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61.
Neil S. Mancktelow Daniel F. Stöckli Balz Grollimund Wolfgang Müller Bernhard Fügenschuh Giulio Viola Diane Seward Igor M. Villa 《International Journal of Earth Sciences》2001,90(3):593-622
Alpine deformation of Austroalpine units south of the Tauern window is dominated by two kinematic regimes. Prior to intrusion of the main Periadriatic plutons at ~30 Ma, the shear sense was sinistral in the current orientation, with a minor north-side-up component. Sinistral shearing locally overprints contact metamorphic porphyroblasts and early Periadriatic dykes. Direct Rb-Sr dating of microsampled synkinematic muscovite gave ages in the range 33-30 Ma, whereas pseudotachylyte locally crosscutting the mylonitic foliation gave an interpreted 40Ar-39Ar age of ~46 Ma. The transition from sinistral to dextral (transpressive) kinematics related to the Periadriatic fault occurred rapidly, between solidification of the earlier dykes and of the main plutons. Subsequent brittle-ductile to brittle faults are compatible with N-S to NNW-SSE shortening and orogen-parallel extension. Antithetic Riedel shears are distinguished from the previous sinistral fabric by their fine-grained quartz microstructures, with local pseudotachylyte formation. One such pseudotachylyte from Speikboden gave a 40Ar-39Ar age of 20 Ma, consistent with pseudotachylyte ages related to the Periadriatic fault. The magnitude of dextral offset on the Periadriatic fault cannot be directly estimated. However, the jump in zircon and apatite fission-track ages establishes that the relative vertical displacement was ~4-5 km since 24 Ma, and that movement continued until at least 13 Ma. 相似文献
62.
The dynamics of thin shells with variable viscosity and the origin of toroidal flow in the mantle 总被引:4,自引:0,他引:4
Neil M. Ribe 《Geophysical Journal International》1992,110(3):537-552
63.
The paleomagnetism of the Late Cretaceous Poços de Caldas alkaline complex (46.6°W, 21.9°S) was investigated through 42 oriented cores from seven sites. Six sites, reversed relative to the present magnetic field of the Earth, yield a pole at 127°W, 82°S (dp = 8°,dm = 13°). This pole is located close to other Late Cretaceous poles for South America obtained by Creer [1] from untreated paleomagnetic samples. The results are significantly different from those for the nearby Early Cretaceous Serra Geral basalt but close to the Triassic pole for South America. The polar wandering path for South America for the Mesozoic seems to be more complicated than anticipated. The available paleomagnetic information may not yet be precise enough to determine the time of opening of the Atlantic. 相似文献
64.
65.
Atoms which escape Titan's atmosphere are unlikely to possess escape velocity from Saturn, and can orbit the planet until lost by ionization or collision with Titan. It is predicted that a toroidal ring of between ~1 and ~103 atoms or molecules cm?3 exists around Saturn at a distance of about 10 times the radius of the visible rings. This torus may be detectable from Earth-orbit and detection of nondetection of it may provide some information about the presence or absence of a Saturnian magnetic field, and the exospheric temperature and atmospheric escape rate of Titan. It is estimated that, if Titan has a large exosphere, ~97% or more of the escaping atoms can be recaptured by Titan, thereby decreasing the effective net atmospheric loss rate by up to two orders of magnitude. With such a reduction in atmospheric loss rates, it becomes more plausible to suggest that satellites previously thought too small to retain an atmosphere may have one. It is suggested that Saturn be examined by Lyman-α and other observations to search for the gaseous torus of Titan. If successful, these could then be extended to other satellites.The effect of a hypothetical Saturnian magnetosphere on the atmosphere of Titan is investigated. It is shown that, if Saturn has a magnetic field comparable to Jupiter's (~10 G at the planetary surface), the magnetospheric plasma can supply Titan with hydrogen at a rate comparable to the loss rates in some of the models of Trafton (1972) and Sagan (1973). A major part of the Saturnian ionospheric escape flux (~ 1027 photoelectrons sec?1) could perhaps be captured by Titan. At the upper limit, this rate of hydrogen input to the satellite could total ~0.1 atm pressure over the lifetime of the solar system, an amount comparable to estimates of the present atmospheric pressure of Titan. 相似文献
66.
Robert E. NovakMichael J. Mumma Michael A. DiSanti Neil Dello Russo Karen Magee-Sauer 《Icarus》2002,158(1):14-23
We present absolute abundances and latitudinal variations of ozone and water in the atmosphere of Mars during its late northern spring (Ls=67.3°) shortly before aphelion. Long-slit maps of the a1Δg state of molecular oxygen (O2) and HDO, an isotopic form of water, were acquired on UT January 21.6 1997 using a high-resolution infrared spectrometer (CSHELL) at the NASA Infrared Telescope Facility. O2(a1Δg) is produced by ozone photolysis, and the ensuing dayglow emission at 1.27 μm is used as a tracer for ozone. Retrieved vertical column densities for ozone above ∼20 km ranged between 1.5 and 2.8 μm-atm at mid- to low latitudes (30°S-60°N) and decreased outside that region. A significant decrease in ozone density is seen near 30°N (close to the subsolar latitude of 23.5°N). The rotational temperatures retrieved from O2(a-X) emissions show a mean of 172±2.5 K, confirming that the sensed ozone lies in the middle atmosphere (∼24 km). The ν1 fundamental band of HDO near 3.67 μm was used as a proxy for H2O. The retrieved vertical column abundance of water varies from 3 precipitable microns (pr-μm) at ∼30°S to 24 pr-μm at ∼60°N. We compare these results with current photochemical models and with measurements obtained by other methods. 相似文献
67.
