Shear-induced graphitization of carbonaceous materials during seismic fault motion: Experiments and possible implications for fault mechanics
Shear-induced graphitization of carbonaceous materials during seismic fault motion: Experiments and possible implications for fault mechanics
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DOI：
10.1016/j.jsg.2011.01.007
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发表时间：
2011-06
影响因子：
3.1
通讯作者：
K. Oohashi;T. Hirose;T. Shimamoto
中科院分区：
文献类型：
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作者：
K. Oohashi;T. Hirose;T. Shimamoto
碳质物质往往集中在泥质岩中发育的断裂带中。在含碳矿物中，石墨被认为是润滑剂，可能在断层的摩擦特性中起关键作用。从滑动局部带中发现的石墨表明，石墨化作用可能发生在孕震断层运动过程中。因此，我们进行了摩擦实验无定形碳和石墨，以调查石墨的形成与断层运动和这些碳质矿物如何影响断层的摩擦性能。实验在空气和氮气气氛中进行，法向应力为0.5- 2.8MPa，滑移率为50 - 1.3m/s。XRD和TEM分析表明，石墨化确实可以发生在孕震断层运动可能是由于大的剪切应变，短暂的闪光加热和应力集中在凹凸接触，即使在低温和缺氧环境下的压力。在低滑移率下，石墨（μss= 0.1）和无定形碳（μss= 0.54）的稳态摩擦系数μ ss相差很大。当滑移率大于10 mm/s时，非晶碳的速度明显减弱，当滑移率为1.3 m/s时，非晶碳的稳态摩擦力降低到与石墨相同的水平。无定形碳断层在低滑动速率下并不弱，但在大地震发生期间，它们可以变得动态弱以促进断层运动。断层带中的富集石墨可以在所有滑动速率下润滑，即使在很深的地方也是如此，因此应该得到更多的关注。
Carbonaceous materials often concentrate in fault zones developed in pelitic rocks. Among carbonaceous minerals, graphite is known as a lubricant and possibly plays a key role in frictional properties of the fault. Graphite reported from slip localized zones suggests that graphitization can occur during seismogenic fault motion. Thus, we performed friction experiments on amorphous carbon and graphite to investigate how graphite forms in association with fault motion and how these carbonaceous minerals affect frictional properties of faults. Experiments were done at normal stresses of 0.5–2.8 MPa and slip rates of 50 μm/s to 1.3 m/s in atmospheres of air and N2gas, using rotary-shear apparatuses. XRD and TEM analyses revealed that graphitization can indeed occur during seismogenic fault motion perhaps due to large shear strain, short-lived flash heating and stress concentration at asperity contacts, even at low temperatures and pressures under anoxic environments. We found large differences in steady-state friction coefficientμssbetween graphite (μss= 0.1) and amorphous carbon (μss= 0.54) at low slip rate. But amorphous carbon exhibits marked velocity weakening at slip rate above 10 mm/s, and its steady-state friction reduces to the same level as that of graphite at a slip rate of 1.3 m/s. Faults with amorphous carbon are not weak at low slip rates, but they can become dynamically weak to foster fault motion during the generation of large earthquakes. Enriched graphite in fault zones can lubricate at all slip rates even at great depths and should receive more attention.