrated in the Ca-rich part of the network. Compositionally, the Na-rich phase of the network is close to the inner non-cellular andesine of the xenocrysts. However, it has a lower Or- and a slightly lower An-content. The Ca-rich phase has the same composition as the inner part of the normally zoned rim, which outwards grades into lower An-contents that overlap the An-content of the matrix plagioclases. The cellular network was developed after the andesine xenocrysts (or andesine mantled K-feldspars) were engulfed in mafic magmatic enclaves during a mixing event. The xenocrysts became heated to a temperature just below the liquidus of the mafic magma. Dissolution of the xenocrysts developed a spongy cellular texture which was penetrated by enclave magma. Ca-rich plagioclase crystallized in the cells in equilibrium with the enclave magma, trapping Fe–Mg-rich melt. As the enclaves cooled the outermost thin rim and matrix plagioclases crystallized from the mafic melt. These processes operated in fairly large enclaves, as the one studied here, which has a diameter of 70cm. Smaller enclaves, on the other hand, were cooled more rapidly to temperatures close to the solidus of the enclave magma, and consequently had no time to dissolve the xenoxrysts. 相似文献
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93.
High-quality seismic data document a Maastrichtian-Paleocene rift episode on the Vøring margin lasting for 20 m.y. prior to continental breakup. The rift structures are well imaged in the Fenris Graben and Gjallar Ridge region in the western Vøring Basin, and are characterized by low-angle detachment faults with variable fault geometries from south to north. The structural restoration has facilitated the division of pre- and syn-rift sediments across the extensional terrain, which is subsequently used to evaluate mode and mechanism for the lithospheric deformation. Extension estimates based on the structural restoration, subsidence analysis and crustal thickness evaluations yield stretching factors ranging between 1.5 to 2.3 across the main fault zone just landward of the early Tertiary flood basalts. The structural restoration also shows that a middle crustal dome structure, observed beneath the low-angle faults, can be explained by extensional unroofing. Thus, the dome structure may represent a possible metamorphic core complex. Calculations of the effects on vertical motion, assuming uniform and two-layer differential stretching models combined with the arrival of the Iceland mantle plume during rifting, indicate that the uniform extension model may account for both observed early rift subsidence and subsequent late rift uplift and erosion. Although the differential model can not be excluded, it implies early rift uplift which is not compatible with our seismic interpretation. The direct and indirect effects of the Iceland mantle plume may have caused as much as 1.2 km of late rift uplift. Comparison of the volcanic Vøring margin and the non-volcanic West Iberian margin shows similarities in terms of structural style as well as in mode and distribution of extension. 相似文献
94.
Phytoliths extracted from loess resting conformably on lacustrine sediments in southeastern North Island, New Zealand, provide a nearly continuous vegetation–climate record spanning the time period from the last interglacial (Marine Oxygen Isotope Stage 5) to the present. Phytoliths reveal changes in vegetation patterns following changes in climate. Correlation between tree–shrub phytolith fluctuations and the SPECMAP oxygen isotope curve, between Marine Oxygen Isotope Stages 1 and 5, suggest that changes in the ratio of arboreal to non‐arboreal phytoliths directly result from changes in climate. These data generally support the existing pollen and diatom record. This study confirms the usefulness of phytolith fossils for providing environmental and climate information from the Quaternary sedimentary record, especially in cases where the deposits contain no fossil pollen or diatoms. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
95.
The paper discusses the seabed shear stresses under random waves, and compares model predictions with results from field measurements near the seabed in the Strait of Juan de Fuca, Washington State, and at EDDA, North Sea. The model predictions as well as the interpretation of data are based on three different representative measures of random wave velocities within 12 minute records: root-mean-square, significant and maximum values. From the two locations the best agreement is found for the Strait of Juan de Fuca data regardless of the random velocity measure used. This agreement is attributed to the absence of suspended sediment at this location. 相似文献
96.
Suspended sediment dynamics during the period 1964–1985 are examined along the mainstem of Changjiang (Yangtze River). The period represents a basin condition prior to major changes in land management policy and dam building on the river's mainstem. The downstream sediment dynamics reflect basin geology and topography and channel morphology. Sediment exchange within the mainstem was calculated by the development of reach sediment balances that reveal complex temporal and spatial patterns. There is relatively little sediment exchange in the upper, bedrock‐controlled reaches, with systematic increases in the downstream alluvial reaches. Degrading, transfer, and aggrading reaches were identified. Relations between input and output in all reaches were significant but no relation was found between sediment exchange and input/output. Comparison between ‘short‐term’ (22 years) and ‘long‐term’ (52 years) records demonstrates the importance of the record length in studying the suspended sediment dynamics in a large fluvial system. The longer record yielded better correlation and different trends than the shorter record. Sediment transfer (output/input ratio) changes downstream: the dominance of the upstream contributing area in sustaining the appearance of net degradation through most of the river system highlights the importance of reach length on characterisation of suspended sediment dynamics in large fluvial systems. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
97.
Bernard G. J. Massart Olav M. Kvalheim Fred Olav Libnau Karl I. Ugland Kjell Tjessem Karl Bryne 《Aquatic Sciences - Research Across Boundaries》1996,58(2):120-138
A method for environmental monitoring using benthic species profiles as input is developed in this work. The method, referred to as projective ordination, utilises local principal component modelling (SIMCA) to obtain a cross-validated model which spans the natural variation in a region around offshore oil-producing installations. The borderline between regions with disturbed and non-disturbed species communities is subsequently decided from the residual distribution. This distribution is used to design an approximate F-test for assessing whether a community at a particular sampling location is disturbed or not. If so, the nature of the disturbance is determined by projecting the data on the PC model.Projective ordination utilises information from previous surveys to define the permissible variation in species communities, i.e. the limit of the natural variation. In addition, the method is dynamic, in a sense that the sampling locations may vary from survey to survey.Furthermore, our analysis shows that the customary number of replicate samples per station can be reduced. Modelling with only four randomly chosen replicates out of the complete set of five for each sampling location, does not affect the model significantly. On the other hand, taking only 3 replicates into account leads to significant divergences.A model based on the 1990 and 1993 surveys at the Statfjord field is presented as an example of the technique. 相似文献
98.
Analysis of magnetic data between the Jan Mayen and Senja fracture zones indicates that the anomaly 24A-B sequence extends from the Lofoten Basin onto the outer Vøring Plateau. Anomaly patterns, including those on the conjugate margin, suggest that the pre-23 sea floor spreading was characterized by an unstable plate boundary between fracture zones. The pre-23 spreading rate was at least 2.5 cm yr-1 which is remarkably high compared with the post-23 rates. An evolutionary model which assumes Cenozoic oceanic crust as far landward as the Vöring Plateau and Greenland escarpments is suggested. 相似文献
99.
100.