Paleomagnetism of Gabbroic Rocks and Peridotites from Sites 894 and 895, Leg 147, Hess Deep: Results of Half-Core and Whole-Core Measurements

Abstract

During Leg 147, in addition to measurements made on minicore samples, remanent magnetization of the archive half of the split core was measured using the cryogenic magnetometer. Magnetic susceptibility was measured on whole cores using the multisensor track aboard the JOIDES Resolution. High intensities of natural remanent magnetization were observed in the gabbroic rocks and the peridotites, which suggests that the lower oceanic crust to upper mantle section contributes significantly to the marine magnetic anomalies. Many of the measurements made on both the gabbroic rocks and the peridotites indicated the presence of secondary magnetizations probably acquired during drilling. The observed secondary magnetization is of mixed polarities regardless of the lithology. Similar values for the average stable inclinations obtained from both sites, which are significantly steeper than the expected value of zero, suggest that both sites experienced a similar post-cooling tectonic rotation with respect to the horizontal axis.

Characterization of the in situ magnetic architecture of oceanic crust (Hess Deep) using near‐source vector magnetic data

“…Solutions derived from potential field modeling are nonunique, and so we use the principle of Occam's Razor to seek the simplest source structure in the total and vector‐VMP modeling that fits the data. In the case of Hess Deep, we have constraints on the inclination value and polarity of magnetization from previous drilling results [ Kikawa et al , ]. Particularly important for our study is the premise, in order to explain the regional tectonic history, that the serpentinization of lower crust and upper mantle predates the rotation of the intrarift ridge [ MacLeod et al , ].…”

“…Within Hess Deep on the intrarift ridge, paleomagnetic and rock magnetic results from 124 discrete gabbro samples recovered at ODP Hole 894G (Figures b and ) show a mean natural remanent magnetization (NRM) of 2.2 A/m [ Kikawa et al , ; Pariso et al , ]. The stable direction of the characteristic remanent magnetization (ChRM) has a median inclination of +45° (averaged values from +51.2° ± 13.9° by Kikawa et al [] and 38° by Pariso et al []). A total of 67 harzburgite and 170 dunite discrete samples from ODP Site 895 yielded mean NRM values of 1.6 and 3.1 A/m, respectively.…”

“…The median Koenigsberger ratio of these samples was about 2.4, consistent with this level of serpentinization and indicating a strong remanent magnetization component. The average ChRM inclination for samples from Site 895 (Figures b and ) is +55.7° ± 22.7° [ Kikawa et al , ].…”

“…Measured mean inclinations for the ODP and IODP rock samples are much steeper than expected at this equatorial location [ Kikawa et al , ]; the expected inclination of the Geocentric Axial Dipole (GAD) is +4.6° and the IGRF‐11 present‐day field [ Finlay et al , ] has an inclination of +17.6° and declination of +8°. Drill cores, in general, lack declination data so that the polarity of the contemporaneous geomagnetic field of the inclination values must be constrained by additional regional tectonic constraints.…”

“…To produce the observed steep inclinations, there are two scenarios based on the polarity of the crust. In the first scenario, the intrarift ridge basement is assumed to have been magnetized with normal polarity and was subsequently tilted northward by 51° (i.e., based on the difference between the averaged inclinations at ODP Sites 894 and 895 and the GAD of +4.6°, Kikawa et al []; Pariso et al []). In the second scenario, the basement was magnetized during a reverse polarity period, and the inclination was steepened by a 55–60° rotation toward the south on a north dipping fault.…”