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

Abstract

[1] 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.

Deep‐tow magnetic anomaly study of the Pacific Jurassic Quiet Zone and implications for the geomagnetic polarity reversal timescale and geomagnetic field behavior

“…Seven subparallel profiles (lines 1, 2‐1, 2‐3, 2‐5, 2‐7, 3‐4, 3‐6, and 3‐9) were also collected in a small area around the Hole 801C drill site (Figures 2 and 3). The purpose of this minisurvey was to confirm the existence of lineated magnetic anomalies in the vicinity of Hole 801C and to examine the correlation of these anomalies with the downhole core and downhole logging results (described by Tivey et al [2005]). Two 125‐km long subparallel lines were extended from Hole 801C south to the RSB (Lines 1 and 3‐9) in order to complete the oldest continuous seafloor record possible in this area.…”

“…This unit is overlain by ∼165 m of presumably off‐axis lavas with ages of 159.5 ± 2.8 and 160.1 ± 0.6 Ma [ Koppers et al , 2003a]. Results of basalt core paleomagnetic and downhole magnetic log measurements from Hole 801C show four zones of alternating polarity in the upper younger extrusive volcanic flows and two zones in the oldest Jurassic aged basement [ Plank et al , 2000; Steiner , 2001; Tivey et al , 2005]. …”

Dipole strength and variation of the time‐averaged reversing and nonreversing geodynamo based on Thellier analyses of single plagioclase crystals

“…Although the relevant marine magnetic anomalies of this interval (M26‐38) have yet to be completely correlated to land sections, it is nevertheless clear than this interval was a time of very frequent geomagnetic reversals [ Opdyke and Channell , 1996]. In some interpretations, this interval represents the highest frequency of reversals of the last 200 Myr [ Tivey et al , 2005]. …”

“…The results averaged by unit yield a mean of 21.0 ± 1.57 μT. Downhole magnetic measurements [ Tivey et al , 2005] suggest the thoeliitic lavas formed at ∼23°S, but this estimate is complicated by dip of the lavas. Given this uncertainty, and the 5–15 Myr time gap [ Koppers et al , 2003] between the thoeliitic and alkalic lavas, we adopt conservative paleolatitude bounds of 10° and 30°, which yield virtual dipole moments (VDMs) of 5.21 ± 0.39 × 10 22 A m 2 and 4.11 ± 0.31 × 10 22 A m 2 , respectively.…”

Paleomagnetic constraints on deformation of superfast-spread oceanic crust exposed at Pito Deep Rift

“…Constraints on magmatic accretion, mechanical deformation, and hydrothermal alteration processes are determined from the internal geologic structure of ophiolites [Moores and Vine, 1971;Penrose Conference Participants, 1972;Cann, 1974;Casey et al, 1981;Varga and Moores, 1985;Nicolas, 1989], marine seismic studies [Christeson et al, 1992;Detrick et al, 1993;Kent et al, 1994;Hallenborg et al, 2003], and limited deep crustal drilling [Alt et al, 1993;Pockalny and Larson, 2003;Tivey et al, 2005;Wilson et al, 2006]. Additional constraints come from studies of dike intrusion events [Delaney et al, 1998;Perfit and Chadwick, 1998], and eruptions along modern spreading centers [Soule et al, 2009, and references therein].…”