HST imaging of the globular clusters in the Fornax cluster: NGC 1379     

Rebecca A. W.Elson  Carl J.Grillmair  Duncan A.Forbes  MikeRabban  Gerard M.Williger  Jean P.Brodie 《Monthly notices of the Royal Astronomical Society》1998,295(1):240-250

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Comments on the development of rifts and transform faults during continental breakup; examples from the Gulf of Aden and northern Red Sea     

Duncan Tamsett 《Tectonophysics》1984,104(1-2)

A number of basins are observed to extend inland from the coasts on both sides of the Gulf of Aden. The basins are orientated at approximately right angles to the spreading direction and intersect the coasts at the meeting of sheared and rifted continental margins. They appear to be grabens, one wall of which is continuous with the half graben of the neighbouring rifted margin. It is suggested that these were once parts of a number of discrete rifts arranged en-echelon along a zone of lithospheric weakness during the early opening of the Gulf of Aden, which became redundant when transform faults formed. The proposed development of rifts and transform faults is similar to that of a spreading centre, transform fault, spreading centre pattern developed in the freezing wax model of Oldenburg and Brune (1975). The Gulf of Suez at the northern end of the Red Sea is interpreted in a similar way since it has a number of features in common with the basins in the continents adjacent to the Gulf of Aden.  相似文献   

Manganese chemistry in rivers and streams     

Duncan P.H. Laxen  William Davison  Colin Woof 《Geochimica et cosmochimica acta》1984,48(10):2107-2111

The speciation of Mn has been determined in 15 rivers and streams representing a wide variety of physico-chemical conditions. Using the technique of anodic stripping voltammetry (asv), specific for reduced Mn(II) species, it is found that a major part of the <0.015 μm Mn size fraction is present in a reduced Mn(II), asv-labile, form. In some waters there is also a significant asv inactive Mn fraction considered to be present as a ‘small colloidal’ species. The soluble (<0.015 μm) Mn fraction represents 15–95% of total Mn and does not appear to be dependent upon pH, alkalinity, specific conductance or humic substance concentration in the water. It is argued that under the dynamic, short residence time, conditions that apply in most rivers the paniculate and soluble Mn fractions are decoupled, their respective presence being dependent principally upon the catchment hydrogeological conditions. This contrasts with a previously held view that the paniculate phase is coupled to the dissolved phase by the pH dependent oxidation of dissolved Mn(II) to highly insoluble Mn(IV) species (Graham et al., 1976). Consideration of manganese speciation in waters which were incubated for five months showed that pH becomes the controlling factor when equilibrium is approached.  相似文献   

Reconstructing the ancestral Yellowstone plume from accreted seamounts and its relationship to flat-slab subduction     

J.Brendan Murphy  Andrew J. Hynes  Stephen T. Johnston  J.Duncan Keppie 《Tectonophysics》2003,365(1-4):185

Recent geodynamic analyses have emphasized the relationship between modern flat-slab subduction zones and the overriding of buoyant oceanic crust. Although most models for the evolution of the Late Mesozoic–Cenozoic Laramide orogeny in the southwestern United States involve flat-slab subduction, the mechanisms proposed are controversial. An examination of the geological evolution of the 60–50-Ma Crescent terrane of the Coast Ranges indicates that it was formed in a shallowing-upward Loihi-type oceanic setting culminating in the eruption of subaerial lavas. Plate reconstructions indicate that the Crescent terrane was emplaced into ca. 20-Ma crust, and the presence of subaerial lavas implies an uplift due to the plume of ca. 4.2 km, which we use to calculate a minimum buoyancy flux of 1.1 Mg s⁻¹, similar to that of the modern Yellowstone plume.Published paleomagnetic data indicate that the Crescent terrane was formed at a paleolatitude similar to that of the Yellowstone plume. The Crescent seamount was accreted within 5 My of the cessation of plume magmatism. Plate reconstructions indicate that it would have originated about 750 km to the west of the North American plate margin if it developed above a fixed Yellowstone plume, and are therefore consistent with the recorded very short interval between its formation and tectonic emplacement.We interpret the Crescent terrane as due to the ancestral Yellowstone plume. Such a plume would have generated an elongate swell and related plateau that would have been overridden by the North American margin. Taken together, the relationship between flat-slab and overriding of oceanic plateau in Laramide times would have been analogous to the relationship between modern Andean flat-slab subduction zones and the Juan Fernandez and Nazca oceanic plateaus.  相似文献   

Tethyan, Mediterranean, and Pacific analogues for the Neoproterozoic–Paleozoic birth and development of peri-Gondwanan terranes and their transfer to Laurentia and Laurussia     

J.Duncan Keppie  R.Damian Nance  J.Brendan Murphy  J. Dostal 《Tectonophysics》2003,365(1-4):195

