Frictional strength, healing, and permeability of pelagic carbonates from the northern Hikurangi margin

The shallow megathrust ( < 15 km) at the northern Hikurangi margin hosts tsunami earthquakes and slow slip events (SSEs), which are proposed to be influenced by pelagic carbonates, high fluid pressure, and stress perturbations associated with seamount subduction. To constrain the rheologic and hydrologic structure of the plate boundary, we measured the strength, healing, and permeability of carbonates from IODP Expedition 375 Site U1520C. Experiments were performed on marl (34% clay) and chalk (2% clay) at effective pressures of 21 and 70 MPa and temperatures of 25 and 75 °C, representing in-situ conditions around the subducting Pāpaku Seamount. We conducted constant displacement rate and long-duration (10 2 − 1 0 6 s) slide-hold-slide experiments, along with flow-through permeability tests before and after a subset of these at 8 and 70 MPa effective pressure. Results show that marl permeability is 1–2 orders of magnitude lower ( ∼ 1 0 − 19 − 1 0 − 21 m 2 ) than chalk ( ∼ 1 0 − 16 − 1 0 − 20 m 2 ). In contrast to chalk, the permeability of marl is more sensitive to changes in effective pressure but less sensitive to shear strain. Because marl overlies chalk, its low permeability may maintain high fluid pressures thought to correlate with SSEs. Higher clay content also corresponds to higher strength under lower effective pressure and elevated temperature conditions, suggesting strain localization into chalk. We attribute this to the inhibition of both mechanical compaction and chemical weakening with increasing clay content. We found a lack of Dieterich-type healing in both lithologies, consistent with the low stress-drops of SSEs.