Showing papers by "Geophysical Survey published in 1967"
TL;DR: In this article, it was shown that the anomaly patterns of the islands in the Southern Cook Group which lie on a suboceanic ridge are not related to topography and must arise in part from sources beneath the ridge.
Abstract: Total magnetic force measurements around several of the Cook Islands have shown that the anomaly patterns of the islands in the Southern Cook Group which lie on a suboceanic ridge are not related to topography and must arise in part from sources beneath the ridge. Bipolar anomalies in the vicinity of the isolated islands of Rarotonga (Southern Cook Group) and Manihiki (Northern Cook Group) can be explained by a more or less homogeneous magnetisation of these extinct volcanoes. Latitudes of virtual palaeomagnetic poles obtained from inferred directions of magnetisation are about 73° S for Rarotonga and about 13° S for Manihiki, indicating a relatively short volcanic history for Raiotonga but a pre-Cretaceous age for Manihiki.
57 citations
TL;DR: In this article, it is suggested that a buried serpentinised ultramafic system might be the most appropriate geological model to explain the anomalies; and that the Dun Mountain Ultramafics may either have been emplaced directly from this system, or may be the surface expression of these anomalies displaced to the east as part of a sheet type movement.
Abstract: Recent magnetic and gravity surveys in the Nelson region permit a fuller discussion of the nature of the long major magnetic anomaly associated with the trend of the Nelson Syncline. The previous view that the anomaly is due to buried Upper Paleozoic igneous rocks such as the Brook Street Volcanics and Rotoroa Igneous Complex is held to be incompatible in some ways with the relationships between gravity and magnetic force values and with the physical properties of these rocks. It is suggested that a buried serpentinised ultramafic system might be the most appropriate geological model to explain the anomalies; and that the Dun Mountain Ultramafics may either have been emplaced directly from this system, or may be the surface expression of these anomalies displaced to the east as part of a sheet-type movement.
35 citations
TL;DR: The magnetic anomaly associated with the North Macquarie Ridge is related to other geophysical features of south-west New Zealand, such as the So lander Trough as mentioned in this paper.
Abstract: Associated with the North Macquarie Ridge is a positive total magnetic force anomaly, of 500–600 gammas amplitude and between 5 and 30 kilometres in width. The course of this anomaly is traced from Lat. 51° S, Long. 163° E, for a distance of 500 km north-north-east to Preservation Inlet, New Zealand, where measurements cease. A subsidiary positive anomaly appears to be associated with the west flank of the So lander Trough. The magnetic anomaly associated with the North Macquarie Ridge is related to other geophysical features of south-west New Zealand.
21 citations
TL;DR: An eruption of mud and steam at Raoul Island on 1964 November 20, was preceded by a swarm of earthquakes, which started on November 10. as mentioned in this paper reported that within four hours of the start of the swarm, the frequency of shocks recorded on the island's seismograph had risen to a maximum of over 80 an hour.
Abstract: An eruption of mud and steam at Raoul Island on 1964 November 20, was preceded by a swarm of earthquakes, which started on November 10. Within four hours of the start of the swarm, the frequency of shocks recorded on the island's seismograph had risen to a maximum of over 80 an hour. Volcanic tremor soon appeared, and became continuous by November 12, masking all but the largest earthquakes. Thereafter, both the level of tremor and the frequency of earthquakes declined until by the time of the eruption only 10–15 shocks an hour were being recorded. Following the eruption, the island was evacuated for two weeks. For four days after the reoccupation on December 6, three additional seismographs were installed and during this time the earthquakes occurred to the west and south-west of the crater, at distances of 4–8 km from it, and at depths of up to 8 km. The amplitude-frequency relationship does not show such a high proportion of smaller earthquakes as has been reported for some other volcanic swar...
12 citations
TL;DR: The magnetic anomalies exhibit characteristic patterns which may be correlated with different physiographic regions as discussed by the authors, and there is an indication of a north-easterly trend in the magnetic anomalies over the Lan Basin between Fiji and Samoa.
Abstract: Total magnetic force and bathymetric profiles totalling 25,000 km have been measured in the South-West Pacific Ocean between New Zealand, Fiji, and the Cook Islands. The magnetic anomalies exhibit characteristic patterns which may be correlated with different physiographic regions. Anomalies of amplitude less than 200 γ prevail over the Norfolk Basin, the flanks of the Tonga-Kermadec Ridge, and the South-West Pacific Basin north of 18° S, including the Manihiki Plateau. Amplitudes from 200 to 500 γ occur over the South Fiji Basin, Three Kings Rise, Colville Ridge, Havre Trough, and the South-West Pacific Basin south of 18° S; and with a marked short wavelength (<10 km) component over the North Fiji Basin, Hunter Ridge, and Lau Basin. The largest anomalies, with amplitudes between 500 and 1,000 γ, are found over the Kermadec Ridge. From a comparison of adjacent profiles there is an indication of a north-easterly trend in the magnetic anomalies over the Lan Basin between Fiji and Samoa. Model studi...
9 citations