Abstract:

The heterogeneity of fault gouges and tectonites is widely recognized as a key factor influencing variations in fault strength and instability during slip events. However, the specific role of gouge heterogeneity during coseismic slip, particularly in modulating slip behavior, strength, and fault stability, remains poorly understood.

To investigate the influence of heterogeneous gouges with differing frictional properties on fault strength, we selected dolomite and quartz—two minerals with contrasting frictional behaviors—and prepared synthetic quartz-dolomite gouge mixtures in varying proportions. A series of rotary shear experiments were conducted across a range of slip velocities(0.000 1~1.0m/s) under a constant normal stress of 1MPa.

At a slip velocity of 0.1m/s, all gouge mixtures exhibited broadly similar frictional behavior, marked by a drop in friction coefficient from peak values(μ_P=0.85-1.0) to steady-state values(μ_ss=0.5-0.6) via slip-weakening. In contrast, at 1.0m/s, the frictional response became more complex, displaying a two-stage weakening pattern: an initial weakening phase, followed by transient frictional recovery, and then a secondary weakening phase. The steady-state friction coefficient(μ_ss)exhibited a non-monotonic relationship with quartz content, peaking at 0.43 for 20wt%quartz, then decreasing with further quartz addition. Concurrently, the slip-weakening distance(D_p) increased exponentially with quartz content, from 4.27m to 13.24m.

At slip velocities ≥0.01m/s, gouges containing 20wt%quartz consistently showed slip-weakening behavior. Compared to monomineralic carbonate gouges, this mixture displayed similar μ_ss values at ≤0.1m/s, but significantly higher μ_ss values(0.33-0.43) at higher velocities(0.5~1.0m/s), diverging from the typical exponential decay trend of carbonate gouges.

Microstructural analyses revealed the development of a 5~10μm-thick nanoparticle layer on the slip surface at high velocities(≥0.5m/s), with a top layer composed of micro-to nano-sized particles(50~200μm thick). In samples with low quartz content(≤20wt%), abundant melt patches(10~20μm in diameter)were observed near the slip surface. At 30wt%quartz, these evolved into interconnected melt films or layers.

We infer that during high-velocity slip, frictional melting of quartz and thermal decomposition of dolomite occur. The initial weakening stage is dominated by nanoparticle lubrication and flash heating, while the subsequent partial strength recovery results from limited quartz melting and nanoparticle bonding. As slip progresses, accumulation of SiO₂ melt forms a continuous film or layer, triggering the second weakening phase via melt lubrication.

These results demonstrate that fault gouge heterogeneity—particularly the presence of frictionally strong and weak minerals—can significantly affect fault frictional behavior during coseismic slip(~1m/s). In carbonate-dominated fault zones, even small amounts of quartz(≤20wt%) can suppress dynamic weakening and enhance fault strength, which has important implications for understanding rupture propagation and stability in natural fault systems.

Key words: heterogeneity, slip weakening, high-velocity frictional experiment, quartz-bearing dolomite gouge

摘要：

断层泥的非均质性是影响或控制断层滑动过程摩擦强度变化的重要因素之一, 但同震滑移过程中断层泥的非均质性在控制断层滑动行为、 摩擦强度和速度依赖性等方面的效应仍不清楚。文中采用人工合成的双矿物相断层泥(石英和白云石以不同比例混合), 在1MPa正应力条件下, 开展了低速—高速(0.000 1~1.0m/s)旋转剪切摩擦实验。结果表明, 在滑动速率为1.0m/s的高速滑动实验中, 石英含量不同的白云石断层泥的摩擦行为呈现出显著差异, 并表现出2个阶段弱化的特征; 随石英含量的增加, 白云石断层泥的稳态摩擦系数μ_ss值呈现出先增加后降低的趋势, 而滑动弱化距离D_c值则呈现出指数型增加趋势。微观构造观测结果显示, 高速滑动(≥0.5m/s)时, 主滑动面发育5~10μm厚的纳米颗粒层; 在石英含量≤20wt%的试样中, 主滑动面附近发育熔体斑块; 当石英含量增加到30wt%时, 主滑动面附近出现相互连通的熔体薄膜或熔体层。总结认为, 含石英组分的白云石断层泥在高速滑动过程中, 纳米颗粒润滑和瞬时生热作用共同主导了第1阶段的弱化, 随后主滑动面上石英颗粒的少量熔融和纳米颗粒黏结效应, 导致摩擦强度得以部分恢复。当SiO₂熔体含量不断增加并形成熔体薄膜/熔体层时, 熔体润滑作用导致了第2阶段的滑动弱化。文中实验结果表明, 在同震滑动(m/s级)过程中, 具有不同摩擦特性的非均质断层泥对断层摩擦强度起着重要的控制作用, 自然界中碳酸盐岩断层岩内含有少量的石英(≤20wt%)有助于抑制同震滑动时的断层弱化。

关键词: 非均质性, 断层弱化, 高速摩擦实验, 含石英的白云石断层泥