Some physical features related to the river development in the Dolgelley district     

A. Austin Miller 《Proceedings of the Geologists' Association. Geologists' Association》1946,57(3):174-203

  相似文献   

Palaeozoic suturing of eastern and western Tasmania in the west Tamar region: Implications for the tectonic evolution of southeast Australia     

A. R. Reed  C. Calver  R. S. Bottrill 《Australian Journal of Earth Sciences》2013,60(5):809-830

A new tectonic model for Tasmania incorporates subduction at the boundary between eastern and western Tasmania. This model integrates thin‐ and thick‐skinned tectonics, providing a mechanism for emplacement of allochthonous elements on to both eastern and western Tasmania as well as rapid burial, metamorphism and exhumation of high‐pressure metamorphic rocks. The west Tamar region in northern Tasmania lies at the boundary between eastern and western Tasmania. Here, rocks in the Port Sorell Formation were metamorphosed at high pressures (700–1400 MPa) and temperatures (400–500°C), indicating subduction to depths of up to 30 km. The eastern boundary of the Port Sorell Formation with mafic‐ultramafic rocks of the Andersons Creek Ultramafic Complex is hidden beneath allochthonous ?Mesoproterozoic turbidites of the Badger Head Group. At depth, this boundary coincides with the inferred boundary between eastern and western Tasmania, imaged in seismic data as a series of east‐dipping reflections. The Andersons Creek Ultramafic Complex was previously thought of as allochthonous, based mainly on associations with other mafic‐ultramafic complexes in western Tasmania. However, the base of the Andersons Creek Ultramafic Complex is not exposed and, given its position east of the boundary with western Tasmania, it is equally likely that it represents the exposed western edge of autochthonous eastern Tasmanian basement. A thin sliver of faulted and metamorphosed rock, including amphibolites, partially separates the Badger Head Group from the Andersons Creek Ultramafic Complex. Mafic rocks in this package match geochemically mafic rocks in the Port Sorell Formation. This match is consistent with two structural events in the Badger Head Group showing tectonic transport of the group from the west during Cambrian Delamerian orogenesis. Rather than being subducted, emplacement of the Badger Head Group onto the Andersons Creek Ultramafic Complex indicates accretion of the Badger Head Group onto eastern Tasmania. Subsequent folding and thrusting in the west Tamar region also accompanied Devonian Tabberabberan orogenesis. Reversal from northeast to southwest tectonic vergence saw imbricate thrusting of Proterozoic and Palaeozoic strata, possibly coinciding with reactivation of the suture separating eastern and western Tasmania.  相似文献   

Formation of lode‐style gold mineralisation during Tabberabberan wrench faulting at Lefroy,eastern Tasmania     

A. Reed 《Australian Journal of Earth Sciences》2013,60(5):879-890

The Lefroy Goldfield in eastern Tasmania is anomalous in southeastern Australia because mineralised fault reefs (i.e. reefs that are also faults) strike in an easterly direction at a high angle to the predominantly northwest strike of bedding and folds. Gold mineralisation is of Early to Middle Devonian age, with reef formation coinciding with a third regionally compressive deformation event (D₃), and a second phase of Tabberabberan orogenesis. Mineralised reefs are hosted by Mathinna Supergroup turbidites of Cambrian to Ordovician age and extend for up to 2 km across the boundary between the sandstone‐dominated Stony Head Sandstone and the shale‐dominated Turquoise Bluff Slate. Ore shoots in the reefs plunge moderately west and, in the Volunteer Mine, coincide with the intersection of the reef and a D₁/D₂ thrust contact. The subvertical orientation and discordant relationship of the mineralised reefs to bedding, as well as the lack of gold mineralisation along bedding and pre‐D₃ structures, indicate that the reefs formed during a period of wrench faulting. In contrast to lode‐style deposits in Victoria, the far‐field minimum compressive stress at Lefroy during reef formation was not vertical but, rather, occupied a subhorizontal orientation.  相似文献   

Magnetic characterisation and correlation of a Younger Dryas tephra in North Atlantic marine sediments     

Clare Peters  William E. N. Austin  John Walden  Fiona D. Hibbert 《第四纪科学杂志》2010,25(3):339-347

A technique for identifying non‐visible basaltic tephra‐rich horizons of Younger Dryas (YD)/Greenland Stadial (GS) 1 age in northeast Atlantic sediments using rapid, non‐destructive magnetic measurements is presented. Three high‐resolution marine sediment cores have been studied in an E–W transect across the Hebridean margin: St Kilda Basin (MD95‐2007), Barra Fan (MD95‐2006) and Rockall Trough (MD04‐2822). Magnetic susceptibilities and remanent magnetisations were measured at contiguous 1 cm resolution on bulk sediments. In all three cores, an interval with higher proportions of hard magnetic minerals coincides with a clearly defined peak in basaltic tephra shard (>250 µm) counts, which can be constrained to the early part of the YD/GS1 based on faunal climate proxies. Electron microprobe analyses of the magnetically distinct basaltic tephra interval, in all three cores, displays the same major element geochemistry as published for the Vedde basaltic (I Tab. 1), i.e. sourced from the Icelandic volcano Katla. The identification of transitional alkalic basaltic tephras within marine sediments could potentially be facilitated by magnetic analysis as a useful chronostratigraphic screening tool. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

Late-Quaternary biogeographic scenarios for the brown bear (Ursus arctos), a wild mammal model species     