A regional geochemical survey using soils, stream sediment and stream water sampling revealed multi-element geochemical anomalies, though of low environmental mobility, associated with Mesozoic sedimentary ironstones in Lincolnshire, UK. The most prominent of these anomalies were of As and V in soils and sediments, but elevated levels of elements such as Cr, La, Ce and Th were also observed. These were initially thought to be part of a residual resistate element or heavy-mineral suite, but careful examination suggests that these too may be primarily associated with the process of Fe oxide precipitation during the initial formation of the ironstones. 相似文献
68.
An 18 million year record of the Ca isotopic composition (δ44/42Ca) of planktonic foraminiferans from ODP site 925, in the Atlantic, on the Ceara Rise, provides the opportunity for critical analysis of Ca isotope-based reconstructions of the Ca cycle. δ44/42Ca in this record averages +0.37 ± 0.05 (1σ SD) and ranges from +0.21‰ to +0.52‰. The record is a good match to previously published Neogene Ca isotope records based on foraminiferans, but is not similar to the record based on bulk carbonates, which has values that are as much as 0.25‰ lower. Bulk carbonate and planktonic foraminiferans from core tops differ slightly in their δ44/42Ca (i.e., by 0.06 ± 0.06‰ (n = 5)), while the difference between bulk carbonate and foraminiferan values further back in time is markedly larger, leaving open the question of the cause of the difference. Modeling the global Ca cycle from downcore variations in δ44/42Ca by assuming fixed values for the isotopic composition of weathering inputs (δ44/42Caw) and for isotope fractionation associated with the production of carbonate sediments (Δsed) results in unrealistically large variations in the total mass of Ca2+ in the oceans over the Neogene. Alternatively, variations of ±0.05‰ in the Ca isotope composition of weathering inputs or in the extent of fractionation of Ca isotopes during calcareous sediment formation could entirely account for variations in the Ca isotopic composition of marine carbonates. Ca isotope fractionation during continental weathering, such as has been recently observed, could easily result in variations in δ44/42Caw of a few tenths of permil. Likewise a difference in the fractionation factors associated with aragonite versus calcite formation could drive shifts in Δsed of tenths of permil with shifts in the relative output of calcite and aragonite from the ocean. Until better constraints on variations in δ44/42Caw and Δsed have been established, modeling the Ca2+ content of seawater from Ca isotope curves should be approached cautiously. 相似文献
69.
Neil S. Mancktelow Djordje Grujic Eric L. Johnson 《Contributions to Mineralogy and Petrology》1998,131(1):71-85
A backscattered and secondary electron SEM study of the grain boundary microstructure in quartz mylonites sampled along the length of the retrograde Simplon Fault Zone established three characteristic components. (1) Fine isolated pores (≤?1?μm diameter) are scattered across two-grain interfaces, preferentially concentrated on surfaces in extension. Pores are uncommon on three-grain junctions and there is no evidence for fluid interconnectivity along three- and four-grain junctions. The fine porosity may develop by accumulation of original, mainly intragranular fluid inclusions to the grain boundary during deformation and recrystallization and by cavitation of grain boundaries during grain boundary sliding. Dynamic cavitation implies that the “ductile” mylonitic deformation is at least locally dilatant and therefore pressure sensitive. (2) Large “vug”-like pores (up to mm-scale) extend along multi-grain boundaries. Observed in all samples, they are most common in the higher initial temperature, coarse-grained samples with a microstructure dominated by grain boundary migration recrystallization. Grains bordering this connected porosity develop perfect crystal faces, undecorated by fine pores or pits. The irregular “lobate” optical microstructure of many migrating grain boundaries actually consists of a series of straight crystal faces. The coarse porosity is probably due to accumulation during dynamic recrystallization of (CO2-rich ?) fluid with a high wetting angle against quartz. (3) In one sample, interconnected sinuous ridges, ≤?0.2?μm high, are observed to follow three- and four-grain junctions and disjoint into more isolated worms and spheroidal globules. On two-grain interfaces, these are transitional to more branching vein-like or convoluted brain-like forms. The brain-like and globular forms have been observed, with varying frequency, through the range of samples, with the globules attaining sizes of up to 60?μm. Vein structures have also been observed on intragranular fractures. These topologies do not match across adjoining surfaces and must have developed into free space. The ridge-vein-brain-spheroid structure is distinctly different to that previously observed on experimentally healed microcracks and its origin is not unequivocally established. They could represent unstable meniscus necking of a thin grain-boundary phase of low viscosity, developed due to quasi-adiabatic shear and/or local stress-induced dilatancy during microcracking. 相似文献
70.
A comparison of estimated and calculated effective porosity 总被引:1,自引:1,他引:0
Daniel B. Stephens Kuo-Chin Hsu Mark A. Prieksat Mark D. Ankeny Neil Blandford Tracy L. Roth James A. Kelsey Julia R. Whitworth 《Hydrogeology Journal》1998,6(1):156-165
Effective porosity in solute-transport analyses is usually estimated rather than calculated from tracer tests in the field
or laboratory. Calculated values of effective porosity in the laboratory on three different textured samples were compared
to estimates derived from particle-size distributions and soil–water characteristic curves. The agreement was poor and it
seems that no clear relationships exist between effective porosity calculated from laboratory tracer tests and effective porosity
estimated from particle-size distributions and soil–water characteristic curves. A field tracer test in a sand-and-gravel
aquifer produced a calculated effective porosity of approximately 0.17. By comparison, estimates of effective porosity from
textural data, moisture retention, and published values were approximately 50–90% greater than the field calibrated value.
Thus, estimation of effective porosity for chemical transport is highly dependent on the chosen transport model and is best
obtained by laboratory or field tracer tests.
Received, March 1997 · Revised, August 1997 · Accepted, August 1997 相似文献