Water-rock Interaction And Fluid Flow In The Hikurangi Subduction Zone

The subduction zone is one of the most active zones on Earth,which has great significance in the evolution of Earth’s surface and interior.In the initial stage of plate subduction,sediments are compressed and dehydrated,and the accompanying low-temperature diagenesis mainly includes ion adsorption and desorption,volcanic ash alteration,and organic matter degradation.With the increase of subduction depth,dehydration of hydrous minerals that can significantly change the pressure field in the subduction zone begins to occur,mainly including the transformation of opal A to opal CT and montmorillonite to illite.Among them,sediment diagenesis and fluid activity are key factors in understanding various geological processes in subduction zones.It controls the geometry of the subduction zone,the type of the subduction zone,and affects the mechanical properties of the subduction zone interface and the depth at which the earthquake occurs.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.At present,many scholars believe that high pore fluid pressure is an important factor for slow slip events.The high pore fluid pressures may be caused by dehydration and fluid activity in the subducting slab.The accumulation of fluid in the pore space can greatly reduce the effective stress,thereby promoting the occurrence of slow slip events.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 thesis presents the results of SO₄^2-,Cl^-,NH₄⁺,K⁺,Na⁺,Ca²⁺,Mg²⁺,Li⁺and Sr²⁺and the radiogenic Sr isotope(⁸⁷Sr/⁸⁶Sr),Li isotopes（δ⁷Li）on the pore fluid and Li content in sediments,Li content of different leaching fractions and their Li isotopes（δ⁷Li）collected from the drillng sites on the subducting plate（Site U1520）,the deformation front（Site U1518）and the overlying plate（Site U1519）of the Hikurangi subduction margin,offshore New Zealand,drilled during the International Ocean Discovery Program（IODP）Expedition 375.The results showed that Ca²⁺and Mg²⁺concentration,Sr²⁺concentration and⁸⁷Sr/⁸⁶Sr ratio are negatively correlated,indicating 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 mbsf 0～8.6 mbsf and 0～37.3mbsf sediments is due to authigenic carbonate precipitation driven by organoclastic sulfate reduction and anaerobic oxidation of methane.In addition,the significant decrease in Mg²⁺concentrations 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 of carbonate recrystallization.The positive correlation between NH₄⁺and Li⁺concentrations in lithological Unit I at Sites U1518 and U1519 is due to ion exchange.NH₄⁺displaces Li⁺from clay minerals into the pore fluid.The negative correlation of the upper half of the lithological Unit I at the Site U1519 site is due to the fact that the Li⁺concentration decreases andδ⁷Li also slightly decreases due to the control of volcanic ash alteration and ion exchange.An unusually high peak of Li⁺concentration（ten times higher than that of seawater）accompanied by minimumδ⁷Li value in the pore fluid at 492 mbsf at Site U1520 is observed.By excluding the possibilities of fluid advection caused by subsurface clay mineral dehydration and high temperature hydrothermal leaching,we propose that the abnormally high peak of Li⁺concentration together withδ⁷Li low in the pore fluid at 492 mbsf at U1520 may be caused by the influence of the localized sediment composition.This is supported by the observations of abundant volcanic ash layers and montmorillonite in which Li are characterized by lowδ⁷Li values.In the lithological Unit V（848.45～1016.24 mbsf）which is dominated by pyroclastic rocks,the SO₄^2-,Cl^-,K⁺,Na⁺,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 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.