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61.
Last glacial hydrological variations at the southern margin of sub‐tropical North America and a regional comparison 
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F. CHOPIN K. SCHULMANN P. ŠTÍPSKÁ J. E. MARTELAT P. PITRA O. LEXA B. PETRI 《Journal of Metamorphic Geology》2012,30(4):347-376
A microstructural and metamorphic study of a naturally deformed medium‐ to high‐pressure granitic orthogneiss (Orlica–?nie?nik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction‐type apparent thermal gradient (～10 °C km?1). The granitic orthogneisses develops three distinct microstructural types, as follows: type I – augen orthogneiss, type II – banded orthogneiss and type III – mylonitic orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I orthogneiss is composed of partially recrystallized K‐feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallized quartz aggregates and interconnected monomineralic layers of recrystallized plagioclase. Compositional layering in the type II orthogneiss is defined by plagioclase‐ and K‐feldspar‐rich layers, both of which show an increasing proportion of interstitial minerals, as well as the deformation of recrystallized myrmekite fronts. Type III orthogneiss shows relicts of quartz and K‐feldspar ribbons preserved in a fine‐grained polymineralic matrix. All three types have the same assemblage (quartz + plagioclase + K‐feldspar + muscovite + biotite + garnet + sphene ± ilmenite), but show systematic variations in the composition of muscovite and garnet from types I to III. This is consistent with the equilibration of the three types at different positions along a prograde P?T path ranging from <15 kbar and <700 °C (type I orthogneiss) to 19–20 kbar and >700 °C (types II and III orthogneisses). The deformation types thus do not represent evolutionary stages of a highly partitioned deformation at constant P?T conditions, but reflect progressive formation during the burial of the continental crust. The microstructures of the type I and type II orthogneisses result from the dislocation creep of quartz and K‐feldspar whereas a grain boundary sliding‐dominated diffusion creep regime is the characteristic of the type III orthogneiss. Strain weakening related to the transition from type I to type II microstructures was enhanced by the recrystallization of wide myrmekite fronts, and plagioclase and quartz, and further weakening and strain localization in type III orthogneiss occurred via grain boundary sliding‐enhanced diffusion creep. The potential role of incipient melting in strain localization is discussed. 相似文献
63.
Progressive formation of modern drumlins at Múlajökull,Iceland: stratigraphical and morphological evidence 下载免费PDF全文
下载免费PDF全文 Ívar Örn Benediktsson Sverrir A. Jónsson Anders Schomacker Mark D. Johnson Ólafur Ingólfsson Lucas Zoet Neal R. Iverson Johann Stötter 《Boreas: An International Journal of Quaternary Research》2016,45(4):567-583
The drumlin field at Múlajökull, Iceland, is considered to be an active field in that partly and fully ice‐covered drumlins are being shaped by the current glacier regime. We test the hypothesis that the drumlins form by a combination of erosion and deposition during successive surge cycles. We mapped and measured 143 drumlins and studied their stratigraphy in four exposures. All exposures reveal several till units where the youngest till commonly truncates older tills on the drumlin flanks and proximal slope. Drumlins inside a 1992 moraine are relatively long and narrow whereas drumlins outside the moraine are wider and shorter. A conceptual model suggests that radial crevasses create spatial heterogeneity in normal stress on the bed so that deposition is favoured beneath crevasses and erosion in adjacent areas. Consequently, the crevasse pattern of the glacier controls the location of proto‐drumlins. A feedback mechanism leads to continued crevassing and increased sedimentation at the location of the proto‐drumlins. The drumlin relief and elongation ratio increases as the glacier erodes the sides and drapes a new till over the landform through successive surges. Our observations of this only known active drumlin field may have implications for the formation and morphological evolution of Pleistocene drumlin fields with similar composition, and our model may be tested on modern drumlins that may become exposed upon future ice retreat. 相似文献
64.
Chronological and palaeoenvironmental context of human occupations at the Buendía rockshelter (Central Spain) during the late Upper Pleistocene in inland Iberia 下载免费PDF全文
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67.
L.M. Cairós B. GarcÍa-Lorenzo N. Caon J.M. VÍlchez P. Papaderos K. Noeske 《Astrophysics and Space Science》2003,284(2):611-614
Preliminary results from a detailed spectrophotometric analysis of the blue compact dwarf galaxy (BCD) Mrk 35 are presented.
We have performed deep UBVRI broad-band and Hα narrow-band optical observations, near-infrared (JHK
s) imaging and long-slit spectroscopy of the galaxy. Mrk 35 is composed of a very young starburst population distributed in
a bar-like structure, placed on top of an underlying, older stellar host galaxy. Using predictions of evolutionary synthesis
models, we estimate the ages of both the starburst regions and the underlying stellar component.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
68.
Sebastian L. Hidalgo Antonio Aparicio David MartÍnez-Delgado 《Astrophysics and Space Science》2003,284(2):595-598
The surface brightness profile in the V band of the Phoenix dwarf galaxy shows two stellar components: an inner one, which
contains all the young stars of the galaxy, and an outer one predominantly populated by red stars. Deep color-magnitude diagrams
(CMDs), based on Hubble Space Telescope (HST) observations and reaching the oldest turn-offs, are used to analyze the inner
and outer stellar components. Results show that, together with an old stellar population, the outer field contains also an
intermediate-age population. These results are compatible with a scenario in which star forming regions are shrinking with
time (the shrinking scenario). It seems more difficult to support a halo-disk scenario, which would require extended structures
populated only by really old stars.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
69.
R. Oreiro F. Pérez Hernández M. Manteiga A. Ulla J.M. González Pérez M.R. Zapatero Osorio R. GarcÍa López J. MacDonald P. Thejll A. Ferriz-Mas R.A. Saffer V. Elkin 《Astrophysics and Space Science》2003,284(1):269-272
Hot subdwarf stars (hot sds) are blue subluminous objects. Only a few determinations areavailable to date regarding the study of such aspects as rotation,microturbulent velocities or the magnetic nature of these objects. Over26 sdBs are known to date to be multiperiodic rapid oscillators. Thisproject presents preliminary results of new observations and models of asample of pulsating and non-pulsating hot sds, includingconsiderations on mass loss and eventual magnetic properties. 相似文献
70.
Resonant absorption of slow MHD waves is studied numerically by using the SGH method and is applied to a model of a coronal arcade in the presence of equilibrium plasma flows. The arcade is approximated by a 1D horizontal magnetic slab that is non-uniform along the vertical direction and which is surrounded by two homogeneous media. While propagating from the photosphere upwards into the corona, the magneto-acoustic waves can be resonantly absorbed in the inhomogeneous region of the arcade. Computational results show that the resonant absorption of the impinging waves strongly depends on the equilibrium model and on the characteristics of the driving wave. The results also indicate that the presence of an equilibrium plasma flow along the magnetic field of the arcade reduces the resonant absorption for the flow speed parameters considered. 相似文献