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doi:10.2204/iodp.proc.320321.213.2013

Introduction

It has been suggested that amplitudes of marine magnetic anomalies globally decrease with age from 0 to 20–30 Ma and then increase with increasing age and remain at elevated levels for the period between 80 and 160 Ma (Bleil and Petersen, 1983; Johnson and Pariso, 1993b). Johnson and Pariso (1993b) compiled rock magnetism data from “normal” upper oceanic crusts from the Atlantic, Indian, and Pacific Oceans and showed that the intensity of natural remanent magnetization (NRM) varied consistently with the marine magnetic anomalies: NRM decays with age from 0 to 30 Ma and then increases prior to 40 Ma. A similar data compilation was independently made by Furuta (1993), and it showed similar results. These compilations were from relevant Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) data that were available at that time.

Gee and Kent (2007) reported that the decrease of marine magnetic anomaly amplitude is only evident at slow-spreading ridges in the Pacific Ocean. They also pointed out that the temporal trend in NRM reported from the Johnson and Pariso (1993b) compilation should be regarded as tentative. That study reported a small number of samples and it did not account for variations in geochemistry and with-flow grain size, among other factors.

Significant problems with this compilation are that it ignored plate motion and was made from NRM data. Also, the compilations were made from NRM data without regard to the oceanic plates where the samples were collected. Igneous basement rocks have been recovered intermittently during ODP and Integrated Ocean Drilling Program (IODP) cruises, and researchers have often measured and reported NRMs of rocks. An increase of igneous basement rock NRM data makes it possible to make a more detailed compilation for individual oceanic plates. Although NRM data are more or less contaminated by spurious overprint fields, such compilation is considered to be useful for several reasons (for example, discussion for magnetic “evolution” of individual oceanic plates).

Because data compilation for individual oceanic plates is laborious, as a case study, we compiled NRM data from DSDP and ODP sites on the Pacific plate. We also conducted rock magnetism and paleomagnetism studies on igneous basement rocks recovered from five sites during IODP Expedition 320/321 in order to provide new data.

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