Low Heat Flow Near the Deformation Front of the Northern Hikurangi Margin Revealed by Deep Borehole Observatories

Deep borehole observatories installed during IODP Expedition 375 at Sites U1518 and U1519 along the northern Hikurangi subduction margin offshore New Zealand provide new constraints on the thermal structure of the frontal prism, where frequent shallow slow slip events occur. Site U1518, located just landward of the trench, targets the frontal thrust near the deformation front, whereas Site U1519 lies farther landward on the hanging wall of the megathrust. We analyze temperature time series from 2020 together with thermal conductivity measurements from adjacent boreholes at each site to estimate heat flow. Mean heat flow is estimated to be 26.93 mW m −2 at Site U1518 and 22.84 mW m −2 at Site U1519. These values lie at the lower end of nearby shallow marine heat flow probe measurements, which also include substantially higher values. At both sites, the deep measurements are lower than estimates from advanced piston corer temperature tool data, and at U1519 they are lower than values inferred from bottom‐simulating reflectors. Because the deep observatories record long‐duration, equilibrated conditions within the forearc wedge, they more reliably capture the slab‐cooled, conductive background than shallow observations that can be influenced by fluid advection, sedimentation, bottom water variability, or uncertainty in estimating formation temperatures. These results provide improved constraints on the thermal state of the northern Hikurangi forearc and help distinguish the slab‐cooled conductive baseline from near‐surface effects.