Geology of ascension Island, South Atlantic Ocean

Nielson, Dennis L.; Sibbett, B.S.

doi:10.1016/0375-6505(96)00018-1

Cited by 44 publications

(20 citation statements)

References 18 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Descriptions of the geology of Ascension can be found in Nielson & Sibbett (1996). A detailed geological map of the island, including the most important geographical features, was reported by Nielson & Sibbett (1996).…”

Section: Continental Platform and Deep Ocean Magmatismmentioning

confidence: 99%

Geochronology (Ar/Ar and K–Ar) of the South Atlantic post-break-up magmatism

Geraldes

Motoki

Costa

et al. 2013

View full text Add to dashboard Cite

This work integrates the available geological information and geochronology data for the Cretaceous-Recent magmatism in the South Atlantic, represented by onshore and offshore magmatic events, including the oceanic islands along the transform faults and near the midocean ridge. The analysis of the igneous rocks and their tectonic settings allows new insights into the evolution of the African and Brazilian continental margins during the South Atlantic opening. Following the abundant volcanism in the Early Cretaceous, the magmatic quiescence during the Aptian-Albian times is a common characteristic of almost all Brazilian and West African marginal basins. However, rocks ascribed to the Cabo Granite (104 Ma) are observed in NE Brazil. In West Africa, sparse Aptian-Albian ages are observed in a few coastal igneous centres. In the SE Brazilian margin, an east-west alkaline magmatic trend is observed from Poços de Caldas to Cabo Frio, comprising igneous intrusions dated from 87 to 64 Ma. Mafic dyke swarms trending NW also occur in the region extending from the Cabo Frio Province towards the Central Brazilian Craton. On the West African side, Early Cretaceous -Recent volcanism is observed in the Walvis Ridge (139 Ma), the St Helena Ridge (81 Ma) and the Cameroon Volcanic Line (Early Tertiary-Recent). Volcanic islands such as Ascencion (1.0-0.65 Ma), Tristão da Cunha (2.5-0.13 Ma) and the St Helena islands (12 Ma) most probably correspond to mantle plumes or hot spots presently located near the mid-Atlantic spreading centre. Within

show abstract

Section: Continental Platform and Deep Ocean Magmatismmentioning

confidence: 99%

Geochronology (Ar/Ar and K–Ar) of the South Atlantic post-break-up magmatism

Geraldes

Motoki

Costa

et al. 2013

View full text Add to dashboard Cite

show abstract

“…Settings included a Yule prior to model lineage birth, a normal distribution of substitution rate (mean 2.13 + 0.065% divergence per million years; see [33]). We also calibrated the tree using an assumed maximum age of separation from the sister taxon F. aquila, of 1 Myr, based on geological dating of the emergence of Ascension Island [34]. BEAST analyses were run for up to 300 million generations, and convergence was checked in TRACER v. 1.5 (available from http://beast.bio.ed.ac.uk/Tracer) and by comparing results from independent runs.…”

Section: (A) Samplingmentioning

confidence: 99%

Long-term isolation of a highly mobile seabird on the Galapagos

et al. 2010

View full text Add to dashboard Cite

The Galapagos Islands are renowned for their high degree of endemism. Marine taxa inhabiting the archipelago might be expected to be an exception, because of their utilization of pelagic habitats-the dispersal barrier for terrestrial taxa-as foraging grounds. Magnificent frigatebirds (Fregata magnificens) have a highly vagile lifestyle and wide geographical distribution around the South and Central American coasts. Given the potentially high levels of gene flow among populations, the species provides a good test of the effectiveness of the Galapagos ecosystem in isolating populations of highly dispersive marine species. We studied patterns of genetic (mitochondrial DNA, microsatellites and nuclear introns) and morphological variation across the distribution of magnificent frigatebirds. Concordant with predictions from life-history traits, we found signatures of extensive gene flow over most of the range, even across the Isthmus of Panama, which is a major barrier to gene flow in other tropical seabirds. In contrast, individuals from the Galapagos were strongly differentiated from all conspecifics, and have probably been isolated for several hundred thousand years. Our finding is a powerful testimony to the evolutionary uniqueness of the taxa inhabiting the Galapagos archipelago and its associated marine ecosystems.

show abstract

“…We have no direct data on seafloor roughness, but the island sits on a young (< 6 Ma) volcanic edifice (Nielson and Sibbett, 1996) and does not have significant sediment cover (van Andel et al, 1973). This would indicate a rough seafloor and an efficient producer of Stoneley waves.…”

Section: Discussionmentioning

confidence: 94%

“…There is a marked increase in the signal loss at ASCN for T-phases arriving from 55" and 80". At this azimuth, radially away from ASCN, there is a large undifferentiated trachyte flow consisting in large part of volcanic ash (Nielson and Sibbett, 1996). This differs from most of the other paths which traverse mafic lava flows.…”

Section: Discussionmentioning

confidence: 94%

Performance of an island seismic station for recording T-phases

Hanson¹

1998

View full text Add to dashboard Cite

As part of the International Monitoring System (IMS) a worldwide hydroacoustic network consisting of 6 hydrophone and 5 island seismic stations has been planned which will monitor for underwater or low altitude atmospheric explosions. Data from this network is to be integrated with other IMS networks monitoring the Comprehensive Nuclear Test-Ban Treaty. The seismic (T-phase) stations are significantly less sensitive than hydrophones to ocean borne acoustic waves. T-phase signal strength-at seismic stations depends on the amplitude of the signal in the water column, the hydroacoustic-seismic conversion efficiency, and loss on the seismic portion of the path through the island. In order to understand how these factors influence the performance of T-phase stations seismic and hydroacoustic data are examined from instruments currently deployed on or around Ascension Island in the South Atlantic Ocean.T-phase recordings for the last 3 years have been collected from the GSN seismic station ASCN on Ascension Island. Surrounding the island are 5 hydrophones which are part of the U.S. Air Force Missile Impact Locating System (MILS). Data from this system have been obtained for some of the events observed at ASCN. Four of the hydrophones are located within 30 km of the coast while the fifth instrument is 100 km to the south.Amplitude spectral estimates of the signal-to-noise levels (SNL) are computed and generally peak between 3 and 8 Hz for both the seismometer and hydrophone data. The seismic SNL generally decays to 1 between 10 and 15 Hz while the hydrophone SNL is still large well above 20 Hz. The ratios of the hydrophone-to-seismometer SNL, at their peak in energy, range between 10 and 100 (20-40 dB) unless a hydrophone is partially blocked by the Ascension Island landmass. This ratio varies with respect to the azimuth of the arriving T-phase due to amplitude variations in both the hydrophone and seismic data. Signal loss at the hydrophones due to island blockage is modeled numerically and then corrected for. Variations between the corrected hydroacoustic amplitudes and seismic amplitudes are 3 compared with physical parameters such as the gradient of the topography at the islandocean interface. T-phases can have various modal structures which will couple into the island differently. Thus events from the same direction have different signal loss.

show abstract

Geology of ascension Island, South Atlantic Ocean

Cited by 44 publications

References 18 publications

Geochronology (Ar/Ar and K–Ar) of the South Atlantic post-break-up magmatism

Geochronology (Ar/Ar and K–Ar) of the South Atlantic post-break-up magmatism

Long-term isolation of a highly mobile seabird on the Galapagos

Performance of an island seismic station for recording T-phases

Contact Info

Product

Resources

About