青藏高原西北缘晚新生代的隆升特征——来自西昆仑山前盆地的沉积学证据

对西昆仑北缘山前盆地新生代沉积特征的研究结果表明,沿西昆仑山前发育的各沉积序列的垂向特征相似:古新世—中新世早期为石膏层、含瓣腮化石的石灰岩和紫红色较细粒的碎屑岩沉积,指示了海相和海陆过渡相较平静的沉积环境;中新世晚期—上新世初期开始出现陆相磨拉石,指示了陆相非平静的沉积环境,砾石的直径由下至上呈增大趋势,可能反映了西昆仑山体不断隆升,其间相对稳定的层段可能是构造运动间歇期或平稳期的沉积,指示了脉动式的隆升模式;磨拉石底部砾石的成分以沉积岩为主,向上火成岩和变质岩砾石逐渐增多,表明剥蚀程度不断加深。根据磨拉石建造的特征,判断剥蚀量和剥蚀强度自西向东有减小和变弱的趋势,可能暗示了西昆仑山晚新生代隆升有自西向东由强变弱的过渡特征。该结论与本区构造地貌学的研究结果一致。     

Gray swans: comparison of natural and financial hazard assessment and mitigation

Even advanced technological societies are vulnerable to natural disasters, such as the 2011 Tohoku earthquake and tsunami, and financial disasters, such as the 2008 collapse of the US housing and financial markets. Both resulted from unrecognized or underappreciated weaknesses in hazard assessment and mitigation policies. These policies relied on models that proved inadequate for reasons including inaccurate conceptualization of the problem, use of a too-short historic record, and neglect of interconnections. Japanese hazard models did not consider the possibility of multiple fault segments failing together, causing a much larger earthquake than anticipated, and neglected historical data for much larger tsunamis than planned for. Mitigation planning underestimated the vulnerability of nuclear power plants, due to a belief in nuclear safety. The US economic models did not consider the hazard that would result if many homeowners could not pay their mortgages, and assumed, based on a short history, that housing prices would keep rising faster than interest rates. They did not anticipate the vulnerability of the financial system to a drop in housing prices, due to belief that markets functioned best without government regulation. Preventing both types of disasters from recurring involves balancing the costs and benefits of mitigation policies. A crucial aspect of this balancing is that the benefits must be estimated using models with significant uncertainties to infer the probabilities of the future events, as we illustrate using a simple model for tsunami mitigation. Improving hazard models is important because overestimating or underestimating the hazard leads to too much or too little mitigation. Thus, although one type of disaster has natural causes and the other has economic causes, comparison provides insights for improving hazard assessment and mitigation policies. Instead of viewing such disasters as unpredictable and unavoidable “black swan” events, they are better viewed as “gray swans” that—although novel and outside recent experience—can be better foreseen and mitigated.     

The dark side of zircon: textural,age, oxygen isotopic and trace element evidence of fluid saturation in the subvolcanic reservoir of the Island Park-Mount Jackson Rhyolite,Yellowstone (USA)

The Island Park-Mount Jackson series in the Yellowstone volcanic field, Wyoming (USA), is a suite of rhyolitic domes and lavas that erupted between the caldera-forming eruptions of the Mesa Fall Tuff (1.3 Ma) and the Lava Creek Tuff (0.6 Ma). Combined zircon U/Pb geochronology, Raman spectroscopy, oxygen isotopic and trace elemental compositions document storage conditions of these magmas between consecutive supereruptions. Based on comparison with co-erupted melt compositions and textural criteria, four zircon compositional groups are identified that record different stages along a continuous magmatic evolution from trace element-poor rhyolite at high temperatures to extremely fractionated rhyolite where zircon trace elements are highly enriched (e.g., >?1000 ppm U). These latter zircon domains are dark in cathodoluminescence images and show broadened Raman peaks relative to near-endmember zircon, indicating that substitution of non-stoichiometric trace elements into zircon leads to distortion of the crystal lattice. Some of these zircon domains contain inclusions of U-Th-REE-phases, likely originating from coupled dissolution–reprecipitation of metastable trace element-rich zircon in the presence of a fluid phase. Rhyolite-MELTS simulations indicate that at the conditions required to produce the observed enrichment in trace elements, a fluid phase is likely present. These findings illustrate that zircons can be assembled from a variety of co-existing magmatic environments in the same magma reservoir, including near-solidus volatile-rich melts close to the magmatic–hydrothermal transition.     

