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31.
PHILIP E. F. COLLINS IAN M. FENWICK D. MICHAEL KEITH-LUCAS PETER WORSLEY 《第四纪科学杂志》1996,11(5):357-375
Marked fluctuations in Late-glacial climate are correlated with significant changes in river behaviour and floodplain environment across much of northwest Europe. Evidence from a new site in the Kennet Valley provides a rare comprehensive picture of varying fluvial regime and local vegetation during this interval. Chronological control for the site is provided by biostratigraphical and morphostratigraphical correlations with other sites in the region, and by radiocarbon dating of carefully selected material. Preservation of the sedimentary sequence was facilitated by its occurrence within a large depression in the underlying bedrock. The origin of this depression is uncertain, perhaps reflecting subsidence or scour. Following the Last Glacial Maximum, a nival regime braided river dominated by floods deposited sandy gravels under stadial conditions. A large channel infilled by fine sediments during the latter part of the Windermere Interstadial indicates a fall in flow competence at a time of increased floodplain vegetation, most notably the appearance of tree birch. Variations in sediments and vegetation suggest fluctuations in the floodplain environment, reflecting complex vegetation dynamics and possibly climate forcing. During the Loch Lomond/Younger Dryas Stadial, a nival, braided regime was re-established, accompanied by the disappearance of tree birch. 相似文献
32.
Thomas M. DAVISON Gareth S. COLLINS Dirk ELBESHAUSEN Kai WÜNNEMANN Anton KEARSLEY 《Meteoritics & planetary science》2011,46(10):1510-1524
Abstract– The majority of meteorite impacts occur at oblique incidence angles. However, many of the effects of obliquity on impact crater size and morphology are poorly understood. Laboratory experiments and numerical models have shown that crater size decreases with impact angle, the along‐range crater profile becomes asymmetric at low incidence angles, and below a certain threshold angle the crater planform becomes elliptical. Experimental results at approximately constant impact velocity suggest that the elliptical threshold angle depends on target material properties. Herein, we test the hypothesis that the threshold for oblique crater asymmetry depends on target material strength. Three‐dimensional numerical modeling offers a unique opportunity to study the individual effects of both impact angle and target strength; however, a systematic study of these two parameters has not previously been performed. In this work, the three‐dimensional shock physics code iSALE‐3D is validated against laboratory experiments of impacts into a strong, ductile target material. Digital elevation models of craters formed in laboratory experiments were created from stereo pairs of scanning electron microscope images, allowing the size and morphology to be directly compared with the iSALE‐3D craters. The simulated craters show excellent agreement with both the crater size and morphology of the laboratory experiments. iSALE‐3D is also used to investigate the effect of target strength on oblique incidence impact cratering. We find that the elliptical threshold angle decreases with decreasing target strength, and hence with increasing cratering efficiency. Our simulations of impacts on ductile targets also support the prediction from Chapman and McKinnon (1986) that cratering efficiency depends on only the vertical component of the velocity vector. 相似文献
33.
The use of settling velocity in defining the initiation of motion of heavy mineral grains, under unidirectional flow 总被引：3，自引：0，他引：3
The results of critical threshold experiments on four commonly occurring heavy minerals are described. The data are presented, in conjunction with comparable quartz data, using the non-dimensional Shields’ Curve and the Movability Number (U*/ws). The results indicate that critical shear stress for material of high density is overestimated by use of Shields’ Curve, under smooth boundary conditions. Grain settling velocity is found to be a good indicator for the critical shear stress for grains of a wide density range. A physical explanation for the results is proposed. 相似文献
34.
