In-situ rock strength of the input materials and prism sediments at the Hikurangi Margin

Abstract:

At the Hikurangi Margin offshore New Zealand, where the Pacific plate subducts underneath the Indo-Australian plate, unique slow slip events (SSEs) have been observed. The International Ocean Discovery Program (IODP) Expeditions 372 and 375 have explored the region where SSEs have occurred at the Hikurangi margin through drilling, coring, logging, and the installation of observatories. Here, we have investigated the rock strength at two sites located on the incoming plate (Site U1520) and in the frontal thrust region (Site U1518) by analyzing drilling parameters and conducting triaxial deformation experiments on the samples from Site U1520. Site U1518 located approximately 70 kilometers from shore drilled through one of the frontal thrust faults, the Pāpaku Fault at ~300 meters below the seafloor (mbsf) and recovered samples from both hanging wall and footwall as well as the fault zone. Site U1520 located 95 kilometers from shore drilled through the incoming Pacific plate including hemipelagic sediment, pelagic sediment, and the volcaniclastic lithological units.

The drilling parameters utilized in this study include: bit depth, weight on bit, torque on bit, and rotation speed recorded every 1 second during drilling operation. We have analyzed the drilling performance curve to investigate how the formation strength evolves with depth, lithology, and structure. We also compare the drilling parameters analysis with logging and core-based data.

The preliminary results indicate that drilling parameters at Site U1520 clearly evolve with lithology and that the data in the volcaniclastic unit shows scatter, implying mechanical heterogeneity within that unit. The drilling parameter analysis at Site U1518 indicates that rock strength generally increases with depth in both the hanging wall and footwall and that it decreases with depth within the Pāpaku Fault zone.