Carbon and oxygen isotope fractionation of carbonate rocks in the fault zone of Wenchuan earthquake:Implications for the mechanism of fault healing
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摘要:
汶川地震断层带北川擂鼓镇赵家沟剖面的露头及显微结构均发现多期次脉体纵横交错，角砾岩被胶结。通过对断层岩相关的碳酸盐矿物同位素分析得知，断层岩角砾和脉体中大量的白云石来源应是断层带内富Mg离子的流体，且碳氧同位素显著分异，角砾的δ18O值和δ13C值与灰岩围岩更接近，脉体和基质显示重同位素亏损。通过“同震热分解”和“水-岩相互作用”2种可能模型的研究分析，同震热分解模型δ13C值明显高于实际，而水-岩相互作用则可形成这种分异结果。故震后深部流体上涌所导致的表层大气水再循环可能是导致震后断层快速愈合的重要原因，同震破裂和间震期愈合则形成完整的断层系统。
Abstract:The outcrop and microscopic structure analysis of Zhaojiagou section at Leigu Town in Beichuan area of Wenchuan earthquake fault zone revealed that multi-phase veins crisscross and the breccia has been cemented.The isotope analysis of carbonate minerals related to fault rocks shows that the source of a large amount of dolomite in the fault breccia and veins should be the Mgrich fluid in the fault zone, and the carbon and oxygen isotopes exhibit significant differentiation.The δ18O and δ13C values of breccia are more close to values of surrounding rocks of limestone, and the veins and matrix exhibit heavy isotope losses. It is found that the δ13C values of the coseismic thermal decomposition model is obviously higher than the real values and the water-rock interaction model can form this differentiation result, as shown by comparison of these two possible models.Therefore, the surface water recirculation caused by the upwelling of deep fluids may be the significant cause of the fault rapid healing after earthquake. The coseismic rupture and inter-seismic healing form a complete fault system.
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Keywords:
- Wenchuan earthquake /
- fault zone /
- carbonate rock /
- isotope /
- fault healing
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致谢: 中国地震局地质研究所陈建业博士在研究课题上给予了指导，山西省地质环境监测中心刘瑾高级工程师在论文修改过程中提出了宝贵意见和建议，审稿专家提出宝贵的修改意见，在此一并表示衷心的感谢。
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图 1 龙门山断裂带地质简图及工作区位置[12]
Figure 1. Geological sketch map and working area location of the Longmen Mountain fault zone
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图 3 同位素分析取样的典型标本(图中黑色、红色及蓝色箭头所指分别为脉体、角砾和基质部分)
Figure 3. Typical samples for isotopic analysis
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图版Ⅰ
a.断层泥和断层角砾岩交界部位；b.典型断层角砾岩；c.断层泥空隙中方解石充填；d.角砾岩为流体高度改造，可见不同期次脉体；e、f.胶结的角砾岩中多期次脉体相互穿插；g.胶结的断层泥和细碎裂岩；h.伟晶脉体，后期产生微裂隙又被愈合
图版Ⅰ.
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图 4 不同部位样品碳氧同位素测试结果
Figure 4. Carbon and oxygen isotope analytical results of different parts
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图 5 基于同位素结果的模拟分析
（黑色粗线为基于同震热分解模型的计算结果；黑色细线及阴影区为水-岩相互作用模型的计算结果）
Figure 5. Simulation analysis based on isotope results
表 1 北川擂鼓镇赵家沟剖面断层岩成分分析结果
Table 1 Fault rock composition analytical results of Zhaojiagou section at Leigu Town, Beichuan area
岩性 全岩成分 粘土组成（相对含量） 石英 长石 白云石 方解石 粘土 蒙脱 伊利 伊/蒙 绿泥 混层比 上盘灰岩原岩 0.5% 0.5% 98% 1% 96% 4% 上盘粗角砾岩 1% 1% 29% 68% 1% 95% 5% 5 上盘细角砾岩 24% 7% 11% 40% 18% 39% 55% 6% 10 灰色断层泥 29% 2% 27% 7% 35% 3% 83% 14% 36 白色伟晶脉体 6% 1% 37% 51% 5% 3% 30% 55% 12% 10 下盘细角砾岩 6% 62% 24% 8% 19% 73% 8% 33 下盘粗角砾岩 42% 33% 8% 17% 51% 49% 5 下盘砂岩原岩 43% 39% 9% 9% 47% 53% 注:混层比为伊蒙混层中蒙脱石的含量 
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