Successive overprinting granulite facies metamorphic events in the Anmatjira Range, central Australia 总被引：4，自引：0，他引：4
The Anmatjira Range and adjacent Reynolds Range, central Australia, comprise early Proterozoic metasediments and othogneisses that were affected by three, and possibly four, temporally distinct metamorphic events, M1–4, and deformation events, D1–4, in the period 1820–1590 Ma. The north-western portion of the range, around Mt Stafford, preserves the effects of ±1820 Ma M1-D1, and shows a spectacular lateral transition from muscovite + quartz-bearing schists to interlayered andalusite-bearing migmatites and two-pyroxene granofelses that reflect extremely low-pressure granulite facies conditions, over a distance of less than 10 km. Orthopyroxene + cordierite + garnet + K-feldspar + quartz-bearing gneisses occur at the highest grade, implying peak conditions of ±750°C and 2.5 ± 0.6 kbar. An anticlockwise P–T path for M1 is inferred from syn- to late-D1 sillimanite overprinting andalusite, petrogenetic grid considerations and quantitative estimates of metamorphic conditions for inferred overprinting assemblages. The effects of M1 have been variably overprinted to the south-east by a c. 1760 Ma M2–D2 event. Much of the central Anmatjira Range, around Ingellina Gap, comprises orthogneiss, deformed during D2, and metapelites that have M1 andalusite and K-feldspar overprinted by M2 sillimanite and muscovite. The south-eastern portion of the range, around Mt Weldon, comprises metasediments and orthogneisses that were completely recrystallized during M2–D2, with metapelitic gneisses characterized by spinel + sillimanite + K-feldspar + quartz-bearing assemblages that suggest peak M2 conditions of >750°C and 5.5 ± 1 kbar. Overprinting parageneses in metapelitic gneisses imply that D2 occurred during essentially isobaric cooling. A third granulite facies event, M3, affected rocks in the Reynolds Range, immediately to the south of the Anmatjira Range, at c. 1730 Ma. A possible fourth event, M4, with a minimum age of c. 1590 My affected both Ranges, but resulted in only minor overprinting of M1–3 assemblages. The superimposed effects of M1–4, mapped for the entire Anmatjira–Reynolds Range area, indicate that only minor or no dislocation of the regional geology occurred during any of the metamorphic and accompanying folding, events. Although the immediate cause of each of the metamorphic events involved advection, the ultimate causes were external to the metasediments and most probably external to the crust. 相似文献
35.
Replenishment, Crystal Accumulation and Floor Aggradation in the Megacrystic Kameruka Suite, Australia 总被引：3，自引：0，他引：3
Detailed field evidence indicates that the Kameruka Suite plutonsof the Bega Batholith, eastern Australia, grew by crystal accumulationon the floor of a magma chamber. Depositional features in theplutons, including mafic enclave channels, asymmetric enclavepillows and exotic rafts, load casts and flame structures, andgraded and trough cross-beds, indicate that the pluton builtprogressively upward. The general eastward dip of depositionalfeatures in the main pluton implies a lower western and uppereastern contact, consistent with a basal granite–migmatitecontact in the west and a sharp hornfelsic sidewall contactin the east. Mafic, felsic and composite dykes, most commonnear and below the basal western contact, are interpreted asconduits for magma chamber replenishment and imply open-systembehaviour during pluton construction. Textural relations arealso consistent with an open-system, cumulate origin. Typically,centimetre-scale grains of quartz, plagioclase and megacrysticalkali feldspar form a touching framework with interstices filledwith smaller biotite flakes and smaller intercumulus quartzand feldspar crystals. Alkali feldspar megacrysts vary fromeuhedral and unzoned, to mantled and partially replaced by plagioclase,to ovoid and completely pseudomorphed by quartz–albiteaggregates. The common occurrence of mantled and pseudomorphedalkali feldspar in mafic enclaves, and in hybrid tonalitic rocksforming the matrix to enclave swarms, suggests that replacementor resorption of granitic primocrysts was associated with maficreplenishments. The occurrence of all megacryst types at outcropscale implies extended alkali feldspar crystallization in differentparts of the chamber, thorough stirring during, or after, periodicreplenishment, and final settling in a cumulate mush. The bulkcomposition of the cumulate mush, represented by granodiorite,cannot represent the emplaced magma. Compositional variationcan be modelled by variable degrees of crystal accumulationfrom a parental, silica-rich melt represented by the silicicdykes. As dykes periodically fed the magma chamber, crystalsaccumulated on the floor, and more evolved melts probably eruptedfrom its roof. Thus, the average composition of the magma, andthe cumulus minerals, may have remained relatively constant,and the sublinear chemical trends that typify the Kameruka Suitesimply reflect differing proportions of melt and cumulate material.Sublinear chemical trends can also be explained by a restitemodel; however, the distinctive Ba, light rare earth elementand Zr spikes at high silica can be explained only by a cumulatemodel, which also explains why the low-silica granites of thesuite share the same chemical characteristics as the high-silicagranites. KEY WORDS: crystal accumulation; magma chamber; open system; granitoids; Kameruka; Australia 相似文献
36.
