排序方式： 共有30条查询结果，搜索用时 109 毫秒
1.
The study investigated the global and regional phylogeography of the yellowtail kingfish Seriola lalandi by examining genetic diversity and population genetic structure of this species at inter-and intra-ocean level and on a regional scale. DNA fragments of two mitochondrial genes, cytochrome b (Cytb) and cytochrome c oxidase subunit I (COI) and one nuclear gene, recombination activating gene 1 (RAG1), were sequenced to investigate the global-scale phylogeography of this species. The population genetic structure within the South Pacific, as well as along the South African coastline, was examined further using six microsatellite markers. Three distinct clades were identified for S. lalandi, which correspond with previously described subspecies of the North-East Pacific, North-West Pacific and the Southern Hemisphere. Within the latter, additional divergence was observed between the South Pacific and the South-East Atlantic regions. Divergence estimates were indicative of a Pacific origin for S. lalandi populations, because of Pleistocene vicariant events. Microsatellite analyses revealed overall significant genetic differentiation between South African and South Pacific samples. This corroborates recent findings on the global phylogeography of the species. No population differentiation was observed within South Africa, indicating high levels of gene flow. 相似文献
2.
summary . In Fishnish, eight dykes of E-W trend are all normally magnetized and all cut dykes of other trends. Normally magnetized dykes form a small minority of the dykes in the British Tertiary igneous province and this is the first reported incidence of a clearly defined group of a single polarity. The balance of the evidence - which is conflicting — favours the E—W dykes being contemporary with the later parts of the centre. 相似文献
3.
This paper extends the evidence that volcanic rocks may contain initial argon, and investigates some of the implications of the results.The incorporation of initial argon with a composition different to that of air at the time of crystallisation may be an important cause of scatter of K-Ar dates in areas such as the British Tertiary igneous province, but in favourable circumstances the correct age may be recovered.An age of about 60 Ma is indicated for the lavas of Northern Ireland. 相似文献
4.
5.
Palaeomagnetism and radiometric dating of the British Tertiary Igneous Province: Muck and Eigg 总被引：1，自引：0，他引：1
Summary. The lavas, dykes and other intrusive bodies of Muck and Eigg have been sampled for palaeomagnetic purposes. All the bodies are reversely magnetized except for four dykes of Muck which are normally magnetized. The lavas show variations of direction of magnetism believed to be a record of secular variation.
The radiometric dates support the geological evidence that the lavas of Eigg and Muck predate parts of the Rhum central igneous complex, the lavas of Rhum and Canna and some of those of Skye. The combined evidence shows that most of the activity took place during an R-N-R-N sequence between 63 and 58 Ma. The younger R-N sequence is the same as the first part of the R-N-R sequence found in Mull but the Sgurr pitchstone of Eigg and parts of the Eastern Redhills of Skye belong to later reversed intervals and correspond to Chron 26r and Chron 26. The igneous activity occurred over a 10 Myr period from about 63 Ma and can probably be associated with Chrons 27r to 23r of the Harland et al. time-scale. 相似文献
The radiometric dates support the geological evidence that the lavas of Eigg and Muck predate parts of the Rhum central igneous complex, the lavas of Rhum and Canna and some of those of Skye. The combined evidence shows that most of the activity took place during an R-N-R-N sequence between 63 and 58 Ma. The younger R-N sequence is the same as the first part of the R-N-R sequence found in Mull but the Sgurr pitchstone of Eigg and parts of the Eastern Redhills of Skye belong to later reversed intervals and correspond to Chron 26r and Chron 26. The igneous activity occurred over a 10 Myr period from about 63 Ma and can probably be associated with Chrons 27r to 23r of the Harland et al. time-scale. 相似文献
6.
7.
8.
9.
10.
Summary. Five hundred and sixteen sites in Arran were sampled. After cleaning by heating and/or alternating fields 87 per cent yielded directions with α95 < 10°, comprising 435 dykes, 12 sills and one granite; polarities could be determined for some of the remainder. Directions are thought to be primary and on average are shallower than corresponds to the mean centred axial dipole, with many shallow reversely-magnetized dykes present.
Seventeen per cent of the dykes are normally magnetized, 5 per cent have intermediate directions. Polarity does not correlate with petrography, thickness or trend, but does depend upon location. In particular, on the NE coast up to 70 per cent are normal, and possibly belong to a different swarm.
Normal polarities were found for both sites in the Northern Granite, for Holy Island and for some of the sills. In conjunction with the known stratigraphy this shows that the rocks were formed during an R-N-R sequence of polarities, or more probably R-N-R-N. Using the radiometric age of 58 Ma for the Northern Granite and the best estimate of the polarity time-scale the duration of activity was probably in the range 0.5 to 3.4 Ma if three polarity periods were involved, and 3.4 to 4.4 Ma if four were involved. 相似文献
Seventeen per cent of the dykes are normally magnetized, 5 per cent have intermediate directions. Polarity does not correlate with petrography, thickness or trend, but does depend upon location. In particular, on the NE coast up to 70 per cent are normal, and possibly belong to a different swarm.
Normal polarities were found for both sites in the Northern Granite, for Holy Island and for some of the sills. In conjunction with the known stratigraphy this shows that the rocks were formed during an R-N-R sequence of polarities, or more probably R-N-R-N. Using the radiometric age of 58 Ma for the Northern Granite and the best estimate of the polarity time-scale the duration of activity was probably in the range 0.5 to 3.4 Ma if three polarity periods were involved, and 3.4 to 4.4 Ma if four were involved. 相似文献