龙门山断层带内方解石机械双晶研究及对断层摩擦系数启示

doi:10.3969/j.issn.0253-4967.2025.01.010

摘要/Abstract

摘要：

摩擦系数作为断层滑动的重要参数, 对断层稳定性和地震危险性评估具有指示意义, 摩擦系数的获取是断层研究的重要内容之一。文中通过统计龙门山断裂带地表断层岩中的方解石机械双晶密度, 估算了断层岩所承受的历史最大差应力, 进一步计算了断层的滑动摩擦系数。统计结果显示, 断层岩中的方解石双晶密度随着与滑动面距离的减小呈现明显的升高趋势, 在破碎带内为(87.39±35)mm^-1, 在滑动面附近为(218.63±36)mm^-1, 计算得到的差应力分别为(182.28±25)MPa和(288.30±25)MPa, 该特征表明断层滑动面附近持续承受差应力。用于双晶统计的样品采集于灰岩质断层岩中, 以往的地质调查资料显示这些断层岩的历史最大埋深约为5km。综合上述深度和差应力值, 以及前人对该区域构造应力的研究结果, 计算得到龙门山断裂带在水平和逆冲滑动时的稳态摩擦系数分别为0.61和0.13。上述计算采用的应力值为断层岩所承受的历史最大应力, 因而给出的摩擦系数为稳态滑动摩擦系数的上限。该结果与实验观察结果及科学钻探研究结果一致, 表明通过机械双晶估算断层稳态摩擦系数的方法具有可行性。

关键词: 龙门山断裂带, 方解石机械双晶, 双晶密度, 摩擦系数

Abstract:

The friction coefficient, a critical parameter governing fault sliding, plays an essential role in evaluating fault stability and assessing seismic hazards. Its accurate determination is a fundamental aspect of fault research, and traditional methods for obtaining this coefficient typically rely on invasive techniques such as scientific drilling or friction experiments. These conventional approaches are often costly, time-consuming, and limited by their ability to capture fault zone conditions in a comprehensive and non-invasive manner. In contrast, this study proposes a more straightforward and practical approach for estimating and constraining the sliding friction coefficient of the Longmenshan fault zone, utilizing statistical analysis of calcite mechanical twin crystal density as an indirect indicator of fault slip behavior.

Calcite mechanical twin crystals, which form as a result of faulting processes, are sensitive to the temperature and pressure conditions within the formation environment. The microstructural characteristics of these twin crystals record the maximum stress experienced by different parts of the fault zone, thereby providing a valuable record of fault activity. As a result, stress values near the fault sliding surface can be used to indirectly estimate the friction coefficient of the fault. To achieve this, high-definition images of calcite twin crystals were captured under an optical microscope, followed by processing to derive the twin crystal density values. These measurements were then used in conjunction with corrected stress meter results from prior research, which were obtained through triaxial compression and torsion experiments. The twin crystal density values were incorporated into calculations of the differential stress experienced by the fault zone, and, using the known relationship between differential stress and friction, the friction coefficient was derived. To ensure the accuracy of the results, stress values for the surrounding rock were provided through regional geological survey data.

The statistical analysis revealed that the twin crystal density within the sliding zone increased markedly as the distance from the sliding surface decreased. Specifically, the twin crystal density increased from (87.39±35)mm^-1 in sample DS-1 to (218.63±36)mm^-1 in the immediate vicinity of the sliding surface. The corresponding historical maximum differential stresses were calculated based on previous stress gauge corrections, yielding values of (182.28±25)MPa and (288.30±25)MPa, respectively. Additionally, the maximum burial depth of the exposed strata was estimated to be approximately 5km, which led to the determination of confining pressure (S_V-P₀) of 83.3MPa on the outcrop. Here, S_V represents the stress of the overlying rock layers, while P₀ denotes the pore pressure. Using these values, the friction coefficients of the Longmenshan fault zone during horizontal and reverse faulting were calculated to be 0.61 and 0.13, respectively. These calculated values correspond to the upper limit of the sliding friction coefficient for the fault zone.

