Downhole magnetic measurements of ODP Hole 801C: Implications for Pacific oceanic crust and magnetic field behavior in the Middle Jurassic

Abstract

Downhole horizontal and vertical magnetic field measurements within the 474 m thick Jurassic crustal section drilled at ODP Hole 801C in the western Pacific show anomalies that are “in phase,” indicating that this site formed in the southern hemisphere and moved across the paleoequator to its present location at 18.6°N. The inclination computed from the horizontal and vertical anomaly logging data varies significantly downhole and can be explained by progressive rotation of the lava sequence as it is buried during accretion. This is compatible with the observed dips in the Formation Micro Scanner (FMS) data, which show up to 42° of rotation toward the fossil spreading axis. We restore the magnetic inclination to prerotated values and obtain an estimated pretilt inclination of 39.9° ± 6.6°, corresponding to a paleolatitude of 22.7° ± 5°S. The magnetic logging data document six polarity units within the drilled volcanic extrusive crust. The upper 132 m thick logged section of lavas contains four polarity units, which apparently formed 7 Myr after the formation of the underlying basement. The lower 212 m logged section of the hole is reversely magnetized with an intervening normal polarity zone. More than one reversal downhole would indicate a rapid reversal rate given the time it takes to form the extrusive section of oceanic crust: ∼45,000 years at a fast spreading rate of 66 km/Myr. Previous results from a contemporaneous section in Spain suggest rapid reversals throughout the late Bajocian–early Bathonian Stage (170–165 Ma). Nevertheless, the complicated formation history prevents quantification of the geomagnetic reversal rate for the 801C crustal section.