Frictional melting processes during subduction zone earthquakes : Inference from pseudotachylytes and high-velocity shear experiments

在日本西南部岛曼都增生杂岩中发现的含假硅质岩的断裂带，是在地震孕育深度的俯冲或底侵过程中发育的。假玄武岩出现在厚度不到几毫米的黑色细脉中，这些脉被源自混杂岩的叶状碎裂岩清晰地包围着。由于摩擦熔体的快速冷却，假玄武岩的显微结构表现为一种充满碎屑的玻璃支撑结构。透射电子显微镜显示玻璃的存在，在玻璃中局部发育了莫来石的自面微晶。准玄武岩基质的成分和未熔融颗粒和微晶的特征表明，摩擦熔融发生在富伊利石滑移带中，最低熔融温度为1100℃。摩擦熔体的粘度由准黄晶基质组成以及未熔化颗粒的体积分数和长径比计算出来。当滑移速率为1m/℃时，沿1 mm厚熔体的剪切阻力为0.1~0.3 Mpa。富伊利石滑移带中熔融层的形成可能会引起较大的应力降，增加滑移速率，增强破裂传播，共同影响俯冲带的震级。

Pseudotachylyte-bearing fault zones, found in the Shimanto accretionary complex, southwest Japan, developed during subduction or underplating at seismogenic depths. The pseudotachylytes occur in narrow dark veins less than a few millimeters thick that are sharply bounded by foliated cataclasites that originated from a melange. The microstructures of pseudotachylytes are represented by a fragment-laden, glass-supported texture resulting from the rapid cooling of the frictional melt. Transmission electron microscopy reveals the presence of glass in which euhedral microcrystals of mullite are locally developed. The compositions of the pseudotachylyte matrix and the characteristics of the unmelted grains and microlites in the matrix suggest that frictional melting occurred in an illite-rich slip zone with a minimum melting temperature of 1100℃. The viscosities of the frictional melt were calculated from the pseudotachylyte matrix composition as well as the volume fraction and aspect ratio of the unmelted grains. The viscosities at 1100℃ range from 85 to 290 Pa s, and the corresponding shear resistance along a 1-mm-thick melt layer at a slip rate of 1 m/s was 0.1-0.3 MPa. The formation of a melt layer in an illite-rich slip zone can possibly induce large stress drops, increase the slip rate and enhance rupture propagation, which together could affect the earthquake magnitude in a subduction zone.