U-Pb geochronology of basement rocks in central Tibet and paleogeographic implications

The ages and paleogeographic affinities of basement rocks of Tibetan terranes are poorly known. New U-Pb zircon geochronologic data from orthogneisses of the Amdo basement better resolve Neoproterozoic and Cambro-Ordovician magmatism in central Tibet. The Amdo basement is exposed within the Bangong suture zone between the Lhasa and Qiangtang terranes and is composed of granitic orthogneisses with subordinate paragneisses and metasedimentary rocks. The intermediate-felsic orthogneisses show a bimodal distribution of Neoproterozoic (920-820 Ma) and Cambro-Ordovician (540-460 Ma) crystallization ages. These and other sparse basement ages from Tibetan terranes suggest the plateau is underlain by juvenile crust that is Neoproterozoic or younger; its young age and weaker rheology relative to cratonic blocks bounding the plateau margins likely facilitated the propagation of Indo-Asian deformation far into Asia. The Neoproterozoic ages post-date Rodinia assembly and magmatism of similar ages is documented in the Qaidaim-Kunlun terrane, South China block, the Aravalli-Delhi craton in NW India, the Eastern Ghats of India, and the Prince Charles mountains in Antarctica. The Amdo Neoproterozoic plutons cannot be unambiguously related to one of these regions, but we propose that the Yangtze block of the South China block is the most likely association, with the Amdo basement representing a terrane that possibly rifted from the active Yangtze margin in the middle Neoproterozoic. Cambro-Ordovician granitoids are ubiquitous throughout Gondwana as a product of active margin tectonics following Gondwana assembly and indicate that the Lhasa-Qiangtang terranes were involved in these tectono-magmatic events. U-Pb detrital zircon analysis of two quartzites from the Amdo basement suggest that the protoliths were Carboniferous-Permian continental margin strata widely deposited across the Lhasa and Qiangtang terranes. The detrital zircon age spectra of the upper Paleozoic Tibetan sandstones and other rocks deposited in East Gondwana during the late Neoproterozoic and Paleozoic are all quite similar, making it difficult to use the age spectra for paleogeographic determinations. There is a suggestion in the data that the Qiangtang terrane may have been located further west along Gondwana’s northern boundary than the Lhasa terrane, but more refined spatial and temporal data are needed to verify this configuration.     

Thermal periodicities in the Venus atmosphere

A search was made for periodic fluctuations in the thermal brightness temperatures recorded by the Pioneer Venus orbiter's infrared radiometer. Data were averaged in 10 × 10° latitude-longitude bins for each of the 72 days the instrument was in operation. This time series of thermal brightness temperatures was then analyzed to determine the amplitude of fluctuations at periods from 2 to 64 days at four levels in the atmosphere (at the cloud tops and at approximately 70, 80, and 90 km). The amplitude of such fluctuations is small at equatorial latitudes and increases to a maximum at 60–70° latitude at most altitudes. The period of the highest amplitude fluctuation is 5.3±0.4 days (at all altitudes) except at 70–80°, where a 2.9-day period which appears to correspond to the polar dipole dominates the cloud-top channel. The amplitude of the periodic fluctuations is a maximum at the cloud tops, decreasing to a minimum at the 80-km channel, and increasing again at the 90-km channel.     

