A geophysical and geologic study of Nagata Seamount, northern Line Islands.

Abstract

Nagata Seamount, located in the northern Line Islands at 12. 5N, 167. 0W, was surveyed and dredged by the Hawaii Institute of Geophysics in the fall of 1979. The seamount is 3. 7 km in height, approximately 45 km at maximum width, and has an estimated volume of 590 km3 Analysis of dredge rocks indicates that the seamount consists of alkali basalts and hyaloclastite breccias. Both acoustic reflection records and magnetic modeling studies indicate that the seamount is draped with a large volume of volcaniclastic debris about its base. At some time in its history Nagata Seamount may have been at or near sea level.The bulk of the seamount is made up of lavas of normal magnetic polarity; however, the uppermost 1, 000 m consists of reversely polarized rock. The virtual geomagnetic pole determined from the seamount falls at 61. 6N, 4. 0E not far from the average Cretaceous Pacific seamount pole. Rock magnetism studies of the dredged basalts indicate a stable magnetization. Three dimensional gravity modeling determined an average density of 2. 5 g/cm3 for the seamount. This value of the density agrees with other similar data from Pacific and Atlantic seamounts; although, unlike some other seamounts studied gravitationally, no large density variations were revealed on Nagata Seamount.K/Ar radiometric ages of 70 Ma and 86 Ma were obtained from the dredged basalts in agreement with the paleomagnetic pole. The younger age may result from alteration, thus the older date is considered more representative. This age is similar to that of nearby Kapsitotwa Seamount.

Isostatic Compensation and Conduit Structures of Western Pacific Seamounts: Results of Three-Dimensional Gravity Modeling

“…This problem is one that has been considered before although this derivation was done independently. Previous efforts in this area include the derivation of the anomaly at points off-axis for a circular disk or vertical cylinder (Parasnis, 1961), for a right circular cone (Schwank and Lazarewicz, 1982), and the work of Moon (1981) and Jenkins et al (1983) which have focused on deriving analyti cal expressions for more complex cases where the density is laterally varying. Our derivation assumes constant density for the structure, although more complex density distributions may be constructed by superposition.…”

“…(1975). andSager et al (1982). The bathymetry for Sio and Ita Mai Tai guyots was approximated with 33 and 11 vertical sided polygonal prisms respectively (Figures 3 and 4).…”

“…A major contribution to the marine geoid is made by seafloor bathymetry, because of the large density contrast at the rock-water interface. Attempts have therefore been made to use satellite altimeter data to predict bathymetry (e.g., Lazarewicz and Schwank, 1982;Dixon et al, 1983). Such a tool would be especially asd'ul in the South and West Pacific where ship crossings are rare.…”

Magnetic Petrology of Basalts from Ninetyeast Ridge

“…While attempts have been made to acquire information about the source by sophisticated inversion techniques (e.g., Harrison, 1971;Sager et al, 1982;Parker et al, 1987) a substantial degree of nonuniqueness is unavoidable. Direct information about the magnetization of seamounts can only be obtained by the study of samples of seamount lithologies potentially capable of contributing to a magnetic anomaly.…”

Magnetization of Erimo seamount.

“…UYEDA and RICHARDS (1966) determined the magnetization vector of the seamount and a planar regional field by inversion of the observed anomalies. This is a conventional and widely used way of seamount paleomagnetism (VACQUIER and UYEDA,1967;FRANCHETEAU et al,1970;SAGER et al,1982;UEDA,1985;MCNUTT, 1986). But the assumption of a planar regional field for removing the effect of the magnetic anomalies caused by bodies other than the seamount should be used with caution (MCNUTT, 1986).…”