Constraints on the paleomagnetic field in the Amami-Sankaku Basin from a three component borehole magnetometer during IODP Expedition 351

Abstract:

Reliable three component magnetic borehole data allow us to constrain the paleomagnetic history in the subsurface, as it can be used to determine the direction of remnant magnetization of drilled layers and aid with the reorientation of drill core samples. The declination of magnetization, which cannot be determined from azimuthally unoriented core samples, is especially important because it yields direct information about the orientation of a tectonic plate at the time of magnetization.

During IODP Expedition 351 in the Amami Sankaku Basin 600 m of sediment between 600 mbsf and 1200 mbsf were investigated with the Göttinger Borehole Magnetometer (GBM) in order to constrain the rotational history of the Philippine Sea Plate. This contributes supplementary information to constrain the initiatial tectonic configuration following subduction initiation in the Izu-Bonin Mariana System and the subsequent plate motions.

In a preliminary processing of the data, the magnetic field vector along the borehole was calculated by using the data of the GBMs orientational sensors to rotate the magnetic field vector from the tool’s reference frame into the geographic reference frame. This provides an initial assessment of the magnetic properties of the environmental structures by comparing the dataset to the magnetic field produced by the natural remnant magnetization and to the induced magnetic flux density influenced by the susceptibility of the drilled layers. Additionally this aids with the evaluation of the data quality and allows an assessment of the future processing strategy to find the magnetization vector.