A tracing study of sediment diagenesis in the Hikurangi subduction zone, New Zealand: Evidence from Sr isotope of pore fluid
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摘要:
俯冲带是地球上地质活动最活跃的地带之一，对地球表面和内部的演化具有重要意义。俯冲带慢滑移事件作为一种重要的断层滑动方式在近十几年才逐渐被地球物理学家所认识。浅源慢滑移可以使浅部断层发生破裂至海底，引发大规模海啸。了解孔隙流体来源和俯冲带沉积物成岩作用有助于认识慢滑移事件的成因机制。以国际大洋发现计划（IODP）375航次在新西兰Hikurangi俯冲板块钻探站位（U1520）和变形前缘逆冲断层钻探站位（U1518）为研究对象，对两个站位沉积物孔隙流体的SO42-、Ca2+、Mg2+和Sr2+浓度以及放射性Sr同位素（87Sr/86Sr）进行了分析。结果显示两个站位Ca2+与Mg2+浓度、Sr2+浓度与87Sr/86Sr呈负相关关系是由于火山灰蚀变作用导致的。两个站位浅层0～14.3和0～37.3 mbsf沉积物孔隙水中的Ca2+、Mg2+浓度同时降低，表明发生了自生碳酸盐沉淀。同时，俯冲板块U1520站位的岩性单元IV（509.82～848.45 mbsf）Mg2+浓度随深度减小，Ca2+、Sr2+浓度则增加，但87Sr/86Sr基本保持不变，显示了碳酸盐重结晶作用。在其下部以火山碎屑岩为主的岩性单元V（848.45～1 016.24 mbsf）沉积物孔隙水的SO42-、Ca2+、Mg2+浓度均趋近海水值，这可能是由于海水在渗透性较好的火山碎屑岩中发生横向流动导致。因此，推测俯冲板片的岩性和成岩作用是高度的不均一，容易促使俯冲板片进入俯冲带后形成特殊的应力场和异常的流体压力，进而可能与Hikurangi俯冲带频发的慢滑移事件有关。
Abstract:The subduction zone, as one of the most active zones on Earth, has great significance in the evolution of Earth's surface and interior. Slow slip events have not been recognized as an important form of faulting by geophysicists until recent decades. Slow slip event occurring in shallow sediments can rupture the seafloor and trigger large-scale tsunamis. Understanding the source of pore fluids and the diagenesis of subduction zone sediments is helpful to understand the formation mechanism of slow slip events. This paper presents the results of SO42-, Ca2+, Mg2+ and Sr2+ and the radiogenic Sr isotope (87Sr/86Sr) on the pore fluid collected from the drilling sites on the subducting plate (Site U1520) and the deformation front (Site U1518) of the Hikurangi subduction margin, offshore New Zealand, drilled during the International Ocean Discovery Program IODP Expedition 375. The results show that Ca2+ and Mg2+ concentrations, Sr2+ concentration and 87Sr/86Sr ratio are negatively correlated, indicating the widespread occurrence of volcanic ash alteration at both sites. In addition, the simultaneous decrease of Ca2+ and Mg2+ concentrations in the pore waters of the shallow 0～14.3 and 0～37.3 mbsf sediments is due to authigenic carbonate precipitation. In addition, the significant decrease in Mg2+ concentration with depth and increase in both Ca2+ and Sr2+ concentrations with depth accompanied by relatively constant 87Sr/86Sr values at the lithologic Unit IV (509.82～848.45 mbsf) of Site U1520 point to the ongoing carbonate recrystallization. In the lithological Unit V (848.45～1016.24 mbsf) which is dominated by pyroclastic rocks, the SO42-, Ca2+, and Mg2+ concentrations in pore fluids are all close to seawater values. This observation likely implies the lateral flow of seawater-like fluid within the more permeable pyroclastic rocks compared to neighboring lithologic units. Therefore, we propose that the lithologic heterogeneity and various diagenetic processes of the subducting slab may lead to the formation of abnormal stress field and high fluid pressure after being subducted, which is likely related to the frequent occurrence of slow slip events in the Hikurangi subduction zone.
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Key words:
- subduction zone /
- diagenesis /
- pore fluid /
- Sr isotope /
- slow slip
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