Borehole Magnetic Field Simulation of Rotated Plates with Special Application for the Philippine Sea and Caroline Plates

Abstract:

The rotation of tectonic plates and basins is not uncommon in the Western Pacific. However, determining the exact amount of rotation over geological time can be quite challenging, especially for the Philippine Sea and Caroline Plates, because paleomagnetic measurements are rare and even if they exist come from sites near the plate boundary which may have suffered local tectonic deformations. Although it is generally thought that the Philippine Sea Plate rotated 90° clockwise during the last 55 Ma and that Caroline Plate likewise began to rotate soon after its formation around 32 Ma, there is no direct evidence for such arguments. While the recent IODP drilling (Leg 351) of the Amami-Sankaku Basin in the northern tip of the West Philippine Basin promises to provide an opportunity to estimate the rotation of the Philippine Sea Plate, in practice, the collection and subsequent reduction of borehole magnetic field data can be quite complicated. Moreover, if the magnetization intensity is low, which is the case for sediments, it is difficult to assess the outcome of magnetic field investigations. Hence, it is important to have the capability to compare the observation with model simulation readily. In this paper, we present magnetic field simulations within the borehole using the analytical methods outlined by Bosum and others (1988) which assumes a circular borehole within uniformly magnetized strata layers and Gallet and Courtilot (1989) which can accommodate inclined layers. The methods are applied to the Philippine Sea Plate and Caroline Plate using geological parameters whose values are derived from previous kinematic models. The results of such modeling can be quite useful and important in planning additional borehole magnetic surveys for the future as well.