Modern Tethyan, Mediterranean, and Pacific analogues are considered for several Appalachian, Caledonian, and Variscan terranes (Carolina, West and East Avalonia, Oaxaquia, Chortis, Maya, Suwannee, and Cadomia) that originated along the northern margin of Neoproterozoic Gondwana. These terranes record a protracted geological history that includes: (1) 1 Ga (Carolina, Avalonia, Oaxaquia, Chortis, and Suwannee) or 2 Ga (Cadomia) basement; (2) 750–600 Ma arc magmatism that diachronously switched to rift magmatism between 590 and 540 Ma, accompanied by development of rift basins and core complexes, in the absence of collisional orogenesis; (3) latest Neoproterozoic–Cambrian separation of Avalonia and Carolina from Gondwana leading to faunal endemism and the development of bordering passive margins; (4) Ordovician transport of Avalonia and Carolina across Iapetus terminating in Late Ordovician–Early Silurian accretion to the eastern Laurentian margin followed by dispersion along this margin; (5) Siluro-Devonian transfer of Cadomia across the Rheic Ocean; and (6) Permo-Carboniferous transfer of Oaxaquia, Chortis, Maya, and Suwannee during the amalgamation of Pangea. Three potential models are provided by more recent tectonic analogues: (1) an “accordion” model based on the orthogonal opening and closing of Alpine Tethys and the Mediterranean; (2) a “bulldozer” model based on forward-modelling of Australia during which oceanic plateaus are dispersed along the Australian plate margin; and (3) a “Baja” model based on the Pacific margin of North America where the diachronous replacement of subduction by transform faulting as a result of ridge–trench collision has been followed by rifting and the transfer of Baja California to the Pacific Plate. Future transport and accretion along the western Laurentian margin may mimic that of Baja British Columbia. Present geological data for Avalonia and Carolina favour a transition from a “Baja” model to a “bulldozer” model. By analogy with the eastern Pacific, we name the oceanic plates off northern Gondwana: Merlin (≡Farallon), Morgana (≡Pacific), and Mordred (≡Kula). If Neoproterozoic subduction was towards Gondwana, application of this combined model requires a total rotation of East Avalonia and Carolina through 180° either during separation (using a western Transverse Ranges model), during accretion (using a Baja British Columbia “train wreck” model), or during dispersion (using an Australia “bulldozer” model). On the other hand, Siluro-Devonian orthogonal transfer (“accordion” model) from northern Africa to southern Laurussia followed by a Carboniferous “Baja” model appears to best fit the existing data for Cadomia. Finally, Oaxaquia, Chortis, Maya, and Suwannee appear to have been transported along the margin of Gondwana until it collided with southern Laurentia on whose margin they were stranded following the breakup of Pangea. Forward modeling of a closing Mediterranean followed by breakup on the African margin may provide a modern analogue. These actualistic models differ in their dictates on the initial distribution of the peri-Gondwanan terranes and can be tested by comparing features of the modern analogues with their ancient tectonic counterparts.  相似文献   

Time-scales of assembly and thermal history of a composite felsic pluton: constraints from the Emerald Lake area, northern Canadian Cordillera, Yukon   总被引：1，自引：0，他引：1  

Ian M. Coulson  Mike E. Villeneuve  Gregory M. Dipple  Robert A. Duncan  James K. Russell  James K. Mortensen 《Journal of Volcanology and Geothermal Research》2002,114(3-4)

Knowledge of the time-scales of emplacement and thermal history during assembly of composite felsic plutons in the shallow crust are critical to deciphering the processes of crustal growth and magma chamber development. Detailed petrological and chemical study of the mid-Cretaceous, composite Emerald Lake pluton, from the northern Canadian Cordillera, Yukon Territory, coupled with U–Pb and ⁴⁰Ar/³⁹Ar geochronology, indicates that this pluton was intruded as a series of magmatic pulses. Intrusion of these pulses produced a strong petrological zonation from augite syenite, hornblende quartz syenite and monzonite, to biotite granite. Our data further indicate that multiple phases were emplaced and cooled to below the mineral closure temperatures over a time-scale on the order of the resolution of the ⁴⁰Ar/³⁹Ar technique (1 Myr), and that emplacement occurred at 94.3 Ma. Simple thermal modelling and heat conduction calculations were used to further constrain the temporal relationships within the intrusion. These calculations are consistent with the geochronology and show that emplacement and cooling were complete in less than 100 kyr and probably 70±5 kyr. These results demonstrate that production, transport and emplacement of the different phases of the Emerald Lake pluton occurred essentially simultaneously, and that these processes must also have been closely related in time and space. By analogy, these results provide insights into the assembly and petrogenesis of other complex intrusions and ultimately lead to an understanding of the processes involved in crustal development.  相似文献   

The Cretaceous Messum igneous complex, S.W. Etendeka, Namibia: reinterpretation in terms of a downsag-cauldron subsidence model     

