3:30 PM - 4:45 PM
[MIS16-P11] Chemical composition distribution of the drilled core across the fault zone of the Nobeoka thrust
Keywords:fluid-rock interaction, NOBELL, thrust
Fluid-rock interaction is one of a very important processes for faulting. We focus on the element composition distribution across the Nobeoka thrust, and thus analyzed chemical composition of the drilled core obtained by Nobeoka thrust drilling project (NOBELL). Major elements and trace elements are analyzed by XRF and ICP-MS, respectively
Results of XRF analysis showed no significant difference between the hanging wall and the footwall despite the difference in lithology and metamorphic grade. Na2O, Al2O3, SiO2, K2O and CaO increase just above the fault core (Depth 41.3~41.8 m). This increase would be caused by the decrease in SiO2, because SiO2 is the dominant component in the analyzed rocks (60~80 wt.%).
Results of ICP-MS analysis also did not show significant difference between the hanging wall and the footwall, except for Li and Cs which are relatively abundant in the footwall. High concentration of Li just above the fault core may suggest Li-rich fluid from external source. The provenance of Li can be attributed to the basalts where significant quantity of the oceanic crust is subducting. Some elements showed increase just above the fault core as observed in the major elements.
Summarizing the results, the divergence in chemical composition is limitedly observed in the vicinity of the upper interface of the fault core. The depletion in Si just above the fault core might be caused by the development of pressure solution resulting Si dissolution and flowing-out in this horizon. Chemical anomalies observed within and just above the fault core suggest high-temperature fluid-rock interaction associated with the faulting. Further characterization of stable isotope analysis (such as Sr, Nb) will provide insights into the provenance of the fluids.
Kondo et al. (2005) Deformation and fluid flow of a major out-of-sequence thrust located at seismogenic depth in an accretionary complex: Nobeoka Thrust in the Shimanto Belt, Kyushu, Japan. Tectonics, 24:TC6008