THE RHEOLOGICAL STRUCTURES OF BRITTLE-PLASTIC TRANSITION IN LONGMENSHAN FAULT ZONE AND SEISMOGENIC MECHANISM OF WENCHUAN EARTHQUAKE

doi:10.3969/j.issn.0253-4967.2014.03.025

Abstract

Abstract:

The seismogenic fault of Wenchuan earthquake is a high-angle reverse-slip fault. It is necessary for the sliding of such a high-angle reverse fault and the triggering of the Wenchuan earthquake on it to have special mechanical conditions at the deep part of fault. In this study, we investigated the deformation mechanism of cataclastic-mylonite rocks in ductile shear zones found in the Yingxiu-Beichuan Fault. The deformation temperature and the flow stress of brittle-plastic transition of fault were estimated by the deformation fabrics of quartz. The water contents and the distribution of major minerals in mylonite were measured using Fourier transform infrared spectroscopy(FTIR). The fluid inclusions were measured using Raman and microprobe. The rehological structures of brittle-plastic transition of the Longmenshan Fault zone under different fluid pressure and strain rate conditions were constructed to discuss the role of the high fluid pressure in the seismogenic and occurrence mechanics of Wenchuan earthquake.
The studies showed that inhomogeneous ductile deformation occurred in the brittle-plastic transition of the Yingxiu-Beichuan Fault. The complex deformation characters of quartz display different deformation temperatures in the ductile shear zone. The quartz in fine-grained mylonite was deformed by the grain boundary migration and recrystallization, implying the deformation temperature is from 500 to 700℃. The quartz in porphyroclastic mylonite was deformed by the subgrain rotation and recrystallization, implying the deformation temperature is from 400 to 500℃. The earlier quartz veins and healed cracks were deformed by the bulges and recrystallization, implying the deformation temperature is from 280 to 400℃. The later quartz veins which cut the earlier quartz veins were deformed by the cataclastics, indicating the deformation temperature is from 150 to 250℃. The deformation of quartz shows that the ductile shear zone experienced multi-phase brittle-ductile transitions. Based on the grain size of recrystallized quartz, the ductile flow stress of the fault is estimated to be 15～80MPa. The trace amount water in quartz and feldspar exists in the forms of hydroxyl in crystals, grain boundaries water and fluid inclusions water, and the water contents are higher with increasing strain of rocks, with a changing range from 0.01wt% to 0.15wt%. A lot of secondary fluid inclusions were found in the quartz in the brittle-plastic transition of fault, which were captured during crack healing. Based on measurement of the fluid inclusions, the capture temperature of the fluid inclusions is from 330 to 350℃, and fluid pressure is about 70～405MPa, the corresponding fluid pressure coefficient is estimated to be from 0.16 to 0.9, which stands for the characters of fluid inclusions captured during cracks healing process related with co-seismic and postseismic slip of fault.
Rheological structure was constructed based on the analysis data and flow law of wet quartz, as well as variation of fluid pressure and strain rate during periods of inter-seismic to earthquake nucleation, and after-slip to post seismic. Rheological structure shows that the strength of fault and depth of brittle-plastic transition change with strain rate and fluid pressure during inter seismic, earthquake nucleation, and after-slip period, and the depth of brittle-plastic transition is fit to the deformation mechanism of quartz, and the depth of transition of velocity weakening to strengthening of fault slip, as well as the focal depth of Wenchuan earthquake, which display that the Yingxiu-Beichuan Fault has the probability of weakening of sliding velocity and qualification of earthquake nucleation. However, the existing high fluid pressure in fault could be the most important factor for the high-angle reverse fault slip and triggering the Wenchuan earthquake.

Key words: Wenchuan earthquake, brittle-plastic transition, rheological structures, high fluid pressure

摘要：

汶川地震发震断层为高角度逆断层，这种断层滑动和发生强震需要断层深部具备特殊的力学条件。发震断层地区地表出露若干韧性剪切带，其中不同类型石英变形具有不同的变形温度。细粒糜棱岩中的石英表现为高温位错蠕变，变形温度为500～700℃；含残斑初糜棱岩中的石英表现为中温位错蠕变，其变形温度为400～500℃；早期石英脉中的石英表现为低温位错蠕变，变形温度为280～400℃；晚期石英脉以碎裂变形为主，其变形温度为150～250℃。石英的这些变形特征显示出断层带经历了多期脆-塑性转化。根据糜棱岩中的重结晶石英的粒度估计的断层塑性流动应力为15～80MPa。石英和长石内的微量水以晶体缺陷水、颗粒边界水和流体包裹体水的形式存在，水含量随岩石的应变增加而升高，变化范围为0.01～0.15wt%。断层脆-塑性转化带内石英含有大量与裂隙愈合相关的次生流体包裹体，其捕获温度为330～350℃，流体压力为70～405MPa，估计的流体压力系数为0.16～0.9，代表强震发生后，断层带内产生的大量微裂隙逐渐愈合过程中的流体特征。在考虑断层带流体压力和应变速率变化条件下，利用石英流变参数建立了从间震期到地震成核阶段断层脆-塑性转化带流变结构和震后快速蠕滑阶段断层脆-塑性转化带流变结构。结果表明，在间震期、地震成核阶段、震后快速滑动阶段，断层强度和脆-塑性转化深度随应变速率和流体压力变化而变化，且脆-塑性转化特征与石英的变形机制、断层速度弱化和强化转化深度、汶川地震震源深度等吻合，显示映秀-北川断层具备摩擦滑动速度弱化和地震成核的基础，而断层带内存在高压流体可能是触发高角度逆断层滑动和汶川地震发生的主要机制。

关键词: 汶川地震, 脆-塑性转化带, 流变结构, 高压流体

CLC Number:

P313

ZHOU Yong-sheng, HAN Liang, JING Chen, HE Chang-rong, DANG Jia-xiang. THE RHEOLOGICAL STRUCTURES OF BRITTLE-PLASTIC TRANSITION IN LONGMENSHAN FAULT ZONE AND SEISMOGENIC MECHANISM OF WENCHUAN EARTHQUAKE[J]. SEISMOLOGY AND GEOLOGY, 2014, 36(3): 882-895.

周永胜, 韩亮, 靖晨, 何昌荣, 党嘉祥. 龙门山断层脆-塑性转化带流变结构与汶川地震孕震机制[J]. 地震地质, 2014, 36(3): 882-895.

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