11:15 AM - 11:30 AM
[SCG55-20] Attempt to investigate vertical magnetic structure of oceanic crust using Marie Celeste transform fault
Keywords:oceanic crust, magnetization structure, transform fault
This study aims to deepen our understanding of the vertical magnetization structure of the oceanic crust by conducting magnetic field observations along the scarp of a transform fault. In marine environments, magnetic field observations are generally conducted by towing a magnetometer at the sea surface or by mounting it on deep-sea equipment such as an autonomous underwater vehicle (AUV). Most of the data obtained from these methods consist of horizontal magnetic anomaly profiles. Tivey (1996) is one of the few examples of observations focusing on vertical magnetic field variatons. This study used the scarp topography of a transform fault to acquire vertical magnetic anomaly profiles and showed that while the uppermost lava layer of the oceanic crust is strongly magnetized, the underlying dike layer is very weakly magnetized. Although this discovery has provided rare evidence for understanding the magnetic structure of the oceanic crust, it has not been verified by observations in other ocean regions, and a unified view that can be generally applied has not yet been reached.
In this study, we conducted near-seafloor magnetic field observations from deep to shallow sections of a transform fault to determine the thickness of the strongly magnetized lava layer, which is considered the uppermost layer of oceanic crust. In addition, rock core samples obtained by ocean floor drilling suggest that the gabbroic layer beneath the dike layer retains partial remanent magnetization (Pariso and Johnson, 1993). Therefore, we also examine the differences in magnetization structures between these two layers.
During the KH-24-4 cruise of the R/V Hakuho Maru, we conducted magnetic field observations near the seafloor targeting the scarp on the south side of the Marie Celeste transform fault in the Central Indian Ridge. The south side of the fault has a slope with a maximum elevation difference of approximately 4km that extends for approximately 210km and continuously exposes cross-sections of oceanic crust likely formed over the past 11 million years. Dredging was conducted to collect deep fault rock samples, and then we simultaneously raised and towed a small wire-mounted three-component magnetometer along the escarpment to ensure it passed close to the escarpment. This approach allowed us to observe magnetic anomalies resulting from the oceanic crustal magnetization as variation in a vertical cross-section. To investigate differences related to age of oceanic crust formation, we made the same observations at eight locations within the fault zone.
We analyzed the data assuming a three-layer structure consisting of the lava layer, dike layer, and gabbro layer, and performed two-dimensional forward modeling based on Talwani and Heirtzler (1964). The magnetization intensities of the three layers and the thicknesses of the lava and dike layers were used as parameters in iterative calculations to determine the magnetization structure model that best explains the observed magnetic anomaly profiles. This procedure was applied to each observation site.
Our data analysis confirmed that large magnetic anomaly fluctuations are created by strong magnetization intensity contrasts especially in the shallow sections at all observation sites. The results of the forward calculations also indicate that there is a highly magnetized layer (corresponding to lava layer) about 300 m thick at the top, followed by a weakly magnetized layer (corresponding to dike layer) about 3,000 m thick, and below that, a region (corresponding to gabbro layer) with a different magnetization intensity that is distinct from the dike layer.
In this study, we conducted near-seafloor magnetic field observations from deep to shallow sections of a transform fault to determine the thickness of the strongly magnetized lava layer, which is considered the uppermost layer of oceanic crust. In addition, rock core samples obtained by ocean floor drilling suggest that the gabbroic layer beneath the dike layer retains partial remanent magnetization (Pariso and Johnson, 1993). Therefore, we also examine the differences in magnetization structures between these two layers.
During the KH-24-4 cruise of the R/V Hakuho Maru, we conducted magnetic field observations near the seafloor targeting the scarp on the south side of the Marie Celeste transform fault in the Central Indian Ridge. The south side of the fault has a slope with a maximum elevation difference of approximately 4km that extends for approximately 210km and continuously exposes cross-sections of oceanic crust likely formed over the past 11 million years. Dredging was conducted to collect deep fault rock samples, and then we simultaneously raised and towed a small wire-mounted three-component magnetometer along the escarpment to ensure it passed close to the escarpment. This approach allowed us to observe magnetic anomalies resulting from the oceanic crustal magnetization as variation in a vertical cross-section. To investigate differences related to age of oceanic crust formation, we made the same observations at eight locations within the fault zone.
We analyzed the data assuming a three-layer structure consisting of the lava layer, dike layer, and gabbro layer, and performed two-dimensional forward modeling based on Talwani and Heirtzler (1964). The magnetization intensities of the three layers and the thicknesses of the lava and dike layers were used as parameters in iterative calculations to determine the magnetization structure model that best explains the observed magnetic anomaly profiles. This procedure was applied to each observation site.
Our data analysis confirmed that large magnetic anomaly fluctuations are created by strong magnetization intensity contrasts especially in the shallow sections at all observation sites. The results of the forward calculations also indicate that there is a highly magnetized layer (corresponding to lava layer) about 300 m thick at the top, followed by a weakly magnetized layer (corresponding to dike layer) about 3,000 m thick, and below that, a region (corresponding to gabbro layer) with a different magnetization intensity that is distinct from the dike layer.