Abstract
The Lovejoy formation consists of undeformed basalt flows distributed across the northern Sierra Nevada, California, and has been dated by the potassium-argon method as early Miocene. Measurements of remanent magnetization of 200 specimens collected at 16 widely separated sites have provided 13 independent determinations of paleomagnetic field direction during the relatively short period of eruption. Of these, 5 are not greatly different from the field of a reversed axial dipole, while 8 correspond to virtual geomagnetic north poles ranging in latitude from 60° north to 8° south. Stability of magnetization was established by a. c. and thermal demagnetization. Viscous magnetization, with intensity of the order of 50% of the natural thermoremanent component, was completely removed by peak alternating fields of 75 to 200 oersteds. The mean of the 13 virtual geomagnetic north poles lies at 76° north latitude and 74° east longitude, but the precision parameter k for these directions is only 4. Attempts to relate the degree of divergence of individual poles from the mean (or from the geographic pole) to the apparent relative intensity of the ancient field were unsuccessful because observed irreversible increases up to five-fold in the susceptibilities of specimens heated to 600°C invalidate any comparison of natural T. R. M. to artificial T. R. M. It is concluded that these basalts were erupted during a period of time, about 24 million years ago, in which the earth's field was predominantly reversed but varied in direction to an unusual degree.
