Magnetic properties and paleomagnetism of some DSDP leg 33 basalts and sediment and their tectonic implications

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Paleomagnetic and magnetic property studies have been made on sediments and basalts recovered from holes 315A (northern Line Island chain) and 317A (Manihiki Plateau) during leg 33 of the Deep-Sea Drilling Project (DSDP). The purpose of this investigation is twofold: (1) to investigate the magnetic properties of basalts from holes 315A and 317A and (2) to determine the origin of the inclination discrepancy between hole 317A carbonate and hole 317A basalt as reported by Cockerham and Jarrard [1976]. Basalts cored at hole 315A, DSDP leg 33, have irreversible Js-T curves (typical of submarine basalts) with Curie temperatures of approximately 300°C. When they are viewed with reflecting light, large (30–100 μ ) anhedral homogeneous titanomagnetite grains of class 1 oxidation are observed. These basalts have acquired a large viscous remanence. The six flow units cored yield an average inclination of −30.9° (σ = 14.4). Hole 317A basalts have reversible Js-T curves and Curie temperatures of about 570°C. Some 317A basalts have a secondary Curie temperature at about 360°C. The two Curie temperatures in a sample are believed to represent two compositionally different titanomagnetites. Under reflected light, class 3–4 deuterically oxidized titanomagnetite grains are observed with abundant ilmenite exsolution lamellae. These basalts are very resistant to AF demagnetization (median destructive field (MDF) values of ∼250 Oe). A mean inclination of −65.2° (σ = 6.1) has been calculated for these basalts, while the overlying carbonate sediments have a 20° shallower inclination (Ī = 46.7, σ = 9.0). Volcaniclastic sediments, 250 m in thickness, separate the basalts from the carbonates. The mean inclination of the volcaniclastics is −62.2° (σ = 2.4); however, the lowest samples of the carbonates together with the uppermost volcaniclastic samples have inclinations that systematically span the 20° difference. The difference in inclinations of the carbonates and the basalts is believed to be a result of tectonic tilting. The remains of shallow water molluscan fauna, volcaniclastics derived from hyaloclastite eruptions, and the magnetic properties of 317A basalts all suggest a subaerially or very shallow water (<200m)eruption of the 317A basalts.