This study focuses specifically on the analysis of type Ⅱ mechanical twin crystals of calcite found in the surface outcrops of the Longmenshan fault zone. A particular emphasis is placed on the statistical analysis of twin crystal density the calculation of differential stress and friction coefficients. By integrating previous geological survey data and research on tectonic stress and fault structure within the study area, the calculated friction coefficient was found to range from 0.13 to 0.61. These results are in close agreement with earlier experimental and observational studies, supporting the feasibility and reliability of the method used to estimate fault zone friction coefficients. Based on calcite mechanical twin density, this approach presents a viable, non-invasive, and effective means of estimating friction coefficients in fault zones, with significant implications for understanding fault behavior and seismic risk assessment.

Key words: Longmenshan fault zone, mechanical twinning of calcite, twins density, friction coefficient

崔娅琪, 党嘉祥, 周永胜. 龙门山断层带内方解石机械双晶研究及对断层摩擦系数启示[J]. 地震地质, 2025, 47(1): 150-166.

CUI Ya-qi, DANG Jia-xiang, ZHOU Yong-sheng. MECHANICAL TWINNING OF CALCITE IN THE LONGMENSHAN FAULT ZONE AND ITS IMPLICATIONS FOR FAULT FRICTION COEFFICIENT[J]. SEISMOLOGY AND GEOLOGY, 2025, 47(1): 150-166.

图/表 7

图1 龙门山断层区域地质图与赵家沟剖面的位置图(修改自Dang et al., 2021)

Fig. 1 Regional geological map of Longmenshan Fault and location map of Zhaojiagou outcrop(Dang et al., 2021).

图2 露头照片及采样示意图箭头指示断层的错动方向

Fig. 2 Outcrop photo and sampling diagram.

图3 断层受力模型图(Rybacki et al., 2011) SH最大水平应力; Sh最小水平应力; SV上覆岩层应力; λ 断层滑动角; γ 断层倾角; β 断层走向与SH的锐夹角

Fig. 3 Stress model diagram of fault(Rybacki et al., 2011).

图4 破碎带内的方解石脉和双晶微观图红色箭头指示双晶的楔形端部, 黑色方框为双晶纹未贯穿的双晶颗粒

Fig. 4 Microscopic map of calcite veins and twinning in the fracture zone.

图5 断层核内的方解石脉和双晶微观图(临近滑动带样品) 带箭头的线段指示同一颗粒内展布的2组不同方向的双晶; 红色箭头指示双晶的楔形端部, 黄色箭头指示台阶状边缘, 黑色方框为双晶纹未贯穿的双晶颗粒

Fig. 5 Microscopic map of calcite veins and twinning in fault core(sample near the sliding zone).

表1 分析样品双晶密度与所受差应力统计计算结果

Table1 The statistical results of twin density and differential stress were analyzed

样品	双晶密度/mm^-1	平均双晶密度/mm^-1	差应力/MPa
DS-1	125.156	87.39±35	182.28±25
	68.212
	52.438
	68.923
	64.851
	70.247
	85.249
	93.196
	67.218
	81.337
	130.039
	72.122
	100.313
	87.655
	74.914
	156.512
DS-2	78.186	156.60±66	244.01±47
	60.643
	69.881
	89.286
	141.326
	80.221
	211.416
	162.966
	144.849
	208.656
	183.415
	387.584
	207.809
	137.497
	164.331
	62.181
	168.646
	125.896
	128.874
	123.154
	114.661
DS-2	165.837	156.60±66	244.01±47
	238.542
	274.369
	138.648
	185.529
	173.913
DS-4	207.970	218.63±86	288.30±25
	182.815
	204.167
	204.918
	293.255
DS-5	128.103	152.26±54	240.60±60
	218.818
	162.317
	202.533
	189.673
	98.328
	109.518
	108.814

图6 断层带不同位置双晶密度值及差应力值分布图横坐标零点指示断层滑动带中心, 负值为断层上盘, 正值为断层下盘

Fig. 6 Distribution of twins density and differential stress at different positions of fault zone.

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