西藏羌塘裂谷带假玄武玻璃的发现及其岩石学特征与构造意义

假玄武玻璃是地震过程中断层滑动产生摩擦熔融并快速冷却的产物,被誉为"地震化石",记录了断层带内地震发生时的物理-化学过程的相关信息,是认识断裂活动历史的重要物质组成。本文以青藏高原腹地羌塘地块SN向正断层——东依布茶卡断层中的假玄武玻璃和碎裂岩等为研究对象,借助偏光显微镜、扫描电镜、粉末X射线衍射分析、原位XRF分析等多种方法对其进行了详细的结构、构造、矿物成分及化学元素分布的分析。野外调查表明该断裂带中假玄武玻璃呈深灰-褐色,与超碎裂岩和碎裂岩相伴生,以断层脉、注入脉和网状脉的形式产出,厚度由几毫米至一厘米不等。显微结构观察显示,假玄武玻璃中残留有摩擦熔融形成的特征构造,如石英的港湾状熔蚀边、蜂窝状气孔构造以及局部发育的长柱状微晶、束状微晶集合体等,指示其为断层滑动摩擦熔融的产物。此外可见假玄武玻璃作为碎屑出现在碎裂岩和角砾岩中,且后期的碎裂岩中包含早期形成的碎裂岩的角砾等,这些特征表明在该断裂带内大地震事件多次重复发生。以该地区正断层初始活动时限13.5Ma、碎裂岩和假玄武玻璃形成深度为10km以下计算,这些断裂岩石从地下深部折返的速率至少为~0.74mm/yr。目前国际上鲜有报导正断层形成的假玄武玻璃,这是我们首次在羌塘地块SN向裂谷正断层中发现熔融成因的假玄武玻璃,对认识区域地震活动及断裂带演化过程具有重要意义。

Discovery of the pseudotachylytes in the Qiangtang Rift, Tibet, and their petrological characteristics and tectonic significance

Tectonic pseudotachylytes are produced by rapid sliding and melting, and then solidified fast in faults during earthquakes, which are considered as earthquake fossils. Pseudotachylytes record the physical-chemical processes related to earthquakes in a fault zone, which are essential materials for understanding the history of fault activity. In this paper, we focus on the pseudotachylytes and cataclastic rocks in the East Yibug Caka fault, SN-trending normal fault in the Qiangtang terrane, in the hinterland of the Tibetan Plateau. Combined optical microscope, scanning electron microscope, powder X-ray diffraction (XRD) and in situ X-ray fluorescence (XRF) analyses, their microstructures, mineral compositions and elemental distributions were analyzed in detail. Field investigations show that the dark gray to brown in color pseudotachylytes, associated with cataclastic rocks, are occurred as fault veins and injection veins with thickness ranging from a few millimeters to one centimeter. Microstructural observations show that multiple lines of evidence, such as embayed quartz fragments, honeycomb-like vesicles and locally developed microcrystallines and cluster aggregates, indicate that the pseudotachylytes were the products of frictional melting during seismic slip. In addition, pseudotachylytes present as clasts in cataclastic rocks and fault breccias, and younger cataclastic rocks contain breccia of earlier cataclastic rocks. These characteristics indicate that large seismic events occurred repeatedly in this fault zone. Considering the initial active time of the normal faults in this area is 13.5Ma, the formation depth of the pseudotachylytes and associated cataclastic rocks is 10km, the exhumation rate of these fault rocks from deep depth is at least ~0.74mm/yr. Pseudotachylytes along normal faults are seldom reported, this is the first time that we find melt-originated pseudotachylytes in the SN-trending normal faults in the Qiangtang Terrane, which is of great importance for learning regional seismic activity and fault evolution process.