John Davison  Simon Y.W. Ho  Sarah C. Bray  Marju Korsten  Egle Tammeleht  Maris Hindrikson  Kjartan Østbye  Eivind Østbye  Stein-Erik Lauritzen  Jeremy Austin  Alan Cooper  Urmas Saarma 《Quaternary Science Reviews》2011,30(3-4):418-430

This review provides an up-to-date synthesis of the matrilineal phylogeography of a uniquely well-studied Holarctic mammal, the brown bear. We extend current knowledge by presenting a DNA sequence derived from one of the earliest known fossils of a polar bear (dated to 115 000 years before present), a species that shares a paraphyletic mitochondrial association with brown bears. A molecular clock analysis of 140 mitochondrial DNA sequences, including our new polar bear sequence, provides novel insights into the times of origin for different brown bear clades. We propose a number of regional biogeographic scenarios based on genetic data, divergence time estimates and paleontological records. The case of the brown bear provides an example for researchers working with less well-studied taxa: it shows clearly that phylogeographic models based on patterns of modern genetic variation alone can be substantially improved by including data on historical patterns of genetic diversity in the form of ancient DNA sequences derived from accurately dated samples and by using an approach to divergence-time estimation that suits the data under analysis. Using such approaches it has been possible to (i) establish that the processes shaping modern genetic diversity in brown bears acted recently, within the last three glacial cycles; (ii) distinguish among hypotheses concerning species’ responses to climatic oscillations in accordance with the lack of phylogeographic structure that existed in brown bears prior to the last glacial maximum (LGM); (iii) reassess theories linking monophyletic brown bear populations to particular LGM refuge areas; and (iv) identify vicariance events and track analogous patterns of migration by brown bears out of Eurasia to North America and Japan.  相似文献   

Discovery of absorption features of the accretion disc corona and systematic acceleration of the X-ray burst rate in XB 1323-619     

M. J. Church  D. Reed  T. Dotani  M. Ba&#;uci&#;ska-Church  A. P. Smale 《Monthly notices of the Royal Astronomical Society》2005,359(4):1336-1344

  相似文献   

In situ cosmogenic exposure ages from the Isle of Skye,northwest Scotland: implications for the timing of deglaciation and readvance from 15 to 11 ka     

David Small  Vincent Rinterknecht  William Austin  Derek Fabel  Maria Miguens‐Rodriguez  Sheng xu 《第四纪科学杂志》2012,27(2):150-158

We present 10 in situ cosmogenic exposure ages from two moraines on the Isle of Skye. The Strollamus medial moraine was deposited during deglaciation of the Devensian ice sheet and yields a mean exposure age from five samples of 14.3 ± 0.9 ka. The moraine age indicates that a significant ice mass existed on Skye at the time of a regional readvance recorded in Wester Ross, northwest Scotland. Taken at face value the ages suggest that deglaciation did not occur until well into Greenland Interstade 1. The Slapin moraine represents the local limit of the Loch Lomond Readvance (LLR) and yields a mean exposure age from five samples of 11.5 ± 0.7 ka, which is consistent with deposition relating to the LLR. These ages suggest that the maximum extent may have been reached late in the stadial and that some glaciers may have remained active until after the climatic amelioration that marks its end. This scenario is considered unlikely given the nature of the climate during this period, which leads us to call for a locally calibrated production rate. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Estimating long-term soil respiration rates from carbon isotopes occluded in gibbsite     

Paul A. Schroeder  Jason C. Austin  John F. Dowd 《Geochimica et cosmochimica acta》2006,70(23):5692-5697

Carbon occluded in the soil gibbsite crystal structure at the Panola Mountain Research Watershed, Georgia, U.S. is presumed to be in isotopic equilibrium with the CO₂ respired from soil organics by microbes and plant roots. Fitting of the stable carbon isotopic data to a Fickian diffusion-based depth function results in an estimate of 47 gC m⁻² y⁻¹ for the long-term soil respiration rate. A numerical model that includes depth-dependent production and diffusion terms results in estimates of 28-12 gC m⁻² y⁻¹. These values range from 15 to 50 times less than the average of modern values for mixed deciduous forests in wet temperate climates. This disparity has several implications for our understanding of the geologic record of climate change, which include: (1) evidence for a cooler and seasonally drier climate during the mid-Holocene in the southeastern U.S., or (2) fluxes of carbon from the soil pool as recorded by soil mineral proxies (i.e., long-term soil respiration rates) under estimate atmosphere annual carbon flux measurements (i.e., short-term measures), and (3) the need to refine soil respiration models used to relate paleosol stable carbon isotopic measurements to paleo-atmospheric estimates.  相似文献   

Time-resolved spectroscopy of the sdBV V338 Serpentis     

R. H. Østensen  J. H. Telting  R. Oreiro  U. Heber  E. De Beck  M. Reed 《Astrophysics and Space Science》2010,329(1-2):167-174

We obtained 1955 high-resolution spectra of the high-amplitude pulsating subdwarf B star V338 Ser with the FIES spectrograph at the 2.5 m Nordic Optical Telescope covering 4 full nights in May–June 2009. Here we present the first results from phase-folding the data on the main pulsation mode, as revealed by simultaneous B-band photometry obtained with the Mercator 1.25 m and Baker 0.4 m telescopes. We compare the results with a sample line-profile model produced with LTE atmospheres and bruce+kylie.  相似文献   

Analysis and modeling of a tropical-like cyclonein the Mediterranean Sea     

R. J. Reed  Y.-H. Kuo  M. D. Albright  K. Gao  Y.-R. Guo  W. Huang 《Meteorology and Atmospheric Physics》2001,76(3-4):183-202

  相似文献