汶川地震假玄武玻璃的特征、成因及其构造意义

Characteristics,genesis and tectonic significance of the pseudotachylyte produced by the Wenchuan earthquake

富流体的断层泥是浅部地震断层带中的特征岩石。一般认为,地震过程中摩擦热会导致粒间孔隙流体热膨胀增压,形成同震断层弱化(热增压机制),从而抑制摩擦熔融的发生。然而我们研究发现,在2008年汶川大地震(M_(W) 7.9)中断层浅部发生了摩擦熔融。汶川地震发生一年后,我们在汶川地震断裂带科学钻探项目一号钻孔(WFSD-1)732.6m深处的断层泥中发现了厚度约2mm的假玄武玻璃(凝固的摩擦熔融物)。该假玄武玻璃形成的位置极浅,且产生于非固结的、富流体的断层泥中。从岩心来看,断层面可见镜面构造和同震擦痕。微构造分析显示,该假玄武玻璃主要由石英碎屑和由长石与黏土矿物熔融的非晶质基质组成,基质中发育众多不规则的微裂隙,并可见流动构造。化学成分分析显示,其基质富Ba且被重晶石(BaSO 4)小细脉切割,为同震及震后流体存在的证据。由于假玄武玻璃在流体存在的条件下会快速蚀变,且龙门山地区大地震复发周期为3000~6000年,因而这些完全未被蚀变的新鲜假玄武玻璃可能是最近一次大地震,即2008年汶川大地震的产物。针对钻孔中断层泥进行的高速摩擦试验,证实了在钻孔732m深度发生地震滑动的条件下确实会产生假玄武玻璃。因此WFSD-1钻孔732.6m的假玄武玻璃被认为是汶川地震的产物,代表了汶川地震主滑动带位置。这是首次在自然界中发现浅部含水断层泥摩擦熔融形成假玄武玻璃的实例,不仅对认识汶川大地震的发震机制、断层强度、应力迁移与破裂传播具有重要意义,而且有助于我们认识断裂带内浅部脆性区域的力学属性、岩石变形环境和变形机制,促进对地震滑动机制和破裂过程的认识。

Fluid-rich gouges are typical rocks at shallow depth in seismogenic faults.It is generally recognized that frictional heat during earthquakes leads to thermal expansion and pressurization of pore fluids,resulting in co-seismic fault weakening(thermal pressurization mechanism),which inhibits the occurrence of frictional melting.However,we show that frictional melting,rather than thermal pressurization,occurred at shallow depths during the 2008 M_(W) 7.9 Wenchuan earthquake,China.One year after the Wenchuan earthquake,an~2mm-thick,glass-bearing pseudotachylyte(solidified frictional melt)was found at 732.6m depth in the first borehole of the Wenchuan Earthquake Fault Scientific Drilling Project(WFSD-1).This pseudotachylyte was formed at an extremely shallow depth and was generated in unconsolidated,fluid-rich fault gouge.Mirror-like structures and co-seismic striations are visible on the fault surface,viewed from the drilling core.The pseudotachylyte consists mainly of quartz fragments and an amorphous matrix.Irregular microcracks are pervasive in the matrix where flow structures are visible.Chemical composition analysis shows that the matrix is Ba-rich and cut by small veins of barite,which provide evidence of co-and postseismic fluid percolation.Because pseudotachylyte can be rapidly altered in the presence of percolating fluids,its preservation suggests that gouge melting occurred in a recent large earthquake.Moreover,since the recurrence interval of large earthquakes in the Longmen Shan area is~3000 years,the completely unaltered pseudotachylyte likely produced in the most recent large earthquake,i.e.the 2008 Wenchuan earthquake.High-velocity frictional experiments on the fault gouge from the drillcore deformed at conditions expected for seismic slip at borehole depths showed the generation of pseudotachylytes.Therefore,the pseudotachylyte at 732.6m in WFSD-1 is considered to be a product of the Wenchuan earthquake and represents the location of the principal slip zone of the Wenchuan earthquake.This is the first report of the pseudotachylyte formed by frictional melting in fluid-rich fault gouge at shallow depth in nature,which is not only of great significance for understanding the seismogenic mechanism,fault strength,stress migration and rupture propagation of the Wenchuan earthquake,but also helps us to recognize the mechanical properties,rock deformation mechanism and environments of the shallow brittle region within the fault zone,and promotes the study of the earthquake faulting mechanisms and rupture processes.