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The samples investigated in this study came from the following two areas: a) The western slope of the East Pacific Rise (EPR) at about 9° N (Holes 420, 421, 422, 423, 427, 428, 428 A, and 429 A). The drill sites form a transect perpendicular to the EPR (see Figure 1). Only Site 427 is off this transect, about 100 km to the south, drilled in a deep trough in the Siqueiros fracture zone. The samples cover an age span from about 1.5 m.y. to 4.6 m.y.; all come from the Pacific plate. The EPR has a spreading rate of about 5.7 cm/y. at the latitude of the transect (this value has been obtained from an interpretation of the magnetic profile recorded during Leg 54, perpendicular to the EPR). b) The area at the Galapagos spreading center at 86° W (Holes 424, 424 A, B, and C, and 425). Sites 424 and 425 are located (Figure 1) on opposite sides of the spreading axis: Site 424 to the south of it on the Nasca plate drilled into an area of geothermal activity (about 0.7 m.y. old) and Site 425 north of the axis on the Cocos plate (about 1.8 m.y. old), in an area of high heat flow (about 5 HFU). The spreading rate at this longitude is 3.25 cm/y. (Sclater and Klitgord 1973). Both areas are situated very close to the equator, and a paleomagnetic correlation between the polarity of remanent magnetization of the drill cores and the sign of the overlying marine magnetic anomaly has therefore not been attempted. The samples are oriented only with respect to the vertical, which means that only the magnetization inclination can be determined. In the Galapagos area the Earth's magnetic field is nearly horizontal, and the sense of magnetization polarity of the drill cores cannot be measured (so it is not possible to tell if a sample is reversely or normally magnetized). At the latitude of the EPR holes the inclination of an Earth-centered dipole field is only 18° which is too shallow to give reliable results, since the secular variation of the Earth's magnetic field may easily exceed this value. In this study, therefore, we have emphasized rock magnetic investigations; in particular, the effect of submarine alteration of the magnetic mineral component has been investigated. MAGNETIC AGES OF THE DRILL SITES

Rock- and Paleomagnetism of Deep Sea Drilling Project Leg 54 BasaltsEast Pacific Rise and Galapagos Rift

Rock- and Paleomagnetism of Deep Sea Drilling Project Leg 54 BasaltsEast Pacific Rise and Galapagos Rift