Boundary value problems for hardening/softening soils, such as Cam-Clay, usually require the extensive use of finite element methods. Here analytical and semi-analytical solutions for the undrained expansion of cylindrical and spherical cavities in critical state soils are presented. The strain is finite, the initial cavity radius is arbitrary and the procedure applicable to any isotropically hardening materials. In all cases only simple quadratures are involved, and in the case of the original Cam-Clay a complete analytical solution can be found. In addition to providing models of the behaviour of displacement piles and pressuremeters these results also provide valuable benchmark solutions for verifying various numerical methods. 相似文献
37.
The Soursat metamorphic complex (SMC) in northwestern Iran is part of the Sanandaj-Sirjan metamorphic belt.The complex is composed of different metamorphic and plutonic rocks,but is dominated by metape... 相似文献
38.
Gareth S. COLLINS H. Jay MELOSH Robert A. MARCUS 《Meteoritics & planetary science》2005,40(6):817-840
Abstract— We have developed a Web‐based program for quickly estimating the regional environmental consequences of a comet or asteroid impact on Earth ( http:www.lpl.arizona.eduimpacteffects ). This paper details the observations, assumptions and equations upon which the program is based. It describes our approach to quantifying the principal impact processes that might affect the people, buildings, and landscape in the vicinity of an impact event and discusses the uncertainty in our predictions. The program requires six inputs: impactor diameter, impactor density, impact velocity before atmospheric entry, impact angle, the distance from the impact at which the environmental effects are to be calculated, and the target type (sedimentary rock, crystalline rock, or a water layer above rock). The program includes novel algorithms for estimating the fate of the impactor during atmospheric traverse, the thermal radiation emitted by the impact‐generated vapor plume (fireball), and the intensity of seismic shaking. The program also approximates various dimensions of the impact crater and ejecta deposit, as well as estimating the severity of the air blast in both crater‐forming and airburst impacts. We illustrate the utility of our program by examining the predicted environmental consequences across the United States of hypothetical impact scenarios occurring in Los Angeles. We find that the most wide‐reaching environmental consequence is seismic shaking: both ejecta deposit thickness and air‐blast pressure decay much more rapidly with distance than with seismic ground motion. Close to the impact site the most devastating effect is from thermal radiation; however, the curvature of the Earth implies that distant localities are shielded from direct thermal radiation because the fireball is below the horizon. 相似文献
39.
In the mingled mafic/felsic Halfmoon Pluton at The Neck, Stewart Island (part of the Median Batholith of New Zealand) some hornblende gabbros and diorites retain magmatic structures, whereas others show evidence of major changes in grain and inclusion shapes, and still others are amphibolite‐facies granofelses with few or no igneous relicts. These mafic to intermediate magmas crystallized in felsic magma relatively quickly, with the result that most deformation occurred at subsolidus conditions. It is suggested that mafic‐intermediate rocks with predominantly igneous microstructures spent less time in the magmatic system. The metamorphism of the mafic rocks appears to be ‘autometamorphic’, in the sense that elevated temperatures were maintained by magmatic heat during subsolidus cooling. Elevated temperatures were maintained because of repeated sheet injection and subconcordant dyke injection of hot basaltic and composite mafic‐felsic magmas, into a dominantly transtensional, km‐scale, outboard‐migrating, magmatic shear zone that operated semi‐continuously for between c. 140 and c. 130 Ma. Complete cooling occurred only when the system evolved to transpressional and the locus of magmatism migrated inboard (southward) between c. 130 and c. 120 Ma, associated with solid‐state mylonitic deformation. Intermingled granitic rocks escaped metamorphism, because they remained magmatic to lower temperatures, and experienced shorter and lower‐temperature subsolidus cooling intervals. However, the felsic rocks underwent relatively high‐temperature solid‐state deformation, as indicated by myrmekite replacing K‐feldspar and chess‐board subgrain patterns in quartz; locally they developed felsic mylonites. The felsic rocks were deformed in the solid state because of their high proportion of relatively weak minerals (quartz and biotite), whereas the mafic rocks mostly escaped subsolidus deformation, except in local high‐strain zones of hornblende‐plagioclase schist, because of their high proportion of relatively strong minerals (hornblende and plagioclase). We suggest that such contrasting microstructural features are diagnostic of long‐lived syntectonic magma transfer zones, and contrast with the more typical complex, batholith‐scale magma chambers of magmatic arcs. 相似文献
40.