A Geophysical and Geologic Study of Nagata Seamount, Northern Line Islands

Abstract

Nagata Seamount, located in the northern Line Islands at 12.5°N, 167.0°W, was surveyed and dredged by the Hawaii Institute of Geophysics in the fall of 1979. The seamount is 3.7km in height, approximately 45km at maximum width, and has an estimated volume of 590km³. 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, 000m consists of reversely polarized rock. The virtual geomagnetic pole determined from the seamount falls at 61.6°N, 4.0°E 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 2.5g/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 70Ma and 86Ma 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.