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.本数据集对应研究《希库朗伊北缘远洋碳酸盐沉积物的强度与渗透性》。 摘要：希库朗伊北缘的浅部逆冲大断层带同时发育海啸地震与周期性慢滑移事件（Slow Slip Events, SSEs），现有研究认为其形成与演化受俯冲远洋碳酸盐丰度以及俯冲海山引发的应力扰动共同调控。为精准约束沉积物流变学特征及其对滑移模式的控制作用，我们对国际大洋发现计划（International Ocean Discovery Program, IODP）375航次U1520C站位回收的远洋碳酸盐沉积物，开展了强度、愈合行为与渗透性测试。 实验分别以含黏土29%的泥灰岩（marl）与含黏土6%的白垩（chalk）为试样，设置有效压力（effective pressure）8、21、70 MPa，温度25与75 ℃的实验条件，该参数覆盖了俯冲帕帕库海山（Pāpaku Seamount）周边的原位环境。实验包含恒位移速率试验与时长100~1,000,000 s的滑停-滑移摩擦实验（slide-hold-slide friction experiments），并在部分试验前后开展了渗流渗透性测试。 实验结果表明：泥灰岩中的黏土组分使其渗透性较白垩低1~2.5个数量级，但对有效压力与剪切应变的敏感性更低。由于泥灰岩覆于白垩之上，其低渗透性可维持较高的孔隙流体压力（pore fluid pressure），而该压力被认为与慢滑移事件存在相关性。更值得关注的是，在低有效压力与高温的实验条件下，黏土含量更高的试样强度也更高，这表明应变可能优先分配至白垩层中。我们将该现象归因于黏土含量升高对机械压实的抑制作用以及化学弱化（chemical weakening）效应。此外，两种岩性均未表现出迪特里希型摩擦愈合（Dieterich-type frictional healing）特征，这与慢滑移事件的低应力降（stress-drops）特征相符。