Rock magnetism on detachment faults: constraints from the Dragon Horn area (49.7E), on the Ultraslow-Spreading Southwest Indian Ridge

The magnetic properties of iron-bearing minerals (especially ferromagnetic minerals) have significant geological and environmental response and can contribute valuable insights into the physical and chemical changes in hydrothermal activities and geological formations at mid-ocean ridges. In our study, the impact of hydrothermal activities and tectonics on detachment faults was better interpreted through the magnetic study of rocks on the Dragon Horn area detachment faults of the ultraslow-spreading Southwest Indian Ridge. Furthermore, by integrating our research on rock magnetism with near-bottom magnetic detection, the exploration capability of seafloor resources can be enhanced, indicating great potential in the understanding of the ocean geodynamics of ultraslow-spreading mid-ocean ridges. Rock samples were collected by the Shenhaiyongshi HOV and TV grabs, which indicated the lithologies on the surface of the detachment fault and adjacent terrains in the Dragon Horn area. Similar to the work of Wang (2020), these rock samples were classified as fresh basalt, partially-chloritized basalt, fully-chloritized basalt, breccia, serpentinized peridotite and sulfide, with the main exposure to chloritization and serpentinization processes. Rock magnetic measurements and microscopic observations including Microscopy, Scanning Electron Microscopy, and Tescan Integrated Mineral Analyzer(TIMA) indicated that the titanomagnetite in the basalt was gradually depleted and the mineral grain size decreases due to a combination of detachment faulting and hydrothermal action, accompanied by a reduction in the magnetization and density of the basalt and an order of magnitude negative trend in the NRM. The ferrimagnetic component was gradually converted to paramagnetic component and the stable NRM was destructed as alteration proceeds. For serpentinized peridotites, the alteration was more uniform, but olivine porphyry remnants were visible. The samples in the detachment fault witnessed strong local plastic shearing, and the alteration environment had been transformed from rock-dominated to fluid-dominated (Maffione et al., 2014). The sulfide contains solely small amounts of magnetite which may be residual. Interacting single-domain (SD) particles dominate in all samples, but ferromagnetic particles would split into finer material as the degree of alteration intensified and disassembly fault friction takes place (Yang et al., 2020). Eventually the physicochemical changes from detachment faulting and hydrothermal alteration were manifested in the magnetic modification of the rocks. It would be conducive to correlate the results of rock magnetism and near-bottom magnetic anomaly detection. In previous researches, the combined effect of residual and induced magnetization were frequently neglected, whereas in practice the measured total anomaly could be substantially amplified by residual magnetization close to the induced magnetic field direction(Maat et al., 2020), resulting in our misinterpretation of the subsurface structure. The conjunction of sophisticated rock magnetics experiments and near-bottom magnetic anomaly detection might remedy this deficiency. In any other case, the study of rock magnetism regarding the degree of hydrothermal alteration may accomplish the work of quantifying the hydrothermal alteration process, locating detailed causes of different magnetic anomalies in the Dragon Horn area, and linking the magnetic properties of rocks to the detachment fault formation. Reference Maat, G. W., Fabian, K., Church, N. S., & McEnroe, S. A. (2020). Separating Geometry From StressInduced Remanent Magnetization in Magnetite With Ilmenite Lamellae From the Stardalur Basalts, Iceland. Geochemistry, Geophysics, Geosystems, 21(2). https://doi.org/10.1029/2019GC008761 Maffione, M., Morris, A., Plumper, O., & van Hinsbergen, D. J. J. (2014). Magnetic properties of variably serpentinized peridotites and their implication for the evolution of oceanic core complexes. Geochemistry, Geophysics, Geosystems, 15(4), 923944. https://doi.org/10.1002/2013GC004993 Wang, S., Chang, L., Wu, T., & Tao, C. (2020). Progressive Dissolution of Titanomagnetite in HighTemperature Hydrothermal Vents Dramatically Reduces Magnetization of Basaltic Ocean Crust. Geophysical Research Letters, 47(8). https://doi.org/10.1029/2020GL087578 Yang, T., Chou, Y., Ferre, E. C., Dekkers, M. J., Chen, J., Yeh, E., & Tanikawa, W. (2020). Faulting Processes Unveiled by Magnetic Properties of Fault Rocks. Reviews of Geophysics, 58(4). https://doi.org/10.1029/2019RG000690