LIU YanJiang,
LIU CaiCai,
ZHANG ZhuQi et al
.2020.Rock magnetic studies of fault rocks from the Laohushan segment of the Haiyuan fault zone and its tectonic implications Chinese Journal of Geophysics(in Chinese),63(6): 2311-2328,doi: 10.6038/cjg2020N0109
Rock magnetic studies of fault rocks from the Laohushan segment of the Haiyuan fault zone and its tectonic implications
LIU YanJiang1, LIU CaiCai1, ZHANG ZhuQi1, LIU Kang1, REN ZhiKun2, ZHANG HuiPing1, LI ChuanYou2, XU HongYan1, LI XueMei1
1. State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China; 2. Key Laboratory of Active Tectonics and Volcano, Institute of Geology, China Earthquake Administration, Beijing 100029, China
Abstract：Fault rocks, especially fault gouge, have recently attracted more and more interests of researchers, but formation mechanism of its magnetic abnormality has not been decided yet. Outcrops of fault rocks with a thickness of several meters to a dozens of meters, has been found on the Jingtai segment of Haiyuan Fault. These fault rocks are good research materials. A fault rock profile in Laohushan mountain of the Jingtai segment has been investigated in this study. Magnetic susceptibility (χ), anhysteresis remanent magnetization (ARM), saturation isothermal remanent magnetization (SIRM), isothermal remanent magnetization (IRM) and magnetic susceptibility versus temperature curve (χ-T curve) and other magnetic parameters combined with particle size, carbon content and XRD analyses have conducted on the fault rocks to explore the formation mechanism and tectonic implication of different colored fault rocks in the Laohushan section of the Haiyuan fault zone. Rock magnetic results show that red, black, and variegated fault gouge show low magnetic susceptibility compared with the surrounding rock and fracture zone, especially the black fault gouge with a mean magnetic susceptibility of less than 10×10-8 m3·kg-1. The thermalmagnetic results combined with carbon content and mineral phase analyses indicate that the black fault gouge has a similar mineral composition to the Carboniferous coal near the fault zone. It is inferred that the Carboniferous coal is the parent rock of the black fault gouge. The Carboniferous coal was involved into the fault zone by active fault, and was continuously grinded under the strong shearing and friction of the fault, forming clay minerals such as illite and promoting the chemical transformation of a part of the paramagnetic iron-containing silicate mineral or other iron-containing minerals. This makes the magnetic susceptibility higher in the black fault gouge compared with the diamagnetism of their parent rock, the carbon layer. The maximum temperature is about 420 ℃, absolutely no more than 450 ℃, when the black fault gouge was formed, calculated from comparing the ferri-χ of the gouge with the χ-T curve of the coal. The presence of this high-carbon and thick fault gouge may be related to the shallow creep phenomenon on this fault.
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