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Element enrichment/depletion during faulting in shale-rich surface exposures of the 2008 Wenchuan earthquake (Mw 7.9) and implications for coseismic temperature
Australian Journal of Earth Sciences ( IF 1.2 ) Pub Date : 2020-11-30 , DOI: 10.1080/08120099.2021.1847734
Y. Y. Wang 1 , X. Q. Gao 1 , S. J. Li 2 , S. Y. Wang 3 , D. Y. Shi 1, 4 , W. B. Shen 5
Affiliation  

Abstract

The coseismic frictional temperature is increasingly recognised as an important factor for the transformation mechanism in shallow crustal faulting. We report changes in the mineralogical and geochemical compositions across the fault zone in the Shaba (SB) exposure of the 2008 Wenchuan earthquake (Mw 7.9), which contained the maximum value of vertical displacement and cut through shale-rich strata by the principal slip surface. Assuming Yb to be an immobile trace element, with respect to those in the adjacent damaged zone and wall rocks, Li, Rb, Cs and most light rare earth elements (LREE) exhibited marked enrichments of 143%, 247%, 475% and >60%, respectively; and a 40% depletion of Sr and <5% depletion of most heavy rare earth elements (HREE) was found, responding to mineralogical transformation such as alteration and dissolution of feldspar and formation of abundant clay minerals. In addition, heterogeneous adsorption of clay minerals could result in fractionation between and within LREE and HREE as well as negative Eu anomalies and positive Ce anomalies. The element enrichment/depletion model of the northern segment of the Yingxiu–Beichuan (YX–BC) fault zone was quite different from the middle/southern segments characterising faults in carbonate and granitic rocks. Through model calculation, the calculated trace element compositions (Li, Rb, Sr, Cs, Ba, La, Sm and Pb) of the fault gouge were obtained at 250 °C, 300 °C and 350 °C, and were produced by frictional heating. By comparing with the observed values, we suggest that high temperatures derived from frictional heating or due to buffering of the slip zone temperature by evaporation of pore water during the earthquake within the principal slip surface in the northern segment of the YX–BC fault zone may not have existed. Mechanical crushing and chemical alteration (not under high-temperature conditions) within the fault zone during the coseismic stage may result in mass removal and element enrichment/depletion.

  1. KEY POINTS
  2. Mineralogical and geochemical compositions were reported in the Shaba exposure of the 2008 Wenchuan earthquake.

  3. Li, Rb, Cs and most LREE exhibited marked enrichments in the fault gouge.

  4. Fractionation between and within LREE and HREE, negative Eu anomalies and positive Ce anomalies occur in the fault gouge.

  5. High temperatures may not have existed within the principal slip surface of the northern segment of the YX–BC fault zone.



中文翻译:

2008年汶川地震(Mw 7.9)富页岩表面暴露中断层时的元素富集/枯竭及其对同震温度的影响

摘要

同震摩擦温度越来越被认为是浅层地壳断层转变机制的重要因素。我们报告了2008年汶川地震(Mw 7.9)的Shaba(SB)暴露断层带的矿物学和地球化学组成的变化,该变化包含垂直位移的最大值,并通过主要滑动面切穿了富页岩层。假设Yb是不可移动的痕量元素,相对于相邻损坏区和围岩中的元素,Li,Rb,Cs和大多数轻稀土元素(LREE)表现出明显的富集,分别为143%,247%,475%和>分别为60%;发现了40%的Sr损耗和<5%的最重稀土元素(HREE)损耗,响应矿物学转变,例如长石的改变和溶解以及丰富的粘土矿物的形成。另外,粘土矿物的非均质吸附可能导致在LREE和HREE之间以及内部以及内部的分馏,以及负Eu异常和Ce正异常。映秀-北川(YX-BC)断裂带北部段的元素富集/耗竭模型与碳酸盐岩和花岗岩岩石断层特征的中/南部段有很大差异。通过模型计算,得出了断层泥在250°C,300°C和350°C下计算出的微量元素组成(Li,Rb,Sr,Cs,Ba,La,Sm和Pb),并通过摩擦产生。加热。通过与观察值比较,我们认为,在YX–BC断裂带北部段的主滑动面地震中,由于摩擦加热或由于孔隙水蒸发造成的滑动带温度缓冲而导致的高温可能不存在。在同震阶段,断裂带内的机械破碎和化学变化(不在高温条件下)可能导致质量去除和元素富集/耗尽。

  1. 关键点
  2. 在2008年汶川地震的沙巴暴雨中报告了矿物学和地球化学成分。

  3. Li,Rb,Cs和大多数LREE在断层泥中表现出明显的富集。

  4. 在断层泥中发生LREE和HREE之间和内部的分馏,负Eu异常和正Ce异常。

  5. YX-BC断裂带北部段的主滑动面内可能不存在高温。

更新日期:2020-11-30
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