Deformation Behavior And Faulting Mechanism Of The Guanxian-Anxian Fault In The Longmen Shan

2008 Mw 7.9 Wenchuan earthquake simultaneously produced surface ruptures of⁸0km and²70km along the Guanxian-Anxian fault?GAF?and Yingxiu-Beichuan fault?YBF?,respectively.The YBF is an oblique thrust fault,while the GAF is a pure thrust fault.A large earthquake as Wenchuan earthquake produces huge ruptures along multiple faults with different kinematic properties at the same time,whihc is not only unexpected by geologists and seismic geophysicists,but also is unable to be explained by the current fracture theories.Although the understanding of the mechanism and seismogenic environment of large earthquakes has always been a major problem,it is also the goal that scientists have been exploring.Therefore,after the Wenchuan earthquake,Longmen Shan fault zone has become a research hotspot in the field of Geosciences to understand earthquakes,and its deformation behavior and faulting mechanism are considered as the key to understand the mechanism of earthquakes and seismogenic environment.At present,scholars have basically focused on the YBF,the main rupture zone of Wenchuan earthquake.They have realized that the YBF is a stick-slip fault where large earthquakes often occur.However,little is known about the GAF.Here we carried out multiscale structural analyses?from field macroscopical to microscopic?,XRD mineral analysis,whole rock geochemical analysis,stable isotope analysis,high resolution magnetic susceptibility tests,XRF non-destructive element scanning and friction experiments of fault rocks from the GAF surface rupture and the 3rd pilot borehole of the Wenchuan earthquake Fault Scientific Drilling project?WFSD-3P?.Based on the above methods,for the first time,the deformation behavior of the GAF with long-term creep is revealed,and its creep mechanism is explained.This has impacted the traditional understanding that creep fault does not occur large earthquakes,and made a breakthrough in the study of seismic fault.The main conclusions are as follows:?1?Analytic results of multi-scale structures and mineral contents show that distributed R1shears,pressure solution seams,partly dissolved clasts and mineral grains?e.g.,quartz and albite?,and newly formed phyllosilicate minerals all prevail in the clay-rich fault rocks of GAF at different depths.The majority of clay minerals in the fault gouges were newly formed by pressure solution.The growth of new phyllosilicates prevents soluble grains from coalescing,thereby maintaining fast diffusive paths along solution seams,which promotes the formation of pressure solution.Both weak phyllosilicate minerals and pressure solution can control aseismic creep.Therefore,the slip behavior of the GAF is dominated by long-term aseismic creep due to the interplay between new phyllosilicate minerals and pressure solution.Based on P wave velocity and strata distribution,we propose a model in which the GAF is creeping at shallow depths and locked at greater depths,as a possible explanation for the Wenchuan earthquake rupture mechanism.?2?Based on the distribution of calcite veins and the variation of mineral and chemical composition,it is found that quartz,albite and carbonate minerals in fault gouge of the GAF are dissolved under the action of fluid and pressure,resulting in obvious mass and volume loss in the fault.Notably lower contents of carbonate minerals,CaO and CO2 compared with elevated contents of clay in the fault gouge when compared to the host rocks;this is consistent with observations that calcite veins only occur in host rocks and damaged rocks,but not in fault rocks.Based on previous studies and regional stratigraphic,topographic and rainfall data,we proposed that the unique environment of acidic CO2-bearing fluids from the deep earth couple with high stress in the GAF as well as the circulation of abundant meteoric water into the fault zone causes carbonate minerals to be dissolved or rarely precipitated in fault rocks.The loss of carbonate minerals maintains open intergranular pores and unobstructed fluid paths in the fault zone,which accelerates fluid-rock interactions along with clay minerals formation and thus results in long-term unsealing and creeping of the GAF.?3?The results of high resolution magnetic susceptibility test,geochemical analyses and microstructural observations show that the magnetic susceptibility values of the fault gouge in the GAF zone are commonly less than the average of the corresponding host rocks;the low magnetic susceptibility of the fault gouge results from the transformation from ferromagnetic minerals to paramagnetic minerals?Fe-sulfides,siderite,or iron-bearing clay minerals?under long-term fluid infiltration during interseismic periods.Newly formed Fe-sulfides and iron-bearing clay minerals form slowly during interseismic periods,which suggests that low magnetic susceptibility of the fault gouge may be an indicator of slowly slip?creeping deformation?of active fault during interseismic periods.The development of pyrite,relatively high contents of Fe²⁺and sulfur element,and the low content of Fe³⁺in the fault gouge imply that the faulting of the GAF is commonly at low temperature and reducing environment.?4?Friction experiments of fault rocks from the GAF under different conditions?slip rate,lithology and fluid?show that weak mineral phases?clay minerals?and fluids have a certain weakening effect on friction strength of the fault.The weakening effect of fluids on fault strength is particularly evident under the conditions of medium and low velocities?0.001m/s,0.01m/s?,which fully reflects the key role of fluid in the creeping process of faults.