Didier Vandamme2 and Jason R. Ali3


During Ocean Drilling Program Leg 152, 779 m of basaltic lavas within the oldest part of the seaward-dipping reflector sequence (SDRS) was drilled in Hole 917A. We present the results of studies of 150 paleomagnetic specimens sampled from 68 igneous units. We tried to reorient basalt pieces using Formation MicroScanner data so that declinations could be determined, but the attempt failed. Using paleointensity experiments, we also tried to test the thermal origin of magnetization of several units that may have recorded a geomagnetic short event. Chemical changes that occurred in minerals in the samples during heating prevented any conclusions from being reached. Conventional paleomagnetic experiments, however, allowed us to determine the inclinations of 74 of the 92 igneous units, directly or indirectly. The reversed polarity of the entire sequence and the presence of two cryptochrons recorded by three (possibly five) units in the Lower Series and one unit in the Middle Series are consistent with an extrusion of the volcanic sequence of Site 917 during the magnetic Chrons 24r, 25r, or 26r, but exclude magnetic Chron 27r.

1Saunders, A.D., Larsen, H.C., and Wise, S.W., Jr. (Eds.), 1998. Proc. ODP, Sci. Results,152: College Station, TX (Ocean Drilling Program).
2Laboratoire des Géosciences de l’Environnement, Université d’Aix-Marseille III, Arbois, CEREGE, BP 80,13545 Aix en Provence Cedex 4, France. vandamme@cerege.fr
3Department of Oceanography, The University, Southampton, SO17 1BJ, United Kingdom.