Remanent magnetization, cooling history, and paleomagnetic record of the Marys Peak Sill, Oregon

Abstract

A paleomagnetic investigation of the middle Oligocene (29.6 m.y.) Marys Peak sill (44°30′N, 123°35′W) demonstrates a relationship between the intrusion's cooling history and its remanent magnetization. The 390-meter-thick sill is vertically differentiated, varying in compositon from a gabbro to a diorite with associated pegmatite and aplite facies. From 7 to 10 cores per sampling site were taken at 26 sites separated vertically by 7 to 10 meters along a traverse through the sill. Directions and intensities of remanent magnetization were measured for all specimens at the original magnetization level and after alternating-field demagnetization at 100−, 200−, and 400-oe peak demagnetizing fields. Demagnetization behavior. Curie temperatures, and polished section examinations indicate that the remanence is stable. The directions of natural remanent magnetization (NRM) at individual sites are well grouped and significantly different from the present field. The site mean direction pattern changes systematically from D ≅ 20°, I ≅ 50°, in the lower portion to D ≅ 10°, I ≅ 50°, in the central region to D ≅ 25°, I ≅ 50°, at the top. A mathematical approximation of the cooling history permits the transformation from the geometric record of the sill to a time record of the earth's magnetic field that existed when the sill cooled. Several models were evaluated and the results indicate that the range of the paleomagnetic record is approximately 2000 to 6000 years. It is concluded that the change in remanent magnetization direction could have been caused by a large nondipole field component or an excursion of the main dipole field.