Lithification of Volcaniclastic Deposits in the Hikurangi Subduction Zone: Preliminary Characterization of Fluid Circulation in the Incoming Plate, and Volatiles Entering the Trench

What is the strength and fluid content of materials entering a subduction zone, that may play a role in slow slip? International Ocean Discovery Program Expedition 375 aimed to investigate processes and in situ conditions of subduction zone slow slip events (SSEs) at the northern Hikurangi Subduction Margin. Two drill sites were cored to characterize the incoming material prior to subduction: the upper 1050 m of sedimentary sequence in the Hikurangi Trough on the flank of the Tūranganui Knoll seamount, and the top of the seamount. Here, we focus on the formation of cements in volcaniclastic deposits recovered at the base of the incoming sedimentary section, and their possible influence on rock mechanical properties and fluid budgets.

The material recovered from the Hikurangi Trough is an Early Cretaceous to Holocene sedimentary sequence comprising, from the seafloor downward, terrigenous turbidites, a pelagic carbonate facies with scattered interbeds of gravity-flow deposits, and a 160-m-thick sequence of Cretaceous granule-size volcaniclastic conglomerate with sub-angular basaltic clasts. The clast-supported volcaniclastics have a tightly packed texture with clasts bounded by pore-filling cements of very fine grained to fibrous calcite and fibrous zeolites.

We identified different types of veins and vein-like features, distinguished as "diffuse" and fracture veins. "Diffuse" veins occur as zones of interstitial cement concentrated in sub-horizontal, bedding-parallel bands. The boundaries of these structures are irregular, and mineralization is not concentrated along discrete fractures. Fracture veins have sharp boundaries with the wall rocks, and blocky to elongate-blocky infillings. The diffuse cementation and different episodes of veining, some of which involved brittle fracturing, suggest complex fluid circulation accompanying lithification and diagenesis. To characterize fluid/rock interaction and the origin of the fluids (seawater vs seamount-derived fluids), we have conducted stable isotope analyses (d¹³C and d¹⁸O) on carbonate samples from pore-filling cement, "diffuse", and fracture veins. The results from this study will provide the basis for further geochemical and paleothermal characterization of both cements and host rock, through LA-ICPMS and clumped isotopes.