Paleomagnetic of Jurassic basalts from ODP Hole 129-801C

The paleomagnetic and rock magnetic properties of 51 Jurassic basalts from Ocean Drilling Program (ODP) Hole 801C have been examined. Magnetic properties vary with lithologic composition; alkalic rocks and hydrothermally-altered tholeiites are much weaker in intensity and generally contain higher coercivity magnetic components than the older and less-altered tholeiites at the base of the hole. For the entire column, the Jurassic basalts have an average initial natural remanent magnetization (NRM) intensity of approximately 1.24 A/m and average median destructive fields (MDF) of 8.31 mT. These values and the mean Koenigsberger ratio of 1.7 are very similar to results obtained for Jurassic basalts from the Atlantic (DSDP Leg 76). The similarities suggest that the basalts of both sites and their remanence characteristics are representative of Jurassic oceanic crust. The most profound discovery in these samples was the presence of 5 inclination zones, each showing consistent positive (or negative) polarity opposite the overlying and underlying polarity bands. We interpret these to represent a record of change in polarity of the EarthÆs magnetic field and, because of the large number over such a short interval (60 m) of crust, we assert that the rapid change in polarity during the Jurassic is the probable reason behind the origin of the Jurassic Quiet Zone.