[S-CG46] New Insights of Fluid-Rock Interactions: From Surface to Deep Subduction Zone

[SCG46-P06] Deformation and hydrothermal alteration processes of the basal peridotites in the central Palawan Ophiolite and their effects on subduction initiation process

*Ken-ichi Hirauchi¹, Hiyori Abe², Julius Pasco³, Betchaida Payot³ (1.Department of Geosciences, Faculty of Science, Shizuoka University, 2.Department of Science, Graduate School of Integrated Science and Technology, Shizuoka University, 3.National Institute of Geological Sciences, University of the Philippines Diliman)

Keywords:hydrothermal alteration, mylonite, olivine, ophiolite, subduction initiation

Numerical simulations of intra-oceanic subduction initiation have shown that a pre-existing fault between oceanic plates is required to be very weak mechanically to allow subduction initiation in intra-oceanic settings (e.g., Gerya, 2022, Geosphere; Izumi et al., 2023, Tectonophysics). However, it remains unclear which rheological weakening mechanisms have operated at the incipient subduction-plate interface. To understand how deformation and hydrothermal alteration at the base of the immature mantle wedge influence intra-oceanic subduction initiation processes, structural and petrological analyses was conducted on basal peridotites from the Eocene central Palawan Ophiolite, Philippines. The central Palawan Ophiolite and underlying metamorphic sole have recorded horizontally-forced subduction initiation that occurred along extinct spreading ridges (Keenan et al., 2016, Proc. Natl. Acad. Sci. U.S.A; Valera et al., 2022, J. Metamorph. Geol.). Located adjacent to metamorphic sole amphibolites, the basal peridotites are mainly composed of dunite and harzburgite, which represent residual mantle peridotites with various degrees of partial melting (Pasco et al., 2023, Int. Geol. Rev.). Grain size reduction of olivine grains occurred by dynamic recrystallization during dislocation creep, and led to the formation of porphyroclastic tectonite, protomylonite, and mylonite. With decreasing distance from the amphibolites, the degree of mylonitization becomes significant, and the crystal-preferred orientation (CPO) pattern of olivine grains transitions from [100]{0kl} (D-type), [100](010) (A-type), or [100](001) (E-type) to [001](100) (C-type). Based on the effect of water and stress on the olivine CPO (Jung et al., 2006, Tectonophysics), this CPO transition was possibly driven by infiltration of a larger amount of water. Since the fabric strength for olivine grains decreases with grain size, we suggest that grain size reduction promoted a transition to a grain-size-sensitive mechanism (i.e., dislocation-accommodated grain-boundary sliding or diffusion creep), and thereby led to a significant decrease in flow strength. At temperatures below ca. 800 °C and places where slab-derived water was added, the mylonite underwent a dissolution-precipitation process, resulting in the formation of anastomosing networks of talc and tremolite above the stability field of serpentine (antigorite). As talc is one of the weakest hydrous phyllosilicates (Hirauchi et al., 2013, Geology), mylonitization and subsequent hydrothermal alteration occurring at the base of the mantle wedge are thought to facilitate underthrusting of the oceanic plate during subduction initiation.