Repeated seismic slips recorded in ultracataclastic veins along active faults of the Arima-Takatsuki Tectonic Line
Repeated seismic slips recorded in ultracataclastic veins along active faults of the Arima-Takatsuki Tectonic Line
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发表时间：
2013
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通讯作者：
K. Yamashita
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作者：
K. Yamashita
众所周知，断裂带内地震的直接证据仅限于假玄武岩的存在(例如，Lin，2008)。除假玄武岩外，先前的研究表明，缺乏寄主岩石原生凝聚力的碎裂脉的中、微观构造特征，包括挤压成因的假玄武岩、断层泥、断层角砾岩和一些方解石脉，可能是孕震断裂带内反复地震滑动造成脆性变形的主要证据(例如，Lin等，2012,2013a，b)。另据报道，在2008年MW7.8汶川地震期间，沿着地震滑移面产生了超碎裂岩脉，并将其注入孕震断裂带内的裂缝中(Lin，2011)。因此，对碎裂脉的研究将为研究孕震断裂带记录的地震滑移的变形过程提供新的思路。本文报道了日本西南部ARIMA-Takatsuki构造线(ATTL)活动断裂中典型的超碎裂岩脉的结构模式，包括挤压成因的假玄武岩和断层泥脉，它们在沿着活动断裂的断裂带内反复形成简单的脉和复杂的网络。文中还从地震断裂事件的角度讨论了这类矿脉的形成机制及其构造意义。野外调查结合中微构造分析表明，在日本西南部ATTL的活动断裂带(<；150m宽)内，广泛发育着大量的超碎裂脉，呈单脉、复杂透镜和网状分布。这些脉主要由挤压成因的假刚玉岩脉和弱固结至松散的断层泥组成，呈黑色、暗棕色、棕色、灰色和褐红色。中微构造特征表明，这些假玄武岩和断层泥形成的脉网是在多个阶段形成的，早期脉一般被年轻脉切割并叠加，表明成脉事件反复发生，超碎屑物质被注入断裂带的断层和裂缝网络中。假玄武岩和断层泥脉以超fiNe至fiNe颗粒基质和不同大小(从亚微米到毫米)的寄主花岗岩的角度到亚角碎屑为特征。扫描电子显微镜-能谱分析和粉末X射线衍射分析表明，所有超碎裂岩脉均以石英和长石为主的结晶物质为特征，与寄主花岗岩相似。这些结果支持了现有的假设，即在地震滑动过程中，由粉碎形成的超fiNe-tofiNe颗粒物质可以被fl活化并快速注入到远离源断层面处的裂缝网络中，因此，这种物质提供了孕震断裂带内反复发生的古地震断裂事件的记录。
It is well known that direct evidence of earthquakes within fault zones is limited to the presence of pseudotachylyte (e.g., Lin, 2008). In addition to pseudotachylyte, previous studies have shown that the meso- and microstructural features of cataclastic veins that lack the primary cohesion of the host rocks, including crush-origin pseudotachylyte, fault gouge, fault breccia and some calcite veins, may represent primary evidence of brittle deformation caused by recurrent seismic slip within seismogenic fault zones (e.g., Lin et al., 2012, 2013a,b). It has also been reported that during the 2008 Mw 7.8 Wenchuan earthquake, ultracataclastic veins were produced along the seismic slip plane and injected into fractures within the seismogenic fault zone (Lin, 2011). Therefore, studies on cataclastic veins would provide new insights into the deformation process of seismic slip recorded in seismogenic fault zones. In this study, we report on the structural mode of typical ultracataclastic veins including crush-origin pseudotachylyte and fault gouge veins that formed repeatedly as simple veins and complex networks within a fault zone along active faults of the Arima-Takatsuki Tectonic Line (ATTL), southwest Japan. We also discuss the formation mechanisms of such veins and their tectonic significance in terms of seismic faulting events. Field investigations, combined with meso- and microstructural analyses, reveal that numerous ultracataclastic veins are widely developed within a fault zone ( < 150 m wide) as simple veins, complex lenses, and networks, along active faults of the ATTL, southwest Japan. These veins comprise mainly crush-origin pseudotachylyte vein and weakly consolidated to unconsolidated fault gouge that is black, dark-brown, brown, gray, and brownish-red in color. Meso- and microstructural features show that these pseudotachylyte and fault gouge veins and networks formed during multiple stages, as earlier veins are generally cut and overprinted by younger veins, indicating that the vein-forming events occurred repeatedly and that ultracataclastic material was injected into networks of faults and fractures in the fault zone. The pseudotachylyte and fault gouge veins are characterized by an ultrafine- to fine-grained matrix and angular to subangular fragments of host granitic rocks of various sizes, ranging from sub-micron to millimeters. SEM-EDS and powder X-ray diffraction analyses show that all the ultracataclastic veins are characterized by crystalline materials composed mainly of quartz and feldspar, similar to the host granitic rocks. The present results support the existing hypothesis that ultrafine- to fine-grained materials formed by comminution can be fluidized and injected rapidly into fracture networks located far from the source fault plane in a solid-fluid-gas system during seismic slip; therefore, such materials provide a record of paleoseismic faulting events that occurred repeatedly within the seismogenic fault zone.