Solar constant secular changes

Our recent model for solar constant secular changes suggests that over extended time intervals, a positive correlation of the solar constant with solar activity results. The positive correlation in this model is not associated with the direct result of active region, which may be in a close energy balance over their lifetimes, but rather is associated with global features - bright global faculae. These exist as both polar and network features on the solar disk. The high latitude faculae enable the solar constant to peak prior to sunspot maximum. Recent solar constant observations support the model's general trend. Using this model, we now calculate a proxy solar constant for: (1) the past four centuries, based upon the sunspot record, (2) the past nine centuries, based upon ¹⁴C observations and their relation to solar activity, and (3) the next decade, based upon a dynamo theory model for the solar cycle. The proxy solar constant data is tabulated as it may be useful for climate modelers studying global climate changes. This could be helpful in disentangling possible solar influences from any anthropogenic changes associated with trace gas increases in the terrestrial atmosphere. An important point for climate modelers to consider in fixing climate model parameters, is that the Sun has been relatively active and thus bright in the latter half of the 20th century, compared with the past few centuries.This paper was presented at the third meeting of the Solar Cycle Workshop, held in Sydney, Australia, January 9–13, 1989.Physics Dept., Univ. of Northern Colorado, U.S.A.     

Short term evolution of coronal hole boundaries

In an examination of the evolution of coronal hole boundaries on a time scale of 1 day, we find that 38% of all the boundaries of coronal holes observed near central meridan passage during the Skylab period shifted in location by >1° heliocentric in 1 day. Of these boundary changes, 70% were on a scale 3 times the average supergranulation cell size. However, large-scale shifts in the boundary locations also occurred, which involved changes in the X-ray emission from these areas of the Sun. X-ray emitting structures on the borders of isolated and evolving holes were less clearly defined than those on the boundaries of well-established, elongated holes. There were generally more changes in the boundaries of the most rapidly evolving holes, but no simple relationship between the amount of change and the rate of hole growth or decay.Skylab Solar Workshop post-doctoral appointee 1975–1976. The Skylab Solar Workshops are sponsored by NASA and NSF and managed by the High Altitude Observatory, National Center for Atmosoheric Research.     

Experiments with scale models of oil collectors for subsea well blowouts — Part II

A previous paper of the same title indicated the feasibility of the collection of oil by an open bottom collector above a blowout with a marine riser above the collector; the whole collection system being driven by gas lift from the blowout gas. That paper was based on small-scale laboratory experiments and it identified the salient dimensionless parameters governing those experiments. This paper describes laboratory experiments on a refinement of the collection system and also describes the results of intermediate scale experiments. The length scale of these experiments was about four times greater than laboratory scale and about one-fourth of full scale. Generally, the intermediate scale results are consistent with the laboratory predictions. Furthermore, two scale-dependent parameters have been identified. The effects of these have been included in an analysis of the results.     

Experiments with scale models of oil collectors for subsea well blowouts

Laboratory experiments on conically shaped oil collectors beneath a marine riser are described. The collection concept involves oil, water and gas entering the collector and being driven into a separating sytem by gas-lift. A parametric study involving various collector shapes, heights above the wellhead, and gas, oil and water flow rates was carried out. The important dimensionless variables were identified and quantified with the conclusion that effective collection is possible if the collector height is sufficiently small and the ratio of water pumped to gas flow is sufficiently large. Increased collector heights can be accommodated at the expense of requiring increased water flows by use of larger diameter risers. The amount of gas required for most efficient oil collection is found to be much less than is expected to come from most blowouts when a single collector and riser system is used. This difficulty can be greatly diminished by the use of a specially designed gas-separating collector which passes most of the gas to the surface through a riser separate from the one which carries the liquids.     

Measurements of the surface flow above round bubble plumes

When a bubble plume exists beneath a free surface, such as after a subsea gas well blowout, a generally horizontal flow occurs in the vicinity of the surface and this flow is influenced by the fact that the surface is free. Two very different theories for such surface flows have been developed in the past and the bases of these theories are reviewed here. The results of measurements of surface flows above plumes of relatively large scale are given. These are compared with both of the existing theories. One theory is found to be accurate at small radii from the plume centre and the other theory is found to be accurate at large radii. The needed boundary condition for the theory that is accurate at large radii is supplied by the results of the theory that is accurate at small radii.