Links between Low-T Dehydration and Recurring Shallow Slow Slip Events in the Northern Hikurangi Subduction Zone

In subduction zones, the depth-dependent release of fluids from compaction and metamorphic dehydration reactions in hydrous lithologies plays a key role in modulating pore fluid pressure, fault strength, and slip behavior along the megathrust. The depth-distribution of fluid release is also the primary control on volatile fluxes through the forearc, and on the residual volatile content of the subducting plate. Here, we investigate the inventory and release of fluids from altered oceanic crust by low-grade dehydration reactions (~50-350 °C) at the Northern Hikurangi subduction zone, where slip on the outer (shallow) megathrust is accommodated almost entirely in frequent, large shallow slow slip events (SSEs).

Regional geophysical surveys and drilling during International Ocean Discovery Program (IODP) Expedition 375 show that the incoming plate of the Hikurangi Plateau carries a thick (>1.5 km) and extensively altered volcaniclastic sediment blanket characterized by an abundance of phyllosilicates (primarily Mg-smectite) and zeolite, and mineral-bound water contents as high as 14-16 wt.%, into the SSE source region. We quantify the distribution of fluid release from this sediment package by combining compaction trends to assess compactive water loss and thermodynamic phase equilibria models using sediment drill-core compositions to compute water release from dehydration reactions.

We find that: (1) compactive dewatering dominates in the outermost 15-20 km of the forearc, where temperatures remain too low (<100 °C) to drive dehydration reactions; and (2) a large volume (~5-8 wt.%) of mineral-bound water is released step-wise over the region spanning from ~30-90 km from the trench (corresponding to depths of 5-15 km below seafloor and temperatures of 150-260 °C), primarily from decomposition of zeolite and phllyosilicate phases. This contrasts with the behavior of Ca- and Na-smectites typically found in detrital marine sediments and altered volcanic ash, which undergo dehydration between 80-150 °C.

Because the majority of compactive dewatering precedes dehydration, mineral-bound water is released where porosity, permeability, and compressibility are reduced, maximizing the potential for excess pore pressure generation along and beneath the megathrust. The broad region of low-temperature metamorphic fluid release directly overlaps the slip zone of recurring SSEs, supporting the idea that dehydration - and associated elevated pore pressures and low effective normal stress - favor SSE as the prevailing mode of strain release on the plate interface. The presence of thick extensively hydrated oceanic crust and persistence of fluid production from clay dehydration to ~260 °C contrasts with other subduction zones, where low-T metamorphism is dominated by the transformation of Ca- and Na-smectites to illite by 120-150 °C. We speculate that this difference may offer an underlying explanation for the lack of a locked seismogenic zone at the Northern Hikurangi margin, whereas at other subduction margins, a lack of significant fluid production from dehydration in the 150-350 °C window may lead to a better-drained megathrust and promote stick-slip behavior.