Strength and Permeability of Pelagic Carbonate Sediments from the Northern Hikurangi Margin

Data for "Strength and Permeability of Pelagic Carbonate Sediments from the Northern Hikurangi Margin". Abstract: The shallow megathrust of the northern Hikurangi margin hosts both tsunami earthquakes and recurring slow slip events (SSEs), which are proposed to be influenced by both the abundance of subducting pelagic carbonates and stress perturbations due to subducting seamounts. To provide better constraints on the rheology and its control on slip mode, we measured the strength, healing behavior, and permeability of pelagic carbonate sediments recovered from IODP Expedition 375 Site U1520C. Experiments were performed on marl (29% clay) and chalk (6% clay) samples at effective pressures of 8, 21, and 70 MPa and temperatures of 25 and 75 °C, representative of the in-situ environment around the subducting Pāpaku Seamount. We conducted constant displacement rate and long duration (100-1,000,000 s) slide-hold-slide friction experiments, with flow through permeability tests prior to and after a subset of these. Our results show that the clay content in marl causes permeability to be 1-2.5 orders of magnitude lower than chalk, but less sensitive to effective pressure and shear strain. Because marl overlies chalk, its low permeability can maintain high pore fluid pressure thought to correlate with SSEs. More surprisingly, higher clay content corresponded to higher strength at multiple conditions that correlated with lower effective pressures and higher temperature, indicating that strain may partition into the chalk. We attribute this to inhibited mechanical compaction and chemical weakening with increasing clay content. We found a lack of Dieterich-type frictional healing in both lithologies, consistent with the low stress-drops of SSEs.