A. Ewart  S. C. Milner  A. R. Duncan  M. Bailey 《Journal of Volcanology and Geothermal Research》2002,114(3-4)

The Messum igneous complex (MIC) lies within the ENE-trending zone of Lower Cretaceous (132 Ma) Damaraland intrusive complexes in Namibia, intruded into Pan-African Damara basement. It is defined by a roughly circular structure 18 km in diameter, the bounding ring fault exposed along the eastern sector. Encircling Messum are the volcanic sequences of the Goboboseb Mountains, comprising a lower basalt series (Tafelkop and Tafelberg types) followed, with intervening basalts, by four voluminous quartz latite (QL) eruptions (Goboboseb and Springbok QL units).Inferred stages of development are: (a) an initial very broad basaltic lava shield, comprising the Tafelberg and Tafelkop basalts, and Messum crater basalts (MCB; possibly ponded in near-vent lava lakes). Embedded within the lower basaltic sequence is a localised rhyolite-dominated eruptive centre (ca. 5 km in diameter), interpreted as a funnel caldera located towards the centre of the MIC. (b) Downsagging, extending northwards from Messum, following the Goboboseb QL eruptions (≥2300 km³). Ponding of overlying basaltic units. (c) Climactic Springbok QL eruption (≥6300 km³) producing further downsag together with the inward radial dip of all volcanic units towards the MIC. Ring fault initiation. (d) Cauldron subsidence emplacement of a granitoid suite, forming the MIC ‘moat’ (area between the ring fault and the core region). (e) Intrusion of gabbroic cone sheets into incompletely solidified granitic melts within the southeastern moat. Resulting hybridisation and magma mingling produced extensive development of heterogeneous granitoid and hybrid dioritic lithologies. (f) Cone sheet intrusions of the eastern gabbros into more highly solidified granitoids of the southeastern moat. (g) Intrusion of thick (1–2 km) western gabbro cone sheets, exhibiting local fine-scale layering, into solidified granitoids, mainly within the western moat. Minor late-stage granitic intrusions. (h) 2–3 Ma quiescent period followed by quartz- and ne-syenite intrusions, and finally basanite dykes, emplaced within the MIC core. Accompanying differential uplift of the core.Uplift/resurgence within the MIC has accompanied intrusion of the moat granitoids and mafic cone sheets, thereby juxtaposing volcanic and intrusive sequences. Phases of both subsidence and uplift have characterised the MIC. The NW Scotland Tertiary central igneous complexes and Messum show evidence of a number of parallel developments, but also important differences. The MIC differs markedly from caldera systems within the western USA and circum-Pacific. Messum is therefore suggested to represent a distinct class of intrusive/extrusive central complex, although probably common in large igneous provinces.  相似文献   

Symplectic integrators for long-term integrations in celestial mechanics     

Brett Gladman  Martin Duncan  Jeff Candy 《Celestial Mechanics and Dynamical Astronomy》1991,52(3):221-240

We discuss the use of symplectic integration algorithms in long-term integrations in the field of celestial mechanics. The methods' advantages and disadvantages (with respect to more common integration methods) are discussed. The numerical performance of the algorithms is evaluated using the 2-body and circular restricted 3-body problems. Symplectic integration methods have the advantages of linear phase error growth in the 2-body problem (unlike most other methods), good conservation of the integrals of the motion, good performance for moderately eccentric orbits, and ease of use. Its disadvantages include a relatively large number of force evaluations and an inability to continuously vary the step size.  相似文献   

Dynamical evolution of globular cluster systems in clusters of galaxies – I. The case of NGC 1404 in the Fornax cluster     

K. Bekki  Duncan A. Forbes  Michael A. Beasley  W. J. Couch 《Monthly notices of the Royal Astronomical Society》2003,344(4):1334-1344

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Last glacial megafaunal death assemblage and early human occupation at Lake Menindee, southeastern Australia     

Matthew L. Cupper  Jacqui Duncan 《Quaternary Research》2006,66(2):332-341

The Tedford subfossil locality at Lake Menindee preserves a diverse assemblage of marsupials, monotremes and placental rodents. Of the 38 mammal taxa recorded at the site, almost a third are of extinct megafauna. Some of the bones are articulated or semi-articulated and include almost complete skeletons, indicating that aeolian sediments rapidly buried the animals following death. New optical ages show the site dates to the early part of the last glacial (55,700 ± 1300 yr weighted mean age). This is close to the 51,200-39,800 yr Australia-wide extinction age for megafauna suggested by Roberts et al. [2001, Science 292:1888-1892], but like all previous researchers, we cannot conclusively determine whether humans were implicated in the deaths of the animals. Although an intrusive hearth at the site dating to 45,100 ± 1400 yr ago is the oldest evidence of human occupation of the Darling River, no artifacts were identified in situ within the sub-fossil-bearing unit. Non-anthropogenic causes, such as natural senescence or ecosystem stress due to climatic aridity, probably explain the mortality of the faunal assemblage at Lake Menindee.  相似文献