Rock Magnetic Record Of Earthquake Faulting Within WFSD-2 Borehole Cores From The Longmen Shan Thrust Belt And Its Implications

Fault belt is a discontinuous narrow band,with large deformation.Fault slipping formes an earthquake,which will change the physical and chemical properties of rocks,including forming fault rocks and new magnetic minerals.Therefore,fault rock magnetism can be used to reveal the structures of the faults,to trace the frictional heating along slip zone,and to investigate deformation process and kinematic behavior and earthquake rupture mechanism.The 2008 Wenchuan earthquake(Mw 7.9)produced two co-seismic surface ruptures along the Yingxiu-Beichuan and AnxianGuanxian faults,respectively.In order to better understand the seismogenic mechanism and mechanical behavior of faults,the Wenchuan Earthquake fault Scientific Drilling(WFSD)carried out six drilling boreholes along two co-seismic surface ruptures.In this thesis,we carried out macroscopic structural and microstructural observations,(ultra-)high temperature experiments,physical and geochemical analyses and rock magnetic measurements of representative cores in WFSD-2.The results with core cataloging and logging data identified the characteristics of the rock and rock magnetic record of large earthquakes,revealed the structures of different rock units and transformations of magnetic minerals in middle part of the Longmen Shan Thrust zone,estimated the temperature of frictional heating along co-seismic slip zone,and investigated earthquake faulting and its magnetic response.The main conclusions obtained are as followings:1.The surface magnetic susceptibility data were measured in the cores(from 500 m to 2283.46 m depth)of the WFSD-2.About 150 thousand magnetic susceptibility values were obtained in 200 days.The magnetic susceptibility results show these values are controlled by the type of lithology.These values of Neoproterozoic Pengguan Complex which consist of granite and volcanic rocks are higher than these of the Upper Triassic Xujiahe Formation sedimentary rocks.The pyroclastic rock has high value in Pengguan Complex,while these values of sedimentary rocks with high clay content or thin grain size are high.Borehole logging,magnetic susceptibility values and lithology indicate that the four parts of the Neoproterozoic Pengguan Complex do not come from the same rock units,and the stratigraphic attribution of the Xujiahe Formation at different depth are different.Five rock units were distinguished in WFSD-2 cores,which formed an imbricated structure.The high magnetic susceptibility values of fault gouge and pseudotachylyte veins indicating the large earthquakes occurred.About 80 high magnetic susceptibility zones were recognized from Yingxiu-Beichuan fault zone in WFSD-2 cores.This finding demonstrate that powerful ancient earthquakes may have occurred repeatedly in the Yingxiu-Beichuan fault belt.2.Twenty-one veins in black,dark-grey and grey,with thicknesses ranging from several millimeters to several centimeters,were found at 579.62-599.31 m-depth in WFSD-2 cores.Most of them are fault veins with the same extension direction.Microstructural analyses,including flow structures,microlites,embayed quartz clasts,spherulites and vesicles were observed in the veins,and the glass showed by powder Xray diffraction spectra demonstrate that the veins are melt-origin pseudotachylyte.Different colored veins and their overprinting relations in macroscopic and microscopic scales,demonstrate that they belong to different staged of pseudotachylyte veins.The young veins injected into old veins and the fragments of early-formed pseudotachylyte in the new layer indicate that numerous earthquakes and subsequent seismic ruptures occurred on the same fracture.21 layers of veins show that earthquakes have occurred repeatedly in this fault belt.The existence of pseudotachylyte veins indicate that multiple fault weakening mechanisms may have been active during a single co-seismic event,and frictional melt lubrication is the most important one.Moreover,the solidified melt(pseudotachylyte)weld fault belt and keep faults strong over a longer time scales.3.High-precision XRF testing,microstructural observations and SEM-EDX analyses indicate that Fe element enriched in pseudotachylyte in WFSD-2 cores.Parts of Fe elements may present as pure iron based on the chemical formula calculation,indicating high temperature reduction environment with few hot fluid.Therefore,it is the first time that we found the geochemical evidence for metallic iron in the nature pseudotachylyte veins in this thesis.4.The(ultra-)high temperature experiments of granodiorite and cataclasite from WFSD-2 cores and rock magnetism results show that magnetite is the main ferromagnetic minerals in the samples from room temperature to 900?,the newformed magnetite contributes to the high magnetic susceptibility values in samples from 400? to 900?,and the pure iron is the main magnetic minerals above 1300°C.Therefore,the metallic iron is formed by the following process: Fe3+(magnetite)?Fe2+?Fe0(metallic iron).The(ultra-)high temperature experiment results verified that metallic iron can be formed during large earthquake employing,which existed in a form of framboids with size less than 10 ?m.5.The rock magnetic measurements in the WFSD-2 cores indicate that the main ferromagnetic minerals are magnetite and pyrrhotite,and abundant paramagnetic minerals are siderite,chlorite and kaolinite in fault gouge.Magnetite and metallic iron are the main magnetic materials in the pseudotachylyte veins.The new-formed magnetite formed by frictional heating and metallic iron formed by frictional melting(temperature is higher than 1300?)induced high values of magnetic susceptibility in pseudotachylyte.The neoformation of magnetite and metallic iron indicate the temperature is as high as 400?-1100? and higher than 1300? in the co-seismic fault zone.The Yingxiu-Beichuan fault belt had experienced high-temperature frictional heating,parts during 400?-1100?,while parts higher than 1300?.