Fluids at slow slip megathrust zone supplied by consolidation of clay-rich, volcaniclastic subducting upper crust

Hydrological and mechanical processes within subduction zones that ultimately control the nature of shallow (<15 km depth) megathrust slip are often attributed to the properties of subducting sediments and topographic irregularities of the oceanic basement (e.g., seamounts). Compared to subducting and accreted marine sediments, the inventory of fluids within upper oceanic crust is typically minor (~1.5-5 wt. %) and not subject to substantial compaction induced dewatering at shallow depths. New results from the NZ3D experiment at the northern Hikurangi margin, a subduction zone renowned for shallow slow slip events, do not follow this concept. Here, the subducting oceanic plate is part of the Hikurangi Plateau, a Cretaceous-age large igneous province with thickened crust and numerous volcanic edifices. P-wave velocities estimated with 3D full-waveform inversion are consistent with the top of mature oceanic crust (4.5 km/s) at ~2 km below the top of low-velocity (2-4.5 km/s), potentially fluid-rich Cretaceous volcanic materials. Clear sedimentary layering within overlapping volcanic cone complexes in the upper crust imply formation by numerous, possibly shallow-water (<1 km b.s.l) explosive eruptions. Layered intra-basement reflections are prevalent within the upper volcanic crust. Many of these layered reflections are adjacent to volcanic cones and upon 3D inspection are interpreted as stacked volcanic intrusions with circumferences on the order of 2-4 km. The Hikurangi Plateau upper crust undergoes dewatering upon subduction; P-wave velocities within the upper 1 km of this unit increase from 2.5 km/s to 3.5 km/s upon subduction to 15 km from the deformation front. On the basis of an empirical relationship between total volumetric water content and P-wave velocity from nearby scientific drilling and laboratory tests we estimate a loss of 16 vol. % water. Both mineral-bound water and remaining pore-bound water will subduct to slow slip source regions. These observations suggest that at the northern Hikurangi margin thick, fluid-rich volcaniclastics have the potential to deliver fluids to the megathrust that may influence slow-slip and that mechanically weak layers within the volcanics may permit deformation through consolidation and shear.